Symposium N: Colloidal Nanoparticles for Electronic Applications--Light Emission, Detection, Photovoltaics, and Transport

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Proceedings to be published in electronic-only format (see MRS Online Proceedings Library at www.mrs.org/opl) as volume 1207E of the Materials Research Society Symposium Proceedings Series.  




November 30 - December 4, 2009

Chairs 

Jonathan Steckel
QD Vision, Inc.
313 Pleasant St.
Watertown, MA 02472
617-607-9703

     

Nicholas Kotov
Dept. of Chemical Engineering
University of Michigan
3074 H.H. Dow Bldg.
2300 Hayward St.
Ann Arbor, MI 48109-2136
734-763-8768

David Norris
Dept. of Chemical
Engineering and
Materials Science
University of Minnesota
421 Washington Ave. SE
Minneapolis, MN 55455
612-625-2043

    

Moungi Bawendi
Dept. of Chemistry
Massachusetts Institute of Technology
Rm. 6-221
77 Massachusetts Ave.
Cambridge, MA 02139
617-253-9796

Masaru Kuno
Dept. of Chemistry and Biochemistry
University of Notre Dame
251 Nieuwland Science Hall
Notre Dame, IN 46556
574-631-0494

     

Symposium Support
ACS Publications
QD Vision
Solaris Nanosciences Inc
 


* Invited paper

SESSION N1: Conductivity in Nanostructures
Chair: Masaru Kuno
Monday Morning, November 30, 2009
Constitution A (Sheraton)

8:00 AM *N1.1
Electrical Studies of Individual Colloidal Semiconductor Nanorods. Matthew Sheldon1,2, Taleb Mokari2 and Paul Alivisatos1,2; 1Chemistry Department, University of California, Berkeley, Berkeley, California; 2Lawrence Berkeley National Laboratory, Berkeley, California.

Single nanostructure electrical measurements directly probe the fundamental limits of semiconductor device miniaturization, providing some of the most precise characterization available of electronic structure resulting from quantum confinement and dimensional control. When the strategy is employed for colloidal semiconductor nanocrystals we also learn the ultimate transport efficiencies of these materials, crucial for determining their utility in photovoltaic applications, as one important example, without the convolution of particle-particle carrier hopping mechanisms or particle size dispersity that are difficult to account for in studies of nanocrystal thin film solids. Electrical studies of nanoscale objects can be dominated by the influence of the contacts. In this talk, we will present recent results, in which the electrical transport through individual CdSe nanorods contacted by different methods is compared. We find that solution-grown Au tips provide the best contacts, and provide a basis for further study of the electrical characteristics of the nanorods.


8:30 AM *N1.2
Quantum Dots, Quantum Wires and Carbon Nanotube Composites for Optoelectronic Applications. Horst Weller and Christian Klinke; Chemistry, University of Hamburg, Hamburg, Germany.

Our presentation describes the syntheses of quantum dots using continuous flow technique allowing a degree of reproducibility which is tremendously better than in conventional batch synthesis. We also report on the attachment of quantum dots to carbon nanotubes. These composites exhibit interesting properties concerning charge carrier injection, transport properties and photoconductivity. We will present appropriate structures for PV and LED applications of these composite materials. Moreover we will show electrical transport measurements on individual InP nanowires which, linked with metallic indium spheres, form readily-made Schottky transistors.


9:00 AM N1.3
Controlling the Effective Surface Work Function of Tethered CdSe Nanocrystals by Ligand Exchange. Andrea M. Munro and Neal R. Armstrong; Chemistry, University of Arizona, Tucson, Arizona.

Colloidal semiconductor nanocrystals are promising chromophores for use in organic light-emitting diodes (OLEDs) and photovoltaics (PVs) due to their size-tunable optical properties, solution processability, and large spin-orbit coupling. While semiconductor nanocrystals have narrower emission bands and higher photostability than common organic dyes, NC-doped OLEDs currently have lower efficiency and higher turn-on voltages than all-organic OLEDs. Colloidal semiconductor nanocrystals are capped by organic ligands that have been shown to control the solubility and photoluminescence efficiency of the nanocrystals. The organic capping layer is often made up of ligands with long alkane chains and thus acts as an insulating barrier to charge and energy transfer between a nanocrystal and its local environment. Additionally, there have been reports that exchanging the ligands in the nanocrystals capping layer can shift the energy levels of certain nanocrystals and can reversibly change a PbS nanocrystal thin film from p-type to n-type. It may be possible to shift the ionization potential and electron affinity of CdSe nanocrystals and the rate of charge transfer to and from a nanocrystal may be controlled by ligand exchange. In order to study the ligand effects on the work function and ionization potential of CdSe nanocrystals, we perform photoelectron spectroscopy on CdSe nanocrystals of varying diameter and with a variety of surface ligands that are tethered to template-stripped gold. The ligands used to cap the tethered nanocrystals are hexanethiol, benzenethiol, and 1,4-fluorothiophenol. We observe surface work function and ionization potential shifts on the order of 0.1-0.3 eV for samples with different capping ligands. We interpret these shifts by modeling the capping ligands as surface dipoles and discuss how the nanocrystal-ligand structure should be modeled to account for shifts in the local vacuum level of the surface and for the final state effects in the nanocrystals. These surface characterization protocols should be easily generalized to all types of semiconductor nanocrystals and provide a clearer understanding of how changes in environment at nanometer length scales affects frontier orbital energies.


9:15 AM N1.4
Effects of Surface Ligands on the Electronic and Optical Properties of CdSe Nanocrystals. Oleksandr Voznyy, Eugene Kadantsev, Marek Korkusinski and Pawel Hawrylak; Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada.

Optical properties of CdSe nanocrystals depend on ligands used to passivate the surface. In the semiempirical tight-binding (TB) [1] or pseudopotential techniques [2] describing low energy excitations of the NC core the surface effects are artificially eliminated. Here we report a density functional theory (DFT) study using the SIESTA and ADF codes of small CdSe nanoclusters with experimentally known both structure and ligands [3]. We develop the understanding of the effects of ligands on the surface and extend our DFT model to ligated NCs of sizes up to 3 nm. We compute excited states within two approaches: (a) the time-dependent DFT approach and (b) configuration-interaction based on the DFT- or TB-computed single particle states. We find that exact adsorption sites and types of ligands influence the ground state electronic structure and localization of the single-particle states. Thus, for small nanoclusters, surface passivation with artificial hydrogen, acting similarly to elimination of surface dangling bonds used in semiempirical methods, should be avoided. However, selective passivation with real hydrogen can predict the correct surface relaxation geometry. For larger nanocrystals the effect of surface is diminished but passivation with pseudo-hydrogen cannot capture the build-up of the internal electric field in NC with real ligands. This field has a strong impact on the localization of electron and holes states and interaction between them and can be added artificially to semiempirical models. Similarly to previous work [4] we observe a significant mixing of NC core and ligand states at high excitation energies which should affect the coupling between excitons and biexcitons and consequently multi-exciton generation efficiency. [1] Korkusinski, M., Zielinski, M., Hawrylak, P., J. Appl. Phys. 105, 122406 (2009) [2] Franceschetti, A.; Fu, H.; Wang, L. W.; Zunger, A. Phys. Rev. B. 60, 1819 (1999). [3] Soloviev, V. N.; Eichhöfer, A.; Fenske, D.; Banin, U., J. Am. Chem. Soc. 123, 2354 (2001). [4] Kilina, S.; Ivanov, S.; Tretiak, S., J. Am. Chem. Soc. 131, 7717 (2009).


9:30 AM N1.5
Transferred to N1.4



10:15 AM *N1.6
Tailoring Electronic Properties of Nanocrystal Solids by Chemical Design of Nanocrystals and Surface Ligands. Dmitri Talapin, Maksym Kovalenko and Jong-Soo Lee; Deparment of Chemistry, The University of Chicago, Chicago, Illinois.

Colloidal nanocrystals are considered promising building blocks for electronic and optoelectronic devices. Potentially, they can combine the advantages of crystalline inorganic semiconductors with size-tunable electronic structure and inexpensive solution-based device fabrication. At the same time, several fundamental problems have to be solved before these materials will compare favorably to the competitive approaches, e.g. organic electronic materials. The bulky and insulating nature of the surface ligands used for nanocrystal synthesis typically results in the poor electronic coupling between individual nanocrystals. In an attempt to address this problem, we demonstrated that molecular metal chalcogenide complexes can serve as versatile ligands for a broad range of inorganic nanomaterials. This new class of nanocrystal colloids provides a set of advantages such as all-inorganic design, small interparticle spacing, outstanding charge transport properties, and diverse compositional tunability for both nanocrystal and ligand constituents. We developed several approaches to electronic doping of nanocrystal solids based on the formation of inter- and intra-nanocrystal charge transfer complexes. Combining semiconductor nanocrystals with other semiconducting materials, metals and magnets either through mixing different nanocrystals or by synthesizing multicomponent nanoscale heterostructures allows adjusting the materials properties. We demonstrate the power of this approach on the examples of prototype thermoelectric and photovoltaic materials.


10:45 AM *N1.7
Conduction with Colloidal Quantum Dots. Philippe Guyot-Sionnest, University of Chicago, Chicago, Illinois.

In the weak coupling regime, favorable for preserving the discrete quantum states of the dots, the conductivity in films of colloidal quantum dots is of a hopping nature, frozen at low temperature and small bias. This generic qualitative behavior is similar to that seen with disordered semiconductors and even with insulating metal nanocrystal arrays but a common description of the scaling behavior with temperature and bias is not settled. I will describe temperature and bias dependent measurements of conductivity on CdSe and other dot systems, as a function of charging and processing. When applying a magnetic field, the conductivity is also affected in several ways. The weakly coupled colloidal quantum dots display a novel magnetoresistive effect which is effectively a spin- blockade. This is proposed to arise principally because of the nondegenerate density of state of the colloidal dots.


11:15 AM N1.8
Coulomb Enhancement of Electron Transport through PbSe/CdSe Core-Shell Quantum Dots. Ingmar Swart1,2, Zhixiang Sun1, Dominika Grodzinska1, Wiel Evers1, Daniel Vanmaekelbergh1 and Peter Liljeroth1; 1Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht, Netherlands; 2Institute for experimental and applied physics, Faculty of Physics, Regensburg, Germany.

Electron - electron effects are important when considering charge transport through nano-scale objects such as quantum dots (QDs). For example, the addition of a single electron to a QD can dramatically change, or even block, the transport of electrons through the dot, due to the charging energy associated with this extra electron [1]. This effect is known as the Coulomb blockade effect. The opposite effect, Coulomb enhancement of electron transport, due to trapping of positively charged holes in the QD, has not been observed to date. In order to observe this effect, experimental access to the individual energy levels of QDs is required. Scanning tunneling microscopy (STM) and spectroscopy (STS) have demonstrated to be capable of providing detailed information on the individual energy levels of single QDs. Our approach to demonstrate the Coulomb enhancement relies on the possibility to trap holes in appropriately designed core-shell nanocrystals and on single-QD electron transport experiments using STM and STS. PbSe/CdSe core-shell QDs with varying shell thickness are synthesized using a cation exchange procedure starting with monodisperse PbSe QDs [3]. This allows tuning the band offsets through quantum confinement and control over a transition from type I to type II band alignment. These particles were then used in low-temperature STM and STS experiments. The measured spectra provide clear evidence of a novel hole-assisted electron transport mechanism through the QD. This involves splitting of the tunneling resonances due to two possible parallel tunneling processes: normal electron tunneling and coulomb enhanced electron tunneling due to the presence of a trapped hole in the PbSe core of the QDs. Based on these measurements, we can extract the electron-hole interaction energy and information on the energy level structure of the core-shell QDs. The experimental findings are supported by a theoretical analysis based on master-equation simulations of electron- and hole-transport through a QD. Our results provide fundamental information relevant for understanding the flow of electrons in quantum dot devices. References [1] Single Charge Tunneling, ed. Grabert, H. & Devoret, M. H., Plenum, New York, (1992) [2] Z. Sun, I. Swart, C. Delerue, D. Vanmaekelbergh, and P. Liljeroth, Phys. Rev. Lett. 102, 196401 (2009) [3] K. Lambert, B. De Geyter, I. Moreels, Z. Hens, Chem. Mater. 21, 778 (2009)


11:30 AM N1.9
Photoconductivity and Gate Effect Studies of Doped and Undoped Semiconductor Nanocrystals. Scott Geyer, Peter M. Allen, Liang-Yi Chang and Moungi G. Bawendi; MIT, Cambridge, Massachusetts.

We have synthesized Cd doped InAs nanocrystals which exhibit p-type conductivity in a field effect transistor. This differs from the intrinsic n-type conductivity of InAs nanocrystals and indicates a successful addition of a stable transition metal electronic dopant prior to deposition. We explore the effects of transition metal doping on nanocrystal film conductivity and investigate synthetic methods to extend this technique to other nanocrystal systems. Photoconductivity studies are employed to further explore the physics of these new systems, specifically looking at the effects of dopants on the distribution of charges in the band gap. A model of compensation doping is proposed for an n type nanocrystal doped with acceptors which involves the ionization of donor states near the conduction band to fill the acceptor states. These states can then act as electron recombination centers.


 

SESSION N2: Optical Processes and New Methods in Nanoparticles
Chair: David Norris
Monday Afternoon, November 30, 2009
Constitution A (Sheraton)


1:30 PM *N2.1
InP/ZnS Nanocrystals for In Vivo Applications. Sudarsan Tamang, Grégory Beaune, Isabelle Texier, Kai Huang, Olivier Renault and Peter Reiss; CEA Grenoble, Grenoble, France.

Indium phosphide quantum dots are promising fluorescent labels for in vivo biological imaging. However, most established synthesis routes do not allow producing nanocrystals with a size larger than 3-4 nm. Therefore the emission color cannot be tuned to the near infrared range, which is especially interesting for in vivo applications. We developed an alternative synthesis method yielding reproducibly larger InP quantum dots with an emission peak in the range of 700-730 nm. In order to obtain a significant fluorescent quantum yield (QY) and photostability, overcoating of the core InP nanocrystals with shell of a larger band gap material, such as ZnS, is indispensable. By means of X-ray photoelectron spectroscopy using synchrotron radiation of variable energy we evidenced that, depending on the reaction conditions for the ZnS growth, either a gradient structure or a core/shell system is obtained. These results demonstrate the possibility to engineer the interface between the InP core and the ZnS shell resulting in less defects and improved fluorescence properties. Finally, a novel approach for the efficient transfer of the InP/ZnS quantum dots to the aqueous phase will be presented. This phase transfer does not affect the QY and the samples are stable for months without any sign of aggregation. The biodistribution of the quantum dots has been studied by fluorescence reflectance imaging upon their tail vein injection into healthy nude mice.


2:00 PM *N2.2
Doped Semiconductor Nanocrystals (D-dots) as Emitters in Lighting. Xiaogang Peng, U of Arkansas, Fayetteville, Arkansas.

Doped semiconductor nanocrystals (d-dots) without any Class A heavy metal (Cd, Pb, and Hg) were synthesized successfully. By changing the dopants and host nanocrystals, the emission peak position of the d-dots can be tuned in an optical window from about 450 to 1150nm. These highly luminescent d-dots will be discussed as emitters mostly for solid-state-lighting applications. This talk discusses only d-dots and the heavy-metal-free intrinsic quantum dots will be discussed in a different talk.


2:30 PM N2.3
Tunable Light Emission from Colloidal Silicon Nanoparticles: Role of Size and Surface Chemistry. Anoop Gupta1, Hartmut Wiggers1,2 and Mark T. Swihart3; 1Institut fur Verbrennung und Gasdynamik, University of Duisburg-Essen, Duisburg, Germany; 2Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Duisburg, Germany; 3Department of Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York.

Aiming for a more practical route to highly stable visible photoluminescence (PL) from silicon, we have developed a novel approach of producing luminescent silicon nanoparticles (Si-NPs). We synthesize single crystalline Si-NPs via pyrolysis of silane (SiH4) in a microwave plasma reactor at very high production rates (0.1-10 g h-1). The size and therefore the emission wavelength of the Si-NPs is controlled by etching them in a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). Emission across the entire visible spectrum is obtained by varying the etching time resulting in different sized Si-NPs. We observed that the oxidation of freshly etched Si-NPs in air profoundly affects their optical properties, causing their emission to blue-shift and diminish in intensity with time. Functionalization of etched silicon nanoparticles was achieved by a photoinitiated alkylation process. This alters the surface chemistry of silicon nanoparticles, while maintaining their size. Despite having similar size, functionalized silicon nanoparticles show a shift in their emission spectrum with respect to freshly etched nanoparticles. Functionalization of silicon nanoparticles with different organic molecules indicates that the shift occurs irrespective of the type of organic ligands on silicon surface. The nature of the shift (red/blue) is dependent on the emission wavelength of the etched Si-NPs. Additionally, the amount of shift depends on the type of organic ligands on the silicon surface and the UV exposure time. The surface modification of Si-NPs with different alkenes results in highly stable PL and allows their dispersion in a variety of organic solvents. This method of producing macroscopic quantities of Si-NPs with very high PL stability opens new avenues to applications of silicon quantum dots in optoelectronic and biological fields, and paves the way toward their commercialization.


2:45 PM N2.4
Polarized Magneto-Photoluminescence from Mn-doped ZnSe/CdSe Core/Shell Nanocrystals. Ranjani Viswanatha1, Scott A. Crooker2, Jeffrey M. Pietryga1, Donald J. Werder1 and Victor I. Klimov1; 1Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico; 2National High Magnetic Field Laboratory, Los Alamos National Lab, Los Alamos, New Mexico.

Colloidal nanocrystal heterostructures provide great flexibility in controlling electronic and magnetic interactions via growth-controlled “engineering” of electron and hole wave functions. Here, we study the magneto-optical properties of colloidal Mn-doped ZnSe/CdSe core/shell nanocrystals using magnetic circular dichroism (MCD) and circularly polarized photoluminescence (PL) as a function of magnetic field (up to 30 T) and temperature (down to 1.5 K). These data are compared with similar measurements of non-magnetic nanocrystals as well as bulk diluted magnetic semiconductors (ZnMnSe). MCD studies of these Mn-doped ZnSe/CdSe nanocrystals reveal giant field- and temperature-dependent Zeeman splittings of the band-edge exciton, demonstrating a strong sp-d exchange coupling of electrons and holes to the embedded paramagnetic Mn atoms [1]. The strength of this sp-d interaction is widely tunable with average Mn concentration, and also by varying the thickness of the CdSe shell. Magneto-PL studies reveal a strong emission from internal Mn transitions at ~2.15 eV, and also a weaker (higher-energy) band-edge emission from samples having low average Mn concentration. Surprisingly, the Mn emission develops a marked degree of circular polarization in applied magnetic fields, which follows the same field- and temperature-dependent (Brillouin-like) magnetization of the Mn spins. Notably, the intensity of the right- and left-circularly polarized Mn emission increases and decreases with applied field, respectively, in strong contrast to similar studies of bulk ZnMnSe epilayers in which both polarizations of Mn emission decrease dramatically with field. We discuss the effects of strong quantum confinement on the coupling between spin-polarized excitons and the local Mn spins. [1] D. A. Bussian, S. A. Crooker, M. Yin, M. Brynda, A. L. Efros and V. I. Klimov, Nature Materials 8, 35 (2009).


3:00 PM N2.5
Carbon Supported CdSe Nanoparticles. Beatriz H. Juarez1, Christian Klinke2 and Horst Weller2; 1IMDEA Nanoscience, Madrid, Madrid, Spain; 2Hamburg University, Hamburg, Hamburg, Germany.

The combined optical and electrical transport properties of nanoparticles and carbon nanotubes respectively may lead to improvements in the efficiency of photoelectric devices. In this work we give some insights to the mechanism by which CdSe rod-like nanoparticles modify their shape and attach to CNTs following the hot injection method. The attachment is observed on both, singlewall and multiwall CNTs and the obtained composite materials exhibit photoelectrical response. It was observed that the presence of water improves the nanotube coverage while dichloroethane (DCE), HCl or in general traces of a chlorine containing medium are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well defined crystallographic facets, and interaction with an organics-free sp2 lattice. Furthermore, this procedure can be extended to decorate graphene by quantum dots.


3:30 PM N2.6
Mechanism of Blinking Suppression in Core/thick Shell CdSe/CdS Nanocrystals. Benoit Mahler1, Piernicola Spinicelli2, Stephanie Buil3, Xavier Quelin3, Jean-Pierre Hermier3 and Benoit Dubertret1; 1LPEM, ESPCI Paristech, Paris, France; 2LKB UMR8552, CNRS, Paris, France; 3GEMaC UMR8635, CNRS, Paris, France.

Colloidal semiconductor nanocrystals or quantum dots are promising photoluminescent sources thanks to their high quantum yield, superior photostability and size tunable spectral properties. Numerous applications have been developed such as photovoltaic devices, lasers, electroluminescent diodes or biological labeling. Despite their unique optical properties, the quantum dots suffer one major limitation for single particle applications due to their emission intermittency. It is commonly admitted that nanocrystal blinking comes from ionization of the nanocrystal. As the Auger recombination lifetime (hundreds of picoseconds) is much shorter than the radiative lifetime (tens of nanoseconds), any electron-hole pair created in the ionized nanocrystal recombines non-radiatively. Attempts to reduce or suppress the blinking have been recently successful using core/shell nanostructures. A first strategy is to overcoat a small core of CdSe (1.5nm radius) with a thick shell of CdS (up to 6nm thick)(1),(2). The second strategy is to use a CdZnSe/ZnSe core/shell structure with a graded composition shell(3). Here we have synthesized CdSe/CdS core/shell quantum dots with thick crystalline shells, which do not blink at low frame rate. At higher frequency (1000Hz) we can see fluctuations between a “bright” and a “gray” state. Analyzing the photon statistics and lifetime of the on state, we first demonstrate that bright periods correspond to single photon emission with a fluorescence quantum efficiency of the monoexcitonic state greater than 95%. We also show that low intensity emitting periods are not dark but correspond to a grey state, with a fluorescence quantum efficiency of 19%. From these measurements, we deduce the radiative lifetime (45 ns) and the Auger lifetime (10.5 ns) of the grey state(4). We can conclude that the observed blinking suppression is associated with a huge reduction of Auger process efficiency in CdSe core thick CdS shell nanocrystals. An Auger lifetime on the order of radiative lifetime induces the formation of continuously emitting nanocrystals making them promising and robust emitters for continuous single molecule tracking or quantum optics. (1) Mahler, B.; Spinicelli, P.; Buil, S.; Quelin, X.; Hermier, J. P.; Dubertret, B. Nat. Mater. 2008, 7, 659-664. (2) Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. Journal of the American Chemical Society 2008, 130, 5026-5027. (3) Wang, X.; Ren, X.; Kahen, K.; Hahn, M. A.; Rajeswaran, M.; Maccagnano-Zacher, S.; Silcox, J.; Cragg, G. E.; Efros, A. L.; Krauss, T. D. 2009, 459, 686-689. (4) Spinicelli, P.; Buil, S.; Quelin, X.; Mahler, B.; Dubertret, B.; Hermier, J.-P. Phys. Rev. Lett. J1 - PRL 2009, 102, 136801.


3:45 PM N2.7
Suppressed Auger Recombination in ``Giant” Nanocrystals Boosts Optical Gain Performance. Florencio Garcia-Santamaria, Yongfen Chen, Ranjani Viswanatha, Javier Vela, Richard D. Schaller, Jennifer A. Hollingsworth and Victor I. Klimov; Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, New Mexico.

Many potential applications of semiconductor nanocrystals are hindered by non-radiative Auger recombination wherein the electron-hole (exciton) recombination energy is transferred to a third charge carrier. This process severely limits the lifetime and bandwidth of optical gain, leads to large non-radiative losses in light emitting diodes and photovoltaic cells, and is believed to be responsible for intermittency (“blinking”) of emission from single nanocrystals. The development of nanostructures in which Auger recombination is suppressed is currently the subject of intense research in the colloidal nanocrystal field. Here, we will present direct experimental evidence that so-called “giant” nanocrystals consisting of a small CdSe core and a thick CdS shell exhibit a significant (orders of magnitude) suppression of Auger decay rates. [1,2] As a consequence, even multiexcitons of a very high order exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds. This demonstration represents an important milestone toward practical lasing technologies utilizing solution-processible colloidal nanoparticles. We also discuss the various possible causes for the long biexciton lifetimes observed, namely, volume scaling effects, charge separation, exciton repulsion and the profile of the interface potential. [1] Garcia-Santamaria, F.; Chen, Y.; Vela, J.; Schaller, R. D.; Hollingsworth, J. A.; Klimov, V. I. Nano Letters 2009, Online as of June 15th (DOI: 10.1021/nl901681d). [2] Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2008, 130, 5026-5027.


4:00 PM N2.8
The Effect of Temperature and Dot Size on the Spectral Properties of Colloidal InP/ZnS Core-Shell Quantum Dots. Arun Narayanaswamy2, Louis F. Feiner2, Pieter-Jan van der Zaag2 and Andries Meijerink1; 1Debye Institute, Utrecht, Netherlands; 2Philips Research, Eindhoven, Netherlands.

The synthesis of colloidal semiconductor nanocrystals (NCs) has seen an impressive progress in the past two decades, fueled by the unique physical properties of these nano-objects arising from quantum confinement. Among all II-VI and III-V semiconductor NCs, InP is probably the most promising compound, combining size-tunable emission in the visible and near-infrared spectral range (as the bulk band gap at room temperature is 1.35 eV) and low intrinsic toxicity. In view of the potential technological applications of semiconductor nanocrystals in biological labeling, lasers, LEDs and solar cells, improvements in the development of these materials are still being made. Apart from the size dependent properties, the band gap of semiconductor nanocrystals has been shown to depend on temperature. Here we report a study of the temperature dependence of both the band gap and photoluminescence (PL) line width in InP/ZnS nanocrystals in the temperature range 4-525 K. Variation of the band gap with temperature gives rise to color changes in absorption as well as in photoluminescence. These color changes may have potential relevance in applications, as visual indicators for the temperature of a surface. Two issues directly related to these color changes are the temperature dependence of both the PL peak energy and the PL line width over a wide temperature range (4-510 K) for QD’s of various sizes (diameter 1.8 - 4.5 nm), and we analyze how this temperature dependence may be described and characterized by a few parameters. The dependence of the PL peak energy and of the exciton energy at 4 K represents the true bandgap. The size dependence of the low temperature bandgap is measured as a function of the size of the QD and compared with recent theoretical predictions. The variation of the zero-temperature PL peak energy, and, more significantly, the variation of the zero-temperature excitonic band gap (as measured by PLE at 4 K) as a function of the size was fitted to Eg(d) = Eg(∞) + Ag/dn. The best agreement was found for a close to inversely linear (n = 1.02) fit which is different from theoretical work where higher values for n are predicted.


4:15 PM N2.9
Orbital and Charge-Resolved Polaron States in CdSe Dots and Rods Probed by Scanning Tunneling Spectroscopy. Zhixiang Sun1, Ingmar Swart1, Christophe Delerue2, Daniel Vanmaekelbergh1 and Peter Liljeroth1; 1Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, Netherlands; 2Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN), Lille, France.

Conduction electrons can interact with the crystal lattice through coupling with lattice vibration modes (phonons), leading to polaron states. In bulk materials, the electronic energy level spacing is negligible and consequently, coupling between phonons and single electronic orbitals cannot be measured directly. Colloidal semiconductor nanocrystals (quantum dots, QDs) have widely spaced, discrete electronic energy levels due to quantum confinement, with a spacing that can be much larger than the phonon energies. This allows us to study electron-phonon coupling at the level of individual electronic orbitals. We present experimental results on electron-phonon coupling of individual CdSe nanocrystals measured by low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). We observed well-resolved polaron eigenstates and report the first quantitative measurements on the electron-phonon coupling strength in spherical and rod-shaped CdSe QDs. The polaron states arise from Fröhlich coupling between electrons and longitudinal-optical phonons. The coupling strength depends on symmetry of the electronic orbital and on the size and dielectric environment of the QD. In addition, by measuring tunneling spectra under electron accumulation (shell-filling), we show that the e-ph coupling strength depends on the number of added electrons in the QD. Our results provide fundamental information required in understanding electron transport in QD devices and superlattices, and the intraband relaxation of hot carriers in QDs. References Z. Sun, I. Swart, C. Delerue, D. Vanmaekelbergh, P. Liljeroth, Phys. Rev. Lett. 102, 196401 (2009).


4:30 PM N2.10
Exploring Trap-Mediated Excitation Dynamics in Colloidal Nanocrystals. Marcus Jones1, Shun S. Lo2 and Greg D. Scholes2; 1Chemistry, UNC Charlotte, Charlotte, North Carolina; 2Chemistry, University of Toronto, Toronto, Ontario, Canada.

Effects due to particle size and shape determine how the optoelectronic properties of nanoscale materials deviate from those of the bulk. A dominant effect is the quantum confinement of photo-generated electrons and holes, which is well understood and results in a wide array of potentially important qualities that have received much attention. For example, CdSe nanocrystals typically undergo highly efficient radiative recombination from photo-generated exciton states with well-defined energies that are predicted by theory; however, when we start to examine the rates at which these processes occur it becomes apparent that photo-excitation dynamics cannot be completely explained with an exciton-only picture. At the nanoscale, interactions with interface states and the surrounding environment are dramatically more important than in the bulk. Since these perturbations involve states that are usually optically inactive, we tend to have only a qualitative understanding of their role and relative impact on the underlying “core” properties. Time resolved fluorescence from CdSe nanocrystalline quantum dots reflects both the radiative recombination rates from intrinsic exciton states and the non-radiative decay rates from interface trap states, but interpretation of fluorescence transients is non-trivial and typical multi- or stretched exponential analyses yield little specific photophysical information. I will describe the construction and application of a simple stochastic model of quantum dot exciton and trap state dynamics, which successfully models quantum dot PL transients over a wide temperature range and for a series of dot sizes. Importantly, we show that the trapping processes, which are really electron transfer reactions, may be discussed in the context of classical Marcus theory. This allows us to directly obtain physically important information such as the reorganization energy and electronic coupling parameters. In this way we are able to assess the influence of traps on the exciton population dynamics and rationalize the way that fluorescence transients evolve in NC samples exposed to a wide range of environmental perturbations.


4:45 PM N2.11
The Eigenstate Spectrum of Multiexcitons in Semiconductor Quantum Dots. Patanjali Kambhampati1, Alberto Franceschetti2, Samuel L. Sewall1, Alex Zunger2 and Ryan Cooney1; 1Chemistry, McGill University, Montreal, Quebec, Canada; 2National Renewable Energy Lab, Goldem, Colorado.

Semiconductor quantum dots can have a four body bound state called a biexciton. The binding energy of the biexciton is strongly dependent upon the size of the quantum dot by virtue of spatial confinement of the charge carriers. An understanding of biexcitonic processes bears relevance to fundamental many body processes in quantum dots, unraveling carrier relaxation dynamics, multiple exciton generation, optical gain, and utilizing quantum dots as single photon sources for quantum devices. The existence of the ground state of the biexciton has been established for some time. The ground state of the biexciton may be represented by a basis of ground state single excitons. Excited states of the biexciton were theoretically predicted to exist, and were subsequently experimentally detected. The difficulty in observing the ground state biexciton is primarily due to its weak binding energy relative to the inhomogeneous broadening; the lifetime is a relatively long 10 - 100 ps. The difficulty in observing the excited states of the biexciton is that the relevant experimental signature can be dynamically cancelled by state filling as the excited exciton cools. Furthermore, the excited state lifetimes of the excitons which are used to construct the biexciton are very short, ca. 100 - 500 fs [1]. While excited states of the biexciton have been observed to exist, a clear and unambiguous measurement of the binding energies of the excited states of the biexciton has remained elusive. We have recently shown that a mixed time/frequency domain spectroscopic approach can yield state-to-state exciton dynamics [1]. Preliminary work has shown clear observation of the size dependence of the splittings between the first excited state and the ground state of the biexciton [2, 3]. Here, we utilize this approach to yield the spectrum of a biexciton in a semiconductor quantum dot. To the extent that an exciton is akin to a hydrogenic atom, the biexciton can be considered akin to a hydrogenic molecule, with a full eigenstate spectrum. [1] “State-to-state exciton dynamics in semiconductor quantum dots”, S.L. Sewall, R.R. Cooney, K.E.H. Anderson, E.A. Dias and P. Kambhampati, Phys. Rev. B., 74, 235328 (2006). [2] “State-to-state exciton dynamics in quantum dots: size dependent biexciton interactions and excited state trapping dynamics”, S.L. Sewall, R.R. Cooney, K.E.H. Anderson, D.M. Sagar, E.A. Dias, and P. Kambhampati, J. Chem. Phys., 129, 084701 (2008). [3] “Intrinsic Stokes shift for biexcitons in semiconductor quantum dots”, S.L. Sewall, A. Franceschetti, R.R. Cooney, A. Zunger, and P. Kambhampati, Phys. Rev. Lett., Submitted (2009).


 

SESSION N3: Light Emitting Devices and Nanoparticle Materials
Chair: Jonathan Steckel
Tuesday Morning, December 1, 2009
Constitution A (Sheraton)


8:00 AM *N3.1
Excitation of the Ligand-free Quantum Dot Light-emitting Active Layer in an All-inorganic Electroluminescence Device. Satoshi Kobayashi and Yuki Tani; R&D Center, HOYA Corporation, Tokyo, Japan.

A thermal nonequilibrium deposition process can put the application of colloidal semiconductor quantum dots in perspective in terms of the development of all-inorganic light-emitting devices. The passivation of the quenching center on the surface of a nanocrystalline quantum dot is essential for achieving highly efficient luminescence originating from quantum confined states in a colloidally dispersed phase. Desorption of the ligand and subsequent re-passivation on the surface are required to achieve an all-inorganic quantum dot luminous active layer in light-emitting devices from the perspective of the durability of the devices themselves. However, molecules of a tertiary alkylphosphine, a primary aliphatic amine, and derivative compounds are the preferred solvents for hot soap synthetic methodologies. The molecules cap the bare surface of the quantum dots efficiently as ligands. The ligands are coordinated to the semiconductor-constituting element on the bare surface with a bonding energy of about 10-1 eV, which corresponds to a temperature of about 103 K. Conventional thermal equilibrium methods for the desorption of the ligands, such as chemical molecular substitution and high-temperature annealing, likely result in the reemergence of a quenching center, carbonization of the organic material, or even interdiffusion of the constituents of the quantum dots. The quantum dot ion beam direct deposition method is uniquely applicable for realizing the desorption of the ligand without degradation of the luminescent spectral characteristics. During energy dissipation, the ion kinetic energy of about 10-1 eV/atom is converted into thermal energy after the collision of the quantum dot ion with the substrate. A localized, short-lasting transient heating effect satisfies both removal of the ligand and preservation of the radiative characteristics. The light-emitting characteristics and mechanisms of all-inorganic electroluminescent devices in which a quantum dot layer is deposited using an ion beam deposition process are discussed. Impact excitation of the ion beam-deposited quantum dots with hot electrons is crucial for the subsequent radiative electron-hole recombination via the quantum confined states when the provided electron energy is comparable to the gap energy between the confined states, i.e., several electron volts. The short-range ballistic flight of the hot electron is not accompanied by avalanche multiplication. Acceleration of the electron is defined by the bias of the electron range and the trap level. This excitation process can be emulated with the tunneling injection of an external electron instead of electron emission from an internal trap state.


8:30 AM *N3.2
Nanocrystal Lasing in the Auger-Recombination-Free Regime Using Engineered Exciton-Exciton Interactions. Victor I. Klimov, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico.

Nanocrystal (NC) quantum dots show high photoluminescence quantum yields and size-dependent emission colors tunable through the quantum-confinement effect. Despite their favorable light-emitting properties, NCs are difficult to use in optical amplification. Because of almost exact balance between absorption and stimulated emission in nanoparticles excited with single excitons, optical gain can only occur due to NCs that contain at least two excitons. A resulting complication is fast optical-gain decay induced by nonradiative Auger recombination, a process in which one exciton recombines by transferring its energy to another [1, 2]. In this talk, I will discuss two approaches for resolving the problem of ultrafast Auger recombination in NCs. In one approach, we utilize core/shell hetero-NCs engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a large, ~100 meV transient Stark shift of the absorption spectrum with respect to the luminescence band. This effect breaks the exact balance between absorption and stimulated emission and allows us to demonstrate optical amplification in the single-exciton regime when Auger recombination is simply inactive [3]. In another approach, we use recently developed “giant” NC quantum dots that comprise a small emitting CdSe core overcoated with a thick shell (up to 20 monolayers) of a wider-gap CdS [4]. These nanostructures exhibit very long biexciton lifetimes (~10 ns), indicating a significant (at least 75-fold) reduction of Auger decay rates. Because of this effect, even high-order multiexcitons exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds [5]. [1] V. I. Klimov et al., Science 287, 1011 (2000). [2] V. I. Klimov et al., Science 290, 314 (2000). [3] V. I. Klimov et al., Nature 447, 441 (2007). [4] Y. Chen et al., J. Am. Chem. Soc. 130, 5026 (2008). [5] F. Garcia-Santamaria et al., Nano Lett., on-line (2009).


9:00 AM N3.3
Narrowband Absorption Enhanced QD/J-aggregate Conjugates and Device Applications. Brian Walker1, Vladimir Bulovic2 and Moungi G. Bawendi1; 1Chemistry, MIT, Cambridge, Massachusetts; 2Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts.

We report hybrid J-aggregate/Quantum dot conjugates that show near-unity energy transfer from the J-aggregates to the quantum dots. The resulting narrowband-sensitized materials have over five times greater absorption at the J-band (the J-aggregate absorption) than QDs alone, indicating that these species are efficient absorbers that retain the tunable emission characteristic of QDs. After discussing the photophysical and morphological characteristics of these versatile hybrid materials, we will discuss their potential application in optoelectric devices.


9:15 AM N3.4
Highly Stable Dispersions of Colloidal CdSe/ZnS Quantum Dots in a ZnS Matrix. Benjamin S. Mashford and Paul Mulvaney; School of Chemistry, University of Melbourne, Parkville, Victoria, Australia.

Luminescent thin films containing colloidal CdSe/ZnS quantum dots (QD) are finding applications in a range of optoelectronic research fields. Of key importance is that the QD emission remains stable for extended periods of time. We report here on a facile method for producing a homogenous dispersion of colloidal QDs in a matrix of nanocrystalline ZnS. Using volatile, short-chain ligands on both the QD and the ZnS components, this technique allows the synthesis of dense films with high QD volume fractions. Compared with previously reported metal-oxide glass matrices, the greater chemical compatibility between the QD shell and the surrounding ZnS matrix provides these films with a high degree of optical and thermal stability. Characterization of these films with electron microscopy, optical spectroscopy and ellipsometry will be presented.


10:00 AM *N3.5
Colloidal Quantum Dot Emitters for Light-emitting Diodes. Y. A. Wang1 and Jian Xu2; 1Ocean Nanotech, LLC, Springdale, Arkansas; 2Penn State Univ., University Park, Pennsylvania.

Quantum dots as promising inorganic fluorophores produced through chemical synthesis have been considered for potential applications in optoelectronics and life sciences. Advantages of using QDs as fluorophores include their high photo-oxidation stability, size dependent optical properties, and flexible synthetic/processing chemistry. As emitters for light-emitting diode applications, QDs offer narrow bandwidths, continuously tunable colors in the entire optical window, and solution based processing. In addition, their emission stability is expected to be much higher than organic emitters due to the inorganic nature. However, based on current research, the performance of QD based LEDs is not as good as the organic emitters. In this talk, the synthesis of high quality QDs and the fabrication of QDLEDs will be reviewed. However, the majority of this talk will focus on some recent experimental results from Ocean NanoTech and its collaborators. Specifically, technical challenges and requirements of QDs emitters for LEDs will be discussed. We will also report the preparation of Cd-free QDs and their performance in QDLEDs.


10:30 AM N3.6
Highly Efficient Light-Emitting Diodes with Monolayers of Energy-Gradient Quantum Dots or Hybrid Films of Energy-Gradient Quantum Dots Dispersed in Conducting Polymers as an Active Layer. Seonghoon Lee1, Changhee Lee2, Kookheon Char3, Minki Nam1, Jeonghun Kwak2 and Wan Ki Bae3; 1Chemistry, Seoul National University, Seoul, Korea, South; 2Electrical Engineering and Computer Science, Seoul National University, Seoul, Korea, South; 3Chemical and Biological Engineering, Seoul National University, Seoul, Korea, South.

The challenge in making an efficient quantum dot light emitting diode is finding an efficient means to inject electronic charges its quantum dots, and to confine these charges long enough for them to recombine to emit light. We took two approaches: 1 ~ 2 monolayers of energy-gradient quantum dots only as an active layer or hybrid films of energy-gradient quantum dots dispersed in conducting polymers as an active layer. In the former case, emission more than 99 % of total electroluminescence originated from QD active layers. Low turn-on voltage of 3.5 V, current efficiency of 5.2 cd A-1, external quantum efficiency of 1.4 % and brightness (1,500 cd m-2) at moderate applied voltage (7.7 V), and the maximum brightness above 10,000 cd m-2 were achieved. In the latter case, QD/conducting polymer hybrid films facilitated LED processing. The hybrid films had uniform QD distribution throughout various solution-based processes and easily were patterned. Direct binding of conducting polymers to QDs facilitated the carrier (electron or hole) injection into the QDs, and, therefore, the devices employing QD/conducting polymer hybrid films as the active layer exhibited a little bit improved device performances. QLEDs with the hybrid films as active layers exhibited moderate turn-on voltage (< 4 V), high external quantum efficiency (> 1.5 %), and high color-purity (FWHM ~ 30 nm). Both approaches and results represent significant progress toward the practical realization of QLEDs, and may also provide a reasonable platform for optoelectronic devices, such as photodetectors or photovoltaic devices, based on organic-inorganic hybridization.


10:45 AM N3.7
Preparation of Conducting Polymer-Grafted Quantum Dots and Their Applications to Light-Emitting Diodes. Wan Ki Bae1,3, Jeonghun Kwak3, Matthias Zorn4, Heeje Woo1, Rudolf Zentel4, Seonghoon Lee2, Changhee Lee3 and Kookheon Char1; 1School of Chemical and Biological Engineering, Seoul National University, Seoul, Korea, South; 2School of Chemistry, Seoul National University, Seoul, Korea, South; 3School of Electrical Engineering and Computer Science, Seoul National University, Seoul, Korea, South; 4Institute for Organic Chemistry, Johannes Gutenberg-Universität Mainz, Mainz, Germany.

We have prepared CdSe@ZnS QDs grafted with poly(para-methyl triphenylamine-b-cysteamine acrylamide) (PTPA-b-CAA) block copolymers. The PTPA-b-CAA block copolymer has a conducting block (PTPA) at one end and a short block (CAA) at the other end for anchoring onto QD surface. The prepared QD/polymer hybrids show good solubility in various organic solvents along with improved colloidal stability derived from the grafted polymer brushes. As a result, the QD/PTPA-b-CAA hybrids demonstrate excellent processing (i.e. spin-coating and drop-casting) and pattern capability (based on unconventional lithography), revealing the TEM morphology with QDs well-dispersed in the conducting polymer matrix. In addition, the QD/PTPA-b-CAA hybrid films were tested for QLEDs as active emission layers, exhibiting low turn-on voltage (< 4 V), high external quantum efficiency (max. = 2.2 %), and high color purity (the contribution of QD emission in total EL spectra > 99 %). The approaches taken in present study represent the significant advancement for practical realization of QLEDs in terms of both processing development and device performance, and would also offer rational guidelines for other optoelectronic devices (i.e. nanobio sensors, photodetectors, and photovoltaic devices) based on such organic/inorganic hybridization.


11:00 AM N3.8
Post-Synthesis Crystallinity Enhancement of Colloidal Nanoparticles. Steven Rutledge1, Abdiaziz Farah1, Jordan Dinglasan2, Darren Anderson2, Anjan Das2, Jane Goh3, Cynthia Goh3 and Amr Helmy1; 1Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada; 2Vive Nano Inc., Toronto, Ontario, Canada; 3Department of Chemistry, University of Toronto, Toronto, Ontario, Canada.

Colloidal nanoparticles (NPs) of numerous shapes, sizes, and compositions have been reported over the past two decades and continue to exhibit exciting optical and electrical characteristics. In particular, II-VI semiconductor CdTe NPs have been demonstrated suitable for use in applications involving efficient solar cells, ultrafast electron transfer, and negative refractive index materials. These nanoparticles require a high degree of structural and optical stability combined with a highly crystalline core for implementation into optoelectronic devices. Such properties are difficult to influence during the initial growth procedure. Post synthesis treatment to effectively manipulate NP characteristics is thus a highly sought after technique. Post growth annealing to influence the crystallinity of colloidal NPs has been investigated with varying levels of success for colloidal NPs. However, the adiabatic nature of and the time constants associated with oven annealing processes have been shown to diminish photoluminescence (PL) efficiencies at temperatures below 200°C. Optical characteristics of colloidal NPs are significantly influenced by the surface ligand layer and its interaction with the semiconductor core. Any post growth manipulation of these components of the colloidal NPs usually proves counterproductive to their properties from an applications stand point. The specific capping material in our system is polyacrylic acid (PAA), which has a glass transition temperature of 106°C. Above the glass transition temperature the polymer chains will be much more mobile and could result in significant reorganization at the NP interface. In order to maintain the attractive optical properties of the NPs, the mobility of the polymer ligands should be limited at temperatures sufficiently high to influence the crystalline properties of the semiconductor core. This can be achieved via an isothermal annealing process with shorter annealing times than those possible in conventional oven annealing. Technologically, this can be obtained via the use of rapid thermal annealing (RTA). RTA is a process in which an incoherent light source is incident uniformly on the sample, limiting thermal fluxes and resulting in a non-adiabatic heating mechanism. RTA has been applied extensively in semiconductor processing and has been demonstrated to provide numerous benefits in traditional semiconductor systems. However, the influence of RTA on the properties of polymer capped nanostructures has not yet been investigated. If harnessed, the influence of RTA on colloidal nanostructures can present an attractive processing technique, since the core material can be influenced while leaving the capping structure relatively unaffected. This communication reports on the ability to influence the degree of crystallinity of polymer stabilized CdTe NPs using RTA while leaving the PL and size of the NPs nominally unaffected.


11:15 AM N3.9
Independent Control of the Shape and Composition of Colloidal Nanocrystals Through Sequential Cation Exchange Reactions. Joseph Luther2,1, Haimei Zheng2, Bryce Sadtler2 and Paul Alivisatos2,3; 1NREL, Golden, Colorado; 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California; 3Chemistry, UC Berkeley, Berkeley, California.

Colloidal nanocrystals display interesting size and shape dependent effects in virtually every measurable chemical and physical property compared to bulk materials of the same atomic composition. The experimental size and shape-controlled growth of nanocrystals is a topic of intense research effort for applications including electro-optic, catalytic, and medical devices. Here we show that nanocrystals, with well-established synthetic protocols for high shape and size monodispersity, can be used as templates to independently control the composition, while preserving the dimensions of the nanocrystal starting materials. Chemical transformations overcome the limitation of traditional colloidal synthesis, where the nanocrystal shape often reflects the inherent symmetry of the underlying lattice. Specifically we show that full or partial conversion between wurtzite CdS, chalcocite Cu2S, and rock salt PbS can occur while preserving anisotropic shapes unique to the as-synthesized material. Highly confined anisotropic PbS nanorods (bandgap ~1 eV) are created which serve as an important material for studying strong 2-dimensional quantum confinement, as well as for near infrared emitting and solar applications. Furthermore, interesting new epitaxial nanoheterostructures of CdS|PbS are obtained by precise control over ion insertion and removal. This work will be presented in a context of the useful applications of these unique new materials.


11:30 AM N3.10
Infrared-emitting Type-II Heterostructured Semiconductor Nanocrystals. Jeffrey M. Pietryga, Doh C. Lee, Istvan Robel and Victor I. Klimov; Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico.

Semiconductor nanocrystal quantum dots represent an important class of nanoscale solution-processible materials. Three-dimensional confinement of carriers within nanocrystals gives rise to novel characteristics such as size-controlled energy gaps and increased Coulomb interactions between charge carriers. In certain applications, e.g., photovoltaic devices, strong electron-hole Coulomb attraction can hamper spatial separation of electrons and holes, and hence, complicate collection of charge carriers. One approach to efficient “splitting” of photogenerated excitons is through introduction of appropriate energy gradients within a heterostructured nanocrystal. Specifically, type-II staggered alignment of electronic states can provide nearly complete separation of electrons and holes between different spatial regions of a nanoheterostructure. Such type-II nanocrystals are also characterized by extended carrier radiative recombination lifetimes which provides an additional advantage for photovoltaic applications as longer times are available for charge extraction. So far, all demonstrated type-II colloidal nanocrystals have a relatively wide energy gap, which is not optimal for solar energy conversion. The purpose of this work is to colloidally synthesize type-II heterostructured semiconductor nanocrystals with band gaps in the vicinity of 1 eV, which yields the maximum power conversion efficiency in single-junction solar cells. Thus, through a multi-step approach, we synthesized PbSe/CdSe/CdS core/shell/shell heterostructured nanocrystals. We examined the dependence of both effective band gap and radiative lifetime on CdS shell thickness through a combination of steady-state and time-resolved infrared photoluminescence spectroscopies. We observed a red shift and a dramatic, order of magnitude increase in lifetime (to as high as 10 μs) for several shell thicknesses, which indicated the onset of a type-II carrier separation regime, at effective band gaps of 0.95 eV, the narrowest band gap of any such system to date. We will present the synthesis and characterization of these nanoheterostructures, including the surprising effect of synthesis conditions on particle shape, emission quantum yields and carrier dynamics.


11:45 AM N3.11
Morphology- and Color-Tunable Bright Fibers with Dispersed CdTe Nanocrystals Self Assembled by Sol-Gel Reaction. Norio Murase, Ping Yang and Masanori Ando; Photonics Research Institute, National Institute of Advanced Industrial Science & Technology, Ikeda, Osaka563-8577, Japan.

Silica is an ideal matrix for dispersing and protecting luminescent nanocrystals (NCs). In addition to these passive roles, a silica matrix provides a distinctive reaction field that can be used to prepare robust NCs with better photoluminescence (PL) properties. Further, silica beads impregnated with NCs self-assemble during preparation, resulting in the formation of bright fibers. We will explain in this presentation such the active aspects of a silica matrix. CdTe NCs (green emitting, 2.6 nm in diameter, PL efficiency 23%) prepared using a normal aqueous method were coated with a thin silica layer (0.6 nm thick) using a sol-gel technique. They were then refluxed in a solution containing Cd2+ and thioglycolic acid (sulfur source), which increased their size to 5.1 nm and their PL efficiency to 78%, accompanied by a significant red shift and spectral width narrowing. Removal of the silica layer by chemical processing revealed that the size of the CdTe NCs remained unchanged. This spectral phenomenon was quite reproducible and was observed in other silica-coated NCs as well. A model calculation based on the complex structure of CdS-like clusters formed close to NCs in a silica shell so that the quantum size effect was reduced explains the red shift quantitatively. Silica beads thus prepared self-assembled during reflux, forming fibers with various morphologies (tubal, rod, or belt-like) determined by the concentration of chemicals during reflux. XRD analysis revealed that the fibers had crystal structures. They showed emission in the orange and red regions with high brightness originating from the high NC concentration (0.002 M) and high emission efficiency (16%), so they have several possible applications.


 

SESSION N4: Light Emitting Devices
Chair: Moungi Bawendi
Tuesday Afternoon, December 1, 2009
Constitution A (Sheraton)


1:30 PM *N4.1
Sol-Gel Based QD-LEDs. Paul Mulvaney, School of Chemistry & Bio21 Institute, University of Melbourne, Parkville, Victoria, Australia.

The fabrication and characterization of solution-processed, all-inorganic light-emitting device incorporating colloidal CdSe/ZnS quantum dots is presented [1]. Using sol-gel synthetic routes, highly luminescent core-shell QDs are embedded between spin-coated p-type NiO and n-type ZnO charge transport layers. The resulting devices show pure QD electroluminescent emissions with a maximum EL brightness of 249 cd/sqm. The emission spectra from the QDs in the device are slightly red-shifted from their solution values. Optimization of the ZnO layer is essential for improved performance and for applications including solar cells and FETs. We present results on PL and conductivity for colloidal ZnO thin films. [1] "All-Inorganic Quantum-Dot Light Emitting Devices Formed via Low-Cost, Wet-Chemical Processing", Benjamin S. Mashford, Tich-Lam Nguyen, Gerard J. Wilson and Paul Mulvaney, J. Mater. Chem. (2009).


2:00 PM *N4.2
New QD-LED Technologies. Vladimir Bulovic, Vanessa Wood, Matthew Panzer and Moungi Bawendi; MIT, Cambridge, Massachusetts.

We demonstrate a series of LED structures that utilize thin films of colloidal quantum dot light emitters inside metal-oxide thin film matrices. These new LED technologies include both field-driven and current-driven structures, covering the entire visible spectrum and exceeding luminance of 1000 Cd/m2. This talk will present methods for room-temperature device fabrication and the mechanisms by which the devices operate.


2:30 PM N4.3
Bright and Stable Quantum Dot Light-emitting Diodes by Solution Processing. Lei Qian, Ying Zheng, Jiangeng Xue and Paul H. Holloway; Materials Science and Engineering, University of Florida, Gainesville, Florida.

Quantum dots light-emitting diodes (QLEDs) have potentials to achieve good color purity, long lifetime, and high efficiency while possessing the similar manufacturing flexibility as organic light-emitting diodes. Existing QLEDs rely on the use of vacuum thermal evaporation to deposit small molecule electron transport layer to achieve good performance. In this study, we report high efficiency QLEDs with a solution processed electron transport layer consisting of ZnO nanoparticles. The device structure was ITO/PEDOT:PSS/poly-TPD/quantum dots emission layer/ZnO nanoparticles layer/Al. Using core/shell quantum dots of CdS/ZnS, CdSe/ZnS, and CdSe/ZnSe, which were synthesized using an hot solution method, as the blue, green, and orange emitters, respectively,. ZnO nanoparticles were synthesized using a sol-gel method and spin coated onto the emissive layer from an alcohol solution In these all-solution processed QLEDs, we have achieved peak brightness values of 31,000 cd/m2 (orange, λmax=596 nm), 65,000 cd/m2 (green, λmax=540 nm), and 2,300 cd/m2 (blue, λmax=470 nm). The full width at half maximum for these emissions ranged from 28 to 40 nm. The corresponding peak power efficiencies of these devices are 3.8 lm/W for orange, 8.0 lm/W for green, and 0.3 lm/W for blue, considerably higher than previously reported highest values for QLEDs of 2.4 lm/W, 5.0 lm/W and 0.2 lm/W, respectively. Furthermore, the ZnO layer also provided passivation to the active layer and significantly prolonged the device lifetime.


2:45 PM N4.4
Highly Efficient Quantum Dot Based Light-Emitting Diodes by Enhancing Carrier Injection and Transport. Jeonghun Kwak1, Wan Ki Bae1,2, Donggu Lee1, Insun Park3, Do Yeung Yoon3, Seonghoon Lee3, Kookheon Char2 and Changhee Lee1; 1School of Electrical Engineering and Computer Science, Inter-university Semiconductor Research Center, Seoul National University, Seoul, Korea, South; 2School of Chemical and Biological Engineering, Seoul National University, Seoul, Korea, South; 3Department of Chemistry, Seoul National University, Seoul, Korea, South.

Recently semiconductor nanocrystal quantum dots (QDs) have been intensively investigated as promising lighting sources, back light units for LCD, or next generation displays. Colloidal nanocrystal quantum dot based light-emitting diodes (QD-LEDs), especially, have advantages such as easy process, superior optical and electrical properties. Although much progress in QD-LEDs have been achieved, the efficiencies of QD-LEDs are still low within practical brightness region (i.e. 500 ~ 5000 cd/m2), and insufficient hole injection into QD cores and unbalanced electron-hole current result drastic efficiency roll-off at higher current density region. In this work, we report color-saturated green QD-LEDs possessing high efficiencies, high maximum brightness, and reduced efficiency roll-off, achieved by modifying the device structure which facilitates both electron transport and hole injection into QDs. As an emitting layer, about 1.5 monolayers of CdSe@ZnS QDs with chemical composition gradient in thick shell are employed. The devices show more than 2 % in external quantum efficiency at 10000 cd/m2, maximum brightness over 30000 cd/m2, which have not ever been reported yet to our knowledge. We also believe that more improvement can be attained by optimization of the layer thickness or material concentration.


3:15 PM *N4.5
Quantum Dot Solid State Lighting Technology and Products. Seth Coe-Sullivan, Sri Sadasivan, John Linton and Peter Kazlas; QD Vision Inc., Watertown, Massachusetts.

Quantum dots semiconductor nanocrystals have been considered in a broad range of applications, from biological tagging to LEDs, lasers, and solar cells. To date, however, the practical application of these materials has largely been limited to biological research and development. Recent advances in the development and application of efficient and stable quantum dots has for the first time enabled them to break into highly valuable commercial products. QD Vision’s Quantum LightTM optics are today addressing color quality and efficiency concerns in LED-based solid-state lighting systems. The presentation will describe how quantum dot technology is being applied to deliver step-change improvements in power efficiency and color quality in commercial lighting, bridging the gap between materials research and commercial products that add value in high growth markets.


3:45 PM N4.6
Colloidal Quantum-dot Light-emitting Diodes with High Injection Efficiency. Kyung-Sang Cho, Eun Kyung Lee, Tae-Ho Kim, Sang Jin Lee, Byoung Lyong Choi and Jong Min Kim; SAIT (Samsung Advanced Institute of Technology), Youngin-city, Gyeonggi-province, Korea, South.

Colloidal quantum-dot light-emitting diodes (QD-LED) have recently received considerable attention due to their ease of colour tunability, high brightness and narrow emission bandwidth. Although there have been rapid advances in luminance, efficiency and lifetime, device performance is still limited by the large energy barriers for hole and electron injection into the quantum-dot (QD) layer. Here, we demonstrated all solution process fabricated QD-LED devices using polymers as HTL and QDs and TiO2 sol-gel films as EML and ETL, respectively. All layers are fabricated by spin-coating based solution method. Based on the conduction band of TiO2, electron injection was efficient. Cross-linking of the QD-layer not only enabled solution process fabrication feasible but also reduced the band-offset between QD layer and HTL which enabled the efficient hole injection. Results of the QD-LED showed high luminance (>10,000 cd/m2) with low turn-on voltage (<2.0 V) and high power efficiency. [1] The effect of various QD structures on the QD-LED performance and the effect of cross-linking on the band-level of QD layer have been tested. QD display devices which are fabricated by QD patterning technique have been demonstrated. Reference [1] Cho, K.-S. et al. High-performance crosslinked colloidal quantum-dot light-emitting diodes. Nature Photonics 3, 341-345 (2009).


4:00 PM N4.7
Air-Stable Operation of Transparent, Colloidal Quantum Dot-based LEDs with a Unipolar Device Architecture. Vanessa C. Wood1, Matthew J. Panzer1, Jean-Michel Caruge2, Jonathan E. Halpert2, Moungi G. Bawendi2 and Vladimir Bulovic1; 1Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts; 2Chemistry, MIT, Cambridge, Massachusetts.

We demonstrate a new unipolar device architecture for colloidally-synthesized quantum dot light emitting devices (QD-LEDs). In the present structure, a QD multilayer is embedded in a transparent, n-type ceramic matrix, which, in contrast to earlier studies, is designed to enable only one type of charge carrier (in this case, electrons) to be injected into the device. We present results indicative of a novel, field driven mechanism for QD electroluminescence and demonstrate both green and red QD-LEDs, with a peak luminance of 1000 Cd/m2 and a luminous efficiency of 1 Cd/A from one face of our most efficient devices. These QD-LEDs exhibit long shelf lives and enable constant luminance over extended operating times in air, unpackaged. Electroluminescence from QDs with different peak emission wavelengths, which was first demonstrated in LEDs with organic charge transport layers [1], indicates the promise QDs hold as the emissive material in LEDs. More recently, QD-LEDs incorporating p- and n-type inorganic charge transport layers enabled air-stable electrical excitation of QDs in structures that operate via direct charge injection [2-3]. In these all-inorganic QD-LEDs, efficient electroluminescence is dependent on favorable energy band alignment between the charge transport layers and the emissive QD film. For this reason, different transport layers are often required to achieve red, green, and blue QD EL of comparable efficiency and luminosity. The unipolar architecture we present here reduces the need for precise band alignment and enables air stable, multicolor QD electroluminescence from the same device structure. [1] Coe, S., Woo, W. K., Bawendi, M. G. & Bulović, V. Nature 420, 800-803 (2002). [2] Mueller, A. H. et al. Nano Lett. 5, 1039-1044 (2005). [3] Caruge, J.-M., Halpert, J. E., Wood, V., Bawendi, M. G. & Bulović, V. Nature Photonics 2, 247-250 (2008).


4:15 PM N4.8
The Role of Surface Morphology in Infrared-emitting, Hybrid Silicon-nanoparticle-organic Light-emitting Devices. Kai-Yuan Cheng1, Rebecca Anthony2, Uwe R. Kortshagen2 and Russell J. Holmes1; 1Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota; 2Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota.

In this work, atomic force microscopy is used to characterize the film morphology of silicon nanoparticles (SiNPs) spun-cast onto a conducting polymer layer. The morphology of this system is of interest for use in efficient, infrared-emitting, hybrid nanoparticle-organic light-emitting devices (NP-OLEDs). Phase images of the SiNP films show increasing surface coverage on the conducting polymer layer as the nanoparticle solution concentration is increased. This trend correlates directly with NP-OLED performance, where efficient electroluminescence and suppression of polymer emission are only realized with uniform film coverage. Multilayer NP-OLEDs using SiNPs with a diameter of ~5 nm show electroluminescence in the infrared at a wavelength of λ~870 nm. A peak external quantum efficiency of (0.7±0.2)% is obtained in the forward viewing direction, and emission from the device originates solely from the nanoparticles up to a current density of 100 mA/cm2.


4:30 PM N4.9
Standards Development for Characterization of Quantum Dot Suspensions and Solids. Lynn Davis1, Seth Coe-Sullivan2 and Mike Leibowitz3; 1RTI International, Research Triangle Park, North Carolina; 2QD Vision, Inc., Watertown, Massachusetts; 3National Electrical Manufacturers Association, Rosslyn, Virginia.

Luminescent nanoparticles such as quantum dots (QDs) are beginning to appear in commercial products such as solid-state lighting (SSL) devices and photovoltaic cells. The allure of QDs in SSL and other emerging applications is the potential to provide enhanced device performance (e.g., improved energy efficiency, better color rendering properties, etc.) than is possible with conventional technologies. When used in SSL and other applications, QDs are typically incorporated into or coated onto a solid organic or inorganic matrix and then are excited using external stimuli (e.g., blue light with a maximum wavelength of 460 nm). This structure is vastly different from the colloidal environment in which QDs are typically synthesized and characterized. Bridging the gap between the measurements typically acquired by QD providers and those needed by potential end-users is currently difficult due to the absence of agreed upon standard test methods. Measurements taken in colloidal QD suspensions often do not translate to solid-phase material characterizations due to a variety of factors including sample preparation methods (e.g., temperature, solvents, etc), QD concentrations, and convolution effects arising from the presence of the solid matrix. Additionally, solid samples are more likely to exhibit diffuse reflectance and/or diffuse transmittance necessitating the use of an integrating sphere and a computer-controlled spectrometer to acquire accurate readings. This paper discusses the development of a standard test method to measure the quantum efficiency of QDs contained in solid organic and inorganic matrices. Examples of the use of this standard in evaluating QD-polymer composites intended for use in SSL devices will be provided and discussed. This standard is being developed under the auspices of the International Electrotechnical Commission, Technical Committee 113 on Nano-electrotechnologies.


4:45 PM N4.10
The Influence of the Electronic Structure of Organic Ligands on the Photoluminescence of Quantum Dot-organic Complexes. Emily Weiss, Kathryn Knowles, Daniel Tice, Eric McArthur and Gemma Solomon; Chemistry, Northwestern University, Evanston, Illinois.

This talk describes the dependence of the photoluminescence (PL) of CdSe colloidal quantum dots on the concentrations of a series of para-substituted anilines, An-X, where X varies in its electron donating or withdrawing character. We model this dependence with a combination of a binding isotherm (to describe the effect of the electronic substituents on the binding affinity of the aniline), and a model for the dependence of PL on the number of anilines bound to the surface of the quantum dots (to describe the effect of the electronic substituents on the ability of the aniline to passivate the surface of the quantum dot). Using the PL data, along with electronic structure calculations to predict the ionization potentials and binding geometries of the anilines, we differentiate ligands that are inductive passivators from those that actively reductively quench the PL of the quantum dots via hole transfer.


 

SESSION N5: Nanoparticle Photovoltaics I
Chair: David Norris
Wednesday Morning, December 2, 2009
Constitution A (Sheraton)


8:00 AM *N5.1
Solar Energy Conversion with Mesoporous Electrodes Prepared with Colloids. Michael Graetzel and Kevin Sivula, ISIC LPI, EPFL Lausanne, Lausanne, Switzerland.

Large surface area electrodes are essential for high efficiency solar energy conversion in dye sensitized solar cells and photoelectrochemical water splitting devices. In addition, the economical fabrication of these devices is required for the global implementation of these technologies. Inexpensive, solution-based techniques enabled by colloidal nanoparticle processing are ideal for this purpose. Here we describe the latest advances in optimizing the photon harvesting and the charge transport in these systems by applying novel nanostructures to traditional materials used in the DSC (TiO2) and solar water splitting (Fe2O3) as well as new materials.


8:30 AM *N5.2
Quantum Dot Sensitized Solar Cells. Manipulating Photoresponse Through Size Control. Prashant V. Kamat1,2, Kevin Tvrdy1,2 and David Baker2; 1Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana; 2Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana.

Semiconductor nanoparticle and nanotube assemblies provide new ways to develop next generation solar cells. Two different approaches have been considered to develop quantum dot sensitized solar cells: (i) By direct deposition of CdS nanocrystallites by successive immersion of TiO2 nanotubes in Cd2+ and S2- solution and (ii) by assembling different size CdSe quantum dots on TiO2 films using 3-mercaptopropionic acid as a linker molecule. Transient absorption spectroscopy and photoelectrochemical measurements have been carried out for establishing the mechanistic and kinetic aspects of interfacial charge transfer processes. Both these quantum dots are energetically capable of sensitizing TiO2 films and generating photocurrents in quantum dot solar cells. Upon bandgap excitation, these semiconductor quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. Maximum external quantum efficiency values of 55% and 26% are observed for CdS sensitized TiO2 nanotube and nanoparticulate architectures respectively. Similar efficiency values were also observed for CdSe quantum dots. The nearly doubling of IPCE observed with the TiO2 nanotube architecture is attributed to the increased efficiency of charge separation and transport of electrons. These composite semiconductor nanostructures can be tailored to tune the photoelectrochemical response via size control of CdSe quantum dots and improve the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. Ways to improve power conversion efficiency and maximize the light harvesting capability through the construction of a rainbow solar cell and carbon nanotube-semiconductor hybrid assemblies will be presented.


9:00 AM N5.3
Hot Electron Transfer from PbSe Nanocrystals. William A. Tisdale1, Kenrick J. Williams2, Brooke Timp2, Eray S. Aydil1, David J. Norris1 and Xiaoyang Zhu2; 1Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota; 2Chemistry, University of Minnesota, Minneapolis, Minnesota.

We use femtosecond time-resolved surface second harmonic generation spectroscopy to study the dynamics of electron transfer (ET) from photoexcited PbSe nanocrystals (nc's). Ultrathin films (1-2 monolayers) of hydrazine or ethanedithiol-treated PbSe nc's supported on a single crystal TiO2 rutile (110) substrate photoexcited far above the first exciton transition are capable of transferring electrons to TiO2 within 50fs. These ET rates are competitive with calculated timescales for electronic relaxation within the excited state manifold of the nc1 and faster than measured 1Pe to 1Se relaxation times2, indicating that ET occurs before complete thermalization of the "hot" electron. These results are consistent with measured PbSe/TiO2 band alignment using Ultraviolet Photoemission Spectroscopy. Strong electronic coupling is attributed to the large Bohr radius of the electron in PbSe and the close proximity of the nc to the substrate following ligand removal. Efficiency of ET is greatly enhanced at low temperatures due to slowing of competitive relaxation channels within the nc. [1] Kilina, Craig, Kilin, Prezhdo, J. Phys. Chem. C 111 (2007), 4871. [2] Schaller, Pietryga, Goupalov, Petruska, Ivanov, Klimov, Phys. Rev. Lett. 95 (2005), 196401.


9:15 AM N5.4
Multi-exciton Generation in Bulk and Nanocrystalline Semiconductors: Mechanism, Efficiency, and Interest for Solar Cells. Christophe Delerue1, Guy Allan1, Joep J. Pijpers2 and Misha Bonn2; 1ISEN, Institut d'Electronique, de Microelectronique et de Nanotechnologie, Lille, France; 2FOM Institute for Atomic and Molecular Physics, Amsterdam, Netherlands.

Using the rate of relaxation by impact ionization calculated for bulk and small quantum dots (QDs) [1], we have developed a simplified method to extend multi-exciton generation (MEG) calculations to larger QDs and bulk. As before, MEG is determined by a competition between impact ionization and intraband multiphonon relaxation. The results of this new procedure are in excellent agreement with our previous fully explicit simulations for Si and PbSe QDs [1,2]. The model is further justified by a comparison with measurements for bulk Si [3] and recent ones for PbSe and PbS [4]. We show that : 1) recent experimental measurements are now interpretable by the generation of excitons by impact ionization ; 2) the number of generated excitons increases with the size of the QDs and is the largest in the bulk semiconductor ; 3) QDs present higher energetic efficiency than the bulk ; 4) QDs of small bandgap semiconductors can be attractive for efficient solar cells. 1 G. Allan and C. Delerue, Phys. Rev. B 73, 205423 (2006). 2 G. Allan and C. Delerue, Phys. Rev. B 77, 125340 (2008). 3 M. Wolf, R. Brendel, J.H. Werner, and H.J. Queisser, J. Appl. Phys. 83, 4213 (1998). 4 J.J.H. Pijpers, R. Ulbricht, K.J. Tielrooij, A. Osherov, Y. Golan, C. Delerue, G. Allan and M. Bonn, (2009).


10:00 AM *N5.5
Quantum Dots for Light Harvesting and Emission in Hybrid Solar Cells and LEDs. David S. Ginger, Chemistry, University of Washington, Seattle, Washington.

Colloidal quantum dots offer many advantages for hybrid organic/inorganic devices in both light-harvesting and light-emitting applications. The energy levels and surface chemistry of quantum dots can have a dramatic influence on their performance. As one example, we describe the use of low-bandgap quantum dots as IR sensitizers of bulk heterojunction solar cells made from blends with conjugated polymers. By understanding why some materials combinations perform poorly, we identify new materials combinations that offer substantially improved photovoltaic performance in the IR. Furthermore, we study how the dots themselves can alter the local environment, affecting long time carrier dynamics on the organic hosts. Finally, we review the effects of surface chemistry, energy level alignment, and local charge transport on the inverse process—light emission from quantum dots in hybrid LED structures.


10:30 AM *N5.6
Full-spectrum Solution-processed Solar Cells. Edward Sargent, Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada.

We review progress in solar cells that harvest infrared as well as visible wavelengths, and that are fabricated via solution processing. We focus in particular on colloidal quantum dot solar cells.


11:00 AM N5.7
Performance Enhancement of Nanocrystal-Conductive Polymer Hybrid Solar Cells Through Chemical Vapor Annealing. Yue Wu, School of Chemical Engineering, Purdue University, West Lafayette, Indiana.

Semiconductor nanostructure-based solar cells are promising candidates for future-generation photovoltaic devices due to their advantages in low temperature processing, scalability, and low cost. Among the different types of nanostructure-based solar cells, hybrid solar cells possess a unique architecture of a solid-state blend of interpenetrating and percolating inorganic nanostructures and organic polymers. In principle, hybrid solar cells should demonstrate a better performance when compared with organic polymer bulk heterojunction solar cells due to the higher absorption coefficient of inorganic semiconductor nanomaterials. However, so far there has been no report of higher performance observed in hybrid solar cells than organic bulk heterojuction solar cells in the real experiment, which can be mainly attributed to the existence of non-conductive surfactant molecules with long alkyl chain, for example, phosphonic acids, on the surface of nanocrystals. A few recent reports on the surface ligand exchange reactions tried to address this issue, however, all of them ended up with aggregated nanocrystals after the replacing of long alkyl chain surfactants with short chain molecules. Once the aggregation happens, the nanocrystals will precipitate out of the solution and can not be mixed uniformly with conductive polymer any more for solution process of hybrid solar cell fabrication. In our research, we have demonstrated that a chemical vapor annealing process can be used to significantly improve the performance of CdSe/P3HT hybrid solar cells though a surface ligand exchange reaction. This method represents a new approach to enhance the charge separation/transport for solution-processed high-performance photovoltaic solar cells by replacing bulky surfactants binded on nanocrystal surface without the sacrifice of solubility of nanocrystals in organic solution and polymer blend.


11:15 AM N5.8
Photoionization in the Carrier Multiplication Regime in PbSe and InAs Nanocrystals. John McGuire, Milan Sykora, Jin Joo, Jeffrey M. Pietryga and Victor I. Klimov; Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, New Mexico.

Absorption of a single photon can yield multiple electron-hole pairs through the process of carrier multiplication (CM). Substantial evidence exists in the literature for the enhancement of CM quantum efficiencies (QE) in quantum-confined nanocrystals (NC) compared to their respective bulk materials, but recently quantitative determination of the CM quantum efficiency (QE) has been a topic of considerable debate. While virtually all studies of CM in NCs have used time-resolved optical spectroscopy to study Auger recombination of multiexcitons in the single-photon absorption regime, reported values of the multiexciton yield have varied by up to an order of magnitude. In recent work, we have shown that photoionization of NCs can yield NC cores with long-lived charges. In part because charged biexcitons are characterized by greater emission rates than neutral ones, the presence of such charged particles in a photoexcited NC ensemble can lead to overestimations of CM yields. Here we present steady-state and time-resolved studies of absorption and emission by solutions of well-passivated PbSe and InAs NCs showing broad evidence of photoionization under excitation by photons with energy above the CM threshold at fluences as low as 0.001 absorbed photons per NC per single-exciton radiative lifetime, which is well below the fluences typically used in time-resolved optical studies of CM. In contrast, these NCs do not show evidence of photoionization at photon energies of 1.5 eV, hundreds of meV above the NC absorption band gap but below the CM threshold. These results are of broad relevance to spectroscopic studies of NCs and particularly to understanding the discrepancies in reported CM QEs.


11:30 AM N5.9
Observation of Exciton and Multiexciton Fluorescence Intermittency in Single CdSe(CdZnS) Core(shell) Nanocrystals. Jing Zhao, Gautham Nair and Moungi Bawendi; Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Semiconductor nanocrystals are known to emit light intermittently, i.e., to “blink”, under continuous illumination. The blinking mechanism is usually attributed to a nonradiative Auger-like process involving excess free carriers. In this work, we study the blinking of exciton and multiexciton of single CdSe(CdZnS) core(shell) nanocrystals by single nanocrystal photoluminescence microscopy. Single nanoscrytals are excited at different power by a pulsed laser. Fluorescence intensity time traces of exciton luminescence and spectrally-resolved triexciton emission are found to be highly correlated. The observation of triexciton blinking provides new insights to the single nanocrystal blinking mechanism.


11:45 AM N5.10
Multiexcitons in a Single Core-shell Colloidal Quantum Dot. Viki Kloper1, Dima Cheskis1, Ruth Osovskly1, Aldona Sashchiuk1, Martin Kroner2 and Efrat Lifshitz1; 1Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute,, Technion, Haifa, Israel; 2Institute of Quantum Electronics, ETH Hönggerberg, Zürich, Switzerland.

The study of multiexcitons in colloidal quantum dots (CQDs) is a timely topic, as multiexcitions in CQDs are the fundamental process in gain devices, photovoltaic cells and a single photon light sources. However, these excitons decay primarily via nonradiative Auger relaxation at a time scale of 10-100 ps, due to the enhancement of many-body interactions inside CQDs cores with a typical diameter of 3-4 nm surrounded by low dielectric medium. Despite the predicted limitations, the present work explicity showed an emission of neutral biexciton (BX), triexciton (TX) and quandraexciton (QX) in a time-integrated micro-photoluminescence (μ-PL) spectrum (at 4.2 K) of a single quasi-type II CdTe/CdSe CQD [1, 2]. The multiexcitons were generated by a sequential filling of the s- and p-electronic shells with the increase of a continuous-wave excitation power. The identification of the multiexciton emission bands was based on a comparison between a plot of the emission pumping power dependence and a theoretical plot, comprised of a calculated probability of multiexciton emission versus the number of excitons created at steady-state conditions in a single CQD. Further on, a contour plot of the emission bands’ intensity versus energy and observation duration showed a remarkable spectral stability, revealing blinking-free behavior. The binding energy of a multiexciton was determined by a standard second order perturbation theory, showing close agreement with the relevant experimental multiexciton emission energy. Finally, based on angular momentum selection rules the lifetime of a BX, TX and QX was evaluated to be shorter than that of the single exciton (~ 800 nsec at 4.2K and 20 nsec at 300K) by a factor of 1/2, 1/6 and 1/4, respectively, thus showing an extension of the multiexciton lifetimes to a nsec range, in a quasi-II CdTe/CdSe core-shell NQDs. The multiexciton emission observed under steady state excitation suggests a significant suppression of the nonradiative Auger process, related to a weakening of Coulomb interaction by carriers’ charge separation, as well as a reduction of the effective dielectric contrast between the core and its immediate surrounding in the CdTe/CdSe core-shell CQDs. This upturn should permit charge extraction in photovoltaic cells and inversion of population in a gain device. [1] V. Kloper et al, “The growth of colloidal cadmium telluride nanocrystal quantum dots in the presence of Cd0 nanoparticles”, J. Phys. Chem. C 111, 10336 (2007). [2] R. Osovsky, D. Cheskis, V. Kloper, A. Sashchiuk, M. Kroner, and E. Lifshitz “Continuous-wave pumping of multiexciton photoluminescence of a single colloidal quantum dot”, Phys. Rev. Lett., (in press) (2009)


 

SESSION N6: Nanoparticle Photovoltaics II
Chair: Masaru Kuno
Wednesday Afternoon, December 2, 2009
Constitution A (Sheraton)


1:30 PM *N6.1
Quantum Dot Solar Cells. M. C. Beard, O. Semonin, Aaron Midgett, M. Law, J. M. Luther, J. C. Johnson, R. J. Ellingson, B. Hughes and Arthur J. Nozik; NREL, Golden, Colorado.

We have constructed simple, all-inorganic QD solar cells from PbS and PbSe QDs that produce large short-circuit photocurrents equivalent to bulk Si solar cells without the need for superlattice order or separate phases for electron and hole transport. However, the photovoltage is small (typically < 0.3 V). The QD films, deposited via layer-by-layer (LbL) dip coating, yield solar power conversion efficiency of up to 3-4 % at AM1.5. Recent results on the improvement of these QD solar cells and experiments to measure photocurrent quantum yields greater than 1.0 created by multiple exciton generation (MEG) and the description of carrier transport in these cells will be described.


2:00 PM *N6.2
Charge Transport in All Inorganic Solar Cells from Colloidal Nanoparticles. Sue Carter, University of California, Santa Cruz, Santa Cruz, California.

All-inorganic solar cells made from colloidal nanoparticles offer many of the cost advantages of organic-based solar cells while enabling greater absorption, higher mobilities, and greater stability. Understanding the charge transport mechanisms in the bulk and at the interfaces is required for device optimization. In this talk, I'll present our groups results on CdTe/CdSe and CdTe-only solar cell devices. The charge transport is strongly impacted by the sintering conditions which modifies both the the trap states, as measured by photothermal deflection spectroscopy, and the grain growth dynamics. Temperature dependence of the short circuit current and open circuit voltage are distinctly different for these device, consistent with CdTe/CdSe being a pn jucntion and CdTe only being Schottky-type junction, with the transport dominated by tunneling enhanced bulk and interface recombination, respectively. By optimizing the sintering conditions and film thicknesses, power efficiencies greater than 3% and 5% have been obtained for CdTe pn and Schottky junction devices, respectively, with the best CdTe nanoparticle solar cells having internal quantum efficiencies approaching 100%. We conclude by comparing these results with our studies of PbS nanoparticle solar cells fabricated using layer-by-layer deposition, where quantum confinement plays a more critical role.


2:30 PM *N6.3
Fabrication of Thin Film Solar Cells from Cu(In,Ga)(S,Se)2 Nanocrystal Inks. Hugh Hillhouse, Rakesh Agrawal and Qijie Guo; Purdue University, West Lafayette, Indiana.

The development of suitable colloidal nanocrystal inks are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photoabsorbing layer. Chalcopyrite structure copper indium diselenide (CuInSe2) and related materials such as Cu(In,Ga)(S,Se)2 are key photoabsorbing materials for existing thin film solar cells due to their near ideal band gap and their serendipitous defect chemistry. Although several methods have been reported which describe the synthesis of CIGS and related nanocrystals, precise control of the composition for these ternary and quaternary compounds has been problematic [1]. Here, we report the solution-phase synthesis of stoichiometric chalcopyrite structured CuInSe2 nanocrystals [2] and Cu(in,Ga)S2 [3]. The syntheses proceed rapidly from elemental and halide reagents via a simple batch reaction without “hot injection” in a single component coordinating solvent. The use of these nanocrystals for solar cells has been demonstrated by fabricating Mo/CIGSSe/CdS/i-ZnO/ITO/Ni/Al devices without using any oxygen-free techniques (after NC synthesis). The nanocrystal inks are used to form nanocrystal coatings on a back contact (Mo coated sodalime glass in this case). The nanocrystal layer is the easily consolidated into large crystalline chalcopyrite domains by a brief thermal treatment. The fabricated cells are robust and increase in efficiency with time exhibiting similar serendipitous defect chemistry as layers formed by vacuum co-evaporation. After 6 months in normal laboratory conditions the CuInSe2 nanocrystal ink based cells have an efficiency of 2.8% under AM1.5 illumination [2] while cells fabricated starting with Cu(In,Ga)S2 are 5.1% efficient [3]. Even more recent improvements have topped 10% power conversion efficiencies. The presentation will focus on the key aspects of the nanocrystal synthesis, nanocrystal consolidation, and device fabrication. References: [1] Hillhouse HW & Beard MC, “Solar cells from colloidal nanocrystals: Fundamentals, materials, devices, and economics.” Current Opinion in Colloid & Interface Science (2009), In press & available online at: http://dx.doi.org/10.1016/j.cocis.2009.05.002 [2] Guo, Q.J., Kim, S.J., Kar, M., Shafarman, W.N., Birkmire, R.W., Stach, E.A., Agrawal, R., Hillhouse, H.W., Nano Letters 8, 9, 2982-2987 (2008). [3] Qijie Guo, Grayson M. Ford, Hugh W. Hillhouse, and Rakesh Agrawal, Nano Letters (2009), ASAP, DOI: 10.1021/nl901538w


3:00 PM N6.4
Solution Processed SnS:PbS Nanocrystal Heterojunction Broadband Solar Cells. Alexandros Stavrinadis, Jason M. Smith, Christopher A. Cattley and Andrew Watt; Materials, University of Oxford, Oxford, Oxfordshire, United Kingdom.

We report the synthesis, characterization and application of Tin Sulfide (SnS) and Lead Sulfide (PbS) nanocrystals in a type-II heterojunction solar cells with a strong IR photoresponse and improved power conversion efficiency as compared to PbS Schottky-junctions. Over the last 2 years PbS nanocrystals have yielded single junction solar cells with photocurrents in excess of 21mA/cm^2 and infrared tunable photoresponses. However their power conversion efficiencies are limited by small open circuit voltages (Voc=0.1-0.3V) generated at the nanocrystal/metal Schottky barrier. In addition, the photoresponse of these devices is limited by the relatively thin (~150nm) depletion region at the Schottky junction. A way of bypassing both problems is to utilize a second material to form a type-II heterojunction with PbS nanocrystals. We demonstrate that solution processed SnS: PbS nanocrystal bilayer solar cell behave as a good type-II heterojunction. SnS nanocrystals were synthesized using an oleic acid/oleylamine coordinated colloidal synthesis method. Nanocrystal size could be tuned using growth temperature. We characterized morphology and structure using XRD, TEM and HREM. Optical spectroscopy shows that SnS has a direct bandgap of 1.44eV creating an infrared optical window. We fabricated thin nanocrystal films with fine thickness control (sub 10nm) using an ethylendithiol solid state ligand exchange dip coating method. SnS nanocrystal films are photoconductive and at 1V applied bias current density increases by 200 times under AM1.5 illumination. SnS/PbS bilayer junctions showed increased Fill Factors and more than two fold increase in Voc compared to PbS control devices. Devices with varying electrodes (ITO/Al and ITO/Au) show that at open-circuit conditions, the polarity of the bilayer is determined by the bilayer order and not by the built-in electric field generated by electrodes. This indicates that a type-II heterojunction is formed and the nature of the junction can be understood by the relative difference between Voc and Isc for different geometries. We conducted experiments with varying SnS and PbS thicknesses to optimize the efficiency of the solar cells and further study the heterojunction characteristics. Voc depends on SnS film thickness and SnS and PbS nanocrystal size. Short circuit current was enhanced by employing a post-synthesis processing step to the SnS nanocrystals. This step aimed to remove impurities from the SnS and effectively doubled the bilayer’s Isc without compromising Voc. Spectral response measurements and charge mobility measurements were conducted in order to further understand the films’ properties and feedback applications. The bilayer devices have more than 10% external quantum efficiency in the infrared and our data suggests a strong prospect of further improvement in Isc whilst retaining high Voc. These devices open the way for multi-junction nanocrystal photovoltaics with tuned broadband light absorption.


3:30 PM *N6.5
Solar Cells Based on Junctions Between Colloidal PbSe Nanocrystals and ZnO. Eray Aydil, Kurtis S. Leschkies, Timothy J. Beatty, Alan Jacobs, Moon Sung Kang and David J. Norris; Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota.

We report a new type of solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We find that the photovoltage depends on the QD size and increases linearly with the QD effective band gap energy. Thus, these solar cells resemble traditional photovoltaic devices based on a semiconductor-semiconductor heterojunctions but changing the size of the QDs varies the band gap of one of the semiconductors and hence the cell’s photovoltage. Under simulated 100 mW/cm2 AM1.5 illumination, these QD solar cells exhibit short-circuit currents as high as 15 mA/cm2 and open-circuit voltages up to 0.45 V, larger than that achieved with solar cells based on junctions between PbSe QDs and metal films. Moreover, we show that incident-photon-to-current-conversion efficiency in these solar cells can be increased by replacing the ZnO films with vertically-oriented array of single crystal ZnO nanowires and by infiltrating this array with colloidal PbSe QDs. The distance between the ZnO nanowires is on the order of the exciton diffusion length in PbSe QD films but the nanowires can be made much longer than the exciton diffusion length to increase light absorption. When illuminated with 100 mW/cm2 simulated AM1.5 spectrum, these nanowire quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin film ZnO devices constructed with the same amount of QDs. Supporting experiments on thin film transistors made from the PbSe QDs and sensitivity of these transistors to nitrogen and oxygen will be shown to argue that the solar cells described above are unlikely to be operating like traditional p-n heterojunction solar cells. All data, including significant improvements in both photocurrent and power conversion efficiency with increasing nanowire length suggest that these photovoltaic devices operate as excitonic solar cells.


4:00 PM *N6.6
Investigations of Dipole Enhancement of Photovoltaic Properties - Use of J2NQDs. David M. Schut1, Thomas Novet1, Thomas Allen1, George M. Williams1, Bruce Parkinson2 and Justin Sambur2; 1Nanophotonics Group, Voxtel, Beaverton, Oregon; 2Chemistry, University of Wyoming, Laramie, Wyoming.

It has been amply demonstrated that ligands can affect and modulate the optoelectronic properties of colloidally synthesized nanocrystalline quantum dots. What we are going to demonstrate is the effects of using J2NQDs (Janus Type II nanocrystalline quantum dots - in which each hemisphere of a 0-dimensional nanoparticle has a different ligand on it) upon the enhancement of photovoltaic properties through increased efficiencies and enhanced charge separation of generated excitons.


4:30 PM N6.7
PbSe Nanocrystal Excitonic Solar Cells. Joshua J. Choi1,2, Yee-Fun Lim4,2, Mitk El B. Santiago-Berrios3, Matthew Oh1, Byung-Ryool Hyun2, Liangfeng Sun2, Adam C. Bartnik2, Augusta Goedhart1, George G. Malliaras4, Hector D. Abruna3, Frank W. Wise2 and Tobias Hanrath1; 1Chemical Engineering, Cornell University, Ithaca, New York; 2Applied Physics, Cornell University, Ithaca, New York; 3Chemistry, Cornell University, Ithaca, New York; 4Materials Science and Engineering, Cornell University, Ithaca, New York.

Colloidal semiconductor nanocrystals (NCs) potentially offer major benefits as photovoltaic materials in next-generation solar cells. Synthetic adjustments in the NC size, shape and composition provide control over electronic and optical properties. Coupled with their high absorption cross section and low-cost solution based synthesis and processing, these materials hold enormous promise for the efficient solar energy conversion in inexpensive, thin film photovoltaic devices. Recent progress with NC-organic hybrid devices, NC bilayer devices and Schottky devices have shown encouraging performance of 1 ~ 3 % 1-sun power conversion efficiency. Rational progress towards more efficient NC based solar cells will require better fundamental understanding of the NC energy levels and interfacial charge transfer mechanisms. The challenge in probing the intrinsic characteristics of the NC active layer stems from the fact that the integration of colloidal NCs into working devices involves chemical and physical treatments that inevitably modify the NC surface and related electrical, optical, and structural properties. For example, although photovoltaic test structures based on metal/NC Schottky junctions have shown encouraging performance and provided initial insights into the extraction of photogenerated charges from NCs, the direct metal/NC contact introduces strong electronic perturbation to the NC and hence partially obscures information about the intrinsic size dependent photovoltaic performance of the NC active layer. We present the design, fabrication and characterization of colloidal PbSe NC based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the ‘intrinsic’ photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4% - ranking among the highest performing NC based solar cells reported to date.


4:45 PM N6.8
Encapsulation of Zinc Oxide Nanorods for Use in Photovoltaic Applications. Jason W. Soares1, Jagdeep Singh2, Jisun Im2, James E. Whitten2 and Diane M. Steeves1; 1US Army Natick Soldier RD&E Center, Natick, Massachusetts; 2Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts.

Metal oxide nanoparticles, nanotubes, nanorods, nanowires and whiskers are finding important applications ranging from catalysis to optoelectronics. Because of its unique semiconducting and optical properties, zinc oxide has been used for light-emitting diodes, photovoltaic devices, heterogeneous catalysis and chemical sensors. In organic-inorganic hybrid devices, it is often desirable to covalently attach organic molecules to metal oxide surfaces such that electrons and holes may be transported across the inorganic-organic interface. Future hybrid devices may require encapsulation of semiconducting inorganic materials within organic matrices. Here, we present a novel method for encapsulating zinc oxide nanorods and nanoparticles within an organic matrix, which consists of stirring and heating an ethanolic nanorod/organothiol mixture. This leads to complete encapsulation of the nanorods within a thick (e.g., 100-500 Å) organic shell comprised of a 1:2 Zn:thiol complex. The thickness and morphology of the encapsulating layer is controllable by choice of thiol and preparation conditions. Furthermore, because a large selection of functionalized thiols is available, it is possible to surround ZnO nanoparticles with a variety of chemical functional groups for subsequent interactions. While previous studies have demonstrated that alkanethiols adsorb on ZnO surfaces and nanoparticles, the present work is the first to demonstrate encapsulation of ZnO nanoparticles. The effect of encapsulation on the photoluminescence of nano-ZnO will be presented. It is observed that encapsulation with an organothiol results in quenching of both the UV and visible emission peaks. This is contrary to previous results achieved with silane modifiers, in which the UV emission was enhanced while the visible emission was maintained. This method may also be useful in future photovoltaic applications in which one wishes to surround ZnO nanorods and whiskers with light-absorbing molecules, which could be achieved by using thiol-terminated dyes. Attachment of one such dye, a thiol-terminated ruthenium-based dye, is demonstrated. Characterization of the electronic structure of the adsorbed dye on ZnO surfaces using ultraviolet photoelectron spectroscopy is presented, and measurements of the electro-optical properties of the complex are discussed.


 

SESSION N7: Poster Session
Chair: Jonathan Steckel
Wednesday Evening, December 2, 2009
8:00 PM
Exhibit Hall D (Hynes)


N7.1
Ferromagnetic Resonance in Single-Crystal and Single-Domain Cobalt Nanoparticles. Jaakko Timonen1, Khattiya Chalapat1, Matti Sarjala1, Robin Ras1, Sorin Paraoanu1, Eira Seppala2, Markku Oksanen2, Mikko Alava1 and Olli Ikkala1; 1Helsinki University of Technology, Espoo, Finland; 2Nokia Research Center, Helsinki, Finland.

Typical ferromagnetic resonance (FMR) spectra measured from magnetic nanoparticles show broad resonances from which the surface contributions to the resonance frequencies are difficult to estimate. The two most apparent origins for the broadening are the magnetic damping and the finite nanoparticle size and shape distributions. In order to clarify the origins of the measured resonance widths and the dependence of them on the surface anisotropy, we measured and analyzed FMR spectra for cobalt nanoparticles with different mean diameters. We followed the Puntes-Alivisatos method and synthesized nearly monodisperse spherical single-crystal epsilon-cobalt nanoparticles that were mixed with polystyrene to form composites. Synthesis and mixing were done in oxygen-free conditions and the composites were exposed to atmospheric oxygen only during molding and measurement. The mean particle diameter was varied from five to twenty nanometers and the nanoparticle volume fraction in the composites from five to fifteen percent. Magnetization in such nanoparticles is expected to be single-domain and to respond to a microwave field by rotating uniformly. Furthermore, exchange modes are not expected. Complex microwave permeabilities in the frequency range 1-18 GHz were extracted from the reflection, transmission, and delay coefficients of a microwave field propagating through the samples inserted in a coaxial airline. From the permeability spectra, it was is shown how the FMR changes as a function of the mean particle size. In addition to demonstrating the nanoparticle size effect, the relative contributions to the observed resonance widths from the damping and the particle size distributions were estimated. Finally, we comment on the applicability of this type of materials for different microwave applications.


N7.2
Preparation of Superfine Copper Particle by Liquid Phase Reduction. Qingming Liu1,2, Kazuaki Nishio1, Takehiro Yasunami1, Debi Zhou2, Ryoichi Ichino1 and Masazumi Okido1; 1Nagoya University, Nagoya, Japan; 2Central South University, Changsha, China.

Preparation of copper nanoparticle is currently an area of intense scientific research, due to a wide variety of potential applications in conductive paste, coating material, powder metallurgical materials and electronic circuit. Superfine copper particles were prepared by liquid phase reduction technique. Cu2+ was reduced to copper particle by adding ascorbic acid into the solution. The effect of different conditions such as Cu2+ concentration, solution pH, additive PVP concentration, and drip rate was examined in contrast experiments. The optimum conditions were found to be as follows: Cu2+ concentration, 0.2 M; solution pH, 3; PVP concentration, 0.75 mM; and drip rate is 3000 ml/min. The SEM image of copper particle prepared under optimum condition indicated that it has spherical or near-spherical appearance and the size distribution showed an average size of 535 nm. Additionally, the effect of driving force of reductant was also explored in this paper. H3PO2, Ti2(SO4)3 and NaBH4 were chosen as reductant, respectively. It can be concluded that there is a firm relationship between the driving force of reductant and size of copper particles. The results showed that the copper particles’ size decreased with the increasing of driving force of reductant; the smallest size of copper particle was 38 nm when NaBH4 was used as reductant at Cu2+ concentration, 0.002 M; pH, 3; PVP concentration, 0.75 mM; drip rate, 3000 ml/min and solution temperature is 333 K.


N7.3
Parameters Influencing Charge Transfer In Quantum Dot Dye-Sensitised Solar Cells (DSSCs). Henry C. Leventis and Saif A. Haque; Chemistry, Imperial College London, London, Greater London, United Kingdom.

Semiconductor quantum dots are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. In addition, ability to generate multiple excitons from a single, high energy, photon represents an extremely desirable property in the optimisation of solar cell efficiencies.(1,2) A significant part of our ongoing research efforts is targeted at the development of dye-sensitised solar cells(3) where semiconductor nanocrystals serve as the light harvesting component. Gaining an understanding of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals high injection yields, and efficient and long-lived charge separation at the interface, as well as high yields of regeneration by the redox mediator/hole conductor. In particular, we focus on the parameters influencing charge transfer in DSSCs using colloidal PbS quantum dots. This material is of particular interest due to the low bandgap of PbS (bulk bandgap ≈ 0.41 eV),(4) and the strong quantum confinement incurred upon photogenerated electrons and holes in PbS quantum dots (QDs) which results in a significant size-dependence of the nanocrystal bandgap. Our aim is to study the impact of varying the thermodynamic driving forces on the yield of both the electron injection and hole regeneration processes occurring within the DSSC by modulating the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn (right figure). Whilst efficiencies approaching 1.5 % under AM 1.5 illumination have been reported for solid-state DSSCs using PbS nanocrystal sensitisers(5) on mesoporous TiO2 films, improvements could be realised by addressing the poor incident photon to current conversion efficiencies (IPCEs) of these devices. This can be achieved by improving the yield of charge separation at the respective interfaces in the DSSC architectures, whilst also minimising interfacial charge recombination losses. 1. R. J. Ellingson, M. C. Beard, J. C. Johnson, P. R. Yu, O. I. Micic, A. J. Nozik, A. Shabaev and A. L. Efros, Nano Letters 2005, 5, (5), 865-871. 2. V. I. Klimov, Annual Review of Physical Chemistry 2007, 58, 635-673. 3. B. O' Regan and M. Grätzel, Nature 1991, 353, (6346), 737-740. 4. S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina and E. H. Sargent, Nature Materials 2005, 4, (138), U114. 5. H. J. Lee, H. C. Leventis, S. J. Moon, P. Chen, S. Ito, S. A. Haque, T. Torres, F. Nüesch, T. Geiger, S. M. Zakeeruddin, M. Grätzel and M. K. Nazeeruddin, Advanced Functional Materials 2009, In Press.


N7.4
Effect of the Microstructure on Light Emission of Luminescent Nanoparticles Based Films: Application to Light Conversion in White LEDs. Amelie Revaux1, Geraldine Dantelle1, Dominique Decanini2, Anne-marie Haghiri2, Thierry Gacoin1 and Jean-Pierre Boilot1; 1Laboratoire de Physique de la Matière Condensée, Palaiseau, France; 2Laboratoire de Photonique et de Nanostructures, Marcoussis, France.

Rare-earth doped oxides are well-known for their applications in light emitting devices. Most systems rely on the light conversion from a UV or blue source of excitation into visible light within films of phosphors deposited on a substrate. The microstructure of these layers controls both the propagation of the light of excitation and emission as well as the extraction of the latter. It has therefore a direct impact on their external efficiency. The effect of the microstructure can be separated into two categories. On the one hand, the structure of the luminescent particles themselves, including the size, shape and concentration of the crystals determines the emission yield, absorbance and diffusion properties of light within the material. On the other hand, the structure of the matrix, including surface texturation, porosity or diffusive inclusions, determines the transport properties of the emitted light in the layer and its coupling to the outside of the film. The optimization of both contributions is crucial to obtain brighter and more efficient films. We propose here to study separately the influence of these two classes of microstructural effects by considering the emission properties of colloidal oxide nanoparticles[1][2], incorporated into sol-gel matrices (SiO2 or TiO2) deposited as thin films. We first use molecular rare-earth chelates as light emitters, which do not induce any microstructure effects and allow us to focus on the structure of the film itself. The surface of the sol-gel films are periodically textured by soft nanoimprint lithography. We observe that the external luminescence yield can be enhanced by the photonic crystal and that the angular distribution of the emitted light is highly dependent on the chosen geometry[3]. The second part of our work investigates the effect of the size and the refractive index of surrounding medium on the emission properties of rare-earth oxides nanoparticles. The emission yield of nanoparticles is lower than the one of the bulk material and the difference, often justified by surface defaults and crystalline distortions, can be explained by the lower effective refractive index of the surrounding medium[4]. Finally we incorporate these rare-earth oxide nanoparticles in sol-gel structured films. When using YAG:Ce nanoparticles, these layers act as light converters resulting in a white light emitting device if deposited on blue LEDs. Emission properties of this device are discussed in relation with microstructural effects. [1]A.L.Pénard et al., Acc.Chem.Res., 2007, 40, 895 [2]R.Kasuya et al., Appl.Phys.Lett., 2007, 91, 111916 [3]T.A.Truong et al., Appl.Phys.Lett., 2009, 94, 023101 [4]G.Mialon et al., submitted to nanoletters


N7.5
Thermal Conductivity of Nanofluids with Well Dispersed and Clustered Nanoparticles. Michael J. Fornasiero, MANE, Rensselaer Polytechnic Institute, Troy, New York.

Nanofluids, engineered colloidal suspensions of nanometer-sized particles in a carrier fluid, have been shown to have great potential as a new class of heat transfer fluids. These systems exhibit increased thermal conductivity which cannot be explained with existing theories for effective media. Currently, the enhanced thermal conductivity is thought to be due to nanoparticle clustering and networking, which could provide a low resistance path to heat transfer. In this context, the thermal conductivity of nanoparticle suspensions with well dispersed and clustered gold particles are measured employing the transient hot wire technique. Nanoparticle clustering is facilitated by EDC reaction of amine and carboxyl functionalized nanoparticles.


N7.6
Optical Investigations of Ultrasmall Alkyl-passivated Silicon Quantum Dots. Tibert van der Loop and Tom Gregorkiewicz; University of Amsterdam, Amsterdam, Netherlands.

Silicon quantum dots were made through a solution-based synthesis method, yielding ± 1.5 nm sized alkyl-capped Si nanocrystals. In this method, crystal growth is controlled by inverse micelles that are purposefully introduced in solution. After growing, the nanocrystals are passivated with alkyl chains of selectable length. The procedure yields free-standing nanocrystals that are relatively insensitive towards oxidation. Upon optical excitation, two separate luminescence bands are observed - one in the UV and one in the blue. These can be selected by excitation wavelength. It is speculated that these bands arise from conduction band splitting at the Γ25 point due to quantum confinement. The current aims of this on-going study are better size-control of the nanocrystals and detailed identification of the microscopic origin of the measured luminescence bands.


N7.7
Influencing Free Carrier Generation and Lifetimes by Varying Dielectric Constants. Kevin Noone, Nick Anderson and David S. Ginger; Chemistry, University of Washington, Seattle, Washington.

One disadvantage that organic photovoltaics suffer compared to their inorganic counterparts results from their low dielectric constants. Whereas absorption of a photon by tradition inorganic solar cells creates free charge carriers, the same event in organic materials results in coulombically bound electron-hole pairs, called excitons. These excitons are separated at the interface between a donor and an acceptor, but the separated charges still remain coulombically bound and this attraction must be overcome to generate free carriers. Raising the overall dielectric of the active layer in an organic solar cell could weaken this coulombic attraction, resulting in higher dissociation efficiencies and reducing recombination losses. Employing colloidal inorganic nanoparticles is one strategy to increase the overall dielectric constant of a donor-acceptor blend while retaining the advantage of solution-processability. In this work, we use quasi-steady state photoinduced absorption spectroscopy to study the effect of the dielectric constant of the electron acceptor on charge carrier lifetimes in polymer blends. We vary the dielectric constant by employing CdSe quantum dots, as well as TiO2 and ZnO nanocrystals, noting that using these materials in place of PCBM increases charge carrier lifetimes. We explore these differences in lifetime and their implications for organic based, solution-processed photovoltaics.


N7.8
Abstract Withdrawn


N7.9
Spectral Switching of Type-II Quantum Dots by Charging. Jiwon Bang, Bonghwan Chon, Nayoun Won, Jutaek Nam, Taiha Joo and Sungjee Kim; Chemistry, POSTECH, Pohang, Korea, South.

Properties of charged semiconductor quantum dots (QDs) attract great interest for their potential applications such as electro-optical devices. However, the studies on charged QDs have been limited to type-I QDs. Type-II QDs can have electron-hole pairs that are spatially separated, and their effective band gaps are heavily governed by their band offsets of the cores and shells. We demonstrate photoluminescence (PL) spectral switching of type-II QDs by chemically controlling transfers of electrons in and out of the type-II heterostructures. When electron charged, CdTe/CdSe (Core/Shell) QDs show huge PL blue shifts, up to ~ 100 nm shifts, whereas PL of CdSe/CdTe (Core/Shell) QDs red-shifts. The degree of PL shifts is dependent on the charging time and the dimensions of type-II characteristics of QDs. Reversible spectral switching of type-II QDs is produced by repeated charging and neutralization processes. As control samples, bare CdSe, CdTe and homogeneous alloyed CdTexSe1-x QDs were prepared. They showed within a few nm spectral shifts upon charging. The huge PL spectral shifts of type-II QDs are due to the interactions between injected spectator electrons and type-II character excitons. Electron charged CdTe/CdSe QDs show PL blue shifts because of the strong repulsions between the shell-localized excitonic electrons and injected spectator electrons. It is opposite for CdSe/CdTe QDs, where injected electrons in surface states attract the holes in the shells. We will discuss solid state electro-chemical type-II QD charging devices for electro-optic modulating applications, which are expected to overcome the colloidal stability limitations of current solution-based charging.


N7.10
Solution-Phase Synthesis of III-VI and I-III-VI Nanocrystals at Low Temperatures Using Dialkyl Dichalcogenide Precursors. Matthew A. Franzman, Michelle E. Norako and Richard L. Brutchey; Department of Chemistry and the Center for Nanoscience and Technology, University of Southern California, Los Angeles, California.

Solution-phase synthetic reactions have proven to be viable routes toward metal chalcogenide nanocrystals; however, these reactions are often reliant upon high temperatures, designer single-source precursors, or environmentally harmful reagents. To overcome these obstacles, we have developed a versatile method for the relatively low temperature synthesis of metal chalcogenide nanocrystals using dialkyl dichalcogenides as the chalcogen source. These precursors decompose in solution to promote the growth of kinetically controlled nanoscale products. Well-defined, 1-D indium sulfide (In2S3) nanorods have been synthesized using di-tert-butyl disulfide as the sulfur source at 180 °C. In a similar fashion, monodisperse wurtzite copper indium sulfide (wz-CuInS2) has also been synthesized by using di-tert-butyl disulfide. To better understand the role of these dialkyl dichalcogenide precursors, the nanocrystal growth mechanisms have been explored for wz-CuInS2 using di-tert-butyl disulfide and c-In2O3 using di-tert-butyl peroxide.


N7.11
Novel Silica Aerogel Panels as Radiators for Cherenkov Detectors. Redouane Begag1, Dong Wenting1, George Gould1, Rhine Wendell1, Craig Woody2 and Sean Stoll2; 1Aspen Aerogels, Inc., Northborough, Massachusetts; 2Brookhaven National Laboratory, Upton, New York.

Aerogels, with their low refractive index and high transmittance of visible light, would be excellent Cherenkov radiation detectors which are used to detect high-energy particles. Unfortunately, aerogels have low light transmission in the UV region and low moisture resistance. It is also difficult to fabricate large sized aerogels panels because of their low strengths. Larger aerogel panels are desired because fewer aerogel panels would be needed within the detector and would also improve the accuracy of particle separation and detection by significantly decreasing the number light scattering interfaces. Aspen has developed new sol-gel chemistry and a drying process for fabricating hydrophobic, crack-free, 2 inch thick monolithic aerogels. Using this new process, hydrophobic aerogel samples with a transmittance of over 85% at 400 nm and a low refractive index in the range of 1.010 - 1.016 were successfully produced. In this paper, the optical and the structural properties of the newly developed aerogels will be presented and discussed.


N7.12
Layer-by-Layer ``Exponential” Polymer Films for Loading, Unloading and Assembly of Nanoscale Materials. Sudhanshu Srivastava, Paul Podsiadlo, Kevin Critchley, Jian Zhu and Nicholas A. Kotov; University of Michigan, Ann Arbor, Ann Arbor, Michigan.

Exponential growth in thickness of the polymers films is a recent advancement in the field of Layer-by-layer (LBL) assembly. Once the exponential LBL films are fabricated, the internal diffusion of the polymer films, offer the ability for the loading of active molecules. Ions, proteins, dyes and drugs are the few molecules that can be incorporated after the LBL films are prepared. The significance of this unique approach was also extended to develop films that can reversibly load-and-unload CdTe nanoparticles (NPs). So, a crucial topic is to identify whether the exponential-LBL films prepared can load long one dimensional (1D) nanoscale materials, such as CdTe nanowires (NWs), single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs), which exhibit specific degree of flexibility similar to polymers. In this approach the exponential LBL films were made from polyacrylic acid (PAA) and poly(diallyldimethylammonium chloride) (PDDA). Once films were made various dispersions of SWNTs, MWNTs and CdTe NWs were kept in contact to the LBL films. The polymer films showed high swelling for SWNTs and no swelling for MWNTs or CdTe NWs. Incorporation of SWNTs with different dispersions in the LBL films was investigated at different time intervals by scanning electron microscopy (SEM) and conductivity measurements. A sequence of results presented SWNTs, MWNTs and CdTe NWs showed different diffusion rates in the LBL films depending upon the surface charge and rigidity of the 1D components. The idea for introducing CdTe NPs was also extended for the assembly processes governed inside the films. Firstly, the NPs were introduced in the already prepared LBL films and then in-situ self-assembly of NPs inside the LBL films due to diffusion lead to NW formation.


N7.13
Phase-Controlled Synthesis of CuInSe2 Nanostructures for Photovoltaic Application. Akshay Kumar1, Edward K. Goo1 and Chongwu Zhou2; 1Materials Science, University of Southern California, Los Angeles, California; 2Electrical Engineering, University of Southern California, Los Angeles, California.

Low-cost, high-efficiency photovoltaic cells have emerged as an important research direction because of ever increasing demand of energy. One of the important material systems for high efficiency solar cells has been CuInSe2/CdS in the form of thin films with highest reported efficiency of 15%. Recently, there has been renewed interest in using colloidally synthesized nanostructured materials instead of rather expensive vacuum deposited thin films because nanocrystals offer the advantages of using inexpensive printing or spin casting processes to assemble the cell. Although nanocrystals belonging to different material classes such as CdSe, PbS, PbSe have been utilized for bilayer and schottky solar cells, there are very few reports on use of CuInSe2 nanocrystals because of difficulties associated with synthesizing high quality materials. In this work, we report the synthesis of CuInSe2 nanorods and nanoparticles in different phases which can be controlled by manipulating reaction conditions. Using simple salts of Cu and In as precursors and elemental Se powder as the Se source, nanocrystals with both the Sphalerite (cubic structure) and Chaclcopyrite (tetragonal structure) phases have been synthesized. Limited solubility of Se powder in the reaction solvent was exploited to control the shape of nanocrystals from nanorods to pyramid shaped nanoparticles. Structural characterization using X-ray Diffraction (XRD) and High-resolution TEM (HRTEM) was carried out to confirm the phases of nanocrystals. Also, electrical measurements indicated that as-synthesized nanocrystals were p-doped and showed weak gate dependence. We fabricated solar cells by blending a mixture of CuInSe2 nanocrystals with region-regular P3HT and spin casting a thin layer between ITO and Al electrodes. The cells showed photovoltaic response with a power conversion efficiency of 0.1 %. Further work on improving the efficiency of the solar cells is in progress. The study is an important advancement towards synthesizing nanocrystal ink for low-cost photovoltaic device development.


N7.14
Nano-Engineered Processing of Copper Nanoparticles for the Fabrication of Copper Sulfide Nanocrystals. Jun Yin1, Paul Chang1, Mark Engelhard2, In-Tae Bae1 and Chuan-Jian Zhong1; 1Department of Chemistry, State University of New York at Binghamton, Binghamton, New York; 2Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington.

Copper(I) sulfide (Cu2S) is a semiconductor with a bulk bandgap of 1.2 eV which has applications in photovoltaic and electrical sensing devices. The ability to engineer Cu2S materials in terms of nanoscale size and shape is important for improving the photovoltaic and electrical properties. This paper describes the results of an investigation of a novel thermal strategy for the preparation of copper sulfide nanocrystals from copper nanoparticles capped with alkanethiolate monolayers. The size and shape are shown to be tunable by manipulating the reaction conditions in the thermal processing solution. Highly-monodispersed Cu2S nanodiscs have been prepared. These nanodiscs display remarkable ordering upon self-assembly. The copper sulfide nanocrystals are characterized using an array of techniques such as HR-TEM, SAXS, XRD and XPS to determine the size, shape, composition, and structural properties. The results from these characterizations will be discussed, along with potential applications of the nanomaterials in photovoltaic devices and chemical sensors.


N7.15
Synthesis and Processing of Metal, Alloy, and Magnetic Core@Shell Nanoparticles. Kun Bao, Lingyan Wang, Jin Luo, Mark Schadt, Stephanie Lim, Derrick Mott and Chuan-Jian Zhong; Department of Chemistry, State University of New York at Binghamton, Binghamton, New York.

The ability to control the size, composition and surface properties of magnetic nanoparticles is important for exploiting potential applications of the nanocomposite materials in chemical and biomedical separation and detection. We have been developing a thermal processing strategy to the preparation of gold nanoparticles, silver nanoparticles, alloy nanoparticles, and magnetic iron oxide nanoparticles with gold or silver shells. This presentation describes recent results of the investigation of the thermal processing of gold, silver, magnetic core@shell nanoparticles in terms of size and monodispersity. In addition to synthesis of magnetic seeds (e.g., iron oxide and MnZn ferrite) and metal nanoparticle precursors, factors controlling the thermal evolution of gold, silver, alloy, and core-shell nanostructure are investigated systematically. Results from spectroscopic and microscopic characterizations of the nanoparticles will be discussed, along with preliminary evaluation of some of the core@shell nanoparticles in probing biological and catalytic reactivities.


N7.16
Ab Initio Thermodynamic Model for the Stability of Nanocrystal Heterostructures. Thomas Sadowski and Rampi Ramprasad; Institute of Materials Science CMBE, University of Connecticut, Storrs, Connecticut.

In recent years, advances in synthesis techniques have led to an increased level of control over the size, shape, and composition of colloidal nanocrystals. The ability to simultaneously control the geometry and composition within a nanocrystal provides a powerful mechanism to engineer the electronic and optical properties through the design of tailor-made interfaces. While the uses of such heterostructure nanocrystals are envisioned in a wide range of applications, it is important to consider how the structural quality of these interfaces is affected at elevated temperatures. In this work, we develop a thermodynamic model built on ab initio density functional theory (DFT) based results to address the stability of two-component nanocrystal heterostructures with an abrupt interface. We consider CdSe-CdTe core-shell nanoparticles and nanowires with interfaces between the two components along the radial direction, as well as CdSe-CdTe nanowires with an axial interface. For these systems, we specifically determine the temperature above which it is no longer favorable for the two components to remain separated. Our model is based on setting up the free energy of mixing of the nanostructures (initially in the unmixed state), and contains the enthalpy of mixing (determined at 0 K using DFT), the temperature-dependent entropic contribution to the mixing process (which is analytic), and a geometry-dependent CdSe-CdTe interface energy contribution (also determined using DFT calculations). Volume fractions in the entire range from pure CdSe to pure CdTe were considered, and the enthalpy of mixing over this range was determined using models of a homogeneously mixed system. The critical temperature above which mixing is expected to occur was determined as a function of the volume fraction and radius (in the case of core-shell nanoparticles and nanowires) or length (in the case of nanowires with an axial heterojunction). For a given volume fraction, the critical temperature was found to increase with radius or length, reaching a geometry-independent limiting value. This limiting value was largest for a 50-50 volume fraction, which displayed a critical temperature for mixing of ~500 K. This estimate is consistent with recent experimental findings in which CdSe-CdTe nanowires exposed to a 600 K environment for an extended period were found to ripen into spherical CdSexTe1-x alloyed nanocrystals. According to our results, a core/shell (axial) heterostructure must be composed of roughly equal amounts of CdSe and CdTe, and have a diameter (length) greater than ~1000 Å (~500 Å) to display high stability and resist mixing (which in these systems is thermodynamically favored at ~ 500 K).


N7.17
Molecularly-Mediated Assembly of Nanoparticles: Probing Selective Mediation in Terms of Size, Shape and Ligand. Hong Yan, Stephanie Lim, De-Lie An and Chuan-Jian Zhong; Department of Chemistry, State University of New York at Binghamton, Binghamton, New York.

The ability to tune interparticle spatial properties of molecularly-mediated nanoparticle assemblies is one of the major challenges for the design and understanding of functional nanostructures. This presentation reports a novel approach to the assembly of gold nanoparticles by selective mediation using rigid methylthio arylethynes of different sizes, shapes and ligands. One important issue is to probe how the shape of mediator and the number of methylthio ligand has affect on the assembly. This issue is addressed by using a series of V, Y, X-shaped methylthio arylethynes for the assembly of gold nanoparticles. These molecules have different number of methylthio groups for coordinating to the binding sites of gold nanoparticles. The results show that nanoparticle assembly can be achieved by selective mediation in terms of size, shape and ligand. Both kinetic and thermodynamic factors are operative in the selective mediation for the nanoparticle assembly. Results form the characterizations of the nanostructured assemblies using various spectroscopic and microscopic techniques will be discussed, along with their potential applications as electrical, optical, and spectroscopic sensing materials.


N7.18
Gas Sensors Over Large Dynamic Ranges with Semiconductor Quantum Dots Integrated onto Nanotube Arrays. Zhouying Zhao1, Teresa M. Dansereau2, Subhendu K. Panda2, Oxana V. Vassiltsova2, Marina A. Petrukhina2 and Michael A. Carpenter1; 1College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany, New York; 2Department of Chemistry, University at Albany, State University of New York, Albany, New York.

Colloidal CdSe quantum dots (QDs) and a nanotube array of anodic aluminum oxide (AAO) were integrated to form nanostructured chemical vapor sensors. The introduction of AAO as a quasi photonic crystal (PC) platform for the QDs is found to provide two advantages for QD based sensing. First, AAO intensifies QD photoluminescence (PL) thus increasing the sensing response toward analyte exposures owing to both the redistribution of high intensity local near-fields for more efficient excitation of QDs and strong scattering effects for enhanced extraction of the resulting QD emission. Second, the nanopores of AAO retards the limiting effect of film wetting and resultant PL inversion and desensitization suffered from QDs or QDs/polymer cast on non-porous substrates under exposures to increased levels of analytes. The combination of these effects has enabled the development of QDs/nanotube-array material for sensing applications across large dynamic ranges, for example, 10 to 9400 ppm for xylenes.


N7.19
Size Dependent Electron Transfer from CdSe Colloidal Nanoparticles to Various Metal Oxide Colloids within Thin Films. Kevin Tvrdy1,2 and Prashant V. Kamat1,2,3; 1Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana; 2Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana; 3Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana.

Colloidal nanoparticles (NPs), due to their size dependent optical behavior, efficient absorption and emission properties, and relative ease of synthesis, have become a popular choice for next generation photovoltaic and optoelectronic devices. Full optimization of NPs in such systems, however, necessitates a comprehensive understanding of their electrochemical and electron transfer behavior. The presented work bolsters that understanding through the determination of electron transfer rates from several different sizes of CdSe colloidal NPs (within the confinement regime) to four different colloidal metal oxide (bulk regime) scaffolds, namely: TiO2 d=30nm, SiO2 d=20nm, SnO2 d=30nm and ZnO d=30nm. These electron transfer rates are then compared with both a Marcus Theory and a Fermi’s Golden Rule model of electron transfer. The prevalence of photovoltaic devices successfully sensitized with CdSe NPs in the literature, coupled with the both laboratory and industrial achievements of these particles in light emitting devices is a clear demonstration of their ability to both donate and accept electrons in a robust fashion. The more fundamental question of the mechanistic nature of the electron transfer itself, however, remains unanswered. Electron transfer rates are strongly influenced by the energy differential between the electron donating state (CdSe conduction band edge) and the electron accepting state (metal oxide conduction band). By varying the size of CdSe NPs under investigation, we were able to modify the conduction band workfunction of the electron donating species. Likewise, by varying the metal oxide to which the CdSe donates its electron, we were also able to modulate the workfunction of the electron accepting species. These two synthetic degrees of freedom, coupled with measurements from an ultrafast transient absorption setup, provided us with a comprehensive insight into the electron transfer rate from colloidal CdSe to metal oxide NPs with picosecond precision. Thin films of colloidal metal oxide materials were made via the doctor blade method, yielding 8 micrometer thick scaffolds of metal oxides NPs which all showed 70+% transmission throughout the visible spectrum. Films were functionalized with the bifunctional “linker” molecule 3-mercaptopropionic acid and then immersed in a CdSe NP solution. Resultant films were then placed in a sample cell which was evacuated to minimize degradation due to NP oxidation. Ultrafast transient absorption was used to identify excited state lifetime parameters and a comparison between electron accepting (TiO2, SnO2, ZnO) and non-electron accepting (SiO2) metal oxide scaffolds was used to determine electron transfer rates.


N7.20
Study of Resonance Elastic Light Scattering and Fluorescence Energy Transfer in Rhodamine B Modified Spherical Au Nanoparticles and Nanorods. Magdalena Stobiecka1, Kaitlin Coopersmith1 and Maria Hepel1,2; 1Department of Chemistry, State University of New York at Potsdam, Potsdam, New York; 2Department of Chemistry, State University of New York at Buffalo, Buffalo, New York.

The unique optical and electronic properties of metal and semiconductor nanoparticles offer a range of new exciting applications in novel bioelectronic devices and energy conversion systems, as well as provide new means of studying biochemical processes, biological signaling pathways and imaging of living cells. In this work, the resonance elastic light scattering (RELS) from gold nanoparticles (AuNP) has been investigated to evaluate the interactions of AuNP with photoluminescent dye rhodamine B (RhB). Strong screening of UV-Vis absorbance of the surface plasmon of AuNP and no aggregation have been observed, which indicates on a tight adsorption of RhB on the surface of AuNP and points to the probability of the fluorescence resonance energy transfer (FRET). The fluorescence quenching measurements using 5 nm diameter citrate-capped AuNP5nm have confirmed that the FRET occurs with high quantum efficiency. We have also found a sharp RELS band of RhB alone (λmax = 566 nm), which can be attributed to the enhanced RELS and resonance fluorescence. The scattering intensity of this mode is linearly dependent on RhB concentration in the low concentration range from 10 to 500 nM. This RELS peak is very sensitive to the presence of AuNP5nm and becomes completely quenched by 0.8 nM AuNP5nm. Moreover, a surprisingly strong RELS band at relatively long wavelength, λmax = 653 nm, has been discovered for citrate-capped AuNP5nm, with a high peak-to-valley scattering intensity ratio, IP : IV = 7.7. The adsorption of RhB does not affect the scattering intensity of AuNP5nm up to RhB concentration of 0.5 μM but decrease afterwards to 75% at 10 μM RhB concentration, at which a recovery of the sharp RhB band at λmax = 566 nm also begins. In contrast to the spherical gold nanoparticles, the gold nanorods (aspect ratio 4:1) do not exhibit the absorbance screening by RhB. Also, the FRET is very limited, most likely only to the RhB interacting with transverse surface plasmon. This is justified by the fact that there is no substantial overlap of the longitudinal surface plasmon absorbance band with RhB photoluminescence spectrum. In this way, by tailoring the shape and aspect ratio of gold nanoparticles and thus the optical properties of their quadrupole surface plasmon oscillations, it becomes possible to fine-tune the quantum efficiency of FRET. Further adjustments of spectral features are attained by gold nanoparticle assembly induced by hydrogen bonding or divalent heavy metal ions providing effective linkages between the core-shell nanoparticles. The molecular dynamics simulations performed indicate that hydrogen bonding and electrostatic interactions are likely to be involved in the ensemble formed by RhB and AuNP/Cit, while further interactions of RhB with Au clusters at close distances may even lead to bending of the rigid xanthene group of RhB, thereby diminishing the high quantum efficiency of fluorescence of the original RhB dye.


N7.21
Preparation of Conductive Color Powders. Chin-Cheng Weng, Yu-Chin Lin, I-Jein Cheng and Ching-Mao Wu; Dept. of Photosensitive Materials and Applications Research, Industrial Technology Research Institute, Hsinchu, Taiwan.

The experimental procedure for the preparation of conductive color powders that were aggregated from color pigment and conductive materials is described. Herein, we first modified the pigment and make the surface to have charges (ex: positive) and modified another conductive materials like ITO, Au, Ag particles or conductive polymer to be opposite charges (ex: negative). Then these two kinds of charge particles will aggregate in appropriate processing by static electricity and form a powder. The aggregation method is popularly used to make chemical toner. Herein, we use a similar concept to generate a conductive color powder that consists with pigment and conductive materials. This is different to mix pigment with conductive materials. The surface charges will attract more particles to aggregate and will improve the interface contact of conductive materials and form well percolation network geometry. After under 200 oC sintering, pigment will provide good color and conductive materials provide the well conductive properties. The powder shape is affected by the solvent, particle concentration, the size of primitive particles of pigment and conductive materials and the surface charge density. If the pigment size is larger than the conductive particles, the conductive color powder will form core-shell structure. The surface charge density is a very important point in this study because it decides how many particles will be attracted together and formed a powder. The size of primitive particles of pigment and conductive materials were obtained on Dynamic light scattering (DLS). The DLS data reveals that the modified pigment size is about 200 nm and the modified ITO particle is about 30nm. After aggregation, the conductive color powder is about 500nm-1um. The real conductive color powder structure was measured by Transmission electron microscopy (TEM) which was carried out on a Hitachi H-600 and a thermal field emission scanning electron microscope (JSM-6500F) with an accelerating voltage of 10 kV. The TEM image shows a core-shell structure of the conductive color powder and the surface resistivity is < 105 Ω/sq, which was carried out on Keithley 6517A system.


N7.22
Study of Surface-related Emission using Time-resolved Spectra of CdSe Quantum Dots. Sang Min Kim1, Kyong Soo Hong2 and Ho-Soon Yang1; 1Physics, Pusan National University, Busan, Korea, South; 2Pusan Center, Korea Basic Science Institute, Busan, Korea, South.

Colloidal CdSe Quantum Dots (QDs) are of increasing interest because of their applications as LED, display and biological senses. Various surface passivation have been considered important during synthesizing the QDs. QDs have a very large surface to volume ratio, and, therefore, the surface passivation has been considered seriously in the study of quantum yield (QY) and dynamic process. Using the chemical colloidal method, we synthesized the CdSe QDs of the various QY as varying the amount of organic materials such as TOP, and ODE or HDA in the preparation. We study the surface-related emission with the time resolved spectra of the CdSe QDs measured using time-correlated photon counter. The spectra are obtained with a few hundred nano second time scale and analyzed to multiexponential processes. We obtain the amplitude ratio of fast process to slow process in CdSe QDs of various QY, and observe that the ratio is related with the QY value of CdSe QDs. We investigate the charge distribution in QDs, and the importance of the surface-related emission.


N7.23
Solution Self-assembly Behavior of Block Copolymer Blends with Shared Hydrophilic Block but Different Hydrophobic Blocks. Jiahua Zhu1, Jeremy W. Bartels2, Ke Zhang2, Karen L. Wooley3 and Darrin J. Pochan1; 1Department of Materials Science and Engineering, University of Delaware, Newark, Delaware; 2Department of Chemistry, Washington University in Saint Louis, Saint Louis, Missouri; 3Department of Chemistry, Texas A&M University, College Station, Texas.

Novel micellar structures due to segregation of unlike hydrophobic domains trapped within the same micelle core have been produced via self-assembly of block copolymer blends in tetrahydrofuran/water solution. The blend is composed of two/or more block copolymers with distinctive hydrophobic blocks but the same poly(acrylic acid) (PAA) hydrophilic block. By taking advantage of the complexation in the hydrophilic corona between the acid side chains of the PAA block and added organoamine molecules, unlike hydrophobic blocks are trapped in the same micelle core and consequently, locally segregate. This segregation gives rise to a class of new micellar nanostructures, such as hemisphere-hemisphere janus micelle particles, sphere-cylinder connectors, etc. The shape and volume of each compartment can be well controlled by changing the solution composition and blending ratio, respectively. The interesting arrangement of hydrophilic PAA block and varied hydrophobic blocks within the micelles makes them potential templates for multi-functional composite nanomaterials via putting varied nanoparticles/quantum dots into targeting domains. Transmission electron, cryogenic transmission electron, and atomic force microscopy along with x-ray and neutron scattering have been applied to characterize the assembled structures.


N7.24
Self-terminating Electroless Gold Plating process for Nanogap Electrodes toward Room Temperature Operational Single-Electron Transistors Victor M. Serdio V.1,3, Seiichi Suzuki1,3, Shinya Kano1,3, Taro Muraki1,3, Yasuo Azuma1,3, Masayuki Kanehara2,3, Toshiharu Teranishi2,3 and Yutaka Majima1,3; 1Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Japan; 2Department of Chemistry, University of Tsukuba, Tsukuba, Japan; 3CREST, JST, Tokyo, Japan.

Nanogap electrodes based single-electron transistors (SETs) with the ability to operate at room temperature are intended to be fabricated with a high precision comparable to a top-down technique. The ability to be mass produced with high yield and using non-hazardous materials is one of the biggest challenge for these SETs. Here, we report the bottom-up technique of self-terminating electroless gold plating (EGP) of nanogap electrodes based on commercially available Iodine tincture and gold sheets. The solution is achieved by oxidizing Au into the form of (AuI2)- and (AuI4)- complex along with L(+)-ascorbic acid acting as a reducing agent [1,2]. This process enables us to achieve a separation of 1~5 nm and uniformity through a very simple but highly controlled development. Before EGP, the nanogap electrodes are fabricated by evaporating a 5 nm layer of titanium followed by a 15 nm layer of gold on a SiO2/Si wafer patterned by electron beam lithography. The nanogap electrodes have an initial separation of 30 nm in average which is consistent with the resolution of the state-of-the-art in-line production photolithography techniques. I-V characteristics were measured and confirmed two cases of single-electron phenomena: full SET operation (Coulomb diamonds) at 80K by using octanethiol-protected 3.3 nm nanoparticles as Coulomb island along with octanethiol / decanedithiol self-assembled monolayers (SAMs) as double-barrier tunneling junctions (DBTJs) and Coulomb staircases at room temperature by using chemisorbed Au nanoparticles. Nanogap electrodes were fabricated at room temperature with a parallel production yield of ~50% and can have their separation modulated to host a variety of Coulomb islands. This study is partially supported by Nanotechnology Network Project of MEXT and World Class University Program, Sunchon National University. [1] A. Umeno, and K. Hirakawa, Appl. Phys. Lett. ,86,143103 (2005). [2] Y.Yasutake et al., Appl.Phys. Lett. , 91, 203107 (2007).


N7.25
Structure and Photoluminescence of Gd2O3:Eu3+ Shell on Self-Assembled Bi2O3 or SiO2 Core for Scintillating Detection. Teng-Kuan Tseng, Jihun Choi and Paul H. Holloway; Department of Materials Science and Engineering, University of Florida, Gainesville, Florida.

To date, scintillation crystals have been made with complex single crystal growth methods such as Czochralski and Bridgeman methods, which frequently result in high costs and small crystal size. Because of these factors, development of low-cost processes for larger area scintillation materials is important. In this study, a facile sol-gel method was used to prepare core-shell nanocomposites for scintillation materials. Based on transmission electron microscopy (TEM) data, ~220 nm mono-dispersed SiO2 cores with an ~15 nm Gd2O3:Eu3+ shell were synthesized. After calcining at 800 C for 2 hr, the Eu3+ ions (~5 at%) emitted at 610 and 622 nm via 5D0-7F2 photoluminescent (PL) transitions, excited primarily by the oxygen to europium charge transfer band (CTB) between 250 and 300 nm. Precipitates of bismuth oxide (Bi2O3), a stable and non-hydroscopic high atomic number material, were also synthesized with and without the addition of polyethylene glycol (PEG). Nano-spheres or micron size bamboo-like or self-assembled flower-like precipitates of crystalline Bi2O3 were formed. This core/shell Bi2O3/Gd2O3:Eu3+ composite showed good PL emission at 610 and 622 nm under near UV excitation. The potential for the application of these materials in scintillation detectors will be discussed.


N7.26
Superparamagnetic Fe3O4(CdTe) Core(shell) Bifunctional Nanoparticles. Haribhau M. Gholap1,2, Anup A. Kale1, Beatrice Hannoyer3 and Satishchandra B. Ogale1; 1Physical & Materials Chemistry Division, National Chemical Laboratory, Pune - 411008, Maharashtra, India; 2Physics, Fergusson College, Pune - 411004, Maharashtra, India; 3Groupe de Physique des Matériaux de Rouen, Université de Rouen, BP 12, 76801 Saint Etienne Du Rouvray Cedex, France.

Abstract Magnetic and fluorescent nanoparticles (NPs) hold some of the most exciting application prospects of nanotechnology. They are also of great scientific interest to the fields of chemistry, biology, physics and engineering. In this work we report amine mediated direct formation of fluorescent semiconductor (CdTe) quantum shell on the surface of magnetic core (Fe3O4) by using Dodecylamine (DDA) as a cross-linking surfactant. DDA is a low molecular weight amine with moderate reactivity with acids. DDA (CH3(CH2)11NH2) bears positive charges at both terminals making it ideal for reacting with two negatively charged moieties. In our case the sequential deposition of oppositely charged DDA and nucleating CdTe quantum dots onto the Fe3O4 nanoparticles was by charge inversion process, finally leading to CdTe shell. These bifuntional nanoparticles were characterized by Optical spectroscopy, Fluorescence spectroscopy, X-ray diffraction (XRD), Fourier Infrared spectroscopy (FTIR), High Resolution Transmission Electron Microscopy (HRTEM), Raman spectroscopy, SQUID magnetometry and Mössbaur Spectroscopy. The nanoparticles were found to have a mean diameter of 15-20 nm. They were also found to be photo stable, fluorescent and could be easily separated from solution by a magnet. The emission peak in the fluorescence spectrum showed blue shift of 18 nm with respect to bulk CdTe suggesting the size of the CdTe shell of less than the exciton radius for CdTe (~6.0 nm). This was confirmed by high resolution transmission electron microscopy. The magnetization curve of Fe3O4 showed the superparamagnetic property at room temperature, with saturation magnetization value (Ms) of ~58 emu g−1. In the case of (Fe3O4) CdTe the saturation magnetization was Ms = 40 emu g−1. Such metal/semiconductor composite nanoparticles retaining the magnetic and optical properties of the subcomponents can be explored as potential optical magnetic reporters for use in bioassays. They can also be used as a multifunctional nano-tool exhibiting combined photodynamic therapy and in-vitro/in-vivo imaging based both on fluorescence and on magnetic resonance.


N7.27
Abstract Withdrawn


N7.28
Metal (Core) / π-conjugated Polymer (Shell) Hybrid Nanoparticles for Nonlinear Optical Properties. Akito Masuhara1, Takahiro Yokoyama1, Yoshihisa Matsuda1, Hitoshi Kasai1,2, Hachiro Nakanishi2 and Hidetoshi Oikawa1; 1IMRAM, Tohoku University, Sendai, Japan; 2PRESTO, JST, Saitama, Japan.

Organic-inorganic hybridized nanomaterials are a major focus of research efforts in recent years. Hybridized nanomaterials have been investigated extensively, especially, from the viewpoints of both fundamental nanoscience and some optoelectronic device applications. In fact, Neeves et al. theoretically predicted the enhancement of nonlinear optical (NLO) properties in core/shell type hybridization of π-conjugated organic materials and metal on nanometer scale [1]. Polydiacetylene derivatives are classified as a one-dimensional π-conjugated polymer, and one of powerful candidates for such NLO devices because of the excellent third-order NLO response, which is attributed to π-conjugated electrons along the main chain [2]. Kasai et al. have established so-called “reprecipitation method” to prepare polydiacetylene nanocrystals [3, 4]. In addition, we have succeeded in preparing hybridized nanocrystals composed of polydiacetylene nanocrystal and Ag nanoparticle by “co-reprecipitation method” [5]. However, the co-reprecipitation method could not be applied to other kind of diacetylene derivatives. In this presentation, we attempted to fabricate a new core/shell type hybridized nanocrystals composed of metal nanoparticle core and π-conjugated polymers shell which are polyalkylthiophene and polydiacetylene by further developing the conventional co-reprecipitation method. Metal nanoparticles water dispersion was fabricated by chemical reduction and used without further purification in the following procedure. Organic material solution was injected into metal nanoparticles water dispersion (10 mL) with vigorous stirring. Then, the dispersion was irradiated with microwave (2.45 GHz, 500 W) for 20-60 s in the closed system. In the case of diacetylene derivative, after cooling to room temperature, solid state polymerization was further performed by UV-irradiation (λ = 254 nm) for 20 min. The obtained hybridized nanocrystals were evaluated with TEM and UV-vis spectroscopy. Typical TEM image show the core-shell type hybridized nanocrystal composed of Ag nanoparticle and polydiacetylene. Ag core was clearly coated with polydiacetylene shell (5 nm). Compare with the UV-vis spectra of the hybridized nanocrystals water dispersion before and after UV irradiation, the very weak excitonic absorption peak appeared at 540 nm after UV irradiation. This peak indicates that polydiacetylene -shell have red-phase, in which the π-conjugated backbone of polydiacetylene was more or less distorted, due to a kind of anchor effect and the curved surface of Ag nanoparticles core. Detail of the preparation method, structures and optical properties of the hybridized nanocrystals will be discussed. [1] A. E. Neeves et al., J. Opt. Soc. Am. B6 (1989) 787. [2] A. Sarkar et al., J. Mater. Chem. 10 (2000) 819. [3] H. Kasai et al., Jpn. J. Appl. Phys. 31 (1992) L1132. [4] H. S. Nalwa et al., Adv. Mater. 5 (1993) 758. [5] A. Masuhara et al., Jpn. J. Appl. Phys. 40 (2001) L1129.


N7.29
Surfactant Influence on Nanoparticles Morphology Synthesized by Solution Plasma. Yasuhiro Nakamura1, Junko Hieda2, Maria-Antoaneta Bratescu2, Nagahiro Saito2,3 and Osamu Takai3,4; 1Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya university, Nagoya, Japan; 2Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya university, Nagoya, Japan; 3Ecotopia Science Institute, Nagoya university, Nagoya, Japan; 4JST/CREST, Nagoya, Japan.

Anisotropic metallic nanoparticles are expected to be used for bioimaging due to their optical properties. The light response properties strongly depend on the nanoparticles shape and size. Many studies have been reported the formation of the anisotropic nanoparticles mainly conducted by chemical reduction methods. However, these methods are carried out under mild conditions and it takes long time to form anisotropic nanoparticles. In this research, we aimed to fabricate the gold nanoparticles with anisotropic shapes using plasma in aqueous solutions (solution plasma). Solution plasma process realizes a rapid reaction due to the products generated from the plasma. In this process we investigated the surfactant influence on the morphology of the nanoparticles. We used a bipolar pulse power supply as plasma power source. The discharge conditions were kept constant ; the discharge voltage was 2 kV, the pulse frequency was 15 kHz and the pulse width was 2.0 μs. Hydrogen tetrachloroaurate(ΙΙΙ) solutions were used as a raw material. Ionic surfactants as Sodium Dodecyl Sulfate (SDS) or Cetyltrimethylammonium chloride (CTAC) were mixed to the solution in order to stabilize colloidal gold. After the discharge, the absorbance spectra of the solutions were measured by UV-visible spectroscopy. The obtained nanoparticles were observed by transmission electron microscopy. The composition of nanoparticles was analyzed by energy dispersive electron microscopy. In the synthesis using SDS surfactant, spherical or polygonal nanoparticles were obtained. Besides, in the synthesis using CTAC surfactant, gold nanorods were formed. This result can be explained by the difference in the adsorption mechanism of the ionic surfactant on the gold nanoparticles surface. The particle size decreased at higher concentration of the surfactant. It is supposed that too much surfactant adsorbed on the gold surface prevented the particle growth. As to the nanoparticles size, smaller particles were obtained as the discharge duration became longer. The effect of discharge duration will be discussed in details.


N7.30
Fabrication of the Cerium Oxide Nanocrystal Film with Heterogeneous Ligand Structures. Daisuke Hojo, Takanari Togashi and Tadafumi Adschiri; Advanced Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan.

The assembling of a highly crystalline metal oxide nanoparticle layer at room temperature has attracted considerable attention. Generally, high crystalline metal oxide films are only obtained through high temperature treatments. If the high crystalline metal oxide nanoparticles are formed elsewhere in advance at high temperature and can be aligned densely on the surface at room temperature, high crystalline film layer can be fabricated even on a heat-sensitive substrate by this method. For this study, the hybrid ceria nanocrystals modified with decanoic acid were covalently adsorbed to the pretreated Si surface to self-assemble mono-layered nanocrystal system. The monolayer of 3-aminopropyltriethoxysilane molecules was formed on the hydroxylated Si surfaces to produce an NH2 terminated surface. 3, 4-dihydroxyhydrocinamic acid (DHCA) was then condensed with NH2 group. On the other hand, DHCA is prone to be exchanged with decanoic acid on the surface of the modified nanocrystals because of equilibriums as found in Fourier transform infrared spectroscopy. Therefore, the bottom surface of the adsorbed nanocrystals is locally modified with DHCA in exchange of decanoic acid and chemical bonds are established between nanocrystals and the substrate. To evaluate these nanostructures dispersed two dimensionally on the surface, scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) were used. After rinsed with cyclohexane, only mono-layered nanoparticles were left on the surface washed away all of physically adsorbed nanocrystals on the nanocrystals chemically attached to the surface. Two-dimensional structures of the nanocrystals were regarded as a continuous thin film from SE; the defects, with a size of ~10 nm, were sufficiently smaller than the wavelength of probe light. The thickness of layered nanocrystals was evaluated to be ~ 10.6 nm from SE; this value is almost the same as the size of nanocrystals. The refractive index of this film was 1.73 at a wavelength of 633 nm. Comparing to the refractive index of cerium oxide thin films obtained by other techniques such as electron beam evaporation at room temperature, this value was found to be sufficiently high. Since the coverage of nanocrystals was estimated to be ~80% from the SEM image, if the surface was completely covered, the refractive index could increase up to ~1.9 theoretically. The fabrication of two-dimensionally aligned nanoparticle systems will be a key technology for layer-by-layer assembly or precisely controlled colloidal superlattices.


N7.31
Dispersion of Tunneling Resistance in Room-temperature Double Barrier Tunneling Junctions by Bottom-up Processes. Shinya Kano1,3, Yasuo Azuma1,3, Masayuki Kanehara2,3, Toshiharu Teranishi2,3 and Yutaka Majima1,3; 1Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Japan; 2Department of Chemistry, University of Tsukuba, Tsukuba, Japan; 3CREST-JST, Tokyo, Japan.

Complementary metal-oxide semiconductor (CMOS) devices have made great progresses in both operation and efficiency following the Moore’s law and the scaling rule. According to the International technology roadmap for semiconductor (ITRS2008), CMOS devices will reach the limit of shrinking the size in around 2022. [1] Beyond CMOS devices recently have attracted great interest of researchers in order to overcome the problem. [2,3] Single-electron transistors (SETs) are one of the candidates of beyond CMOS devices. Because SETs can operate with only one electron using Coulomb blockade phenomena, SETs are the devices to reduce the power consumption to the limit. For practical application, SETs must operate at room temperature (RT-SETs). Toward the fabrication of RT-SETs, double barrier tunneling junctions (DBTJs) have to be in a few nm-scales. Here we demonstrate small dispersion of tunneling resistance of DBTJs fabricated by bottom-up processes.DBTJs were prepared by using Au nanoparticle as a Coulomb island, which consist of scanning tunneling microscopy (STM) tip/vacuum/Au nanoparticle/acidic self-assembled monolayer (SAM)/Au(111) substrate. The Au nanoparticle is protected by basic ligands and is chemically absorbed onto the acidic SAM. Clear Coulomb blockade phenomena have been reproducibly observed at room temperature in the current-voltage (I-V) characteristics by scanning tunneling spectroscopy (STS). Theoretical curves by the “Orthodox theory” are good agreement with the experimental results. [4] The tunneling resistance between Au core and Au(111) surface (R2) have been estimated from the theoretical fitting circuit parameters. The average tunneling resistance R2 is 10 GΩ from the histogram of R2 The standard deviation σ of common logarithm R2 is estimated as 0.44, which means the dispersion of R2 can be within 20 times (=10±3σ) from the mean value. Consequently, we can utilize these bottom-up fabrication processes for the preparation of SETs, which show small dispersion of tunneling resistance. This work was partially supported by World Class University Program, Sunchon National University. [1] International Technology Roadmap for Semiconductors 2008 Edition, 2008, http://www.itrs.net/ [2] Zhang, H.; Yasutake, Y.; Shichibu, Y.; Teranishi, T.; Majima, Y. Phys. Rev. B 2005, 72, 205441. [3] Matsumoto, K.; Ishii, M.; Segawa, K.; Oka, Y.; Vartanian, B. J.; Harris, J. S. Appl. Phys. Lett. 1996, 68, 34. [4] Hanna, A. E.; Tinkham, M. Phys. Rev. B 1991, 44, 5919.


N7.32
Coulomb Blockade at Room Temperature by Using Gold Nanoparticles and Electroless Gold Plated Nanogap Electrodes. Taro Muraki1,3, Victor M. Serdio V.1,3, Seiichi Suzuki1,3, Shinya Kano1,3, Yasuo Azuma1,3, Masayuki Kanehara2,3, Toshiharu Teranishi2,3 and Yutaka Majima1,3; 1Material and Strucutures Laboratory, Tokyo Institute of Technology, Kanagawa, Japan; 2Department of Chemistry, Tsukuba University, Tsukuba, Japan; 3CREST-JST, Tokyo, Japan.

Bottom-up electronics enable us to fabricate the single electron devices with high precision of sub-nanometer order such as electroless gold plating method and self-assembled process. We have established the simultaneous fabrication method of multiple nanogap electrodes using electroless gold plating with common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C)[1]. We also formed a double barrier tunneling junction (DBTJ) by introducing self-assembled monolayers (SAMs) as a tunneling barrier layer and inserting alkanthiol-protected Au nanoparticles as Coulomb islands in between the nanogap electrodes. We have been observed Coulomb staircase at liquid nitrogen temperature on the DBTJ. The junction parameters such as tunneling resistances and capacitances obtained by the theoretical fitting of the Coulomb staircase were in good agreement with those mentioned in our previous measurements by means of scanning tunneling spectroscopy (STS) [2]. Here, we demonstrate clear Coulomb blockade in the DBTJs with nanogap electrodes and Au nanoparticles at room temperature. The initial electrodes pattern (source, drain, and two side gates) with ~30 nm gap separation was fabricated using electron-beam lithography and a lift-off process on a SiO2/Si substrate. The sample is simply immersed to achieve nanogap electrodes with 5 nm or less in separation into the gold plating solution which consists of gold-dissolved iodine tincture and L(+)-ascorbic acid as a reducing agent. The DBTJs were fabricated by introducing SAMs and Au nanoparticles which were chemically immobilized into the nanogap. The I-V characteristics between source and drain electrodes showed clear Coulomb blockade at room temperature. From the tunneling resistance between the Au nanoparticles and the source and drain electrodes, we discuss the number of Au nanoparticles that work as coulomb islands will be discussed. This study is partially supported by Nanotechnology Network Project of MEXT, Japan and World Class University Program, Sunchon National University. [1] Y. Yasutake, K. Kono, M. Kanehara, T. Teranishi, M. R. Buitelaar, C. Smith, and Y. Majima, Appl. Phys. Lett., 91, 203107 (2007). [2] H. Zhang, Y. Yasutake, Y. Shichibu, T. Teranishi, and Y. Majima, Phys. Rev. B., 72, 205441 (2005).


N7.33
Study of Magneto-optical Properties and Carrier Dynamics in Mn2+-doped CdS Nanocrystals. Seiji Taguchi1, Atsushi Ishizumi2, Takeshi Tayagaki1 and Yoshihiko Kanemitsu1; 1Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan; 2Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara, Japan.

There have been extensive studies on the fabrication and characterization of semiconductor nanocrystals (NCs) doped with functional impurities. Doped semiconductor NCs show unique multifunctional properties beyond those of undoped NCs [1]. Transition-metal ions (e.g. Mn2+) are one of the most important dopants because they act as luminescence centers and localized spins. Mn-doped semiconductor NCs are model materials for the physics of doped semiconductor NCs, and many efforts have been made for doping of Mn ions into semiconductor NCs. However, it is not clear whether Mn ions have been incorporated into semiconductor NCs and all doped Mn ions in heavily doped NCs act as magnetic and optical impurities. We have prepared Mn-doped CdS NCs coated by ZnS shell layer (CdS:Mn/ZnS core-shell NCs) by a reverse micelle method to achieve high concentration Mn doping [2]. In this paper, we study the magneto-optical properties and carrier dynamics in heavily Mn-doped NCs up to 10 mol %. From magnetic circular dichroism (MCD) spectroscopy, we can obtain the magnetic susceptibility of Mn ions incorporating into NCs. Moreover, to clarify the interaction between the doped Mn ions and photoexcited carriers, we studied the dynamics of photoexcited carriers in undoped and Mn-doped CdS NCs by femtosecond pump-probe spectroscopy. From MCD spectroscopy, the Zeeman splitting energy of the lowest exciton is obtained as a function of Mn concentration. The Zeeman splitting energy increases with the Mn concentration up to a few mol %, and starts decrease rapidly. The Mn-related PL intensity also shows the same Mn concentration dependence. These behaviors indicate that in heavily doped NCs, Mn—Mn interactions play an important role in reducing the MCD and PL intensities. This Mn-concentration dependence of the MCD intensity can be explained by the formation of Mn—Mn pairs in heavily doped nanocrystals. We found that in heavily Mn-doped samples, the antiferromagnetic coupling of Mn ion pairs causes a reduction in the magneto-optic signal intensity [3]. Moreover, we study the carrier dynamics under the high-density photoexcitation in the Mn-doped NC samples, and clarified that the dynamics of the carrier recombination in Mn-doped NCs is different from that in the undoped NCs. Our findings provide useful information in the design of magneto-optical materials based on semiconductor NCs. [1] D. J. Norris, Al. L. Efros, and S. C. Erwin; Science 319, 1776 (2008). [2] A. Ishizumi and Y. Kanemitsu, Adv. Mater. 18, 1083 (2006). [3] S. Taguchi, A. Ishizumi, T. Tayagaki, and Y. Kanemitsu, Appl. Phys. Lett. 94, 173102 (2009).


N7.34
The Effect of Cation Modification on TiO2 Nanoparticles for Dye Sensitized Solar Cells. Halil I. Yavuz and Ahmet M. Ozenbas; Metallurgical and Materials Engineering, Middle East Technical University, Ankara, Turkey.

Dye-sensitized solar cells (DSSC) based on nanocrystalline inorganic oxides such as TiO2, ZnO and SnO2, are a relatively new class of low-cost solar cells. They are based on a semiconductor formed between a photo-sensitized anode and an electrolyte. A photo electrochemical system (PECS) extracts electrical energy from light, including visible light. Dye-sensitized solar cells are extremely promising and currently attracting widespread interest for the conversion of sunlight into electricity because of their low cost and high efficiency and do not need elaborate apparatus to manufacture. For this purpose; TiO2 nanoparticles and TiO2 films were synthesized using sol-gel technique. In order to achieve smallest size of TiO2 particles, small amount of cation is added such as Sr+2 or Zr+4. Glass, ITO and FTO-ITO coated glass are used as substrates. To keep the narrow size distribution of nanoparticles, the effect of stabilization agents such as ethylene glycol and polyethylene glycol were examined. The optimum film porosity and particle size distribution were obtained by mixing polyethylene glycol and ethlylene glycol in 90:10 weight ratio. The best film adhesion was observed on ITO coated substrate, and the minimum particle size of 3 nm was obtained by adding Zr+4. TGA analysis was used to determine the weight loss of powder and gel as a function of temperature. Structural, topographical and chemical analysis were carried out using XRD, SEM and EDS.


N7.35
Naturally Dispersed Sb2Se3 0-D Nanoparticles with Blue Photoluminescence. Willinton Farfan1,2, Edgar Mosquera1,2, Rajasekarakumar Vadapoo1,2, Sridevi Krishnan1,2 and Carlos Marin1,2; 1Physics, University of Puerto Rico, San Juan, Puerto Rico; 2Institute for Functional Nanomaterials, San Juan, Puerto Rico.

The photoluminescence (PL) of Nanoparticles has been of great interest in diverse fields, such as: biological applications (detection and imaging), optoelectronic applications (light emitters devices, lasers) and others. In the present work, we reported the synthesis of Antimony Selenide (Sb2Se3) 0-D nanoparticles using a sonication process in Sb2Se3 nanowires for first time. Transmission Electron Microscopy (TEM) images show naturally dispersed Sb2Se3 0-D nanoparticles with diameters in the range between (2.0 ± 0.1) nm and (6.0 ± 0.1) nm. According to the size distribution histogram, the average size mainly produced of the nanoparticles is of ~ (3.8 ± 0.1) nm. High Resolution Transmission Electron Microscopy (HRTEM) studies show the crystalline nature of the 0-D nanoparticles. Sb2Se3 0-D nanoparticles in suspension (methanol and deionized water) exhibited room temperature blue PL under UV excitation with one PL peak at 422 nm (2.94 eV). This result is very important since, nanoparticles emitting in blue are of great use in light emmiting diodes (LEDs) and lasers.


N7.36
Robust Bonding of Cu Wires with Low Melt Point Nano Ag Solder for Power Chips and Flexible Electronics. H. Alarifi, Anming Hu, M. Yavuz and N. Zhou; Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada.

Lead free low melt point alloys working at 140oC to 250oC are widely used soldering materials for microelectronic and optoelectronic packaging. Under certainly surrounding, such as, for power chips in cars near the engine or CPU at high energy consuming, these interconnection has to bear high operating. Hence the usage of these low melt point solders is suffering high risks to degrading and ageing. To improve the long term reliability new solders which are cured at low temperatures while stably work at higher temperatures are highly desired. On the other hand, the next generated flexible electronics based on plastics/polymers are calling novel solders which can work at lower temperatures. These are motivation for us to develop novel solders with Ag nanoparticles. There are extensively studies on the application of Ag nanoparticles for solders. There are few reports to use aquatic Ag nanosolution as solders because the concentration of Ag nanoparticles is generally low. To improve the concentration also results in the coarsening of Ag nanoparticles. Therefore, Ag nanoparticles are obtained from decomposition of metalo-organic compounds in order to improve the enough concentration or coating Ag nanoparticles with organic shells. Here we report a method to improve Ag nanoaprtciels through centrifuge and successfully solder Cu wires to Cu foils. SEM results show that there are no remarkable coarsening of the Ag nanoparticles before and after centrifugalization. The average size of Ag nanoparticles is around 30 nm. TG results show that there are two thermal issues at 100oC and 165oC for obtained nanoAg paste, which corresponds to water and the fusion of nano Ag respectively. The fusion of Ag nanoparticles at 160oC is confirmed by SEM observation. After aneealing at 160oc or 200oC for 30 min. both thermal issues diappear. The tensile shear test is examined with 250 µm Cu wires soldered to Cu foil at 200oC and 250oC, respectively. The bare Cu wire is shown for comparison. It can found that the maximum pull strength of 250 µm Cu wire about 13 MPa. Soldering at 250oC reached a higher bonding intensity so that the wire is broken during test. Soldering at 200oC results in a low bond strength of 4 MPa where the breaking occurs at bonded interfaces. If the tensile test is carried out while keeping the soldered Cu wire at 250oC 8MPa tensile intensity can be reached. This evidences that Ag nano-soldered bonds can be operated at a higher temperature than curing temperature.


N7.37
Functionalization of Colloidal Crystals Using Photoswitchable Molecules. Yoonho Jun and Paul V. Braun; Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois.

Colloidal crystals have been used as templates for photonic band gap crystals and functionalized porous materials and masks for lithography. Optical properties of colloidal crystals can be controlled by material itself or introducing structural defects. However, incorporation of other functional materials, for example nanoparticles in a site-localized fashion is difficult or impossible with these methods. Photoswitching of molecules grafted to the sphere surface of colloidal crystals followed by localized binding of functional materials is a versatile and unique method to manipulate the properties of photonic crystals. Specifically, photoinversion of surface charge of specific areas enables site-localized deposition of various materials such as quantum dots and metal nanoparticles in the photoswitched area. Nitrobenzenes have been used widely for photocleaveable protecting groups. We used 4-bromomethyl-3-nitrobenzoic acid as a photocleavable linker enabling photoinversion of surface charges deep inside colloidal crystals. However, photocleavage of this molecule requires a long exposure at 351 nm because of the very low absorption at this wavelength. We attached dansylamide to the molecule in order to increase the absorption at the wavelength of the laser, reducing the exposure time by about an order of magnitude. We demonstrate charge inversion upon UV exposure by two beam interference, followed by site-localized deposition of gold nanoparticles and subsequent electroless gold deposition. This generated either a transmission or reflection diffraction grating depending on the beam arrangement. Using a reflection grating, it may be possible to obtain patchy particles by controlling the spatial intensity of the interfering light relative to the particles comprising the colloidal crystal.


N7.38
Abstract Withdrawn

N7.39
Quantification of the Optical Properties of Semiconductor Quantum Dots. Iwan Moreels1, Karel Lambert1, Christophe Delerue2, Guy Allan2 and Zeger Hens1; 1Ghent University, Gent, Belgium; 2Institut d’Electronique, de Microélectronic et de Nanotechnologie, Villeneuve d’Ascq, France.

The optical properties of colloidal semiconductor quantum dots (Qdots) at energies around the band gap are at present well understood. The blue shift of the band gap transition due to quantum confinement has been thoroughly investigated for nearly all types of semiconductors, with good correspondence between experiment and theory. Much less is known however about the oscillator strength of this transition, and about the optical properties at energies far above the band gap. We use the example of colloidal PbS and PbSe nanocrystals, Q-PbS(e),[1,2] to discuss the quantification of the quantum dot optical properties and make a detailed comparison of both materials. First, from the Qdot size, determined with transmission electron microscopy, and the Qdot band gap transition, determined with UV-vis-NIR absorption spectrometry, a sizing curve is constructed. For both materials, this curve is in excellent agreement with theoretical tight binding calculations. Second, after the determination of the Qdot concentration using elemental analysis (inductively coupled plasma mass spectrometry and/or Rutherford backscattering spectroscopy), a procedure is derived to calculate the Qdot molar extinction coefficient ε and absorption coefficient µ from the Qdot absorbance spectrum. At high energies, we find that the Qdot absorption agrees well with calculated values obtained using the bulk semiconductor dielectric function and the Maxwell-Garnett effective medium theory. This demonstrates that quantum confinement plays no role in this spectral range, and it highlights the importance of the local field factor fLF when calculating the Qdot optical properties. At the band gap, the Qdot oscillator strength fif is determined from the energy integrated molar extinction coefficient εgap. For both materials, fif scales linearly with size, in contrast with calculations performed within the effective mass approximation, and results are in excellent agreement with tight-binding calculations. Values for Q-PbS are 34% smaller than for Q-PbSe. Interestingly, despite the smaller fif, both materials have still the same εgap, as fLF is 38% larger for Q-PbS. The oscillator strength is finally related to the Qdot exciton lifetime. Calculated values range between 1 and 3 µs, in agreement with literature data obtained from time-resolved luminescence measurements. This again demonstrates that the local field factor (i.e. dielectric confinement) plays an important role in the quantification of the Qdot optical properties. [1] I. Moreels, K. Lambert, D. De Muynck, F. Vanhaecke, D. Poelman, J.C. Martins G. Allan and Z. Hens, Chemistry of Materials 2007, 19, 6101-6106. [2] I. Moreels and Z. Hens, Small, 2008, 4, 1866-1868.


N7.40
The Dielectric Function of Colloidal Lead Chalcogenide Quantum Dots. Iwan Moreels1, Bram De Geyter1, Christophe Delerue2, Guy Allan2 and Zeger Hens1; 1Ghent University, Gent, Belgium; 2Institut d’Electronique, de Microélectronic et de Nanotechnologie, Villeneuve d’Ascq, France.

We have recently demonstrated that the optical properties of colloidal semiconductor quantum dots (Qdots) can be quantified using the Maxwell-Garnett (MG) effective medium theory.[1,2] Most importantly, for the calculation of the Qdot molar extinction coefficient, or equivalently the absorption coefficient, a local field factor needs to be taken into account (dielectric confinement). At high energies, the Qdot absorption coefficient agrees well with calculated values using the bulk semiconductor dielectric function, demonstrating the absence of quantum confinement effects in this spectral range. For colloidal PbS, PbSe and PbTe quantum dots, we now exploit this bulk-like nature of the optical properties to expand the Qdot absorption coefficient with calculated bulk values in the wavelength range of 0-400nm. This way, the Qdot absorption coefficient is known over the entire spectral range, and the Kramers-Krönig (KK) relations are applied to calculate the real and imaginary part of the Qdot dielectric function. As the combination of the KK relations with the MG effective medium theory yields a set of nonlinear relations, a direct calculation of the dielectric function is not possible. However, we have elegantly solved this by developing an iterative approach to the problem. The validity of this method is confirmed by a calculation of the bulk dielectric function of PbS and PbTe. The resulting Qdot dielectric function allows us to study the effects of quantum confinement in more detail than previously possible, due to the removal of the local field factor (which leads to a strongly rising absorption at short wavelengths, hereby cloaking optical transitions[2]). We find that both the E0 (band gap) transition, at the L point in the Brillouin zone, and the E1 transition, along the Σ direction, shift to shorter wavelengths with decreasing Qdot size. This is accompanied by an increase of the oscillator strength (per unit volume). The E2 transition however, along the Δ direction, remains bulk-like for all three materials. We then use the f-sum rule and the static limit sum rule to calculate the band gap oscillator strength and static dielectric constant, respectively. The oscillator strength agrees well with values obtained from the Qdot absorption coefficient.[1] Surprisingly, both our experimental data and tight-binding calculations show that the static dielectric constant is bulk-like for all three materials. In summary, our approach to calculate the dielectric function offers both an improved understanding of the Qdot optical properties and a tool to obtain valuable data for further Qdot applications. The optimization of Qdot lasers and solar cells for instance may benefit greatly from knowledge of the Qdot refractive index. [1] I. Moreels, K. Lambert, D. De Muynck, F. Vanhaecke, D. Poelman, J.C. Martins G. Allan and Z. Hens, Chemistry of Materials 2007, 19, 6101-6106. [2] I. Moreels and Z. Hens, Small, 2008, 4, 1866-1868.


N7.41
The Effect of Reduction Time on the Growth and Luminescence Properties of Hexadecylamine -capped CdSe Nanoparticles Synthesised via an Aqueous Method. Oluwatobi S. Oluwafemi, Physics, Nelson Mandela Metropolitan University, Port -Elizabeth, Eastern Cape, South Africa; Chemistry, University of Zululand, Kwadlangezwa, Kwazulu-Natal, South Africa.

Cadmium selenide (CdSe) is an interesting II-VI semiconductor material because its band gap can be tuned across the visible spectrum by varying the size of the materials. Its unique chemical and electronic properties give rise to its potential use in many applications which includes; biological label, display, diodes and lasers, solar cells and gas sensors. The quality of the nanoparticles synthesized via an aqueous route is controlled by subtle variation in synthetic parameters such as reactant ratio, pH of the solution and nature of the solvent. In addition, the type of reaction(s) (i.e. oxidation, reduction, precipitation, decomposition, displacement reaction, etc.) occurring between the reacting precursors also contribute to the nanoparticle size, quality and luminescence properties. We herein report the synthesis of Hexadecylamine (HDA) capped CdSe nanocrystals via a facile, non-Organometallic approach, without the use of any additional stabilizer or acid. The synthetic approach involved the reduction of selenium powder in water to produce selenide ions which act as selenium source followed by addition of CdCl2 which acts as the cadmium precursor. Thermolysis was carried out in HDA to passivate the surface. The temporal evolution of the optical properties, of the growing nanocrystals was monitored in detail by varying the reduction and reaction time between 2 to 6 hrs and 2 to 30 mins respectively. The particle size and Photoluminescence properties were found to decrease as the reduction time increases The reduction time of 6 hrs gives the slowest growth stage. The as-synthesised nanocrystals show fast growth during the early reaction time ( 2-15 mins) and slow growth in the later growth in the later stage ( 15-30 mins). All the as- synthesised samples were of high quality i.e. sharp absorption feature and strong band-edge emission and were stable upon aging for several months. The nanoparticles show narrow emission and broad excitation spectra, a distinct advantage over conventional organic dye for possible use as fluorescent probe. The structural properties were investigated using x- ray diffraction, transmission electron spectroscopy, scanning electron microscope and energy dispersive spectroscopy (EDS). The EDS spectra confirm the presence of the corresponding elements. The insight gained from this study enable a synthetic approach for the synthesis of high quality water soluble nanoparticles with high synthetic reproducibility.


N7.42
Electro-responsive Photonic Bandgap of Crystalline Colloidal Arrays. Tae Soup Shim, Shin-Hyun Kim, Jae Young Sim, Jong-Min Lim and Seung-Man Yang; Chemical & Biomolecular Eng., KAIST, Daejeon, Korea, South.

Three dimensional photonic crystals have been widely studied for a decade due to their unique optical characteristics of photonic bandgap. In particular, self-organization of monodisperse colloids is one of the most promising methods to prepare these bandgap materials due to fast and inexpensive fabrication process. When the charged particles are dispersed in polar medium, they assemble to various crystalline phases at thermodynamic equilibrium state depending on the particle volume fraction and strength of interparticle potential. In order to stabilize and control the crystal lattice, many researchers have introduced polymeric network into the colloidal crystals in liquid phase which is responsive to various external stimuli such as pressure, pH and humidity. However, the polymerized crystalline colloidal arrays (PCCA) exhibit slow response due to the high viscoelasticity of polymeric network and external stimuli such as pressure, pH, or humidity cannot be easily modulated. In this study, we report a reversible photonic bandgap tuning of colloidal crystals using electric field to overcome the above mentioned problems of PCCA. To achieve this, we used highly charged polystyrene particles of ca. 180 nm in diameter which were synthesized through emulsion polymerization by adding ionic comonomer, 3-allyloxy-2-hydroxy-1-propanesulfonic acid (COPS-1) and 2-amino ethyl methacrylate hydrochloride (AEMH) for negative and positive charges, respectively. The charged particles formed colloidal arrays (CCAs) even in low volume fractions under the strong electrostatic repulsion. Appling the AC electric field to CCAs confined between two transparent electrodes, the lattice constant oscillated along the direction of electric field due to the electrokinetic force on particles. The periodic modulation of the lattice constant induced the bandgap shift of which the magnitude was controlled by electric field intensity. The CCAs kept their crystallinity for at least 12 000 cycles, and actuation voltage and response time of bandgap tuning were as small as 2 V and 50 ms, respectively, thanks to high colloidal mobility in liquid medium of low viscosity. The present prototype system is easily achievable and useful in the most of mobile electronic devices. With these advantages as well as high reflectivity of CCAs, finally, we could tune the local reflection color with patterned transparent electrodes, demonstrating the possibility of potential applications such as reflective mode displays, optical switches, and tunable mirrors.


N7.43
Organosoluble and Reactive Nanoparticles of Oxides Ready for Nanocomposite. Bruno Boury, Hubert Mutin, Abdelhay Aboulaich and Olivier Lauret; Chemistry, UM2, Montpellier, France.

The preparation and properties of nanoparticles of SiO2, SiO2/TiO2, TiO2 and SnO2 is presented with the idea of their incorporation in nanocomposite. The ether route of non-hydrolytic sol-gel method is used for their preparation and the whole characterization agree with the formation of well dispersed and uniform rounded shape nanoparticles (XRD, TEM, AFM, Raman, FTIR, UV-vis, BET, TGA). Two important points make them suitable for nanocomposite applications, their organosolubility and their high reactivity. Indeed these nanoparticles are synthesis in dichloromethane and can also be dissolved in THF or Toluene. Due to the presence of an important part of metal-chloride functions at the surface, they reactivity is very low on themselves but very high toward an hydroxylated surface or a nucleophile agent. They can also be specifically reacted with phosphonic acid before or after direct graphting on silicon wafer.


N7.44
Conductivity and Adhesion Enhancement in Low-temperature Processed Indium Tin Oxide (ITO)-Polymer Nanocomposites. Ilja Maksimenko1, Michael Gross1, Tobias Koeniger2, Helmut Muenstedt2 and Peter J. Wellmann1; 1Department of Materials Science and Engineering 6, University Erlangen-Nuremberg, Erlangen, Bavaria, Germany; 2Department of Materials Science and Engineering 5, University Erlangen-Nuremberg, Erlangen, Bavaria, Germany.

We report on the conductivity and adhesion enhancement of indium tin oxide (In2O3:Sn; ITO) nanoparticle films by the application of polymers as matrix material. We fabricated ITO layers at a maximum process temperature of 130°C by modifying and spin-coating nanoparticulate ITO dispersions. Dispersions containing the organic film-forming agent polyvinylpyrrolidone (PVP) and the organofunctional coupling agent 3-methacryloxypropyltrimethoxysilane (MPTS) have been developed to obtain transparent and conducting coatings on substrates which do not withstand high process temperatures like polymers or already processed glasses. The layers were cured by UV-irradiation as well as by low-temperature heat treatment (T = 130°C) in air and forming gas. The influence of the additives on the electrical, optical, morphological and mechanical layer properties is presented. Compared to best pure ITO layers (3.1 Ω-1cm-1), the nanocomposite coatings exhibit a conductance of up to 9.8 Ω-1cm-1. Stable sheet resistances of 750 Ω/square, which are already sufficient for first test devices like electroluminescent lamps, at a coexistent transmittance of 86% at 550 nm for a layer thickness of about 1.3 µm were achieved. The conductance enhancement is a consequence of the consolidation of the ITO nanoparticle network due to the acting shrinkage forces caused either by drying in the case of PVP or UV-irradiation induced condensation and polymerization reactions in the case of MPTS.


N7.45
Ion and pH Sensing with Colloidal Nanoparticles - the Influence of Surface Charge on Sensing and Colloidal Properties. Feng Zhang, Zulqurnain Ali, Faheem Amin and Wolfgang J. Parak; Physics, Philipps University of Marburg, Marburg, Germany.

Ion sensitive colloidal nanoparticles are typically charged in order to be colloidally stable. We demonstrate that this charge can significantly change the binding constant of the ions to the nanoparticles and thus change the read-out. The sensor read-out is not determined by the ion concentration in the bulk, but by the ion concentration in the local nano-environment of the nanoparticles.


N7.46
Abstract Withdrawn


N7.47
Colloidal Photonic Crystals with Controlled Sizes and Shapes for Advanced Photonic Devices. Shin-Hyun Kim1,2, Seog-Jin Jeon1,2 and Seung-Man Yang1,2; 1Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Korea, South; 2National Creative Research Initiative Center for Integrated Optofluidic Systems, Korea Advanced Institute of Science and Technology, Daejeon, Korea, South.

Photonic crystals have emerged as one of the most promising materials for manipulating the lights because they have photonic bandgaps, doing for photons what energy bandgaps of semiconductors do for electrons. The periodic modulation of refractive index in space with subwavelength periods produces the photonic bandgaps; the photons with energies in the gaps are prohibited in the materials. For creating 3D photonic crystals, the self-organization of colloidal particles has been considered as the most simple and economical approach. However, colloidal crystals have intrinsic defects such as vacancies, cracks and faults, which deteriorate the optical performance of the crystals. Also, the most of colloidal crystals or their derivatives have low physical rigidity and weather resistance. More importantly, the exact control of the bandgap is difficult and tailoring the crystals into desired shape requires additional complexity in fabrication steps. In the present work, using silica particles dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) resin, we successfully fabricated various photonic structures with negligible cracking via evaporation-free colloidal self-assembly. Due to the disjoining pressure of solvation film on the surface of the particles and weak electrostatic repulsion, the monodisperse silica particles crystallized into the face-centered cubic (fcc) lattice in the photocurable ETPTA resin with high polarity. For practical applications, we prepared spherical photonic crystals (called photonic balls) using emulsion droplets as templates. Monodisperse photonic balls could be generated by using simple and high-throughput optofluidic devices which were composed of microfluidic emulsion generator and UV exposure unit. The color of photonic balls was controlled by particle size and concentration, precisely. In addition, photonic Janus balls with electrical anisotropy could be generated for color pigments in rotating ball type display (i.e., Gyricon display). On the other hand, integration of photonic crystals with different bandgaps was achieved by molding technique of the photocurable silica suspensions. Twenty photonic crystal strips, of which bandgaps covered entire visible range, were patterned to a single thin film. Using the integrated photonic crystal patterns, we could make miniaturized spectrometer of new paradigm which can be used in Lab-on-a-Chip system of a stand-alone platform.


N7.48
Non-Hydrolytic Synthesis, Characterization, and Electronic Structure of Colloidal Early Transition Metal Oxide Nanocrystals. Sean W. Depner1, Kenneth R. Kort1, Cherno Jaye2, Daniel A. Fischer2 and Sarbajit Banerjee1; 1Chemistry, University at Buffalo, State University of New York, Buffalo, New York; 2Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland.

Interest in early transition metal oxide nanoparticles has grown in recent years due to their many potential applications in electronics and energy storage. Colloidal HfO2, ZrO2, and solid-solution HfxZr1-xO2 nanocrystals are of interest for use in high-κ dielectric films and high-refractive-index coatings. CeO2 nanocrystals are of interest as UV absorbers and catalyst supports for CO oxidation and the water-gas-shift reaction. A non-hydrolytic sol-gel approach has been used to prepare colloidal metal oxide nanocrystals based on the condensation of a metal alkoxide and a metal halide proceeding via a SN1 reaction mechanism. Nanocrystalline solid solutions of HfxZr1-xO2 have been synthesized, and control over crystal structure and stoichiometry has been established through the choice of the R group on the metal alkoxide precursor. For R groups with a higher degree of branching, the nanocrystal crystallizes in the monoclinic phase, whereas lower degree of branching of the metal alkoxide yields the tetragonal phase. In contrast, the sterics of the R group on the metal alkoxide precursor is found to influence the relative Hf:Zr ratios in solid-solution HfxZr1-xO2 nanocrystals. For R groups with longer carbon chain lengths, the concentration of Hf is decreased, while shorter carbon chain lengths have higher concentrations of Hf in the HfxZr1-xO2 solid solutions. This choice of metal alkoxide also has a significant influence on the synthesis of nanocrystalline CeO2. When an alkoxide with more branching is used, the cubic CeO2-γ product is formed, while less branched alkoxides yields the matlockite CeOCl product. The effect of varying the halide precursor has also been studied; the Cl precursor has been found to yield smaller nanocrystals with higher concentrations of Ce3+ and oxygen vacancies, while the Br and I precursors lead to larger nanocrystals with decreased oxygen vacancy concentrations. Solid-solution nanocrystals of both CexLa1-xO2-γ and CexLa1-xOCl have also been synthesized using varying amounts of LaCl3 and La-(III)-isopropoxide as the precursors. The electronic structure of the prepared CeO2-γ nanocrystals has been systematically examined by synchrotron-based near edge X-ray absorption fine structure spectroscopy at the Ce L and O K-edges. The prepared metal oxide nanocrystals have been structurally characterized by high-resolution transmission electron microscopy, X-ray diffraction, optical absorption, FTIR, Raman, inductively coupled plasma optical emission spectroscopy. The integration of these nanocrystals in flexible high-κ nanocomposite films has also been explored. 1Depner, S. W.; Kort, K. R.; Banerjee, S.; CrystEngComm. 2009, 11, 841-846. 2Depner, S. W.; Kort, K. R.; Jaye, C.; Fischer, D. A.; Banerjee, S.; 2009, submitted for publication.


N7.49
Synthesis of Crystalline Silicon Nanoparticles by Gas Phase Reactions using Inductive Coupled Plasma. Bo yun Jang1, Chang Hyun Ko2 and Jeong Chul Lee3; 1Energy Conversion and Storage Research Center, Korea Institute of Energy Research, Daejeon, Korea, South; 2Greenhouse Gas Research Center, Korea Institute of Energy Research, Daejeon, Korea, South; 3Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon, Korea, South.

Silicon nanoparticles were synthesized by passing monosilane through the quartz tube wrapped with Inductive Coupled Plasma (ICP) coil. When ICP was applied to tube reactor, the shape of plasma region was very sensitive to the process conditions. Therefore, for controlling the plasma shape, additional inner tube was inserted in quartz tube reactor. To evaluate this double tube reactor, microstructures of silicon nanoparticles were investigated with various process conditions. This additional tube reduced the length of plasma zone and made highly dense plasma formed only near ICP coil. Injection of reactive gas through inner tube resulted in formation of crystalline nannoparticles. For formation of single crystalline silicon phase, ICP power and partial pressure of monosilane were key parameters. Controlling those parameters, we achieved single crystalline silicon nanoparticles with diameters of 10 ~ 15 nm. Silicon nanoparticles in this study can be applied for light absorber material of the solar cell and wavelength down-converter material of Light Emitting Diode (LED).


N7.50
Mixed Ag/Pd Nanoparticle Substrates for Sensitive Detection of NO and CO by Surface Enhanced Raman Spectroscopy. Imran Khan and Steven I. Rae; Materials Science Research Department, AWE, Reading, United Kingdom.

Surface enhanced Raman spectroscopy (SERS) can achieve sensitive and selective analysis of analytes adsorbed to SERS substrates. In this paper we demonstrate sensitive SERS analysis of analytes that do not adsorb to traditional SERS substrates. Mixed Ag/Pd nanoparticle substrates are shown to adsorb NO and CO gases and generate strong SERS from the adsorbed species. The SERS enhancement factor for both NO and CO was calculated to be 5 orders of magnitude and comparable to the SERS enhancement factor measured from benzenethiol, a "model" SERS analyte that chemisorbs to SERS substrates. The desorption and displacement of NO and CO from the Ag/Pd substrate is also presented, demonstrating the potential of SERS substrates for continuous, long lifetime gas analysis.


N7.51
Thermally Activated Photoluminescence in PbSe and PbSe/PbS Colloidal Quantum Dots. Georgy I. Maikov, Roman Vaxenburg, Diana Yanover, Gal Grinbom, Meirav Saraf, Aldona Sashchiuk and Efrat Lifshitz; Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute and Solid State Institute, Technion, Haifa, Israel.

This work describes the influence of thermal activation processes on the ground-state exciton emission of PbSe cores and PbSe/PbS core-shell colloidal quantum dots (CQDs) [1, 2]. In general, the core-shell CQDs showed a long-term chemical robustness and a higher luminescence quantum yield (70%) at room temperature, in comparison with that of the corresponding core (~40%) CQDs. The activation processes were monitored by recording the continuous-wave and transient photoluminescence (PL) spectra over a temperature range from 1.4 K to 300K, viewing sudden changes in the photoluminescence band intensity, emission peak energy, and full width at half maximum. A low temperature activation process was seen between 5-7K in the core samples, and between 10-20K in the core-shell structures, both cases are related to assisted dark exciton recombination by an acoustic photon coupling. A second activation process occurred between 100-200K, depending on core size, as well as on the core-radius/shell-thickness ratio. This activation overcame an energy difference between a dark and a-bright exciton state, when the activation temperature decreased with the increase of the dots’ size or the shell thickness. A dark-bright states manifold is created by electron-hole exchange interactions and by an induced valley-valley interaction among four-fold degenerate minima in PbSe semiconductors at the L-point of the Brillouin zone. The transient PL measurements of PbSe cores (with diameters between 3-4 nm) showed a single exponent decay process, with a dark exciton lifetime of ~6μs as monitored at 4K, however, a bright exciton lifetime of ~450 ns at 300K. The corresponding core-shell CQDs (with similar core size) showed a stretch exponential PL decay, with a bright exciton effective lifetime of ~ 1μs, obviously longer than that of the core. The variation between the core and core-shell samples are related to a certain degree of delocalization of at least one carrier wavefunction across the core-shell interface (depending on the band-edge offset), thus inducing coupling with PbSe and PbS phonons, and a partial electron-hole separation. The electronic configuration of the core-shell CQDs was further examined by scanning tunneling microscopy, revealing a shrink of the band-gap energy with respect to the cores, suggesting a certain degree of carriers delocalization into the shell regime, which increased with the increase of the shell thickness. [1] Brumer M, Kigel A, Sashchiuk A and Lifshitz E 2005 Adv. Fun. Mat. 15 1111 [2] Kigel A, Brumer M, Maikov G I, Sashchiuk A and Lifshitz E 2009 SMALL in press


 

SESSION N8: New Methods In Nanoparticles and Superstructures
Chair: Nicholas Kotov
Thursday Morning, December 3, 2009
Constitution A (Sheraton)


8:00 AM *N8.1
Engineering Binary Nanocrystal superlattices for photonic applications. Chris Murray1,2, Xingchen Ye1, Jun Chen2, Angang Dong1, Dong-Kyun Ko2, Weon-Kyu Koh1, Danielle Reifsnyder1 and Don-hyung Ha2; 1Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania; 2Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania.

Colloidal nanocrystals with controlled crystal shape, structure and surface passivation provides a rich family of nanoscale building blocks for the assembly of multi-component solid thin films. The tunability of the electronic and optical properties of the nanocrystals has lead to them being compared to a set of "artificial atoms"(1). This talk will briefly share our “best practices” in preparation, isolation and characterization of monodisperse semiconducting quantum dots plasmonic nanocrystals and nanophoshors. I will next discuss the organization of monodisperse nanocrystals in to single component and multi-component superlattices that retain and enhance many of the desirable mesoscopic properties of individual nanocrystals of which they are composed. The potential to design new materials expands dramatically with their assembly into superlattices. Differently sized PbS, PbSe, PbTe, Ag2Te, CdSe, CdTe, Au, Ag, NaYF4: RE (where RE is a family of rare earth dopants). Binary superlattices with AB, AB2, AB3, AB4, AB5, AB6 and AB13 stoichiometry and with cubic, hexagonal, tetragonal and orthorhombic packing symmetries have been grown. The final portion of the talk describes the techniques to integrate these materials in functional devices for photonic applications. Although modular nano-assembly approach has already been extended to a wide range of nanocrystal systems and directed self-assembly has provide a route to integrate many of these into novel devices, we are confident that we have produced only a tiny fraction of the materials that will soon be accessible.


8:30 AM N8.2
Formation of Large Scale 3D Colloidal Nanoparticle Superlattices in Spatially Controlled Locations. Chenguang Lu, Austin Akey and Irving P. Herman; Applied Physics and Applied Mathematics, Columbia University, New York, New York.

A multiple solvent system consisting of a high boiling point solvent (decanol) and a low boiling point solvent (toluene, xylene) was found to greatly aid the self-assembly of colloidal nanoparticle superlattices in lithographically defined capillaries. Large (up to micrometer scales), single crystalline 3D supercrystals were formed at spatially controlled locations using this technique. The ordered nature of the sample was probed by high-resolution SEM and other methods. This technique is versatile and has been applied to various types and sizes of colloidal nanocrystals, including those composed of CdSe and FexO, to form 3D supercrystals. The underlying mechanism of this technique was investigated and the capillary effect and the slow evaporation rate of the high boiling point solvent were found to promote the formation of such supercrystals.


8:45 AM N8.3
Large-Area Ordered Superlattices from Magnetic Wustite/Cobalt-Ferrite Core/Shell Nanocrystals by Doctor Blading. Maryna I. Bodnarchuk1,4, Maksym V. Kovalenko1,4, Stefan Pichler1, Gerhard Fritz-Popovski2, Guenter Hesser3 and Wolfgang Heiss1; 1Institute for Semiconductor and Solid State Physics, University Linz, Linz, Austria; 2Institute of Physical Chemistry, Karl-Franzens University Graz, Graz, Austria; 3Center of Surface- and Nanoanalytics, University Linz, Linz, Austria; 4Department of Chemistry, The University of Chicago, Chicago, Illinois.

Although the formation of ordered arrangements from particles upon drying from colloidal solutions is known since a century, and self-assembly of colloidal nanocrystals was demonstrated to lead to a large diversity of single-component, binary and quasi-ternary superstructures, there are almost no applications of nanocrystal superstructures, due to a lack of control of the self assembly over large scales. A reel-to-reel compatible large area coating technique for solution processing is given by doctor blading. Here we demonstrate doctor blading of colloidal nanocrystal solutions on various substrates, to obtain nanocrystal superlattices with high homogeneity and large-scale ordering over coherence lengths, outperforming those obtained by alternative methods like drop casting, spin casting or ink-jet printing. The self-assembly process is demonstrated for magnetic nanocrystals, having a high potential for applications in magnetic memory devices. In particular, shape controlled and monodisperse nanocrystals with a Wustite core and a Cobalt ferrite shell were used for the large scale deposition because of their simple synthesis, their environmental compatibility, and their adjustable magnetic properties [1]. The latter arise from the hard-magnetic Cobalt ferrite shells and to some part from their exchange interactions with the antiferromagnetic cores. The doctor blading of the colloidal solutions results in films exhibiting hexagonally closely packed nanocrystal arrangements, which are formed by a top-down growth, as is evidenced by cross sectional transmission electron microscopy. The ordering in the topmost layer is extending over large areas, although some defects like grain boundaries, vacancies and other point defects are found. The degree and quality of self assembly is quantified by analysing the microscopic plan view-images of the assemblies by means of the decay of their autocorrelation function. This analysis is demonstrated to be valuable for different types of superlattices - single component superlattices formed either from spherical or cubic shaped nanocrystals and binary superlattice structures. For the latter, however, much shorter coherence lengths are achieved than for single-component superlattices. Furthermore, the results for the coherent lengths deduced from the autocorrelation analysis is shown to be consistent with those from grazing-incidence small-angle X-ray scattering, giving coherence length in the order of 1000 nm. [1] M. I. Bodnarchuk, M. V. Kovalenko, H. Groiss, R. Resel, M. Reissner, G. Hesser, R. T. Lechner, W. Steiner, F. Schäffler, W. Heiss, Small 2009 (in print)


9:00 AM N8.4
Supra- Quantum Dots: Parallelism Between Quantum Dots and Supra- Quantum Dots in Growth, Assembly and Device Applications. Juwon Park1, Jiwon Bang1, Ji Hwang Lee2, Jutaek Nam1, Kilwon Cho2 and Sungjee Kim1; 1chemistry, POSTECH, Pohang, Korea, South; 2chemical engineering, POSTECH, Pohang, Korea, South.

A supramolecule can be defined as a system of two or more molecular entities held together and organized by means of intermolecular bindings. Analogous to this, we introduce supra- quantum dots (SQDs) that consist of a few hundreds of primary CdSe or CdTe quantum dots. The primary QDs assemble into three-dimensional aggregates by dipole induced oriented attachments. The kind and amount of surfactants are carefully controlled to induce the primary QDs to have high surface energy facets that can be subsequently used for SQD building blocks. SQDs nucleate as tetrahedral shaped primary QD seeds self-assemble into larger tetrahedral aggregates. Time-resolved SQD growth studies reveal the fast growth by face-oriented attachments among the tetrahedral aggregates which are followed by the continuous and spherical growth at the expense of smaller aggregates. This can be paralleled by ‘magic size’ QDs in their early growth stage and the subsequent Ostwald ripening. ‘Size-focusing’ and ‘size-defocusing’ behaviors of QDs are reproduced by SQDs upon multiple injections of primary QDs instead of QD precursors. SQDs can be obtained size-tunable ranging 40 ∼ 200 nm with as narrow size distribution as ∼ 5 % relative deviation. This unique size control ability over tens or hundreds nm sized supra-nanoparticles is further demonstrated by self-organization of SQDs into three dimensional superlattices. SQDs are flexibly compatible with solution-process, and can be incorporated into conducting polymers. CdSe SQD composite photovoltaic devices show the energy conversion efficiency reaching ∼ 1 %, which is over 3 times higher than the control QD devices with same inorganic weight contents. We will discuss potential applications of 3D assemblies of nanoparticles for various opto-electronic devices.


9:15 AM N8.5
All-inorganic Approach to Ligand Coating, Encapsulation and Linking of Colloidal Nanocrystals. Maksym V. Kovalenko and Dmitri V. Talapin; Department of Chemistry, University of Chicago, Chicago, Illinois.

Colloidal nanocrystals are recognized as promising building blocks for electronic and optoelectronic devices, which offer a unique combination of size-engineered properties, diverse compositions and morphologies, and solution processability. However, many practical implementations of these materials are hindered by the poor electronic coupling between the nanocrystals in close-packed nanocrystal films, caused by the presence of bulky insulating organic surface ligands. Since most of high quality nanocrystals are synthesized using these ligands, the problem is a real puzzle for materials science. Despite several successful examples of improved conductivity in nanocrystal solids using small organic linking molecules, there is a real need to find a general approach to electronic coupling between chemically-synthesized nanocrystals. To address this problem we demonstrate that molecular metal chalcogenide complexes can act as versatile ligands for a broad class of colloidal nanocrystals [1]. We developed a simple ligand-exchange procedure for complete replacement of original organic ligands by metal chalcogenide complexes such as SnS4(4-), SnTe4(4-), In2Se4(2-), Sn2S6(4-), SbSe4(5-) etc. These complexes exhibit high affinity to various semiconductor (CdSe, PbS, PbTe, Bi2S3 etc.) and metallic (Au, Pd, FePt etc.) nanocrystals rendering them soluble in water and polar organic solvents. This novel ligand coating provides an “electronic glue” for colloidal nanocrystals. Nanocrystal solids prepared from this new class of colloids show a set of advantages such as all-inorganic design, small interparticle spacing and greatly improved transport properties. As examples, we show metallic conductivity (~200 S/cm) in arrays of Au nanocrystals capped with Sn2S64- ions and operation of field-effect transistor based on CdSe nanocrystals. Furthermore, we show several other attractive aspects of this approach. First, ligands can be converted into host semiconductors by gentle thermal treatments. Second, these ligands can react with the nanocrystal core forming useful semiconducting nanocrystalline compounds such as BixSb2-xTe3 thermoelectric solid solutions. Third, polydentate nature of metal chalcogenide complexes enables linking of nanocrystals with various metal ions to tune the electronic structure of nanocrystal solids. [1] M. V. Kovalenko, M. Scheele, D. V. Talapin, Science 2009, 324, 1417-1420


9:30 AM N8.6
Modular Inorganic Nanocomposites from Nanocrystal Superlattices. Ravisubhash Tangirala, Robert Y. Wang and Delia J. Milliron; The Molecular Foundry, Lawrence Berkeley Natl Lab, Berkeley, California.

Amenable to large area and low cost processing, colloidal nanocrystals are now poised to enable of a wide range of electronic devices. Already prototype transistors, photodetectors and photovoltaic cells have been demonstrated using nanocrystal superlattices as the active semiconducting material. Recent leaps ahead in performance metrics have been facilitated by the replacement of bulky, insulating organic ligands with small, surface passivating molecules which allow greater electronic interaction between adjacent nanocrystals. We are pursuing a new generation of nanocrystal-based electronic materials which are purely inorganic nanocomposites. We now report the modular construction of such nanocomposites from two classes of building blocks: colloidal nanocrystals and soluble metal chalcogen molecular clusters. Various materials combinations can be selected and systematically varied to understand and manipulate the role of nanostructuring in transport properties relevant to diverse electronic devices.


9:45 AM N8.7
A Robot Called WANDA: An Automated Platform for High-Throughput Investigations into Nanoparticle Nucleation and Growth. Emory Chan1, Jonathan S. Owen2,3, Alvin W. Mao1, Chenxu Xu1 and Delia J. Milliron1; 1Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California; 2Dept. of Chemistry, University of California, Berkeley, California; 3Dept. of Chemistry, Columbia University, New York, New York.

Electronic applications of colloidal nanoparticles demand robust, reproducible, and scalable synthetic methods for producing nanostructures with precisely defined physical properties. Traditional synthetic methods, however, are often irreproducible due to these schemes’ inability to control critical reaction parameters such as reagent injection rates, temperature, and mixing. The uncertainty in these parameters hinders the systematic study of nanoscale reaction mechanisms, while the poor scaling of heat and mass transport prevents the scale up of these reactions for mass integration. We present a robotic platform for the rapid, reproducible synthesis and screening of colloidal inorganic nanomaterials. Using WANDA - the Workstation for Automated Nanomaterials Discovery and Analysis - we demonstrate the automated production and characterization of colloidal semiconductor, metal, insulator, and doped nanoparticles and nanowires. Liquid-handling robotics prepare reagent solutions and initiate reactions via the injection of precursors into an array of independently-controlled, high-temperature batch reactors. Aliquots are sampled from reaction mixtures over time and are characterized in parallel using microplate-format absorption spectroscopy, photoluminescence spectroscopy, and wide-angle X-ray diffraction. Because WANDA utilizes the same set of reaction conditions as those used for traditional flask-based syntheses, we can screen a diverse set of reactions and can directly transfer WANDA’s results to non-automated syntheses. We demonstrate WANDA’s fine control over reaction conditions by synthesizing CdSe nanocrystals with highly reproducible and predictable diameters, size distribution, concentration, and yield. Over a series of 40 reactions, the mean particle diameter across reactor array elements and across five separate runs exhibits a coefficient of variation (<1 %) within the error of our sizing techniques - a precision that far exceeds that of the analogous manual procedure. Using WANDA’s ability to screen multi-dimensional phase space, we demonstrate the optimization of CdSe nanocrystal diameter and size distribution - critical parameters for nanoparticle assemblies and devices. Applying WANDA’s precise control over reaction parameters, we systematically investigate the kinetics of CdSe nanocrystal nucleation and growth over a wide range of temperatures, precursor concentrations, and surfactant concentrations. Aliquot absorption spectra are used to calculate particle concentrations, diameters, and reaction yield, which then reveal nucleation rates, growth rates, rate orders, and thermodynamic constants for the CdSe nanoparticle reaction. The breadth and precision of the data produced by this automated platform enable the development of a mechanistic model that integrates nanoparticle nucleation, precursor conversion, and surfactant-mediated crystallization.


10:15 AM N8.8
Atomically Controlled Nanoplatelets of CdSe: A New Class of Nanoparticles. Sandrine Ithurria and Benoit Dubertret; LPEM, CNRS ESPCI, Paris, France.

Well documented procedures to grow zero dimensional systems, dots, and one dimensional systems, wires and tubes, as colloidal particles in solution have been reported. In contrast, there are no methods of preparation that yield optically active two dimensional soluble particles. Yet, ultra thin films (quantum wells) of II-VI and III-V semiconductors epitaxially grown on substrates by molecular beam epitaxy for example have proven extremely useful for both fundamental studies and a wealth of applications in optoelectronics. We show that II-VI cadmium selenide platelets, with thicknesses tuned at the atomic level, can be synthesized in solution. We describe the method for the preparation of these new colloidal nanocrystals and characterize them structurally and optically. We identified three platelets populations with emission maximum at 462nm, 513nm and 550nm with corresponding thicknesses estimated at 1.9nm, 2.2nm and 2.5nm respectively. Despite the fact that the platelets aspect ratio within a population can range from 4 to several hundreds, the emission spectra full width half maximum of each population is < 10nm at room temperature with quantum yields that can reach 30%. The platelets we have synthesized are an extension of the quantum wells epitaxially grown on substrates, with the advantages that they can be easily synthesized in solution at low cost and used as building blocks for more advanced structures, have uniform thickness that can be tuned within one CdSe monolayer, and finite lateral dimensions ranging from 10nm to few 100 nm.


10:30 AM N8.9
Rational Synthesis of Semiconductor Quantum Dots. Christopher M. Evans and Todd D. Krauss; Chemistry, University of Rochester, Rochester, New York.

Colloidal semiconductor nanocrystals represent an important class of nanostructures that serve as model systems for fundamental investigations of quantum confinement and that also have potential for significant breakthroughs to advance applications over a broad range of fields from optoelectronics to biology. Over the past two decades, significant advances have been made regarding the synthesis of high quality nanoparticles with a controllable size, shape and composition. However, these syntheses are notoriously dependent on ill-defined details of the starting materials, reaction yields are poor, and exact reproducibility is extremely difficult to achieve, primarily because the fundamental reaction mechanism responsible for the initial stages of nanocrystal growth is largely undetermined. We will present studies of the chemical reaction mechanism describing the general synthesis of metal chalcogenide semiconductor quantum dots using a combination of optical spectroscopy, electron microscopy, mass spectrometry and nuclear magnetic resonance spectroscopy. In particular, we have identified specific chemical reactants critical for mediating the synthesis of semiconductor nanoparticles. These highly reactive species are typically found as impurities in the starting reagents, but are completely responsible for driving the kinetics of the nanocrystal formation. These findings have allowed us to determine a reaction pathway general to all phosphine based chalcogenide nanocrystal syntheses. Further, we also propose a complete reaction mechanism that provides a complete picture of how simple organometallic molecules can be transformed into nanoscale semiconductors comprising thousands of atoms. We have utilized these findings to improve reaction yields for CdSe and PbSe nanocrystals by over an order of magnitude, while producing nanocrystals of exceptional quality with respect to control over size, size distribution, and fluorescence efficiency. These results should allow for future identification of cheaper, safer, and greener precursors for large-scale preparations of high-quality nanocrystals.


10:45 AM N8.10
Self-assembly of Nanoparticles into Nanowires, Helical Ribbons and Hybrid Nanocomposites. Sudhanshu Srivastava, Kevin Critchley, Aaron Santos, Paul Podsiadlo, Kai Sun, Sharon Glotzer and Nicholas A. Kotov; University of Michigan, Ann Arbor, Ann Arbor, Michigan.

Self-assembly of inorganic nanoparticles (NPs) has created a range of exceptional structures including nanowires (NWs), nanoribbons (NRs), and nanosheets (NSs). The biggest challenge in the field of nanoscience is to develop unique synthetic approaches and to understand the assembly mechanisms of NPs into complex morphologies for material and biological applications. Creating structures inspired from nature using assembly approach will lead to design architectures with more tunability and unique physical responses. In this approach NP assembly was used to fabricate 1D wires, 2D sheets and 3D helical ribbons using NP dipole-dipole interactions. The helicity in the 3D NRs was shown to be modulated with the intensity of light. The structures generated were compared with the alpha-helix and beta-sheet structures present in biomolecules. Introducing the biomolecular components with inorganic NP assemblies is another step in assembly process to add the versatility of the system for generating hybrid materials. Natural proteins/DNA is at same length scale as inorganic NPs, and combination of these lead to incorporate native properties of both biomolecules and inorganic crystals. In this study, 1) assembly of CdTe NPs into 1D, 2D and 3D architectures based on the capping groups used is demonstrated, 2) insertion of DNA/proteins in the NP assembly via dipolar interactions is investigated and 3) Finally, the structures generated were compared to nature and how to generate dynamic assemblies. The choice of metallic/semiconductor/magnetic NPs and natural bio-components used can lead to design hybrid nanocomposites to be applied in the field of photonics, optical sensors and delivery agents.


11:00 AM N8.11
Colloidal Synthesis of Amorphous Germanium Telluride Nanoparticles and Characterization of Structural Phase Stability. Marissa Caldwell1, Jeffrey J. Urban2, Simone Raoux3, Robert Y. Wang2, Mark J. Polking4, R. Ramesh4, A. Paul Alivisatos5,2, Delia Milliron2 and H.-S. Philip Wong6; 1Chemistry, Stanford University, Stanford, California; 2The Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California; 3IBM Watson Research Center, Yorktown, New York; 4Materials Science and Engineering, University of California - Berkeley, Berkeley, California; 5Chemistry, University of California - Berkeley, Berkeley, California; 6Electrical Engineering, Stanford University, Stanford, California.

While colloidal syntheses of metal chalcogenide nanocrystals are widely known, few examples of colloidal routes to amorphous nanoparticles have been reported. Here we present a colloidal route for producing as-synthesized amorphous germanium telluride nanoparticles. In the rhombohedral crystalline phase, GeTe is a binary ferroelectric, and it is also a known phase-change memory (PCM) material due to the bistability of the amorphous and crystalline phases and the high resistivity contrast between these. The synthesis is a high-temperature injection of a separate tellurium source into a preheated germanium solution. The synthesized nanoparticles are 2-7 nm in diameter, and they were characterized by TEM, EDAX, and XRD to determine their size, composition and structure. The stability of and transitions between different structural phases of GeTe are crucial to both ferroelectric and PCM applications. GeTe exists in three different structural phases: a disordered amorphous state, a rocksalt crystalline phase stable at high temperatures, and a distorted rhombohedral phase responsible for its ferroelectricity and stable at lower temperature. In order to investigate the phase stability of the nanoparticles, we used both as-synthesized amorphous GeTe nanoparticles and as-synthesized GeTe nanocrystallites. Using X-ray diffraction with in-situ heating, we were able to thermally-induce two distinct phase transitions: crystallization and the rhombohedral-to-rocksalt transition. Remarkably, the nanoparticles’ amorphous structure is stable at temperatures exceeding 300 degC, above which we observed their crystallization at >150 degC above the bulk crystallization temperature. Using GeTe nanocrystallites, we also observed the reversible rhombohedral to cubic transition and investigated the dependence of the transition temperature on nanocrystal size. In addition, we report resistivity versus temperature measurements demonstrating that solution-processed nanoparticle films retain the high resistivity contrast required for PCM applications. Finally, we compare the temperature of the drop in resistivity with the crystallization temperature determined from the in-situ XRD studies.

11:15 AM N8.12Gold Nanoparticle-Microgel Composites for SERS Detection. Luis M. Liz-Marzan1, Ana Sanchez-Iglesias1, Isabel Pastoriza-Santos1, Jorge Perez-Juste1, Ramon A. Alvarez-Puebla1, Rafael Contreras-Caceres2,1 and Antonio Fernandez-Barbero2; 1Departamento de Quimica Fisica, Universidade de Vigo, Vigo, Spain; 2Departamento de Fisica Aplicada, Universidad de Almeria, Almeria, Spain.

Nanocomposite materials consisting of a colloidal metal nanoparticle within a synthetic polymer hydrogel shell have attracted great attention due to potential applications in several fields such as catalysis, photonics, electronics, optics and biomedicine. For these applications, a precise control over the size and shape of the core particles, as well as a low dispersity, are crucial issues. Among the ususal polymer shells, stimuli-responsive materials are particularly interesting because of the possibilities they offer for external switching and manipulation. A common example is poly(N-isopropylacrylamide) (pNIPAM), a thermoresponsive polymer that undergoes a phase transition from a hydrophilic, water-swollen state to a hydrophobic, globular state when heated above its lower critical solution temperature (LCST) which is about 31-32 C, in water. Addition of co-monomers has additionally been proposed as a means to add responsiveness toward different stimuli such as temperature, pH, ionic strength or light. We have developed a novel and efficient method to coat CTAB-capped metal nanoparticles with pNIPAM, involving initial coating with a thin polystyrene shell and subsequent emulsion polymerization of NIPAM monomers on the coated nanoparticles. The resulting core-shell structure was undoubtedly confirmed through detailed TEM, AFM and UV-vis spectroscopy analysis. A temperature-driven, reversible swelling-deswelling transition was identified in the core-shell system, with a transition temperature similar to that of the pure microgel system, which can be easily monitored through (reversible) surface plasmon shifts. Remarkably, the metallic cores can be grown within the microgel, leading to different morphologies as a function of CTAB concentration, which allows simple tuning of the corresponding optical response and environmental sensitivity. All these results demonstrate the accessibility of the metal cores, which is crucial for applications such as catalysis or detection. Demonstration of the potential of these systems for detection was carried out through capture of analytes from solution and SERS analysis. The thermosensitive properties of the microgel shell allowed capture of various types of analytes, including phenol, which could be readily identified from their SERS spectra.

 

SESSION N9: Optical Processes in Quantum Dots
Chair: Moungi Bawendi
Thursday Afternoon, December 3, 2009
Constitution A (Sheraton)


1:30 PM *N9.1
Semiconductor Nanocrystals as Color Conversion Phosphor on LED Device. Shinae Jun, Eunjoo Jang and Hyosook Jang; Basic Material Lab, Samsung Electronics, Institute of Advanced Technology, Yongin-si, Gyeonggi-do, Korea, South.

The luminescent semiconductor nanocrystals (NCs) would be an attractive candidate of color conversion phosphor material on LED device, because of NCs broad absorption range, easy color tenability and high purity compared to inorganic phosphor. For the practical application of NC to LED, NCs should be embedded in the matrix with maintaining its optical properties and enhancing stability under thermal and photo-activation condition in air. In this presentation, we will show how to stabilize NCs in host matrix homogeneously with maintaining its optical properties. We also report here on the successful integration of NC as color converting materials on blue LED.


2:00 PM *N9.2
Ultrafast Electron and Hole Transfer from Quantum Dots: Towards Multi-exciton Dissociation. Tianquan Lian, Department of Chemistry, Emory UNiversity, Atlanta, Georgia.

Charge transfer to and from quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as solar cells and light emitting diodes. Recent reports of multiple exciton generation (MEG) by one absorbed photon in some QDs offer an exciting new approach to improve the efficiency of QD-based solar cells. In addition to clarifying the efficiency, mechanism and generality of this process, it is also important to investigate potential approaches to separate multiple excitons before the exciton-exciton annihilation process, which occurs on the 10s to 100s ps time scale. We have recently started to investigate ultrafast exciton dissociation in QDs by charge transfer to adsorbed molecules on surface. In this presentation we report a series of studies of exciton dissociation dynamics in quantum dots by electron or hole transfer to adsorbed electron or hole acceptors, respectively. We showed that excitons in CdS and CdSe can be dissociated on the a few picosecond time scale to various adsorbates. We will discuss the dependence of these rates on the size and the nature of the quantum dots and possible approaches for multiple exciton dissociation.


2:30 PM N9.3
Exciton Nonlinearities and Optical Gain in Colloidal CdSe/CdS Dot/rod Nanocrystals. Michele Saba, Francesco Quochi, Marco Marceddu, Agnieszka Gocalinska, Andrea Mura and Giovanni Bongiovanni; Dipartimento di Fisica, Universita di Cagliari, Monserrato, Italy.

Semiconductor colloidal nanocrystals have been proposed as optically-active media for solution-processable optoelectronic devices, because they combine inexpensive, wet-chemistry synthesis with high photoluminescence quantum yield, large oscillator strength and size tuneability of optical transitions. We evaluated the potential as lasing medium of a recently-introduced class of CdSe/CdS nanocrystals where electrons and holes are not segregated, but their confinement is intermediate between type I and type II heterostructures. Our main goal was to measure the exciton-exciton interaction in such structures. Ultrafast nonlinear spectroscopy revealed repulsive exciton-exciton interaction in CdSe/CdS dot/rod nanocrystals, a very beneficial feature to achieve lasing under cw or quasi-cw pumping, especially when coupled with the antenna effect provided by the CdS rod and the large photoluminescence quantum yield. Engineering the sign and strength of the interaction between confined excitons has important applications beyond lasing, like the realization of tailor-made nonlinear optical materials and design of two-bit logic gates for quantum information processing. The 1D shape of CdS rods surrounding CdSe nanocrystals can be exploited in unique ways to control optical nonlinearities with applied electric fields: experiments exploring such applications will be discussed.


3:15 PM *N9.4
Non-blinking Semiconductor Nanocrystals: Suppression of Nonradiative Auger Processes. Alexander Efros, Naval Research Laboratory, washington DC, District of Columbia.

Colloidal nanocrystals (NCs) randomly turn their photoluminescence (PL) “off” and “on” under continuous light illumination, despite intensive research efforts aimed at suppressing this phenomenon. Today there is a consensus that the blinking is caused by extra electrons or holes that repeatedly charge, and then neutralize, the NC. When a charged NC is excited by a photon the additional energy is not re-emitted as PL, but instead triggers a process known as "non-radiative Auger recombination" during which this energy is acquired by an extra electron or hole. The rate of Auger recombination is orders of magnitude faster than the rate of radiative recombination that produces PL in neutral NCs. As a result, PL is completely suppressed, or "quenched," in charged NCs. Recently, the soft-confinement (CdZnSe/ZnSe) nanocrystals have been grown that show complete absence of single molecule photoluminescence blinking [1]. Other remarkable photophysical properties these nanocrystals exhibit include unique multi-peaked photoluminescence spectra, and unusually short photoluminescence lifetimes. These properties are consistent with the novel observation of charged exciton recombination in colloidal nanocrystals, and thus are quite unlike any of the typical nanocrystals currently being studied. We will explain why Auger processes are so efficient in standard NCs and how they have been recently suppressed in the non-blinking NCs. [1] X. Wang, X. Ren, K. Kahen, M. A. Hahn, M. Rajeswaran, S. Maccagnano-Zacher, J. Silcox, G. E. Cragg, Al. L. Efros, and T. D. Krauss, Nature v. 459, 686 (2009)


3:45 PM N9.5
Inter-molecular Electronic Transfer. Karel Kral, Institute of Physics, ASCR, v.v.i., Prague, Czech Republic.

The inter-molecular transfer of electrons or holes is considered theoretically. The mechanism can apply to molecular crystals, polymer solids, DNA molecule, etc. The mechanism of the transfer is studied on the model with inter-molecular electron tunelling mechanism and on the coupling of the charge carrier to the intra-molecular atomic vibrations. The electron-phonon interaction is considered in the self-consistent Born approximation to the nonequilibrium Green's function self-energy. The corresponding kinetic equation shows us the diffusion mechanism of electron along the molecular solid. This mechanism will be compared to the well-known Marcus theory of electronic transfer. With using the above mechanism we shall discuss earlier experimental data on photoconductivity in charge-transfer crystals, and also rather recent data on electric conduction of DNA molecules.


 

SESSION N10: Poster Session
Chair: Jonathan Steckel
Thursday Evening, December 3, 2009
8:00 PM
Exhibit Hall D (Hynes)


N10.1
Abstract Withdrawn


N10.2
Abstract Withdrawn


N10.3
Dynamic Control of Quantum Dots Photoluminescence by External Fields. Rafal Korlacki and Stephen Ducharme; Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska.

Owing to quantum confinement effects the optical properties of semiconductor nanostructures (nanocrystal quantum dots) are strongly size-dependent and thus may be tailored in a very broad range by choosing the desired size during the synthesis. Once synthesized, however, their properties are essentially fixed, and cannot be easily tuned further, which is a major limitation for many applications requiring dynamical control of the optical and electronic properties. Electrical control, which is technologically convenient and allows for easy integration with the existing electronics, can be achieved through the Stark effect, the shift of electronic energy levels due to an electric field. With sufficiently large electric fields, the Stark effect contributions to the electronic states induce spectral shift of 10s of nanometers [1], a value of the order comparable with effects of the nanoscale quantum confinement. This early work demonstrated the possibility of controlling large shifts in the emission spectrum, but this method requires application of a high voltage. The main challenge to practical application of this effect is to apply very high electric fields with moderate voltages. This capability will eliminate the need for high-voltage electronics and prevent the avalanche breakdown. We embedded CdSe colloidal quantum dots in ultrathin polymer films made by Langmuir-Blodgett deposition. This enabled apply extremely high electric fields of a few 100s MV/m, but requiring only moderate voltages of up to 25 V. We were able to observe repeatable shifts of the photoluminescence peak by nearly 10 nm at room temperature. Moreover, this new composite material can easily be manufactured in low cost facilities, and can potentially be used for developing tunable devices, like for example broadband QD lasers and triggered single-photon sources. This work was supported by the Nebraska Research Initiative. CdSe quantum dots were purchased from Ocean Nanotech and NN-Labs. [1] S. A. Empedocles and M. G. Bawendi, Science 278, 2114 (1997)


N10.4
First-Principles Theoretical Analysis of Doping in II-VI Compound Semiconductor Nanocrystals. Tejinder Singh, Triantafillos J. Mountziaris and Dimitrios Maroudas; Chemical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts.

Doping of II-VI compound semiconductor nanocrystals allows for precise control of their optoelectronic properties. However, in spite of its feasibility, nanocrystal doping has been an extremely difficult task. In colloidal Mn-doped ZnSe nanocrystals, it has been observed that increasing the anion:cation ratio leads to higher doping efficiencies. This has been attributed to adsorption of Mn dopant atoms onto the ZnSe nanocrystal surface facets, in particular onto the ZnSe(001)-(2×1) surface. In this context, we aim at obtaining a fundamental quantitative understanding of dopant adsorption and diffusion on II-VI semiconductor nanocrystal surfaces toward elucidating the mechanisms of dopant incorporation into growing nanocrystals. In this presentation, we report a systematic analysis of dopant adsorption and diffusion on surface facets of ZnSe and ZnSe1-xSx nanocrystals based on first-principles density functional theory (DFT) calculations within the generalized gradient approximation (GGA). In our DFT calculations, we have employed slab supercells, plane-wave basis sets, and the projector-augmented wave method. We have made comparisons of our theoretical findings with experimental reports on doping efficiencies. We have computed the surface energies of the (001), (110), and (111) surfaces of ZnSe, examined the various surface reconstructions, and constructed the equilibrium crystal shape (ECS) of ZnSe nanocrystals as a function of the Se chemical potential. We found that anion-rich reconstructed surface facets are present in the ECS. In addition, we have computed the binding energies of Mn dopant atoms onto all the nanocrystal surface facets and found them to be strongly dependent upon the surface morphology and nanocrystal shape. From our analysis, we conclude that all anion-rich surfaces contribute to dopant adsorption onto nanocrystal surface facets and we have established a relationship between doping efficiency and stable nanocrystal surface structure. Our DFT calculations also indicate that the binding energy for Mn adsorption onto various sites of all surface facets increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration provides an interpretation for the doping difficulties during nanocrystal growth. Furthermore, we have analyzed several dopant migration pathways for Mn diffusion on the ZnSe surfaces and calculated the corresponding activation barriers and their dependence on dopant surface concentration. Finally, the above analysis of dopant adsorption and diffusion on nanocrystal facets has been extended to ZnSe1-xSx nanocrystals in order to examine the effects of nanocrystal alloying on the doping efficiency.


N10.5
Single-electron Transistor Made From a Cadmium Selenide Nanocrystal. Qian Miao, University of Cambridge, Cambridge, United Kingdom.

By using self-assembly and Dielectrophoresis (DEP) techniques, single spherical 5nm CdSe nanocrystal particle could be attracted into a nano-scale highly n-doped GaAs-GaAs gap, to form a GaAs-nanocrystal-GaAs sandwich structure, such structure could function as a two-terminal single nanocrystal device. Such kind of devices could perform negative differential resistance when voltage raises at some point when the GaAs energy levels line up with the CdSe nanocrystal energy levels, thus, could be used as the potentially next generation of nanocrystal memory device. The spherical CdSe nanocrystals were initially covered by TOPO when dispersed in toluene, and the GaAs material was initially had a GaAs-AlAs-GaAs sandwich structure, the 6nm AlAs layer can be removed using Hydrofluoric(HF) Acid Etching, then the GaAs material would have a 6nm gap. Leave GaAs material into the 1-6 hexanedithiol (dispersed in IPA) solution, the 1-6 hexanedithiol molecules would self-assembled on to the GaAs surface , then dip the GaAs device into CdSe solution while also applying a high frequency voltage at drain and source at the same time, the electric field could efficiently attract the nanocrystal particle, once a particle fit into the gap, its outside TOPO would be replaced by a sulfur atom at the end of 1-6 hexanedithiol molecule, forming a firm tunneling barrier. Since the device connected in series with a resistor, when one nanocrystal has been trapped, the voltage would suddenly drop down and no more nanocrystals would be attracted. This process has been proved efficient and reliable, similar devices with metal leads have also been made and showed staircase shape Coulomb blockade in their I-V characteristics. This kind of devices are much more smaller than current CMOS devices and it is also a good way of understanding the energy level spectra of semiconductor nanocrystal particles.


N10.6
Surfactant-free, Low Temperature Synthesis of ZnO Nanocrystals and Study of Their Photoluminescence Properties. Sanjaya Brahma and Srinivasrao A. Shivashankar; Material Research Centre, Indian Institute of Science, Bangalore, Karnataka, India.

We report a soft chemical route to synthesize monodispersed nanoparticles of ZnO by using microwave as the energy source. A variety of synthesis techniques or methods such as hydrothermal process, solvothermal process, chemical coprecipitation, sol-gel technique, microimulsion, and polyol process have been employed for the synthesis of ZnO nanoparticles. Most of these processes are energy- and time-consuming and require need post synthesis-processing, such as annealing at elevated temperatures, washing and purification, control of pH and need of a surfactant. The present synthesis is quick and efficient in producing nanoparticles, without the need for any surfactant and post- synthesis processing. This powerful method, based on microwave irradiation, is simple and uses inexpensive equipment, and is capable of producing nanoparticles within one minute. The synthesis involves a pure metalorganic precursor of zinc i.e, zinc acetylacetonate, commonly designated as Zn(acac)2¬, a stable crystalline solid at room temperature (prepared in our laboratory). This precursor is dissolved in an appropriate amount of decanol, the solution then being subjected to microwave irradiation in a domestic microwave oven operating at 2.45 GHz, with power varied from 160 -800 W. A few minutes of irradiation leads to the formation of ZnO nanopowders. The morphology and size of ZnO nanostructures by employing other long chain alcohols, such as hexanol, butanol, and their mixtures as solvents. For example, using decanol as the solvent, ZnO nanospheres of ~150 nm have been obtained, each comprised of smaller spheres (~20 nm), whereas monodispersed ZnO nanoparticles (~15-20 nm) has been obtained using a mixture of decanol, with ethanol or methanol, as the solvent. In each case, the as-prepared powder material was found to be pure and well-crystallized ZnO, requiring no post-synthesis processing (such as annealing). The small particle size leads to a very high specific surface area, making it a useful sensor material, as will be reported. These nanoparticles have been characterized by x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Optical properties have also been studied by photoluminescence measurements and it is found that there is a possibility of white light emission from these ZnO nanocrystals.


N10.7
Theoretical Analysis of Core/Shell-Like Structure Formation through Equilibrium Surface Segregation in InxGa1-xAs and ZnSe1-xSx Nanocrystals. Sumeet C. Pandey, Tejinder Singh and Dimitrios Maroudas; Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts.

Compound semiconductor nanocrystals with dimensions over the range from 2 to 10 nm can be designed optimally for pertinent photovoltaic applications, light-emitting devices, and as luminescent biological labels. This is based on engineering of the band structure by controlling variables such as composition, morphology, and size. Of particular importance is the synthesis of core/shell quantum-dot structures, which produces materials that are more stable against photo-oxidation and have improved photoluminescence efficiencies. Various colloidal synthesis routes to core/shell structures exist, including the coating of a narrower-band-gap semiconductor core with a shell of a wider-band-gap material in a two-step process. However, one-pot single-step processes for synthesizing such core/shell structures remain elusive. Ga1-xAs and ZnSe1-xSx slabs. We report simulation results for the equilibrium concentration distributions in slabs of InxGa1-xAs and ZnSe1-xSx as a function of composition, x, slab thickness, and slab surface crystallographic orientation, as well as in InxGa1-xAs and ZnSe1-xSx nanocrystals with well-defined {001} and {110} surface facets as a function of x and nanocrystal size. The results identify the parameter range for the formation of compositionally inhomogeneous equilibrium nanocrystals that constitute core/shell-like structures.>

N11.8
Spectroscopic Characterization of Bridge-Mediated Electron Transfer Processes in Tethered Quantum Dot-Metal Oxide Assemblies. Rachel S. Dibbell, Diane G. Youker, Kathleen M. Coughlin and David F. Watson; Chemistry, University at Buffalo, Buffalo, New York.

Semiconductor quantum dots (QDs) may be attractive alternatives to molecular chromophores for applications in photocatalysis and solar energy conversion. Our research involves the spectroscopic characterization of interfacial electron transfer within tethered assemblies of QDs and electron-accepting metal oxide nanoparticles. This presentation will highlight our efforts to understand and control excited-state deactivation pathways of QDs and interfacial electron-transfer reactions by controlling the distance and electronic coupling between nanoparticles. Cadmium chalcogenide QDs are tethered to metal oxide surfaces through bifunctional molecular linkers, which are adsorbed to both materials through coordinate covalent bonds. Equilibrium binding experiments have revealed surface adduct formation constants (Kad) of 103 - 105 M-1 for both surface-attachment interactions. Our emission quenching, time-resolved emission, and nanosecond transient absorption data have shown that the efficiency of interfacial electron transfer varies dramatically with the distance and electronic coupling between nanoparticles, both of which are, in principle, broadly tunable by varying the nature of the molecular linker. A first set of experiments focused on electron transfer from photoexcited CdS QDs to TiO2 nanoparticles through mercaptoalkanoic acid linkers.1 Electron transfer between nanoparticles occurred on multiple timescales. A fast component (< 10-8 s) accounted for the majority of electron transfer, while the remainder occurred on the microsecond timescale. The multiexponential nature of the kinetics suggests that electron transfer may occur from a distribution of conduction-band and trap states. Notably, the electron transfer yield decreased dramatically with increasing chain length of mercaptoalkanoic acid linkers; therefore, well-established models of bridge-mediated intramolecular electron transfer may be relevant to electron transfer between molecularly-linked nanoparticles. Ongoing experiments in our laboratory are focused on promoting efficient electron transfer between tethered inorganic nanoparticles. To increase the electronic coupling between donor and acceptor nanoparticles, we have incorporated aromatic groups into molecular linkers. Electron transfer yields increased with favorable electronic coupling, potentially enabling excited-state electron transfer over relatively long distances between tethered nanoparticles. 1. Rachel S. Dibbell and David F. Watson J. Phys. Chem. C 2009, 113, 3139-3149.


N10.9
Light Emission Processes in Single Silicon Nanoparticles. Jan Valenta1, Anna Fucikova1,2,4, Katerina Dohnalova2, Katerina Kusova2, Ivan Pelant2, Anton Fojtik3, Frantisek Vacha4 and Frantisek Adamec4; 1Dept. Chemical Physics & Optics, Charles University, Prague 2, Czech Republic; 2Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic; 3Department of Physical Electronics, Faculty of Nuclear Physics and Physical Engineering, Czech Technical University, Prague, Czech Republic; 4Institute of Physical Biology, University of South Bohemia, Budweis, Czech Republic.

Mechanisms of efficient light-emission from silicon nanostructures remain a subject of controversy even after two decades of investigation. In our contribution we present application of high-sensitivity luminescence micro-spectroscopy to investigation of individual silicon nanostructures (at room and cryogenic temperatures) [1]. The samples are prepared by several technological approaches resulting in Si nanocrystals of various shapes (spherical or elongated), well separated or connected in clusters, passivated by SiO2 or organic compounds. Especially the shape and surface passivation have strong influence on luminescence spectral shapes (phonon side bands), on-off blinking statistics, excitation and emission polarization anisotropy etc. The single nanocrystal spectroscopy experiments are combined with ensemble measurements in order to reveal limits of light-emission performance [2] and to estimate application prospects of silicon nanostructures in optoelectronic devices and in biology. [1] J. Valenta & J. Linnros: Optical Spectroscopy of individual silicon nanocrystals, in Silicon Nanophotonics: Basic Principles, Present Status and Perspectives, Ed. L. Khriachtchev, World Scientific Publishing Co. Pte. Ltd. 2009, p. 179. [2] J. Valenta et al., Advanced Functional Materials 18 (2008) 2666.


N10.10
Band Alignment in PbSe|CdSe Core/Shell Quantum Dots. Bram De Geyter1, Yolanda Justo1, Karel Lambert1, Arjan J. Houtepen1, Iwan Moreels1 and Zeger Hens2; 1Ghent University, Gent, Belgium; 2Delft University of Technology, Delft, Netherlands.

Covering colloidal semiconductor quantum dots (QDs) with an inorganic shell has become a popular approach to tailor their optical properties. If the energy levels of the shell encompass those of the core (type 1 band alignment), this results in nanoparticles with an enhanced and stable luminescence quantum yield. A staggered band alignment (type 2) offers much more possibilities since it leads to a spatially separated exciton which features, for instance, a lower threshold for light amplification [1]. Recently, the formation of core/shell quantum dots has been extended to IV-VI materials for the cases of PbS|CdS, PbSe|CdSe and PbTe|CdTe[2,3]. This means that the advantages offered by QD heterostructures in terms of tunable electro-optical properties can be extended from the visible to the near IR (1000-3000 nm), provided that they exhibit the required staggered band alignment. Here, we present an extensive structural and optical analysis of PbSe|CdSe QDs that leads to a model for the PbSe-CdSe band alignment. PbSe|CdSe QDs are made by cationic exchange. With increasing exchange time, the CdSe shell formation leads to a blueshift and a broadening of the absorption spectrum. High resolution TEM lattice images resolving both the PbSe core and the CdSe shell show that they form a coherent interface, preferentially along (111) planes[3]. We analysed the luminescence of PbSe|CdSe QDs on a variety of samples with core sizes varying between 2.5 and 4.5 nm and shell thicknesses from 0.3 to 1.4 nm. First, we find that PbSe|CdSe QDs systematically show a larger Stokes shift than PbSe QDs. Second, no systematic increase of the luminescence quantum yield is observed after CdSe shell growth if PbSe/CdSe core/shell QDs are compared with PbSe cores of the same core size. Finally, addition of thiols to a PbSe|CdSe suspension (hole scavenger) leads to an increase of the luminescence intensity while addition of methylviologen (electron scavenger) completely quenches the PbSe|CdSe luminescence. These results are incompatible with a type 1 band alignment that confines the conduction band electrons and valence band holes to the PbSe core. Especially the quenching of the luminescence upon addition of methylviologen shows that the wavefunction of a conduction band electron extends to the CdSe surface, while the absence of quenching upon hole addition demonstrates that holes are confined to the PbSe core. This means that the PbSe conduction band is close to or above the CdSe conduction band (type 1½ or type 2). The great advantage of type 2 colloidal QDs is the flexible tuning of their electro-optical properties. This result is the first, necessary step to achieve this in the technologically important near IR wavelength range. [1] Klimov, V. I. et al., Nature 447, 441-446 (2007). [3] Pietryga J. M. et al., J. Am. Chem. Soc. 130, 4879-4885 (2008). [2] Lambert K., et al., Chem. Mat. 21, 778-780 (2009).


N10.11
Abstract Withdrawn


N10.12
Fluorescence from Single PbSe Magic-sized Clusters. Julie A. Smyder1, Christopher M. Evans1 and Todd D. Krauss1,2; 1Department of Chemistry, University of Rochester, Rochester, New York; 2Institute of Optics, University of Rochester, Rochester, New York.

Colloidal semiconductor magic-sized clusters (MSC) are intermediate precursors to semiconductor quantum dots (QD) that have strongly molecular-like characteristics. Like fullerenes, MSCs exist as several distinct families of highly stable structures with an exact number and arrangement of atoms, as opposed to the continuum of possible sizes and structures for the larger QDs. Thus, MSCs provide a unique opportunity to study the fundamental effects of quantum confinement for materials with nominally an exact number of atoms. MSCs have been reported in several semiconductor nanocrystal materials, but PbSe MSCs are notably interesting due to their simple synthesis in air at room temperature, small size, and versatile solubility in both organic and aqueous environments. Importantly, their fluorescence in the near-infrared (NIR) region of the spectrum with unexpectedly excellent quantum yields of up to 90%, and good photostability, make PbSe MSCs extremely intriguing for fundamental studies and highly attractive for applications such as in vivo biological imaging. However, critical aspects of the fundamental photophysics of these ultrasmall particles remain unexplored. We will present studies of the fluorescence properties of single PbSe magic-sized clusters using epifluorescence confocal microscopy. Because a method for separating different sizes of the polydisperse as-synthesized clusters is yet to be optimized, single molecule studies are critical. Using continuous wave laser excitation at 594 nm, we are able to observe fluorescence from clusters up to ~950 nm using a Si CCD. To minimize the effects of photodegredation due to oxidation, samples were processed and measured under a nitrogen enriched environment. Fluorescence intermittency, or blinking, has been observed and occurs on time scales similar to that of PbS QDs. Details on single particle spectra, and the implications for the homogeneous line width of the PbSe MSCs will be discussed.


N10.13
Correlation Between Strength of Interdot Coupling and Transport Mechanism in Silicon Quantum Dot Assembly. Mitsuru Inada1, Manabu Gibo2, Hiroshi Yamamoto2, Ikurou Umezu2, Tadashi Saitoh1 and Akira Sugimura2; 1Pure and Applied Physics, Kansai University, Osaka, Japan; 2Physics, Konan University, Kobe, Japan.

An Electric structure of coupled quantum dots (QDs) is of considerable interest both fundamentally and technologically, because such electric structure can be described by Hubbard model. In addition, spin relaxation time for silicon is reported to be longer than that for other typical IV and III-V group semiconductor materials. This suggests that silicon QD is a good candidate for quantum dot devices that are based on the many-body effect. In this study, we report on the conduction mechanism of silicon QD assembly and discuss the correlation between the sample structure and the conduction mechanism. We prepared silicon QD assembly by pulsed laser ablation of silicon target in hydrogen gas atmosphere. The structure of the QD assembly changes by the hydrogen gas pressure during deposition. At 270 Pa, the QD assembly has a columnar structure. We have to note that silicon QDs are dispersed individually without aggregation at the initial stage of the deposition. On the other hand, silicon QDs are connected and formed chain-like structure at 1100 Pa. Note that the mean diameter of silicon QD is 4.6 nm, which value is independent of hydrogen gas pressure. Electrical transport measurements were curried out as a function of temperature from 30 K to 300 K. For columnar QD assembly, the I-V curves have nonlinear shape with temperature-dependent voltage threshold, indicating Coulomb blockade behavior. All the I-V curves collapsed on each other and described by a power functions as I=A(V-Vt)ζ. This behavior is explained as collective transport of tunnel junction networks whose transport is dominated by Coulomb blockade. This suggests that interdot coupling is weak in this system. On the other hand, the I-V curves, for chain-like QD assembly, are highly symmetric and the conductance drops monotonically with decreasing temperature. Temperature dependence of the conductance G is expressed as G = G0exp(-T0/T) 1/2. From our experimental data, we identify the conduction mechanism as Efros-Shklovskii variable range hopping (ES-VRH) with strong interdot coupling. One possible reason for explains the difference of transport mechanisms between columnar and chain-like structure is the strength of the interdot coupling. In the columnar structure, electrons are strongly localized in individual QDs because each QD is electrically separated. On the other hand, in the chain-like structure, strong interdot coupling can be expected because QDs are directly connected each other. We thus point out that the difference in the transport behavior is attributed to the magnitude of interdot coupling between QDs. The results suggest this QD system is one of the candidates to realize electrically coupled systems, with competition between confinement and coupling effects, which is expected to provide outstanding properties necessary for quantum information processing.


N10.14
Synthesis in Aqueous Solution of Nano-textured Manganese Oxides for Lithium Battery Electrodes. David Portehault1,2, Sophie Cassaignon1,2, Emmanuel Baudrin3 and Jean-Pierre Jolivet1,2; 1Lab. Chimie de Matiere Condensee de Paris, UPMC Univ Paris 06, PARIS, France; 2Lab Chimie de la Matiere Condensee de Paris, CNRS, UMR 7574, PARIS, France; 3Lab de Réactivité et Chimie des Solides, Université de Picardie-Jules Verne, UMR 6007, Amiens, France.

During the last years, research on lithium batteries has evolved toward nanoscaled electrode materials. Among them different oxides were reported to present new reactinities and/or improved electrochemical behavior upon Li reaction in terms of rate capabilities and reversibility when the surface area is high and the crystallite size is sufficiently decreased. Within this context, manganese oxide nanomaterials are of particular interest from a fundamental point of view to investigate structure and size effect on the electrochemical behavior owing to the richness of this family. Nevertheless, tailoring the electrode materials structure/texture remains a significant challenge. Our group develops the synthesis of manganese oxide nanoparticles by precipitation in aqueous medium at low temperature. This soft chemistry route is well known to provide a versatile control of the solid formation. This presentation is focused on a low temperature aqueous route for precipitation of Mn(IV), Mn(III) or mixed valence Mn(III)-Mn(IV) manganese (oxyhydr)oxides using MnO4- and/or Mn2+ precursors. Soft conditions permit fine tuning of the reaction path and different pure phases are obtained depending on the synthesis conditions (i.e. acidity, oxidation state, temperature and aging). In particular, nanowires of cryptomelane K0.11MnO1.85.(H2O)0.75, manganite γ-MnOOH and pyrolusite β-MnO2 are formed through self-assembly or topotactic processes, while γ-MnO2 hollow nanocones originate from heterogeneous oriented attachment. In parallel, heterogeneous nucleation or heteroepitaxy can be controlled in order to obtain hierarchical nanoheterostructures such as core-corona architectures of lamellar birnessite K0.19MnO1,77(H2O)0.30 or cryptomelane-birnessite nanocomposites. Soft chemistry is therefore a valuable tool to investigate and to control nucleation-growth processes, as well as the resulting morphological characteristics. Using these well defined structures, shapes, sizes and hierarchical orders, some effects of the nanoscale on the electrochemical properties versus lithium are emphasized.


N10.15
Uniformly-Assembled Metal Nanoparticles on Anodic Aluminum Oxide (AAO) Applied in Surface-Enhanced Raman Spectroscopy. Boon Loo1, Zhixun Luo2 and Jiannian Yao2; 1Chemistry, Towson University, Towson, Maryland; 2Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.

We extend the applications of the anodic aluminum oxide (AAO) templates onto the filtration and assembly for metal nanoparticles. It was found that the colloidal Au nanoparticles can be arranged like a nanonet along the edge of the 100-nm AAO pores. But, they form monolayer assembly on the membrane with smaller AAO pores, such as 20 nm. Similar behavior was also observed for the Ag nanoparticles. Both of the supported Au and Ag nanoparticles on the AAO membranes are closely packed and exhibited localized surface plasmon resonance (LSPR). As a result, taking fullerene molecules C60 & C70 as the probe molecules, and the filtrated Au nanoparticles as the substrate, high quality surface-enhanced Raman scattering (SERS) spectra were obtained. It is demonstrated that the AAO membrance filtrated with noble metal nanoparticles is a highly SERS active substrate. The spherical fullerene molecules C60 & C70 have also been employed as probe molecules for SERS by some groups. However, the measured SERS spectra of fullerene C60 and C70 from the net-assembled Au nanoparticles not only show the sharp differences from their usual SERS, but also differ much from SERS obtained from the layer-coating assembly of the same Au nanoparticles on the AAO templates. The SERS spectra from the monolayer coatings of Au nanoparticles on AAO templates show small differences from those observed from the gold-C60/C70 clusters on various substrates. However, it differs substantially when compared to the SERS from the net-assembled Au nanoparticles. Especially for the C60 system, the two SERS spectra show substantial differences with each other. It is noted that the fullerene C60 and C70 are stable molecules which did not form chemical bonds with the Au nanoparticles. The appearance of additional modes resulted from a symmetry lowering and selection rule relaxation by the adsorption of C60 & C70 on the Au-coated substrates. In addition, the possible weak interaction (Coulombic force) in the ground state of C60 & C70 on the net-like assembly with respect to monolayer coatings of Au nanoparticles may also contribute to the observed difference in the SERS spectra.


N10.16
Abstract Withdrawn


N10.17
Electron Transfer from Colloidal PbS Quantum Dots into SnO2 Nanoparticles. Byung Ryool Hyun1, Adam C. Bartnik1, Liangfeng Sun1, Joshua Choi2, Tobias Hanrath2 and Frank W. Wise1; 1Applied physics, Cornell University, Ithaca, New York; 2Chemical Engineering, Cornell University, Ithaca, New York.

The electron transfer time from PbS QDs to TiO2 nanoparticles was reported to be a couple of hundred nanoseconds.[1] This electron transfer time is at least one order magnitude slower than that of a CdSe QD-TiO2 system. This relatively slow electron transfer time invokes interest in its physical mechanism. To that end, injection of photoexcited electrons from colloidal PbS quantum dots into SnO2 nanoparticles is investigated. The PbS QD-SnO2 composite system is supposed to have a faster electron transfer time because of the lower electron affinity of SnO2, which produces one of the largest energy differences between Pb salt QDs and metal oxide materials. Based on the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into SnO2 for all QD sizes. Continuous wave fluorescence spectra and fluorescence transients of coupled PbS quantum dots are measured to verify the electron transfer. When coupled to SnO2, electron transfer is still seen in the largest available QD diameter. The electron injection times from small PbS QDs into SnO2 nanoparticles are obtained from fluorescence lifetime and transient absorption spectroscopy and found to be a few tens of nanoseconds. This is still slower than that of a CdSe QD-TiO2 system, which is hundreds of picoseconds.[2] Therefore, the relative slow electron transfer time could an intrinsic physical feature of Pb salt QDs. The size-dependent electron transfer times from PbS QD to SnO2 are presented, including the possible physical origin of the slow transfer time. [1] B.-R. Hyun et al., ACS Nano 2, 2206 (2008). [2] I. Robel, M. Kuno, and P. V. Kamat, J. Am. Chem. Soc. 129, 4136 (2007).


N10.18
Adsorption of Aqueous Quantum Dots to Nanocrystalline TiO2 Thin Films: Influence of Surfactant Structure on Surface Coverage and Electron Injection Efficiency. Jeremy S. Nevins, Kathleen M. Coughlin and David F. Watson; Chemistry, University at Buffalo, Buffalo, New York.

Semiconductor quantum dots (QDs) may be attractive alternatives to molecular chromophores for applications in photocatalysis and solar energy conversion. One strategy involves the use of QDs to sensitize electron-accepting metal oxide semiconductors, such as TiO2. This presentation will focus on the fabrication QD-sensitized TiO2 films using “green” aqueous chemistry, and on the photoinduced electron transfer reactivity of such materials. CdSe QDs were synthesized by mixing aqueous solutions of cadmium sulfate and sodium selenosulfate in the presence of L-cysteine, 3-mercaptopropionic acid (MPA), or mercaptosuccinic acid (MSA) under ambient conditions. These reactions yielded stable aqueous dispersions of CdSe QDs. Cysteine-capped QDs exhibited narrower and more intense excitonic absorption bands than MPA- and MSA-capped QDs, indicating greater monodispersity. QDs were adsorbed to nanocrystalline TiO2 films through deprotonated carboxylate groups of the surfactants. Equilibrium binding experiments revealed that the saturation surface coverage of cysteine-capped QDs was approximately twice that of MPA-capped quantum dots. Furthermore, the surface adduct formation constant (Kad) for adsorption of cysteine-capped QDs to TiO2 was an order of magnitude greater than for MPA-capped QDs. These data and the results of desorption measurements suggest that cysteine may coordinate to TiO2 through both carboxylate and amine groups, giving rise to enhanced stability on the surface. Electron injection was characterized using photoelectrochemistry and time-resolved spectroscopy. Incident photon-to-current efficiencies (IPCEs) of QD-sensitized solar cells incorporating cysteine-capped QDs were significantly greater than for cells incorporating MPA-capped QDs. Nanosecond transient absorption spectroscopic data suggest that electrons are injected efficiently from photoexcited QDs to TiO2 via cysteinate linkages. Taken together, our data suggest that cysteine may tether QDs to metal oxide surfaces more efficiently than related capping groups, while also promoting efficient interfacial electron injection.


N10.19
Spectral Dynamics of CdSe Nanocrystals on Nanosecond to Millisecond Timescales. Lisa Marshall, Xavier Brokmann and Moungi Bawendi; Chemistry, M.I.T., Cambridge, Massachusetts.

The emission spectrum of a single emitter can be artificially widened and blurred due to fluctuations in emission energy, i.e. spectral diffusion. This spectral diffusion can be much more rapid than the time required to collect sufficient photons to measure a spectrum. We use a new method, Photon Correlated Fourier Spectroscopy (PCFS), to “freeze” spectral diffusion and obtain spectral information of single CdSe nanocrystals on timescales comparable to the lifetime of the emitter. This method cross-correlates the two outputs of a Michelson interferometer, providing a histogram of frequency shifts between two photons separated by a given amount of time. We apply PCFS to single nanocrystals and also combine PCFS with Fluorescence Correlation Spectroscopy (FCS) to resolve single nanocrystal linewidths from a solution of nanocrystals flowing under a microscope objective.


N10.20
Transport Characteristics of Nanoscale Metal-Oxide Interfaces. Ramsey Kraya, Dawn A. Bonnell, Doug Yates and Lolita Rotkina; Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania.

The issue of charge transport across nanoscale metal-semiconductor interfaces is of primary importance in the downscaling of components for future electronic devices. The present work provides new information on the conduction mechanisms involved across such barriers, namely between SrTiO3 and gold nanoparticles, as a function of contact size. SrTiO3 is a common perovskite oxide with an atomically smooth surface, and is used as a substrate for superconductors, in thin film electronics, and potentially in future VLSI systems. To determine the barrier heights and transport characteristics as a function of contact size, a well defined interface structure between the SrTiO3 and gold nanoparticles was fabricated. This was achieved by controlling the surface structure of the substrate at the atomic scale and controlling the morphology and orientation of the nanoparticles on the surface. A scanning probe technique was developed to quantify frequency dependent transport across individual interfaces and high resolution TEM was used to characterize the interface structure. The effects of interface size and structure on contact barrier and transport will be discussed.


N10.21
Abstract Withdrawn


N10.22
Synthesis of Nanoparticles in Agarose Gel. Erwan Faoucher and Mathias Brust; Chemistry, University of Liverpool, Liverpool, Merseyside, United Kingdom.

In past decades, research efforts were concentrated on the synthesis of metal particles especially the preparation of gold colloids. A large number of synthetic methods are known and have been reviewed extensively1. In most cases, these rely on wet chemical processes which are most suitable for obtaining narrow size distributions and the desired shape. The synthesis of the same metallic particles in complex fluids such as gels or extremely viscous media adds several other parameters to be taken in consideration, notably the diffusion of the chemical compounds through the matrix fluid. This project originated from an investigation of the influence of the medium viscosity on the properties (size, shape) of the resulting particles. Based on this approach, further studies have been carried out on the use of hydrogel networks as three dimensional molecular templates for the formation of metallic nanostructures. UV-visible spectroscopy, X-ray diffraction, and different methods of Electron Microscopy (TEM, STEM and HAADF-STEM) were used to characterise the samples produced. Our results indicate that the viscosity of the medium has little effect on particle size and size distributions. Interestingly, we have found that gels are excellent matrices for the formation of nanoparticles from 0.5 to 4nm, which tend to decorate the internal molecular network structure of the gel. This synthesis works for Au, Pd, Pt, Ru and Ag nanoparticles. The effect of metal loading on the electrical transport properties of the gel has been studied. Owing to their tuneable charge transport properties the materials prepared show promise for the development of sensors . Finally, the use of Ag loaded agarose gel as substrate for ultra-sensitive detection of molecules via SERS has been studied. Because the gel can collapse upon drying and recover when rehydrated, it can be foreseen as an excellent mechanical molecular trap. Additionally it gives rise to dynamic hot spots as the network volume decreases and the silver particles get close to each other, thereby generating the huge electromagnetic fields that are needed for ultra-detection. Reference (1) D.Astruc, M-C. Daniel Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size -Related Properties, And Applications toward Biology, Catalysis, and Nanotechnology, Chem. Rev. 2004, 104, 293-346


N10.23
Germanium Nanocrystal Thin Films for Electronic Applications. Zachary Holman and Uwe Kortshagen; Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota.

Colloidal semiconductor nanocrystals have garnered much attention for their size-dependent optical properties; however, in order to be used in traditional thin film devices, nanocrystal films with tunable properties must be designed. The Group II-VI and IV-VI nanocrystal communities have had some success in achieving this goal by drying and then chemically treating colloidal particles, but Group IV nanocrystals have proven more challenging. We report on thin films of germanium nanocrystals (Ge NCs) that are synthesized in an RF plasma and deposited using two different techniques. Germanium tetrachloride is dissociated in the presence of hydrogen in a nonthermal plasma to nucleate Ge NCs. Transmission electron microscopy and X-ray diffraction indicate that the particles are nearly monodisperse (standard deviations of 10-15% the mean particle diameter) and the mean diameter can be tuned from 4-15 nm by changing the residence time of the Ge NCs in the plasma. The particles can be made either crystalline or amorphous without otherwise altering their characteristics by controlling the plasma power. In the first deposition scheme, a Ge NC colloid is formed by dispersing hydrogen-terminated Ge NCs in 1,2-dicholorobenzene (DCB) without further surface modification. These “bare” NCs quickly agglomerate and flocculate in nearly all non-polar solvents, but remain stable in DCB. We believe that this occurs because DCB has a similar dielectric constant to Ge, significantly reducing the effective Hamaker constant of the van der Waals attraction. Films are formed by spinning the Ge NC colloid onto substrates. Thin-film field-effect transistors (FETs) have been fabricated with this technique and have been subjected to various annealing procedures. The devices show n-type, p-type, or ambipolar behavior depending on the annealing conditions, with Ge NC films annealed at 300°C exhibiting electron saturation mobilities greater than 10^-2 cm^2/Vs and on-off ratios of 10^4. We have verified by X-ray diffraction that this performance is not due to sintering of the NCs. We believe this is the first report of FETs based on Ge NCs. The second deposition scheme involves the impaction of Ge NCs onto substrates directly downstream of the synthesis plasma via acceleration of the particles through an orifice. This technique produces highly uniform films with densities greater than 50% of the density of bulk Ge, as determined by cross-sectional scanning electron microscopy and Rutherford backscattering. By varying the size of the Ge NCs, we have measured films with band gaps ranging from the bulk value of 0.7 eV to over 1.1 eV for films of 4 nm Ge NCs. As far as we know, this is the first report Ge NC films with tunable optical properties. This work was supported by NSF under grant CBET-0756326, IGERT grant DGE-0114372. Support was also provided by the UMN Center for Nanostructure Applications.


N10.24
Blue Luminescence and Superstructures from Magic-Size Clusters of CdSe. Frank S. Riehle1,2, Ralf Thomann1, Gerald Urban2 and Michael Krueger1,2; 1Freiburg Materials Research Centre (FMF), University of Freiburg, Freiburg, Germany; 2Institute for Microsystems Technology (IMTEK), University of Freiburg, Freiburg, Germany.

We present a new type of an ultra-small CdSe nanoparticle family with exceptional narrow blue emissions between 437nm and 457nm and full width at half-maxima (FWHM) of 19nm obtained without further size selection. The narrow FWHM of the photoluminescence (P.L.) signal and the constant peak position in the photoluminescence excitement (PLE) spectra recorded at different detection energies indicate a well-defined homogeneous species. The ultra-small CdSe nanoparticles with a mean diameter of 1.6nm were found to be in equilibrium with conventional larger CdSe nanocrystals (NCs) of 1.9nm. The ultra-small nanoparticles can be separated from the larger NCs due to their different surface reactivity: After ligand exchange the ultra-small nanoparticles assemble into nanowires with close diameters of 1.6nm and lengths of about 100nm and precipitate, whereas the larger NCs remain in solution. This controlled one-dimensional aggregation can already occur in the synthesis matrix depending on the initial precursor concentration or in very-diluted samples. The ultra-small nanoparticles act as building blocks for elongated structures such as nanowires, which might be advantageous in photovoltaic devices, as well as growth material or even nuclei for the formation of NCs. Due to their sharp and strong P.L. signal the ultra-small CdSe nanoparticles might also have a possible application in blue LEDs.


N10.25
CdS, CdSe Nanoparticles in Silica Matrix by Sol Gel method. Nilima Hullavarad and Shiva Hullavarad; Office of Electronic Miniaturization, Univeristy of Alaska - Fairbanks, Fairbanks, Alaska.

‘Quantum dots’ or nanoparticles of diverse semiconductor materials are extensively studied because of their interesting size dependent properties. It is further interesting to organize the quantum dots in the form of superlattices thin films, monolithics, ordered arrays for fruitful applications. Various applications such as sensors, displays, recordings, communications etc. require condensed organic or inorganic tunable material in the ultra violet to visible range [1]. This work discusses the synthesis of CdS and CdSe nanoparticles in a silica matrix. The UV absorption measurements of CdS nanoparticles indicated sharp absorption at 260 and 350 nm for different precursor compositions. The particle size of CdS nanoparticles was estimated to be 1.5-2 nm. Silica gel containing the CdS nanoparticles was spin coated onto substrates to form thin film samples. Scanning Electron Microscope (SEM) measurements revealed formation of CdS nanoparticles within the branches of gel-network. Depending upon the mole ratio of additives and drying method, fibrous or monolithic tablets of CdS nanoparticles embedded in silica matrix could be produced. The paper discusses the effect of precursors in obtaining the single size distribution of CdS nanoparticles in the silica gel matrix. The matrix consisting of monodispersed CdS nanoparticles in a silica gel matrix has potential applications in sensors, tunable waveguides, and sensitive photon counting systems, luminescent displays, and laser micro cavities. [1] I. Finkelstein, S. Ruschin, Y. Sorek, R. Reisfeld, Optical Materials 7, 9 (1997)


N10.26
On Doping CdS/ZnS Core/Shell Nanocrystals with Mn. Ou Chen, Yongan Yang, Alexander Angerhofer and Y. Charles Cao; University of Florida, Gainesville, Florida.

The ability to precisely control the doping of semiconductor nanocrystals can create an opportunity for producing functional materials with new properties, which are important for applications. This opportunity has stimulated research efforts to develop synthetic methods to incorporate dopants into a variety of colloidal semiconductor nanocrystals. Herein, we report a new doping method using a three-step synthesis to make high-quality Mn-doped CdS/ZnS core/shell nanocrystals. This approach allows precisely controlling the Mn radial position and doping level in the core/shell nanocrystals. Importantly, this synthetic advance provides us a great opportunity to study the chemical and physical properties of Mn-doped nanocrystals with different dopants radial positions. Based on this synthesis, we have prepared Mn-doped CdS/ZnS core/shell nanocrystals with Mn photoluminescence quatum yield up to 56%, which is important to applications such as biomedical diagnosis.


N10.27
Shell-Dependent Blinking of Zinc-Blende CdSe-Based Type-I Core/Shell Nanocrystals. Seung Koo Shin1, Bonghwan Chon1, Sung Jun Lim1, Wonjung Kim1, HyeongGon Kang2, Taiha Joo1 and Jeeseong Hwang2; 1Bio-Nanotechnology Center, Department of Chemistry, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea, South; 2Physics Laboratory, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland.

Blinking of zinc-blende CdSe-based core/shell nanocrystals are studied as a function of shell materials and surface ligands. Zinc-blende CdSe/ZnS, CdSe/ZnSe/ZnS, and CdSe/CdS/ZnS core/shell nanocrystals are prepared by colloidal synthesis and converted into water-soluble ones by ligand exchange with 3-mercaptopropionic acid. The zinc-blende lattice structure is verified by powder X-ray diffraction, and the size and shape distributions are examined by high-resolution transmission electron microscopy. Optical properties, such as the absorption and emission spectra as well as the photoluminescence lifetime, are obtained in solution to characterize the bright states of nanocrystals. The time trace of blinking is recorded for single nanocrystals on a borosilicate cover glass. Both on- and off-time distributions are fit to a power law. The off-time exponents are clustered around 1.43-1.72 for organic-soluble nanocrystals and 1.40-1.67 for water-soluble nanocrystals, whereas the on-time exponents are scattered in the range of 1.91-2.26 for organic-soluble nanocrystals and 2.19-2.54 for water-soluble nanocrystals. The on-time duration of blinking significantly varies with shell materials, thickness, and surface ligand. Of the core/shell nanocrystals, CdSe/CdS/ZnS offers the least variation in both the on- and off-time exponents. In all core/shell nanocrystals, the thiolate conjugation on the surface zinc atom greatly reduces the on-time duration regardless of shell thickness and materials, confirming that the rate of photoinduced on-off transition increases as the number of thiolate-based surface hole-trap states increases.


N10.28
Light Harvesting Action Spectroscopy of Single CdSe/CdS Nanocrystals. Nicholas J. Borys1, Manfred J. Walter1, Dmitri V. Talapin2, Jing Huang2 and John M. Lupton1; 1Physics, University of Utah, Salt Lake City, Utah; 2Chemistry, The University of Chicago, Chicago, Illinois.

Nanocrystal heterostructures offer unique systems to study the migration of excitation energy on nanometer length scales, while enabling accurate control over physical shape and thus electronic structure. Given a semiconductor nanostructure consisting of a wider gap (CdS) and a narrower gap (CdSe) material, we address the question of how relaxation occurs to the CdSe following excitation in CdS. Nanocrystal tetrapods are particularly versatile systems to investigate this question as a large amount of absorbing material (CdS) can be grown around a small emitting nucleus (CdSe). We vary the excitation energy and study the luminescence of the single particle, at low temperatures. Such photoluminescence excitation (PLE) spectra reveal a step-like onset of the CdS absorption. Remarkably, single particle PLE spectra appear in two distinct but universal shapes, quite in contrary to conventional single particle luminescence spectra. In addition, spectrally resolving the luminescence as a function of excitation energy reveals correlated (driven) spectral diffusion, arising from excitation energy-induced band gap renormalization. Effectively, the experiment probes light harvesting action - exciton migration as a function of exciton energy - with conceptual similarities to single particle electrical transport (blockade) spectroscopy. Excitation spectroscopy reveals information on the overall shape of the nanoparticle which is not accessible in conventional luminescence experiments. Overcoming disorder in single particle excitation spectroscopy identifies the most efficient light harvesting structures with near perfect coupling. Ultimate control over macroscopic device performance requires such classification of each individual particle in the ensemble, and will necessitate a maximization of the fraction of particles with the most efficient characteristics.


N10.29
Large Scale Electric Field Directed Assembly of Metal Nanoparticles for the Next Generation Nanoscale CMOS Interconnects. Cihan Yilmaz1,2, Taehoon Kim1,2, Sivasubramanian Somu1,2 and Ahmed Busnaina1,2; 1Mechanical Engineering, Northeastern University, Boston, Massachusetts; 2NSF Nanoscale Science and Engineering Center for High-rate Nanomanufacturing (CHN), Boston, Massachusetts.

The current trend in semiconductor technology toward smaller device features has led to narrower line widths and smaller interconnects in integrated circuits. Many advances have been made to decrease the RC delay associated with high resistance of densely packed interconnect layers and high inter-layer and intra-layer capacitance. Although these efforts have played a key role in the development of the better circuit performance and the current generation of the technology, further improvements are essential to address the potential challenges of decreasing RC delay in the interconnect lines for the smaller node sizes (at 22 nm and beyond). One way of improving the RC delay is to increase the aspect ratio of the present interconnects. However, producing smaller size, very high aspect ratio interconnects by utilizing electroplating process is very challenging. To overcome this issue, ITRS has outlined the necessity for the development of manufacturable and cost-effective alternative filling techniques. In this paper, we present a fast highly scalable environmentally benign process for fabricating interconnects between different layers in an integrated circuit. Metallic nanoparticles are precisely assembled into the prefabricated vias by applying a controlled dynamic electric field between the electrodes at the bottom of the vias and a reference electrode placed far away from the vias. By using 5 nm gold (or other metal) nanoparticles, we have successfully fabricated interconnects down to 30nm in diameter with an aspect ratio (height/diameter) of 3.0. The dimensions of these interconnects can be controlled by dimensions of the vias and assembly parameters. The process meets the manufacturability requirements while also being chemical free, environmental friendly method as compared to electrochemical deposition in a damascene process. Electrical characterization of produced interconnects for various dimensions will be presented. These results show that this novel approach could be an excellent candidate to overcome some of the issues of next generation interconnect technology.


N10.30
Synthesis of PbSxSex-1 Alloyed Nanoparticles. Javeed Akhtar, Mohammad A. Malik and Paul O'brien; School of chemistry, The university of manchester, Manchester, United Kingdom.

Lead chalcogenides(PbE= S,Se.Te) are narrow band gap compound semiconductors and find applications as biological labels, and light harvesters. We describe here the synthesis of ternary PbSxSex-1nanoparticles by colloidal method. These nanoparticles were prepared in olive oil using TMS and TMSe .The as-prepared nanoparticles were characterized by XRD, energy filtered transmission electron microscopy and optical spectroscopy. The size and composition of PbSxSex-1 nanoparticles can be controlled by injection temperature, growth time and concentration of precursors.


N10.31
Co-CoO-Au Core-multi-shell Nanoparticles. Stephanie H. Johnson1, Craig L. Johnson1, Steven J. May1,3, Samuel Hirsch2, M. W. Cole2 and Jonathan E. Spanier1; 1Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania; 2US Army Research Laboratory, Aberdeen Proving Ground, Maryland; 3Materials Science Division, Argonne National Laboratory, Argonne, Illinois.

We report on the chemical synthesis and characterization of the structure, composition, and hierarchal organization of Co-core, concentric CoO-Au multi-shell nanocrystals. On the basis of electron microscopy and micro-analysis, variable-temperature magnetometry, and optical and ultraviolet absorption and Raman scattering spectroscopy, we present evidence challenging the common assumption that the reduction reaction to form Au shells on Co nanoparticle cores in the syntheses of CocoreAushell nanoparticles can only occur if the cobalt suface has not oxidized. Our findings suggest that the presence of a metal shell surrounding a transition-metal core nanoparticle following such a reduction reaction cannot be taken as evidence that the transition metal oxide is absent from the surface of the nanoparticle core. On the basis of our experimental data and images, we propose that the employed method of Au shell growth can produce Au shells possessing five-fold symmetry through self nucleation and coalescence. Work supported by the ARO under W911-NF-08-1-0067


N10.32
Size-Controlled Monodisperse Colloidal Magnesium Oxide Nanocrystals: Direct Synthesis and Their Optical Properties with Efficient Blue Luminescence. Hoi Ri Moon, Jeffrey J. Urban and Delia J. Milliron; Lawrence Berkeley National Laboratory, Berkeley, California.

The development of solvent-processible, size-tunable alkaline earth oxide nanocrystals could potentially open up new opportunities in rational catalysis, gas separations, and nano-optics. Here, we present the direct synthesis and optical characterization of colloidal magnesium oxide nanocrystals of narrow size distribution, tunable from 2-8 nm in diameter by incorporating catalytic amounts of water into the organic reaction medium. The resulting nanocrystals are single crystalline and exhibit size-dependent efficient blue luminescence with quantum yield up to ca. 20%. The impact of bound surfactants on their optical properties is investigated through chemical exchange of their surface ligands. The bright blue emission observed in these colloidal MgO nanocrystals, not present in bulk MgO crystals, offers an inexpensive, attractive alternative for optical applications. Future work will involve studies of networks of these crystals and their gas sorption (H2, CO2) properties as a function of size, matrix, and surface chemistry.


N10.33
Abstract Withdrawn


N10.34
Abstract Withdrawn


N10.35
Carrier Dynamics in Systems of Strongly Interacting Semiconductor Nanoparticles. Jeremy A. Rowlette1,2, Rohan D. Kekatpure1,3, Matt A. Panzer2, Aaron C. Hryciw3, Mark L. Brongersma3 and Kenneth E. Goodson2; 1Electrical Engineering, Stanford University, Stanford, California; 2Mechanical Engineering, Stanford University, Stanford, California; 3Materials Science and Engineering, Stanford University, Stanford, California.

It is well-known that dense systems of semiconducting nanoparticles (NPs) can exhibit electrical and optical properties differing greatly from those of sparse systems as a consequence of NP interaction at close range (< 10 nm). The ability to control NP interactions is therefore an essential step in developing NP-based devices. Long-range Coulombic interactions such as the Förster dipole-dipole type [1], can be particularly challenging to control because host materials tend to have low refractive indices. In the case of Förster resonant energy transfer (FRET), a donor electron-hole pair (EHP) in one NP spontaneously couples to the ground state of a neighboring acceptor NP or impurity having equal or lesser band gap energy via the Coulomb potential -e2/r. The coupling leads to nonradiative recombination of the donor EHP and subsequent transfer of energy to the acceptor NP creating a new EHP in the process. The total number of EHPs is conserved and the coupling strength scales as r^-6. A similar mechanism, arising from the same dipole-dipole coupling, exists between two excited NPs but has received far less attention in the literature [2]. In this case, which we refer to as Förster-Fröhlich-Mahr RET (FFM-RET), both the donor and acceptor sites are initially in an excited state; the donor EHP recombines transferring a bandgap of energy to an acceptor EHP, resembling the three-body Auger process in bulk semiconductors. The interaction strength also scales as r^-6 but unlike FRET, the FFM-RET process reduces the total number of EHPs and is a potentially devastating source of efficiency loss in NP-based photonic devices. Another important distinction is that energy can be efficiently exchanged from small to large bandgap NPs in FFM-RET whereas such a process is forbidden in FRET. Our talk addresses the impact of FFM-RET on NP devices and on methods for its observation. Our talk begins with a brief review of dipole-dipole coupling mechanisms in colloidal NP systems and proceeds with the presentation of recent theoretical and experimental evidence which show that FFM-RET plays an essential role in the carrier dynamics of high-density NP systems. Our conclusions stem from recent optical pump-probe studies [3,4] which were performed on concentrated (2x1018 cm-3) systems of Si NPs (5 nm mean diameter) embedded in Si oxide glass matrices. We will discuss the key signatures of the FFM-RET mechanism that are observable in these measurements and how these signatures are modified as the dimensionality of the system changes from 3D to quasi-0D. We conclude by comparing device architectures which optimize the performance of colloidal NP photonic devices having high NP concentrations. [1] Th. Förster, Ann. Physik 2, 55 (1948). [2] D. Fröhlich and H. Mahr, Phys. Rev. 148, 868 (1966). [3] J. Rowlette, R. Kekatpure, M. Panzer, M. Brongersma, and K. Goodson, Phys. Rev. B (in press) (2009). [4] R. Kekatpure and M. Brongersma, Nano Lett. 8 3787 (2008).


N10.36
Abstract Withdrawn


N10.37
Direct Electron Beam Nanopatterning Using Functionalized Semiconductor Nanocrystal Quantum Dots. Sung Jin Kim1,2, Teng-Yin Lin1, Alexander N. Cartwright1,2 and Paras N. Prasad1,2,3; 1Electrical Engineering, University at Buffalo, State University of New York, Buffalo, New York; 2Institute of Lasers, Photonics and Biophotonics, University at Buffalo, State University of New York, Buffalo, New York; 3Chemistry, University at Buffalo, State University of New York, Buffalo, New York.

We report an approach for nanopatterning with electron beam lithography using functionalized semiconductor nanocrystal quantum dots (NQDs) by incorporation of the functional ligand t-butoxycarbonyl (t-BOC) which has an acid-labile moiety. This approach enables direct writing of nanoscale patterns/structures of nanocrystals using e-beam lithography. The energy from the electron beam breaks the t-BOC protection and results in the surface changing from a hydrophobic state to a hydrophilic state. In order to obtain nanostructures with low doses of electron beam energy, a photo acid generator (PAG), which uses electron energy to generate the H+ catalyst, was added for the necessary amplification process. After mild heat treatment subsequent to e-beam exposure, the e-beam written nanocrystal structure was fixed in place by using a developing process with properly chosen solvents. As a preliminary result, we have demonstrated a line width of ~100 nm using CdSe nanocrystals and an electron dose of dose of 100 μC/cm2. The resulting structure using CdSe nanocrystals was developed with hexane after post baking process for 1 minute at 120°C. This directly patterned nanocrystal structures can be used for nanoscale optoelectonic and electronic devices.


N10.38
Self-organization of Colloidal Structures via Capillary Interactions of Silica Nano-particles in Organic Solvents. Garfield T. Warren and Dobrin P. Bossev; Physics, Indiana University, Bloomington, Indiana.

We have investigated the evolution of structures formed by colloidal silica dispersed in an organic solvent. These nano particle structures interacting through capillary bridges are responsible for the formation of a 3-D fractal network and the strong viscoelastic properties induced in the solvent. The capillary force dominates the electrostatic and Van der Waals interaction between the particles and can bridge multiple colloids. The volume of the bridging fluid determines the morphology of the structures formed. We have investigated the mobility of these structures by measuring the impedance of the organic solvents while sweeping the frequency at different temperatures. We have also used Static and Dynamic Light Scattering techniques to investigate the significance of water as a bridging fluid in organic solvent with submicron colloidal particles. Small-angle neutron scattering (SANS) was used to study nanoparticles with an average diameter of 10 nm in polar and non-polar organic solvents. The SANS intensity as a function of the scattering vector was analyzed as a product of the form factor and structure factor. This dependance on the particle shape and structure factor characterizes the interparticle interactions. The form factor is experimentally determined in methanol after addition of simple electrolyte to suppress the interparticle interactions. The SANS intensities are divided by this form factor to produce the structure factor at different concentrations. The interaction of particles in polar solvents is considered to be through electrostatic repulsion and the data was successfully fitted by Hayter-Penfold mean spherical approximation (HPMSA). We have continued to investigate the structures formed by colloidal particles suspended in organic solvents at volume fractions below 10% by using computer simulations. The pearl necklace-like chain of spheres was modeled to explain the structure factor when capillary bridges are present. Alternatively, we have analyzed the slope of the intensity at low scattering vector in a double logarithmic plot to determine the dimension of the fractal structures formed by the particles at different volume fraction of the bridging fluid. The focus of this study was to explore the significance capillary forces have as a tool to engineer new colloidal structures and sol-gel material while optimizing their viscoelastic properties.


N10.39
Synthesis of CuS on Micropillar Arrays for Wave Guiding Applications. Yolanda Vasquez and Joanna Aizenberg; Harvard University, Cambridge, Massachusetts.

Organisms have developed creative solutions to utilize light for communication, camouflage, and energy harvesting. These are often actuatable structures that tune their conformation in response to optical stimuli. The goal of this work is to fabricate a responsive light capture system in which arrays of polymer nanoposts or microposts enhance photon collection by reconfiguring to the source. Polymer post tips were functionalized with Cu2-xS (x= 1, 0.2, 0.03) by solution synthesis. The pH, precursor concentration, and temperature were varied in order to control the size and chemical composition of the Cu2-xS (x= 1, 0.2, 0.03) on the micropost arrays. Particle crystallinity was analyzed by XRD and the morphology of particles was analyzed by SEM and TEM.

N10.40
Charge Separation Dynamics in Type-II Semiconductor Nanorods. Stoichko Dimitrov, Chad Dooley and Torsten Fiebig; Chemistry, Boston College, Chestnut Hill, Massachusetts.

The photophysics of semiconductor quantum rods was examined with the intent of studying their capabilities and limitations as they pertain to photovoltaic technologies. Specifically, the results from femtosecond pump-probe measurements on the charge separation dynamics in type-II CdSe/CdTe donor-acceptor nanorods are presented. The photoinduced electron transfer from CdTe to CdSe conduction bands was estimated to happen in the 400 fs timescale, while the direct optical electron transfer from the valence band of CdTe to the conduction band of CdSe on the timescale of our pump pulse (~50 fs). Based on the determined injection rates, a carrier separation efficiency of > 90% has been calculated suggesting these materials are sufficient for use in solar energy capture applications. Model photovoltaic devices were fabricated, and their power conversion efficiency and photon-to-current generation efficiency characterized. In devices based of CdSe and heteromaterial quantum rods we observed fill-factors on the order of 10-20% though with power conversion efficiencies of < 0.02%. It was discovered that using a high temperature annealing step, while critical to get electrochemically stable photoelectrodes, was detrimental to quantum confinement effects.


N10.41
Effect of Surface Ligands in Hybrid Solar Cells. Yeng Ming Lam1, Jun Yan Lek1, David Cooke2 and Lydia Helena Wong1; 1School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore, Singapore; 2Department of Photonics Engineering, Technical University of Denmark, Lyndby, Denmark..

Nanoparticles has the advantage of higher charge carriers mobility as compared to organic molecules and also the absorption range can be tuned by changes to the dimensions. This advantage can be exploited in the solar cells application in the form of hybrid solar cells. Hybrid solar cells architecture is similar to that of bulk heterojunction concept in organic solar cells where the two components are blended together, except that now one of the components is the semiconducting nanoparticles. To date, various inorganic nanoparticles have been blended with semiconducting polymers to form BHJ hybrid solar cells, such as cadmium selenide (CdSe), cadmium sulphide (CdS), lead sulphide (PbS), etc. During the synthesis, surfactants or ligands are usually present as stabilizing agents to prevent aggregation, mediate the growth and also passivate the surface electronic states after the synthesis. However, these ligands with bulky alkyl chains act as insulting layer which impede the charge transfer and transport within the nanocrystals. Due to the presence of these ligands, such as tri-octylphosphone oxide (TOPO) and alkyl phosphonic acids, the active layer conductivity may be affected. Hence, ligand exchange is widely carried out in preparation of CdSe nanocrystals. One of the most commonly used ligands is pyridine. It has been shown by others that the efficiency of the cells can go up to 1.8%. In this work, we will show how the chemical structures of the ligands affect the optoelectronic, luminescence properties and also the efficiency of the cells. We have seen that in most cases, the absorption properties do not change but the conductivity may change and the surface states of the nanoparticles might be affected by the presence of the ligands. Together with the steric effect of the ligands, the charge transport between nanoparticles can be tuned. The benefit of improved absorption and mobility due to the presence of nanoparticles in the film will be retained.


N10.42
Synthesis and Scintillation Characterization of Bismuth Oxide Nanoparticle Polymer Composites. Qi Chen1,2, Qibing Pei1,2, Tuling Lam1,2, Nerine Cherepy1,2 and Stephen Payne1,2; 1MSE, UCLA, Los Angeles, California; 2Lawrence Livermore National Laboratory, Livermore, California.

There is a growing demand for better and less-expensive scintillators for medical imaging and detection of radioactive substances. Current scintillator materials are largely characterized into three categories: inorganic crystals such as NaI(Tl) and LaBr3, ceramic and plastic scintillators. The plastic scintillators can be made inexpensively into large sizes and various shapes. Their low effective atomic number and low density, however, lead to low energy resolution, unsuitable for gamma ray spectroscopy. We report polymer composites with high-loading content of bismuth oxide nanoparticles as a new scintillator that leverages the high atomic number of bismuth with the processing advantage of plastics. The composites contain bismuth oxide nanoparticles “dissolved” into polymer matrix with high uniformity and negligible agglomeration. The synthetic chemistry that leads to transparent composites will be discussed. We will also describe scintillation measurement results using various radiation sources.


N10.43
Fabrication of Two Dimensional Array of Silica Nanospheres on GaN Using Spin Coating Method and Nomarski Image Processing. Kyoungnae Lee and Dimitris Korakakis; Lane Department of Computer Science & Electrical Engineering, West Virginia University, Morgantown, West Virginia.

Interest in self assembly of nanoparticles has increased significantly during the past decades because of their various applications in data storage, sensing, optoelectronics, etc [1-4]. A number of techniques have been used to fabricate two and/or three dimensional ordered arrays of nanoparticles on substrates but there is a lack of literature data about the spreading dynamics of silica nanospheres on GaN substrates, in spite of its practical importance in GaN based light emitting diodes for example[5]. Self assembly of silica nanoparticles on GaN substrate has been employed as nanomask for fabrication of photonic crystal structures and it is a simple and economic technique for the large scale fabrication of nanoscale periodic patterns. Spin coating is the most popular method employed to produce a uniform distribution of nanoparticles over large areas by centrifugal force[6]. The particle density on the surface can be controlled by the concentration of nanoparticle colloids or spin coater parameters. Recently, wafer-scale assembly of silica nanoparticles on silicon wafer has been demostrated using a spin coating technique [6]. Also, spin coating has been employed to fabricate two dimensional ordered arrays of silica nanospheres on surface of nitride based LED structures or on GaN substrates for fabricating photonic crystal structures. However, large scale fabrication of two dimensional ordered arrays of silica nanosphers on GaN substrates is challenging because the contact angle of the solvent used in the particle suspension is desirable to be less than approximately 10 degree but the contact angle between the GaN substrate and deionized water used in silica nanospheres suspension is higher than 10 degree. We have developed an image processing technique and analysis using Nomarski microscopy to investigate the mechanism of silica nanosphere ordering on GaN surface. Nomarski microscopy offers higher contrast than the other microscopy techniques and is very sensitive to surface features. We achieved large scale monodispersed silica nanospheres of around 70 x 100 micrometer square on GaN substrates by spin coating. The roles of spin speed, waiting time, and SDS as a surfactant on the process of array formation are investigated by direct microscopic observations and image processing and the results will be discussed. 1. S. H. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science, 287, 1989 (2000) 2. K. Burkert, T. Neumann, J. J. Wang, U. Jonas, W. Knoll, H. Ottleben, Langmuir, 23, 3478 (2007) 3. V. Rastogi, O. D. Velev, Biomicrofluidics, 1, 014107 (2007) 4. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, C. B. Murray, Nature, 439, 55 (2006) 5. C. H. Chao, S. L. Chuang, T. L. Wu, Appl. Phys. Lett. 89, 091116 (2006) 6. P. Jiang, T. Prasad, M. J. McFarland, Appl. Phys. Lett. 89, 011908 (2006).


N10.44
Hybrid Solar Cells Based on Non Ligand-exchanged CdSe Quantum Dots and Poly(3-hexylthiophene). Yunfei Zhou1,2, Frank S. Riehle1,2, Ying Yuan1,2, Gerald A. Urban1,2,3 and Michael Krueger1,2; 1Freiburg Materials Research Center (FMF), University of Freiburg, Freiburg, Germany; 2Institute for Microsystems Technology (IMTEK), University of Freiburg, Freiburg, Germany; 3FRIAS, School of Soft Matter Research, University of Freiburg, Freiburg, Germany.

Organic or organic/inorganic hybrid solar cells based on solution-processable conjugated polymers have the promising advantages of potentially low-cost, easy to process, and large-area fabrication on flexible substrates. Inorganic semiconductor nanocrystals (NCs) such as CdSe, with tunable bandgaps and high intrinsic charge carrier mobilities act in a similar manner as good electron acceptors and can also be incorporated into conjugated polymers such as e.g. poly(3-hexylthiophene) (P3HT) to form bulk-heterojunction hybrid solar cells. In our group, highly reproducible synthesis methods for CdSe quantum dots (QDs) have been developed, leading to monodisperse QDs with excellent photophysical properties. The photoluminescence emission colors start from blue, resulting from ultrasmall cluster-like CdSe nanoparticles [1] with diameters of about 1.6nm, to red (ca. 5.5 nm in diameter) [2]. Current research is undertaken to control the shape and the surface functionalization of NCs. Bulk-heterojunction hybrid solar cells based on blends of spherical CdSe QDs (ca. 5.5 nm in diameter) and P3HT were fabricated and investigated. The QDs were treated after synthesis by a simple, fast and reproducible washing procedure [3]. No additional ligand exchange was performed thus reducing the possibility of post-synthetic surface defects. Solar cells with optimized ratios of QDs to P3HT exhibited the best power conversion efficiencies exceeding 2% up to 2.28% under AM1.5G illumination at intensity of 100 mW/cm2. This is among the highest reported value for a spherical CdSe QDs based photovoltaic device so far. Further investigations to improve the materials and device performance are currently in progress using elongated structures of various NCs based on CdSe, CdTe, PbS, PbSe etc. and by controlling the surface properties of NCs. Ref. [1] F. S. Riehle et al. Nano Lett. 9, 514 (2009). [2] Y. Yuan et al. J. Nanosci. Nanotech., accepted (2009). [3] Y. Zhou et al. submitted.


N10.45
Synthesis and Characterization of Nanoparticles of CoO, MnO, and Hollow Oxides. Kwangjin An and Taeghwan Hyeon; School of Chemical and Biological Engineering, Seoul National University, Seoul, Korea, South.

We describe the synthesis of uniform-sized metal oxide nanocrystals with controlled sizes and shapes including hollow nanostructures. The synthetic method is based on a colloidal synthetic approach by the thermal decomposition of metal-oleate complexes, which are prepared from the reaction of metal chlorides and sodium oleate. In this presentation, CoO, MnO, and hollow oxides nanoparticles are discussed. Firstly, uniform-sized pencil-shaped CoO nanorods with various aspect ratios were synthesized by the thermal decomposition of a Co(II)-oleate complex. It was found that the CoO nanorods have an extraordinary wurtzite ZnO crystal structure and these uniform-sized nanorods self-assembled to form both horizontal parallel arrangements and perpendicular hexagonal honeycomb superlattice structures. The reduction of the nanorods by heating under a hydrogen atmosphere generated either hcp Co or Co2C nanorods. Secondly, uniform-sized MnO nanocrystals having various sizes and shapes were synthesized from thermal decomposition of Mn(II)-oleate complex in the presence of oleylalcohol and different types of carboxylic acids. It was found that the shape of MnO nanocrystals was evolved from cube to octahedron, finally to sphere by the thermal decomposition followed by aging at reflux condition. The particle size could be tuned by the hydrocarbon chain length of carboxylic acids. Anisotropic multi-branched MnO nanocrystals by oriented attachment and tetrahedron-shaped MnO nanocrystals were also synthesized. Finally, various hollow oxide nanoparticles of iron and manganese were generated via a controlled nanoscale etching of metal oxide nanocrystals in the presence of trioctylphosphine oxide (TOPO) and alkylphosphonic acid. The formation mechanism for the hollow oxide nanoparticles was elucidated by using various characterization methods. It was revealed that the hollow nanoparticles were amorphous metal oxides containing significant amount of phosphorous and showed spin relaxation enhancement effect for magnetic resonance imaging (MRI). This synthetic method was highly reproducible and could be generalized to synthesize hollow oxide nanoparticles of various sizes, shapes, and compositions.


N10.46
A Novel Inkjet Printing Method: Drop and Synthesis (DAS) - Application to Synthesis of ZnS:Mn. Hongki Cha, Sung Il Ahn and Kyung Cheol Choi; Electrical Engineering and Computer Science, KAIST, Daejeon, Korea, South.

Inkjet printers are known for their compatibility with various materials and devices including OLEDs, conducting polymers, ceramics and other materials [1]. In this study, patterned colloidal nanoparticles are formed in a single-step process via micro-chemical reactions between reactive ink-drops from an inkjet printer at least two inkjet heads. Each of the two inkjet heads in the present study prints different inks, both of which are in a complete solution state. These inks react with each other in a synthesis reaction to form colloidal nanoparticles after they are dropped precisely to a set point. Thus, the novel scheme proposed here is termed Drop and Synthesis (DAS.) Unlike conventional inkjet printing, colloidal nanoparticles do not exist in any of the inks; consequently, the probability that the inkjet head will clog is reduced significantly. In particular, the synthesis of ZnS:Mn is introduced as one of the applications for this scheme. Two different inks were prepared. The first contains Zn acetate and Mn and the second contains sodium sulphide as the reactants. While using DI water as the main solvent, ethylene glycol and 2-methoxyethanol were added to prevent drying and clogging. Finally, an appropriate amount of polyvinyl pyrrolidone (PVP) was added as a capping agent for each ink. Upon an examination, the photoluminescence spectra indicated that the three inkjet-printed ZnS:Mn samples with different amounts of PVP had two peaks (590 nm and 430 nm). The magnitudes of the peaks at 590 nm and 430 nm decreases and increases, respectively, as PVP is added. Such a difference is discussed in conjunction with the 4T1 to 6A1 transition of Mn2+ ions [2] and the “self-activated” colloidal ZnS that arise due to sulphur vacancies in the lattice [3]. Additionally, the X-ray diffraction pattern identifies the colloidal ZnS:Mn nanoparticles as having a nearly stoichiometric hexagonal shape. It was also verified that additional XRD peaks apart from Wurtzite-2H (ZnS) reference peaks are attributable to the layered structure that forms due to the residual chemicals and the PVP. Finally, scanning electron microscopy images showed that the colloidal nanoparticles have an average size of 10 nm. At the same time, the layered structure could be observed in the SEM pictures. REFERENCES [1] P. Calvert, Chem. Mater. 2001, 13, 3299-3305 [2] R. N. Bhargava, D. Gallagher, X. Hong, and A. Nurmikko, Phys. Rev. Ltr. 1994, 72, 416-419 [3] W. G. Becker, A. J. Bard, J. Phys. Chem. 1983, 87, 4888-4893


N10.47
Colloidal ITO for Fast-Switching and Stable Contrast in Tungstate-Based Electrochromic Thin Films. Yong Tae Park and Jaime Grunlan; Mechanical Engineering, Texas A&M University, College Station, Texas.

Layer-by-layer (LbL) assembly of ultrathin films is a powerful technique, which provides nanometer-scale control of film thickness and composition. In the present work, tungstate (WO4 2-) anions have been alternately deposited with cationic poly(4-vinylpyridine-co-styrene) [PVP-S], using the LbL technique, to generate electrochromic thin films that transition from transparent to dark blue in their oxidized and reduced states, respectively. Tungstate is advantageous as an electrochromic material because it is colorless in its deposited state, while most other electrochromic materials exhibit some type of color in their natural state. The primary drawback of tungstates is long switching time (> 30s), which is common amongst ceramic electrochromic species due to lack of electrical conductivity in at least one of the two states. We have deposited incorporated indium tin oxide (ITO) nanoparticles into these tungstate-based assemblies to decrease the coloration time in these LbL assemblies. In the absence of ITO, these films grow linearly during deposition, achieving a thickness of approximately 80 nm with 40 PVP-S/ WO42- bilayers. These films take 30 - 60s to completely switch and exhibit reduced transmittance with each switch (i.e., never go back to fully transparent. ITO-containing films, with ITO in every other bilayer, fully switch in 3 - 10s and do not exhibit the same drift in transmittance with repeated switching (i.e., full oxidation and reduction are achieved with each cycle). Film growth, microstructure, electrochemical properties and electrochromic efficacy will be described in this presentation. These thin films hold promise for use in smart windows (or blinds) and flexible displays.


N10.48
ZnX (X = O, S, Se, Te) Nanowires Under Uniaxial Strain: A Pathway for Band Gap Engineering. Satyesh K. Yadav, Thomas Sadowski and Rampi Ramprasad; Chemical, Materials & Biomolecular Engineering, Institute of Materials Science, Storrs, Connecticut.

Cd based II-VI semiconductors being toxic, attention is being shifted to Zn based II-VI semiconductors for several applications, including solar cells, light emitting diodes, etc. However, progress has been limited due to the large band gaps (well above the visible spectrum) of ZnX (X = O, S, Se, Te) systems, which are expected to further increase in ZnX nanostructures due to quantum confinement. In this ab initio work, we consider wurtzite ZnX in both bulk and nanowire forms. We show that both forms exhibit the following universal behavior: under uniaxial compressive strain along the wurtzite c-axis, the band gap of the system initially increases, goes through a maximum, and then decreases abruptly, allowing for a wide range of possible uniaxial strain-induced band gaps. Our ab initio calculations were based on density functional theory (DFT). Uniform uniaxial strain in bulk wurtzite ZnX along the (0001) direction was achieved by systematically varying the c-axis lattice parameter, and optimizing the a-axis lattice parameter and the atomic coordinates. In the case of nanowires, optimization of all atomic coordinates was performed for each choice of c-axis lattice parameter. Since ZnX is a covalent sp3-hybridized systems displaying tetrahedral bonding, the band gap increases with small amounts of compressive strain, owing to increased splitting of the “bonding” valence band energies from the “anti-bonding” conduction band energies. We however find that for larger uniaxial compressive strains (> 4%), both bulk and nanowire systems display the onset of a phase transformation to a graphite-like phase: each Zn atom abruptly becomes coplanar with the 3 X atoms below in one layer; likewise, in the adjacent layer, each X atom becomes coplanar with the underlying Zn atoms. This sp3 to sp2 phase transformation is accompanied by a drastic drop in the band gap values. Subtle differences between the bulk and nanowire behavior, as a function of the identity of X was observed, but the qualitative trends were similar in both systems. Although DFT has the well-known deficiency of underestimating band gaps, trends in changes in the band gaps, and structural distortions due to strain are predicted accurately. We thus believe that this new discovery may lead to important applications.


N10.49
Synthesis of Organic Capped Colloidal Zinc Oxide Quantum Dots and Their UV Dominant Emission Property. Takahisa Omata, Kazuyuki Takahashi, Shinichi Hashimoto, Yasuhiro Maeda, Katsuhiro Nose and Shinya Otsuka-Yao-Matsuo; Graduate School of Engineering, Osaka University, Suita, Japan.

Previously reported organic capped zinc oxide quantum dots (QDs), which were prepared by non-aqueous solution process, showed intense emission in visible (VIS) region. Depending on conditions, the exciton-recombination ultraviolet(UV)-emission could be hardly detected especially for the small QDs below 5 nm in diameter. Because the VIS-emission relates the oxygen vacancies and metallic impurities, to reduce these defects is essential to decrease the VIS-emission. We have developed a novel solution synthesis route to colloidal ZnO QDs that show clear quantum size effect on their optical absorption and photoluminescence in UV region. Typical synthesis procedure is as following: Zinc alkoxide with 99.99 % of metallic purity is hydrolyzed at 60 oC by dilute water in benzylamine, the water concentration in the present study is ~500 ppm in mass; and the insoluble residue is then centrifugally separated from the solution. The resulting clear colorless solution contains colloidal ZnO capped with hydroxyl group; the size of the ZnO is 2~3 nm in diameter. Successively, oleylamine is added into the hydroxyl capped colloidal ZnO QDs solution, and then it is heated above 100 oC. After the heating, the capping ligand of the ZnO QDs is exchanged from hydroxyl group to oleylamine. Therefore, the resulting ZnO QDs are extractable by addition of lower alcohol and dispersible in organic solvents such as toluene and chloroform. The resulting ZnO QDs shows quantum size effect on the optical band gap, i.e., the optical band gap can be controlled in the range between 3.5 eV for 5.2 nm QDs and 3.7 eV for 3.1 nm QDs in the present study. The UV-emission is clearly observed for the resulting QDs; the emission energy is 3.3 eV for 5.2 nm QDs and 3.5 eV for 3.1 nm QDs. The intensity of the UV-emission is more than twice of the VIS-emission in peak height; we have successfully synthesized ZnO QDs whose UV-emission is dominant. To increase UV-emission is now underway.


N10.50
Insights into the Formation of Nanoscale Spatially Indirect Excitons: Evolution of the Type-II Optical Character of CdTe/CdSe Heteronanocrystals. Celso de Mello Donega and Esther Groeneveld; Chemistry, Debye Institute for Nanomaterials Science - Utrecht University, Utrecht, Netherlands.

Semiconductor heterostructures can show different carrier localization regimes after photoexcitation, depending on the energy offsets between the valence and conduction band levels of the materials that are combined at the heterointerface. In the type-I case both carriers are primarily confined in the same part of the heterostructure, while in the type-II case electron and hole are spatially separated on different sides of the heterojunction, leading to the formation of a spatially indirect exciton. The relative energy offsets in semiconductor heteronanoparticles can be tuned by controlling the composition, size and shape of each component. This offers the possibility of directly controlling the electron-hole wavefunction overlap, and consequently the material optoelectronic properties. Understanding the size-, shape- and composition-dependence of the properties of indirect excitons in heteronanoparticles is thus of great scientific interest, from both fundamental and applied viewpoints. Despite the advances made in recent years, a comprehensive fundamental understanding of nanoscale spatially indirect excitons has yet to emerge. In this contribution, we systematically investigate the evolution of the optical properties of nanoscale spatially indirect excitons as a function of the size, shape, and composition of the heteronanocrystal, using highly efficient CdTe/CdSe colloidal nanoparticles as a model system. Emphasis is given to quantitative aspects, such as the absorption cross section of the lowest energy exciton transition, the Stokes shift, transition linewidths, and the exciton radiative lifetime. It is demonstrated that the hole localizes in the CdTe core already for CdSe volume fractions as small as 1%, while the electron is initially delocalized over the whole heteroNC (type-I1/2 regime), and gradually localizes in the CdSe segment as the growth proceeds, until the spatially indirect exciton transition becomes the lowest energy absorption transition (type-II regime). This leads to a progressive shift of the optical transitions to lower energies, accompanied by a decrease of the oscillator strengths at emission energies and an increase of the exciton lifetimes, linewidths and non-resonant Stokes shift. The indirect exciton formation leads primarily to redistribution of the oscillator strength of the lowest energy transition over a wider frequency range, rather than to a pronounced reduction. Further, two radiative decay rates are observed, and ascribed to fine-structure exciton states. The partial lifting of the indirect exciton degeneracy is attributed to shape anisotropy. The results presented in this work provide novel fundamental insights into nanoscale spatially indirect excitons. This knowledge will have a positive impact on the design of colloidal heteronanoparticles for optoelectronic applications.


N10.51
Increasing PbSe Optical Stability through Shell Formation. Yu Zhang1,2, Quanqin Dai1, Xinbi Li1, Ong-art Thanetnit1,4, Yiding Wang2, Bo Zou3, Supawan Tantayanon4 and William W. Yu1; 1Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts; 2College of Electronic Science and Engineering, Jilin University, Changchun, China; 3State Key Laboratory of Superhard Materials, Jilin University, Changchun, China; 4Department of Chemistry, Chulalongkorn University, Bangkok, Thailand.

Infrared-emitting semiconductor nanocrystals (NCs) are of increasing interest in potential applications such as telecommunication, photoelectronic device, and biomedical labeling. Lead chalcogenide NCs, especially PbSe, provide strong confinement effect and high quantum yield in a wide spectral range of near infrared. However, the applications of PbSe NCs have been limited by their instability of optical properties under ambient conditions. Therefore, several approaches have been developed to stabilize it. PbSe/PbS and PbSe/SiO2 core/shell structures have been developed but with excessive loss of quantum yield. Hollingsworth group has developed an ion exchange method using Cd to replace Pb in outlayer of large PbSe plain NCs to form PbSe/CdSe core/shell structures. The limitation of this method is that it is difficult to growth thick CdSe layer and it damages lattice structure of PbSe NCs. In our lab, the successive ion layer adsorption and reaction was employed to form the air-stable PbSe/CdSe NCs. The thickness of the CdSe layer could be well controlled. The nanocrystal surface was treated with a small amount of sodium borohydride in hexanes at room temperature. After the surface treatment, the PbSe/CdSe NCs showed higher quantum yield than the PbSe plain core. Further, these NCs could also be covered with another ZnSe shells and became biocompatible optical labels for near infrared wavelength.


N10.52
Polystyrene/Gold Nanocomposite Containing Well-Dispersed Gold Nanoparticles. Raymond C. Tsiang, Chemical Engineering, National Chung Cheng University, Chiayi, Taiwan.

The thermal and spectroscopic properties of polystyrene/(gold particles) (PSSH/Au) nanocomposite materials containing uniformly dispersed nanoparticles in a polymer matrix have been studied. The nanocomposite comprises the thiol-terminated polystyrene (PSSH) which was produced by capping a living polystyrene with ethylene sulfide and the nano-sized gold particles (AuNPs) which were in-situ formed in the presence of PSSH. The PSSH was characterized by NMR and the bonding between thiols and AuNPs was verified by X-ray Photoelectron Spectroscopy. Based on the transmission electron microscopy and the UV-vis absorption spectra, PSSH with a lower molecular weight leads to the formation of bigger AuNPs. The TD of the nanocomposite (PSSH/Au) increases with an increase in the Au content and at a constant Au content the PSSH with a higher molecular weight affords PSSH/Au a higher TD. While PSSH/Au has a higher Tg than the corresponding pristine PSSH, the difference reduces with an increase in Au content (i.e., a decrease in the PSSH:Au weight ratio) because of the formation of larger AuNPs which provide a smaller total surface area for PSSH to anchor or to cover.


N10.53
Colloidal Silicon Nanoparticle Dispersions for Inkjet Printing. Anoop Gupta1, Ahmed S. G. Khalil2, Markus Winterer2 and Hartmut Wiggers1,3; 1Institut fur Verbrennung und Gasdynamik, University of Duisburg-Essen, Duisburg, Germany; 2Nanoparticle Process Technology, University of Duisburg-Essen, Duisburg, Germany; 3Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Duisburg, Germany.

Colloidal nanoparticles as a functional building blocks are of growing interest in many areas of chemical, biological and microelectronic industry from scientific and technological point of view. Therefore, a high industrial demand exists on stabilized colloidal dispersions of nanoparticles in aqueous as well as non-aqueous solvents. Silicon has many advantages compared to other nanoparticle systems in terms of toxicity, availability, cost effectiveness and compatibility with the well-established semiconductor technology. The stability of colloidal dispersions of silicon nanoparticles is essential for their use in drug delivery applications as well as for the manufacture of electronic and optoelectronic devices using printing technologies. In this study, we examined the stability of silicon nanoparticles in aqueous medium at different pH. The silicon nanoparticles show high zeta potential values within pH= 6.5-8.5. In addition, the silicon nanoparticles do not show any isoelectric point in the pH range studied. The effect of different additives on the stability of the dispersion is also investigated. In order to stabilize silicon nanoparticles in organic solvents, their surface is functionalized with alkyl groups via a thermally induced alkylation process. The functionalized silicon nanoparticles form optically clear dispersions in a variety of organic solvents and no sedimentation of functionalized sample was observed over any period of time. Fabricating films of silicon nanoparticles using ink-jet printing is currently under investigation.


N10.54
Nanoscale Cu2Cl(OH)3 and Cu2(NO3)(OH)3 - Synthesis and Use for Cu(0)-based Thin-film Electronics. Silke Wolf and Claus Feldmann; Universitaet Karlsruhe (TH), Karlsruhe, Baden-Wuerttemberg, Germany.

Copper materials are highly relevant due to their antibacterial effects, their colour and the potential reduction to elemental conductive copper metal. Moreover, printed electrodes and electronics are required for many different applications such as thin-film transistors, membrane keyboards and solar cells. Currently silver is mostly applied for those devices [1]; replacement by copper would be beneficial concerning the increasing price of silver and the concerns regarding its biocompatibility [2]. The aim of this work was to synthesize nanoscale copper compounds at moderate conditions which exhibit low lattice energy. This strategy allows an easy reduction to elementary copper. Accordingly Cu2Cl(OH)3 and Cu2(NO3)(OH)3 have been selected, which are available as 200 nm sized plates [3] and microcrystals [4] till now. We have developed the polyol-mediated synthesis of nanoscale Cu2Cl(OH)3 and Cu2(NO3)(OH)3. The polyol method uses high-boiling, multivalent alcohols, such as diethylene glycol, as a reaction medium and a surface stabilising agent [5]. According to scanning electron microscopy as-prepared particles exhibit diameters of 51 nm (Cu2Cl(OH)3) and of 37 nm (Cu2(NO3)(OH)3). These are in according with dynamic light scattering values of the resuspended particles. X-ray diffraction evidences the particles to be crystalline after synthesis without further thermal treatment. The parameters affecting size, morphology and yield of the products, such as pH-value, concentration of the reactants, reaction temperature and time, were studied. As-prepared nanoscale Cu2Cl(OH)3 and Cu2(NO3)(OH)3 finds application in thin-layer deposition on paper. Therefore suspensions of Cu2Cl(OH)3 and Cu2(NO3)(OH)3 in ethanol were spin coated or stamped. Subsequently these thin layers were reduced to elementary copper by NaBH4 in water. This process was carried out at room temperature in a water-based system. The sheet resistances of the layers were measured by four-point probing and resulted in a sheet resistances of up to 4 kΩ [6]. [1] T. H. J. van Osch, J. Perelear, A. W. M. De Laat, u. S. Schubert, Adv. Mater. 2008, 20, 343. [2] J. Rickerby, J. H. G. Steinke, Chem. Rev. 2002, 102, 1525. [3] Y.-H. Luo, J. Huang, J. Jin, X. Peng, W. Schmitt, I. Ichinose, Chem. Mater. 2006, 263, 473. [4] H. Niu, Q. Yang, K. Tang, Mater. Sci. Eng., B 2006, 135, 172. [5] C. Feldmann, Adv. Funct. Mater. 2003, 13, 101. [6] S. Wolf, C. Feldmann, submitted.


N10.55
Effects of Surface Chemistry on Nonlinear Optical Properties of Lead Sulfide Nanocrystals. Daniel Asunskis1, Igor L. Bolotin1, Joy Haley2,3, Augustine Urbas2 and Luke Hanley1; 1Chemistry, UIC, University of Illinois @ Chicago, Chicago, Illinois; 2Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio; 3UES Inc., Dayton, Ohio.

The nonlinear optical properties as well as surface properties of lead sulfide (PbS) nanocrystals have been analyzed using the Z-scan method and through femtosecond pump-probe experiments. The synthesis of the nanocrystals was carried out using two strategies to generate nanocrystals with different surface properties. The first used a surfactant based synthesis to grow oleic acid capped (OA) lead sulfide nanocrystals and the second strategy used polymers as the sole growth limiting agent for the nanocrystals. PbS nanocrystals were grown in poly(1-butene) (PB) and poly(1-decene) (PD), two optically clear hydrocarbon polymers, as well as poly(vinyl alcohol) (PVA), which has hydroxyl groups capable of interaction with the nanocrystals during growth. The open aperture Z-scan results indicated a clear difference in nonlinear optical properties between the OA and PVA composites in comparison to PB and PD. The nanocrystal materials that have strong covalent bonding at the surface, PVA and OA, did not show nonlinear absorption behavior in Z-scan measurements at 532 or 1064 nm. The PB and PD composites did however show strong reverse saturable absorption at these wavelengths. The fitted βeff values were 5.4(±0.2) and 9.3(±0.5) cm/GW for the PD and PB composites, at 1064 nm, a full order of magnitude lower then previously reported values of 160(±30) and 120(±20) cm/GW at 532 nm[1]. The transient absorption measurements further separated the PVA and OA composites from the PB and PD samples. The decays for each of the samples indicated up to three contributing events. First are two short decay processes, with lifetimes τ1 ~1 ps and τ2 ~20 ps, that were due to the intraband and shallow trap state relaxations, respectively. The third component observed in the absorption decay created the main differences in the samples and was attributed to the relaxation from the deep trap states. The value for its lifetime, τ3, was determined to be >6 ns in PB and PD. This component was visible in the OA sample, but its lifetime was much shorter <2 ns and the results for the PVA showed no traces of this component. The differences in excited state lifetime seen in the OA and PVA composites in comparison to PB and PD mirror the differences that were seen in the Z-scan experiment. [1]. Asunskis, D.J., Bolotin, I.L., Hanley, L., Journal of Physical Chemistry C, 2008, 112(26), 9555.


N10.56
Synthesis and Linear Electrogyration Property of Eu3+-doped In2O3 Nanocrystals. Zhaoyong Sun1, I. V. Kityk2 and Jiye Fang1; 1Chemistry, State University of New York at Binghamton, Binghamton, New York; 2Electrical Eng., Technological University of Czestochowa, Czestochowa, Poland.

Colloidal Eu3+-doped In2O3 nanocrystals (NCs) were successfully synthesized in a noncoordinating solvent using Indium (III) and Europium (III) acetates as precursors. The NCs were treated with pyridine to remove surface-bound dopant ions prior to an ICP-MS composition evaluation. The concentration of doped Eu3+ was varied up to 2.85 at% based on the ICP-MS result. Linear electrogyration (LEG) induced by coherent circularly-polarized light was observed from samples in which Eu3+-doped In2O3 NCs were embedded in photopolymer oligoethracryalte matrices. The result on ~2.5 at% Eu3+-doped sample shows that the maximal LEG could reach to 12 deg./mm at an electric field of 120V/cm for He-Ne laser with a wavelength of 1150 nm. The microscopic configuration of the polarized light beam confirms the occurrence of the optically induced axial grating interferometric rings corresponding to formation of the axial-like grating.


N10.57
Tuning Functionality of Nanoparticles with Self Assembled Monolayers of Phosphonates. Eric Bruner, Aculon, Inc., San Diego, California.

Aculon, Inc. specializes in inventing and commercializing unique molecular-scale surface and interfacial coatings leveraging nanotechnology discoveries made at Princeton University. These coatings can be classified into three functional areas; non-stick, pro-stick/adhesion, and anti-corrosion. The company has formulated coating solutions and processes for numerous markets including optical, display, electronics, consumer products and industrial coatings. These specialized coatings outperform all known alternatives in characteristics such as adhesion, stain resistance, and scratch resistance. Fueling the company’s commercialization efforts is its proprietary Self-Assembled Monolayer of Phosphonates (SAMP) technology. The commercialization of SAMP treatments can be used for a variety of applications including coating oxide surfaces, including metal oxide and yttria nanoparticles. This technology, along with other surface modification techniques, coating methods and materials developed at Aculon, serve as a platform on which nanoparticle surfaces can be tailored for a broad range of uses. One application of coating yttria particles has immediate commercial potential to effectively compete with Quantum dots for anti-counterfeit and labeling systems. Yttrium oxide (Y2O3) is an air-stable, colorless substance widely used to make YVO4Eu and Y2O3Eu phosphors that give the red color in TV picture tubes. This readily available material has recently been used for medical diagnostics and bioimaging. Utilizing self-assembled monolayers of phosphonates (SAMPs), Researchers at Princeton University were able to disperse yttria into solutions of water and solvents.2 By taking advantage of the luminescent properties in doped nanocrystalline yttria, yttria nanoparticles were used to detect and image cells. Furthermore, better dispersivity of nanoparticles using SAMP treatments into organic and aqueous solvents enables the production of stable ink-jet solutions for anti-counterfeit and labeling systems.


N10.58
Abstract Withdrawn


N10.59
Electron Transport in Nanostructured TiO2 Electrodes Using Supporting Ti Metal Substrates. Tomohiro Nin, Qing Shen and Taro Toyoda; Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Japan.

Dye sensitized solar cells (DSSCs) are expected as an alternative to silicon solar cells due to their low cost and high efficiency. The nanostructured TiO2 electrodes in the DSSCs are usually deposited onto FTO transparent conducting glasses. However, FTO glass substrates have high sheet resistance and high cost [1]. On the other hand, Ti metal has very high conductivity and are much cheaper. Thus, it is expected that the application of Ti sheet as a substrate of TiO2 electrodes in the DSSCs can not only reduce the cost of solar cells, but can also contribute to improve the performance of solar cells by reducing internal resistances. In this study, we prepared two kinds of TiO2 nanostructured electrodes with various thicknesses on Ti substrates using two different methods. One is the attachment of TiO2 nanoparticle (average size: 15 nm) paste onto Ti substrate (named as TiO2-1). The other is the in-situ deposition of self-assembled TiO2 nanoparticles prepared from the precursor solution of TiCl4 (named as TiO2-2) [2]. The latter can be expected to reduce the resistance of TiO2 grain boundary and TiO2/Ti interface resistance. We characterized the crystal structures, optical absorption, incident photon to current conversion efficiency (IPCE) and electron transport properties of the TiO2 nanostructured electrodes using XRD, photoacoustic, photoelectrochemical and transient photocurrent measurements, respectively. From the XRD patterns, anatase structures were confirmed and the average sizes of TiO2 nanoparticles in the two kinds of TiO2 electrodes were determined to be almost the same from the full-width at half maximum of diffraction peaks. From the optical absorption spectra, band gaps of about 3.2 eV could be determined for all of the TiO2 electrodes, which were consistent with the band gaps of anatase TiO2. From the IPCE spectra, broad peaks can be observed at photon energies of about 3.7 eV. The IPCE values became larger as the TiO2 thickness decreases. For the TiO2 electrodes with same thicknesses, the IPCE values of TiO2-2 were larger than those of TiO2-1. For the photocurrent transients, the shapes and the times corresponding to the photocurrent peak both depend on the film thickness and the preparation method of the nanostructured TiO2 films. Using a diffusion model, chemical diffusion coefficients D for the electrons in the TiO2 electrodes can be determined. We found that D was 1x10-4 cm2/s for TiO2-2 but 4x10-6cm2/s for TiO2-1 under our experimental conditions. The larger values of IPCE and D for TiO2-2 were attributed to the increased electron conductivity due to the smaller resistances at the TiO2 grain boundaries and the TiO2/Ti interfaces. It indicates that the preparation methods of the TiO2 electrodes affect the electron transport greatly. [1] K. Onoda et al., Solar Energy Materials and Solar Cells, 91, 1176 (2007). [2] R. Vogel et al., Chemical Physics Letters 174, 241 (1990).


N10.60
Autoreduction of Metallic Species on the Surface of Silica Nanoparticles by Surface Functionalization. Jorice Samuel, Olivier Raccurt and Olivier Poncelet; Department of Nano-Material (DRT/LITEN/DTNM/LCSN), CEA, Grenoble, France.

Silica nanoparticles with metallic nanoclusters are of great interest in many applications from bio imaging to optical devices. The Nanometric size of metallic particles induces specific absorption properties due to surface plasmon resonance. This absorption mainly depends on the morphology of the nanoparticles [1,2]. If the encapsulation of metallic nanoparticles into a silica shell is now well developed [3], there is a great activity on the synthesis of either silica nanoparticles covered by metallic nanoparticles or silica cores with metallic shells. Two main ways are described in the literature to bind metallic nanoparticles onto the surface of silica nanoparticles. The first way consists on the mixing of a metallic colloidal sol with a sol containing silica nanoparticles bringing at their surface suitable chemical functions able to properly interact with the metal nanoparticles [4]. The second way consists on the use of a reducing agent to reduce metallic ions adsorbed at the surface of the silica nanoparticles [5,6]. In this last case, the reducting agent and the metallic ions are introduced successively into a suspension of silica nanoparticles. Herein is proposed an original third way enabling to wrap to the surface of silica nanoparticles by metal colloids. This way is based on an in-situ reduction of metallic ions by two chemical functions (amino and thiol) previously grafted onto the surface of the silica nanoparticles. Silica nanoparticles are synthesized by a reverse micro-emulsion sol-gel reaction. This synthesis gives monodispersed silica nanoparticles of 40 nm diameter. The functionalisation of the silica surface by two silane coupling agents owning the chemical functions thiol and diamino is performed by a sol-gel reaction during the micro-emulsion process. By this way a functionalized silica shell provides a reactive surface on the nanoparticles. The capability of this surface to reduce metallic ions depends on the chemical functions used. Two examples are given: the case of diamino functions which reduce copper ions and the case of the combination of diamino and thiol functions in a silver nitrate solution. In the second case, small silver nanoparticles (4-5 nm) grow on the silica nanoparticles’ surface. References: [1] Y. Hu et al., OPTICS EXPRESS, Vol. 16, No. 24, 2008, 19579-19591. [2] J. Perez-Juste et al., Coordination Chemistry Reviews 249 (2005) 1870-1901. [3] S. Cavaliere-Jaricot, M. Darbandi, T. Nann, Chem. Commun, (2007), 2031-2033. [4] J.C. Flores and al., Journal of Non-CrystallineSolids 354 (2008) 5435-5439. [5] J.C. Flores and al., Colloids and Surfaces A : Physicochem.Eng. Aspects 330 (2008), 86-90. [6] Y.L. Shi et al., Langmuir (2007), 23, 9455-94.


N10.61
Effect of Doping on the Structural and Optical Properties of Microwave-Assisted Synthesis of ZnSe@ZnS Core-Shell Quantum Dots. Sonia J. Bailon-Ruiz1, Oscar Perales-Perez1,2, Surinder P. Singh2 and Maharaj Tomar3; 1Chemistry, University of Puerto Rico, Mayaguez, Puerto Rico; 2Engineering Science & Materials, University of Puerto Rico, Mayaguez, Puerto Rico; 3Physics, University of Puerto Rico, Mayaguez, Puerto Rico.

In past decades semiconductor quantum dots have attracted much attention because of their interesting application in cell imaging, cancer nanomedicine and other bio-applications. We report here the microwave-assisted synthesis of water-soluble ZnSe/ZnS core-shell quantum dots from zinc chloride and selenide aqueous solutions in presence of different types of thiol species. The effect of the pH of the reacting solutions and the concentrations of the thiol ligands, 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA) were systematically investigated. The formation of the ZnSe@ZnS quantum dots was confirmed by X-ray diffraction, transmission-electron microscopy, X-ray photoelectron spectroscopic and optical characterization techniques. UV/Vis absorbance spectra for synthesized nanocrystals showed an excitonic peak at around 320 nm. The sizes of nanocrystals were dependent on the pH; below pH 6.5, the corresponding emission peaks were blue-shifted whereas a red-shift was observed for pH above 7.5. Quantum dots prepared in presence of TGA exhibited a strong emission centered at 360 nm with a FWHM ~ 20 nm. The emission peak was centered at 400nm (FWHM 40-55 nm) when the quantum dots were synthesized in presence of MPA; low-intensity bands, attributed to surface defects (trap emission peaks), were observed at ~ 450 nm. The effect of doping of the ZnSe@ZnS quantum dots with Mn2+ and Cu2+ species (atomic fractions between 0.001 and 0.5) on the morphological, structural and optical properties as well as the corresponding quantum yields will be presented and discussed.


N10.62
Synthesis and Properties of Monometallic and Bimetallic Silver and Gold Nanoparticles. Xavier E. Guerrero Dib1, Ubaldo Ortiz Mendez3, Miguel Jose Yacaman2, Domingo Ferrer2 and Selene Sepulveda3; 1Engineering & Design, Universidad del Valle de México, San Nicolás de los Garza, Nuevo León, Mexico; 2Texas Materials Institute, University of Texas, Austin, Texas; 3Mechanical & Electrical Engineering, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico.

Au, Ag monometallic, and Au-Ag bimetallic nanoparticles have been synthesized by successive reduction of metal salts with ascorbic acid on pre-made seeds in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), with this method the coverage of the seeds is extremely uniform, although in some cases deviations from a spherical shape are observed with the formation of nanorods or nanoprisms. The evolution of the optical properties as further metal layers are deposited is very dramatic and can be modelled using Mie theory for multilayer spheres. However, preliminary results using high-resolution STEM-XEDS elemental mapping suggest that the actual distribution of the two metals within the multilayer spheres may involve (partial) alloying of the metals.


N10.63
Linear and Nonlinear Optical Properties of Er3+/Yb3+ Doped NaYF4 Nanocrystals. Vamsi K. Komarala1, Ningning Xu1, Yunjun Wang2 and Min Xiao1; 1Physics, Univeristy of Arkansas, Fayetteville, Arkansas; 2Mesolight LLC, Little Rock, Arkansas.

Photoluminescence (PL) and nonlinear optical properties of NaYF4 nanocrystals (NCs) doped with Er3+ and Yb3+ ions are investigated. The Yb3+ ions are added in the host lattice as ‘sensitizing ions’ to increase the upconversion process via effective excited-state absorption and energy transfer processes, and the Er3+ ions are ‘acceptors’. Our main aim in this work is to experimentally understand the role of Yb3+ ion concentration on linear and nonlinear optical properties of ion-doped NCs. The NaYF4 NCs are synthesized by doping with 2% Er3+ ions and different concentrations (5, 10, 20 30 and 70%) of Yb3+ ions. The upconverted PL measurements were carried out with the excitation wavelength of 980 nm from a CW diode laser. The PL spectrum exhibits three distinct Er3+ emission peaks. The blue, green and red emission bands are due to the electronic transitions in Er3+ ions from 2H9/2 ® 4I15/2, (2H11/2, 4S3/2) ® 4I15/2 and 4F9/2 ® 4I15/2, respectively, which conform the multi-photon absorption processes. The relative intensities of the blue, green and red emission peaks depend on the Yb3+ ion concentration in the NCs. Variations in the relative peak intensities can be explained by considering modification of distances between the Er3+ and Yb3+ ions in the host lattice, which can promote the back energy transfer from Er3+ to Yb3+ ions. The back energy transfer suppresses the excited-state populations of the higher energy states, which can lead to decreases of the blue and green emission intensities with the increase of red emission intensity. With such binary dopants, one can control the color outputs from the NCs for desired applications. Third-order nonlinear optical properties are investigated by using Z-scan technique. The femtosecond laser pulses are from a mode-locked Ti:Sapphire laser with a high repetition rate of 82 MHz at the wavelength of 800 nm. The sample doped with 2% Er3+ and 70% Yb3+ ions, shows a nonlinear absorption coefficient of 19.04 cm/GW and a nonlinear refractive index of -564.8 X 10-5 cm2/GW. This observed large nonlinearity is attributed to two-photon-absorption-generated electrons and its modified decay dynamics in the presence of the NC hosts due to the surface-related states, modified phonon modes, and also the surrounding medium. The increase in the doped Yb3+ ion’s concentration causes accompanied increases of nonlinear coefficients due to the efficient energy transfer from Yb3+ to Er3+ ions in the NCs. The nonlinear absorption and refractive index values from the NCs doped with Er3+/Yb3+ ions are quite high compared to some of the semiconductor NCs. The observed results show potential usefulness of such doped NCs for opto-electronic, biophotonic and solar cell applications.


N10.64
Study of Dynamics and Static Fluorescence Quenching of Fluorescent Molecules Adsorbed on Gold Nanoparticles. Jimmy A. Castillo, Hector M. Gutierrez, Irving Marquez and Alberto J. Fernandez; quimica, Universidad Central de Venezuela, Caracas, Venezuela.

Gold nanoparticles has unique properties and have been motivated the development of different Studies including the ability to be usefully like sensors in medicine and biology. The local oscillations of the electrons in the valence shell induced by interaction with a light beam (Plasmon Resonance effect) can be used join to fluorescent probe molecules to enhanced the selectivity and sensitivity and performing a very specific sensor for different diseases. The energy transference between the nanoparticle and bounded fluorescence molecule could cause different effects depending of the kind of molecule-particule interaction forces and distances. For molecules at distances correspondent to van der waals interactions a quenching effect is observed, low energy interaction with long particle-molecule distances exhibit an enhancement in the fluorescence signal. In the present Works we synthesize gold nanoparticles using the laser ablation techniques. These nanoparticles were stabilized with sodium dodecilsulphate solutions to permit generate a controlated size particles. The stabilized nanoparticles were adsorbed with a Fluorescein, a typical fluorescent molecule and the interaction between the Systems was followed by UV-visible ,fluorescence spectra and Dynamic Light scattering measurements. We found a dynamic and static quenching effect in the system.


N10.65
Hexagonal Arrays of Gold Nanoparticles by Block Copolymer Micelles for the Control of Coupling Between Surface Plasmon and Fluorescence. Jeong-Hee Kim1,2, Ki-Se Kim1, Jin-Hyung Kim1 and Byeong-Hyeok Sohn1,2; 1Department of Chemistry, Seoul National University, Seoul, Korea, South; 2NANO System Institute, Seoul National University, Seoul, Korea, South.

Coupling between fluorescence and surface plasmon of noble metal nanoparticles can be used for many potential applications such as optical devices and biosensors. For example, fluorophores near noble metal nanoparticles can show the surface-plasmon-enhanced fluorescence due to the coupling. Depending on the distance between fluorophores and metal nanoparticles, however, the fluorescence can be also quenched. In this study, we fabricated near-perfect two dimensional arrays of gold nanoparticles on a solid substrate by a non-lithographic method for the control of coupling between surface plasmon and fluorescence. PS-P4VP diblock copolymers in toluene, a selective solvent for the PS block, self-assembled into nanometer-sized spherical micelles, consisting of an insoluble P4VP core and a soluble PS corona. When a thin film of PS-P4VP micelles was exposed to THF vapor, a highly-ordered hexagonal array of the micelles was created. After incorporating precursors of gold nanoparticles into the P4VP cores, a highly-ordered array of gold nanoparticles were synthesized by oxygen plasma treatment. The hexagonally-ordered array of gold nanoparticles was characterized by AFM and FE-SEM, and then was employed as a substrate for the control over the coupling of the surface plasmon of gold nanoparticles with the florescence of organic dyes which were nano-encapsulated into the P4VP cores of PS-P4VP micelles on the top of the array of gold nanoparticles.


N10.66
Effects of the Surrounding Medium on the Photoexcited Electron and Phonon Dynamics of Au Nanoparticles Characterized Using Transient Grating Technique. Qing Shen1, Kenji Katayama2, Tsuguo Sawada3 and Taro Toyoda1; 1The University of Electro-Communications, Tokyo, Japan; 2Chuo University, Tokyo, Japan; 3Japan Science and Technology Agency, Tokyo, Japan.

Metal nanoparticles, such as Au and Ag, have attracted much attention because they show interesting optical, electronic and chemical properties. In particular, they have a characteristic feature of localized surface plasmon resonance (LSPR) due to the collective oscillations of the electrons at the surface of the nanoparticles. Such metal nanoparticles have potential applications in biological sensors and optoelectronic devices. Recently, it is found that photoinduced charge separation based on LSPR is possible between the interface of Au nanoparticels and TiO2 electrodes. Thus, a new type of metal nanoparticle-sensitized solar cells is promising. For these applications, optical absorption and photoexcited electron dynamics are very important. Especially, the issue of how the environment affects the electron and phonon dynamics is very significant. However, it has not been systematically studied to date. In this study, we investigate photoexcited electron dynamics of Au nanoparicles with different environment, i.e., (a) Au colloids (average size: 20 nm) in ethanol, (b) Au nanoparticles (average size: 20 nm) adsorbed onto mesoporous TiO2 films, by using an improved transient grating (TG) technique [1,2]. The TG technique is a powerful time-resolved pump-probe method and has been proved to be powerful for measuring various kinds of dynamics of semiconductor and metal, such as carriers and phonon dynamics, by monitoring the refractive index changes of the sample versus time. The TG measurements were carried out using titanium/sapphire laser with the wavelength of 775 nm, the repetition rate of 1 kHz and the pulse width of 150 fs. The pump pulses were 530 nm in wavelength. The probe pulses were 775 nm in wavelength. Optical absorptions due to the LSPRs around 530 nm were confirmed for the both kinds of samples. So, the TG signals resulted from the optical absorption of the LSPRs in the Au nanoparticles. From the TG responses, two decay processes are observed and they can be fitted satisfactorily with two exponential functions. For the samples (a) and (b), the decay times of the fast decay processes are 2.4 and 1.3 ps, and the decay times of the slow ones are 125 ps and 410 ps, respectively. The fast and slow decay processes are attributed to the electron-phonon coupling (energy transfer from the electrons to the lattice) and phonon-phonon coupling (energy transfer from the Au nanoparticles to the environment), respectively. Our experimental results indicate that the surrounding medium affects the electron and phonon dynamics greatly. Further detailed studies are in progress now. [1] K. Katayama, M. Yamaguchi, and T. Sawada: Appl. Phys. Lett. 82 (2003) 2775. [2] Q. Shen, M. Yanai, K. Katayama, T. Sawada, and T. Toyoda, Chem. Phys. Lett. Vol. 442 (2007) 89.


N10.67
Photoexcited Carrier Dynamics of ZnO Nanostructured Films Characterized Using Transient Grating Technique. Qing Shen1, Kenji Katayama2, Tsuguo Sawada3 and Taro Toyoda1; 1The University of Electro-Communications, Tokyo, Japan; 2Chuo Univeristy, Tokyo, Japan; 3Japan Science and Technology Agency, Tokyo, Japan.

A great deal of attention has been devoted to ZnO nanoparticles for photocatalytic activity and solar energy conversion. Photoexcited carrier dynamics in ZnO and charge transfer processes between ZnO and adsorbed molecules are important factors affecting the photocatalytic activity and solar energy conversion efficiency. It is known that nanoparticle size of ZnO is very important for these applications. So, it is necessary to understand the relation between photoexcited carrier dynamics and the ZnO sizes. In this paper, we studied ultrafast carrier dynamics of ZnO nanostructured films made from ZnO nanoparticles with different average sizes ((a) 20 nm, (b) 500 nm) using a transient grating (TG) technique [1,2]. For sample (b), the TG signal decreases within a very short time (< 2 ps) and then a slow decay remains for a few hundreds of ps. For sample (a), however, only the slow decay process is observed. The TG responses reflect the changes in the numbers of photoexcited electrons and holes as a function of time [2]. The fast decay process in sample (b) relates to the photoexcited hole relaxation, since it disappeared when the TG measurement was carried out in 2-propanol, which is known as a hole scavenger. The great difference in the ultrafast carrier dynamics between (a) and (b) probably resulted from the changes of surface band bending with the decrease of the ZnO nanoparticle sizes. Our results indicate that the nanoparticle size affects the electron and hole dynamics greatly. [1] K. Katayama, M. Yamaguchi, T. Sawada, Appl. Phys. Lett. 82, 2775 (2003). [2] Q. Shen, M. Yanai, K. Katayama, T. Sawada, T. Toyoda, Chem. Phys. Lett. 442, 89 (2007).


N10.68
Characterization of Electron and Hole Dynamics in CdSe Quantum Dots Adsorbed onto Nanostructured TiO2 Electrodes Using Near-Filed Heterodyne Transient Grating Technique. Qing Shen1, Kenji Katayama2, Tsuguo Sawada3 and Taro Toyoda1; 1The University of Electro-Communications, Tokyo, Japan; 2Chuo University, Tokyo, Japan; 3Japan Science and Technology Agency, Tokyo, Japan.

Recently, semiconductor quantum dots (QDs) sensitized solar cells (QDSSCs) have received much attention as a candidate of the third-generation solar cells [1-3]. The photoexcited carrier dynamics of the semiconductor QDs, and the electron transfer processes from the semiconductor QDs to the TiO2 electrodes are important factors affecting the photovoltaic conversion efficiency. However, only a few reports are reported on these issues. In this study, we investigate the ultrafast carrier dynamics of CdSe QDs adsorbed on nanostructured TiO2 electrodes using near-filed heterodyne detection transient grating (NF-HD-TG) method [4,5]. Compared to other pump-probe techniques such as transient absorption (TA) spectroscopy, NF-HD-TG technique exhibits higher sensitivity because the NF-HD-TG signals are background-free. The CdSe QDs were adsorbed onto nanostructured TiO2 electrodes using a chemical bath deposition (CBD) [2,3,6]. We have studied ultrafast photoexcited carrier dynamics in the QDs in air and in electrolyte of Na2S. We have found that photoexcited hole and electron dynamics can be measured simultaneously for CdSe QDs with the same probe beam using the NF-HD-TG technique. The hole relaxation occurs within a few ps, which is due to surface trapping and/or relaxation into intrinsic nanocrystal states. It becomes faster in the Na2S electrolyte than in air, because of the scavenging of holes by S2- in the electrolyte. By comparing the hole relaxation times in the CdSe QDs in air and Na2S electrolyte, we can estimate the hole injection rate into the electrolyte. On the other hand, the electron relaxation occurrs from a few tens ps to hundreds ps, which results from electron transfer and/or trapping. We can estimate the electron transfer rate from the QDs to TiO2 by comparing the electron relaxation of the QDs adsorbed on glass substrate in the NF-HD-TG responses. Relationships between the CdSe QDSSC photovoltaic conversion efficiency and the carrier (electron and hole) dynamics will be discussed. [1] A. J. Nozik, Physica E 14, 115 (2002). [2] Q. Shen and T. Toyoda, J. Photochem. Photobiol., A 164, 75 (2004). [3] Q. Shen, J. Kobayashi, L. J. Diguna, T. Toyoda, J. Appl. Phys. 103, 084304 (2008). [4] K. Katayama, M. Yamaguchi, and T. Sawada, Appl. Phys. Lett. 82, 2775 (2003). [5] Q. Shen, M. Yanai, K. Katayama, T. Sawada, T. Toyoda, Chem. Phys. Lett. 442, 89 (2007). [6] S. Gorer et al., J. Phys. Chem. 98, 5338 (1994).


N10.69
Iron Oxide/Gold Core/Shell Nanoparticles Synthesized through Organometallic and Aqueous Methods. Wegdan R. Osman1 and Mona Baker2; 1Physics Department, Faculty of Science-Alexandria University, Alexandria, Egypt; 2National Institute of Laser Enhanced Sciences, NILES, Cairo University Egypt, Cairo, Egypt.

Bifunctional nanostructures offer novel ways of realizing the desired set of physical and chemical properties in nanosystems that are not possessed by either of the separate compounds. Fe3O4/Au core/shell structures provide an effective platform where nanoparticles can be detected and manipulated in two ways: magnetic and optical, thereby making them strong candidates for use in biosensors and biomedical applications. Different fabrication techniques have been reported in the literature to grow such composite nanoparticles. Although the organometallic route offers a good control over nanoparticles size and thickness of the shell, it involves the use of expensive and non-green solvents. On the other hand, the aqueous methods are environmentally friendly, simple with particles dispersed in water. However, control over size is not guaranteed. In this work we have synthesized Fe3O4/Au core/shell structures adopting two different routes. The first is based on organometallic precursors in which Fe3O4 NP around 10 nm was done through the thermal decomposition of Fe (acac) at Olyelamine and Oleic Acid at 300οC. The Au shell was then reduced from HAuCl4. The second is water soluble Fe3O4/Au nanoparticles thus synthesized at room temperature starting with FeCl3.6H2O and Ascorbic Acid as reducing agent. Then gold shell was also formed from the reduction of HAuCl4. TEM and UV-Vis were used for structural characterization. XPS is used to examine the complete Au shell formation with the magnetic phase at the core. SQUID magnetometry was used for magnetic measurements. It showed reduction in the saturation magnetization, Ms, with the increase of the gold shell thickness. However, thick Au shell was reported to enhance the system’s durability, a desirable attribute when applications are concerned. The difference in the magnetic properties for the core/shell structures resulting from the two different routes will be discussed.


N10.70
Gold on the Tips of ZnO Hexagonal Pyramids. Ming Yang1, Kai Sun2 and Nicholas A. Kotov1,2,3; 1Chemical Engineering, University of Michigan, Ann Arbor, Michigan; 2Material Sciences and Engineering, University of Michigan, Ann Arbor, Michigan; 3Biomedical Engineering, University of Michigan, Ann Arbor, Michigan.

Hybrid nanoparticles (NPs), constructed from domains of different materials, have been studied with great interest over the past few years. They are technologically advantageous due to the performance of multifunctional tasks based on the specific properties which are characteristic of each domain. Furthermore, the modified properties (optical, electrical and magnetic) of hybrid NPs distinct from their individual components or physical mixture of the two components are usually anticipated which is attributed to the strong electronic structure coupling between different materials such as metal-semiconductor, semiconductor-semiconductor, and metal-magnet. In particular, metal-semiconductor hybrid NPs are technological important regarding the photocatalytic processes due to the tendency of light-induced charge separation and the catalytic properties of the metal islands. Also, the introducing of metal domains can bring opportunities for the preferential attachment of organic molecules or biomolecules which can direct further assembly process. The hybrid structures are usually formed by spherical domains of two different materials or can contain linear or branched nanostructured as one of the components. In the latter case, the anisotropic NPs (nanorods or tetrapods) offer the additional possibility to selectively position the second constituent at a specific location producing the anisotropic hybrid NPs which can be potentially wired onto electrical circuitry under directed self-assembly. Here, we report the synthesis of a new type of hybrid NPs-gold on the tips of ZnO hexagonal pyramids (HPs), which is the first example of anisotropic hybrid NPs with pyramids as components. The preferred growth of gold NPs on the tips of ZnO HPs agrees well with the quantum calculation result of HOMO of ZnO HPs atomic model. The strong interaction between ZnO and gold is indicated by optical property change and electron energy loss spectroscopy (EELS) is further used to study the electron transfer at the interface. Such unique hybrid structures provide a new model system for further investigation into their physical attributes and should be a good candidate for self-assembly.


N10.71
Effect of Solvent Treatment on Solution Processed Nanocrystal Infrared Photodetectors. Galileo Sarasqueta, Kaushik Roy Choudhury and Franky So; Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida.

Inorganic nanocrystalline materials from II-VI and IV-VI groups have shown great promise as candidates for optoelectronic devices due to their simple colloidal synthesis, tunable band gaps, and low cost processing. There have been several attempts to fabricate devices for photo-detection employing nanocrystals in polymer-inorganic hybrid bulk-heterojunction composites or in pure photoactive layers. The polymer based composite devices often have limited absorption range, low carrier mobility, inferior polymer stability, and sensitivities significantly lower than in crystalline semiconductor photodetectors. On the other hand, the photoconductive nanocrystal only devices suffer from slow modulation response and exceedingly high dark currents. In this study we fabricated infrared photodiodes from colloidal nanocrystals and chemically treated the thin films to achieve significantly low dark currents. Here, we employed PbSe nanocrystals due to their tunable absorption in the infrared region. Processing good quality nanocrystal films can be challenging depending on several factors such as the choice of solvent, nanocrystal capping groups, desired thickness, solution concentration, etc. At the same time, charge transport and electrode contact properties of the nanocrystal films also depend on capping groups and processing conditions. Since the electrical properties of the nanocrystal films are determined by the nature of the capping groups, we studied the effect of different nanocrystal capping groups, including oleic acid, octylamine, ethanedithiol, ethanethiol, and benzenedithiol, on the PbSe nanocrystal photodiode electrical characteristics. The spectral responsivity of the photodetector devices and the quantum efficiency were measured. Thiol capping groups were found to enhance the rectification of the photodetectors and, at the same time, significantly reduce the dark current in the devices by effectively passivating the surface traps on the nanocrystals and improving the nanocrystal packing. The responsivity of these devices showed a direct correlation between the dark current and the photo current values, which affects the quantum efficiency of the devices. Furthermore, the physical quality of the films seemed to be very sensitive to different nanocrystal capping groups and to the specific processing conditions under which the capping groups were exchanged.


N10.72
Abstract Withdrawn


N10.73
Step-Wise Synthesis of InP/ZnS Core-Shell Quantum Dots and the Role of Zinc Acetate. SungWoo Kim and SangWook Kim, Molecular Science and Technology, Ajou University, Suwon, Korea, South.

Considerable research has been carried out on colloidal semiconductor nanocrystals(NCs) on account of their size dependent electronic and optical properties, which are the key issues in many applications, such as biomedical fluorophores, LEDs and photovoltaic devices. Among them, a group of II-VI semiconducting NCs e.g. CdSe quantum dots (QDs), exhibit excellent photo-stability, quantum yield (QY), and a tunable emission wavelength. However, CdSe QDs have limited applications owing to their intrinsic toxicity. It is believed that III-V QDs, particularly InP NCs, are the most desirable alternative. However, the photo-luminescent quantum yield tends to be very low due to nonradiative surface recombination sites and high activation barriers for carrier detrapping. A core-shell structure appears to be essential for surface passivation but a limited number have been reported, including InP/ZnS and InP/ZnCdSe core/shell QDs. HF etching and low reaction temperature methods have also been attempted. In particular, the recent reports by Peng’s group on highly luminescent InP/ZnS core-shell quantum dots in the visible range using fatty amine and Reiss on the one-pot synthesis of InP/ZnS are quite impressive. We report the step-wise synthesis of InP/ZnS core-shell quantum dots and the role of zinc acetate during the reaction. Zinc acetate was used as a precursor for zinc and acetic acid. Highly luminescent InP/Zn-palmitate was obtained as an intermediate.


N10.74
Preparation of Monodisperse Nanoparticles by Membrane Emulsification Using Highly Ordered Anodic Porous Alumina. Takashi Yanagishita1,2,3, Yukari Maejima1, Kazuyuki Nishio1,3 and Hideki Masuda1,3; 1Tokyo Metropolitan Univ., Tokyo, Japan; 2JST-PRESTO, Saitama, Japan; 3KAST, Kanagawa, Japan.

The preparation of monodisperse nanopatricles with uniform shapes and sizes has been increasing interest due to the utilization for various types of functional application fields, such as biosensors and drag delivery systems, catalysis and so on. So far, there have been a large number of techniques for preparing monodisperse particles, including liquid phase process, gas phase process, and template process. However, the uniformity in shape and size has not been satisfied in most techniques. Membrane emulsification, in which emulsion droplets are formed by passing liquid through a porous membrane, is one of promising technique for preparing monodisperse emulsion droplets. In addition, monodisperse solid particles can be obtained by solidifying the droplets. In this process, the size of emulsion droplets can be controlled by changing the size of the holes in the porous membrane. In our previous report, we describe the preparation of monodisperse nanoparticles by membrane emulsification using anodic porous alumina [1]. Anodic porous alumina, which is formed by anodization of Al in an acidic solution, is a promising nanohole array material for the preparation of monodisperse emulsion droplets on nanometer scales by membrane emulsification because of its ordered fine structure with cylindrical holes of uniform diameter [2,3]. In the present report, we describe the preparation of monodisperse inorganic nanoparticles by membrane emulsification using ideally ordered anodic porous alumina. In the experiment, monodisperse emulsion droplets of TiO2 sol were prepared by membrane emulsification. Uniform sized TiO2 nanoparticles were prepared by solidifying the droplets of a sol solution. The obtained monodisperse nanoparticles will be applied to several types of functional nanodevices. [1] T. Yanagishita et al., Langmuir, 20, 554 (2004). [2] H. Masuda et al., Science, 268, 1466 (1995). [3] H. Masuda et al., Appl. Phys. Lett., 71, 2770 (1997).


N10.75
Electrical Characterization of Charging Capacitor in Au Nanoparticle Organic Memory Device. Sung-Mok Jung1, Hyung-jun Kim1, Bong-Jin Kim1, Il Seo2, Tae-Sik Yoon2, Yong-Sang Kim2 and Hyun Ho Lee1; 1Chemical Engineering, Myongji Univ., Yongin-si, Gyeonggi-do, Korea, South; 2Nano Science & Engineering, Myongji Univ., Yongin-si, Gyeonggi-do, Korea, South.

Nanoparticle (NP) of semiconductors and metals using monolayer binding with self-assembly chemicals are currently the focus of intense device researches. Electrical properties of such structures can be tailored for particular application, such as memory device. Recently, chemical self-assembly of Au NPs has been reported to show great potential in memory applications. In this study, Au NP and gluing layer were fabricated through a new fabrication of a chemical bonding or gluing. In this study, a new nanpoparticle memory system was fabricated by using organic semiconductor, i.e., pentacene or MDMO-PPV (poly-2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) as the active layer, evaporated Au as electrode, SiNx or dielectric polymer as the gate insulator layer on silicon wafer. In addition, Au NPs coated with binding chemicals were used as charge storage elements on a APTES (3-amino-propyltriethoxysilane) as a gluing layer. In order to investigate chemical binding of Au NP to the gate insulator layer, citrate or GPTMS (3-glycidoxy-propyltrimethoxysilane) were coated on the Au NPs. As a result of that, a layer of gold nanoparticles has been incorporated into a metal-pentacene/polymer-insulator-semiconductor (MPIS) structure. The MPIS device with the Au NP exhibited a hysteresis in its capacitance versus voltage analysis. Charge storage in the layer of nanoparticles is thought to be responsible for this effect.


 

SESSION N11: Metallic Nanoparticles and Superstructures
Chair: Nicholas Kotov
Friday Morning, December 4, 2009
Constitution A (Sheraton)


8:00 AM *N11.1
The ‘Optical Diode’ Response in Plasmonic Heterodimer Nanoparticle Complexes. Naomi J. Halas, ECE Dept.- MS-366, Rice University, Houston, Texas.

In reduced symmetry plasmonic nanoparticles, the plasmon modes that couple to the far field, known as bright modes, are permitted to couple to the nonpropagating, “dark” modes of the nanostructure. This interaction gives rise to a host of interesting coherent phenomena such as sub and superradiance, Fano resonances, and electromagnetically induced transparency (EIT) in the nanoparticle complex. We have developed a method for assembling plasmonic nanoparticle dimers with detuned resonances that give rise to these interesting effects in their optical response. Characteristic of coherent phenomena in plasmonic nanocomplexes is that their resonant behavior is strongly dependent upon the direction of incident light that excites the coupled plasmon oscillators of the nanoscale complex. We examine this “optical diode” property in the context of plasmonic heterodimers and the strong Fano resonant and EIT phenomena characteristic of their linear optical response.


8:30 AM *N11.2
Transferred to *N8.13


9:00 AM *N11.3
Metal Nanoparticle Arrays for Enhanced Absorption and Photo-current in Thin Film Solar Cells. Harry A. Atwater, Applied Physics, California Institute of Technology, Pasadena, California.

Metallic nanostructures can couple sunlight into guided modes, dramatically increasing the optical path length in thin active photovoltaic layers to enhance overall photoabsorption. This has potential for not only cost and weight reduction with thinned layers but also for increased conversion efficiency associated with increased carrier excitation level in the solar cell. We have investigated the design of incoupling structures for plasmonic waveguide-based thin film solar cells, focusing on single subwavelength scatterers. Quantitative full-field electromagnetic simulations were used to calculate the incoupling cross section for various groove-like incoupling structures across the solar spectrum, and to determine the angular dependence of the absorption enhancement effects. We find that the spectral features of the coupling efficiency originate from different resonant phenomena, and the incoupling can be tuned and enhanced through modifications of the scatterer shape, layer thicknesses, and materials choices. We demonstrate that a single 100 nm-wide groove can enhance absorption in angle insensitive manner, and a groove array enhances absorption by a factor of up to 2.5 over a 10 micron area. We have observed short-circuit current and efficiency enhancements under AM1.5G solar spec-trum for thin GaAs solar cells with dense arrays of Ag nanoparticles deposited through porous alumina membrane masks, relative to reference GaAs cells with no metal nanoparticle array. This photocurrent enhancement is attributed to the scattering effects of metal nanoparticles for light incident into photovoltaic layers. A simple optical model representing metal nanoparticle surface plasmon resonances and multi-angle scattering has been developed and well explains the spectral behavior of the experimental photocurrent enhancement. Also, an ultrathin GaAs solar cell structure with a metallic back layer shows significant enhancements in short-circuit current density and efficiency relative to reference GaAs cells with an absorbing GaAs back layer due to a Fabry-Perot resonance in the air/semiconductor/metal heterostructure. We will also discuss recent results for enhancement of absorption and photocurrent in thin film amorphous Si solar cells with nanoscale metallic structures that couple sunlight into guided modes in the thin film cell.


9:30 AM N11.4
Monodisperse Ag-Au Alloy Nanoparticles with Independent Control of Morphology, Composition, Size and Surface Chemistry, and their 3-D Superlattices. Qingbo Zhang and Jim Yang Lee; Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore.

Ag and Au nanoparticles, with their highly customizable optical and electronic and properties, are rich resources for nanotechnology applications. By combining the two metals into a common entity to form alloy nanoparticles, these properties can be further modified through composition tuning, leading to new possibilities in applications and enhancements of application performance. As nanoparticle properties are strongly dependent on the particle attributes, it is imperative to develop synthesis procedures where monodisperse Ag-Au alloy nanoparticles with controllable morphology, composition, size, and surface chemistry may be produced reliably and in large quantities. Herein we report a strategy for synthesizing highly monodisperse Ag-Au alloy nanoparticles. The synthesis uses a step-wise procedure to deliver control of morphology, composition, size and surface chemistry of the nanoparticles in discrete and independent steps. Using this procedure, Ag-Au alloy nanoparticles with two different types of morphologies were produced: truncated octahedral single-crystalline nanoparticles and icosahedral multiply twinned particles (MTPs). For each type of morphology, the composition, size and surface properties of the nanoparticles could be individually varied while keeping the other attributes constant. The successful preparation of monodisperse alloy nanoparticles with independently tunable particle attributes is valuable to the study of the physicochemical properties of the particles as a function of morphology, composition, size and surface chemistry. Due to the uniform morphology and narrow size distribution, all the as-synthesized Ag-Au alloy nanoparticles could easily assemble into 2-D and 3-D superlattices on a substrate upon solvent evaporation. The superlattices were characterized by transmission electron microscopy and electron diffraction. It was found that the single-crystalline Ag-Au alloy nanoparticles in the superlattices have both translational and orientational ordering while the multiply twinned icosahedral alloy nanoparticles only display translational ordering. The packing pattern of the nanoparticles in the superlattices is dependent on the native morphology of the nanoparticles. The 2-D and 3-D superlattices may be used to study the effects of the close coupling of proximal alloy nanoparticles on application performance.


9:45 AM N11.5
A Tunable Nanoparticle Ionic Fluid with Reversible Plasmonic Response. Rama R. Bhattacharjee2, Ruipeng Li1,2, Emmanuel P. Giannelis2 and Aram Amassian1; 1Materials Science and Engineering, King Abdullah University of Science and Technology, Jeddah, Saudi Arabia; 2Materials Science and Engineering, Cornell University, Ithaca, New York.

Nanoparticle ionic materials (NIMs) are a new class of hybrid organic-inorganic fluidic materials consisting of a hard inorganic nanoparticle corona and a soft organic canopy. NIMS possess high nanoparticle content, zero vapor pressure, as well as inexpensive and green mass production capabilities. Their design provides multiple avenues for tuning properties of nanomaterials. In this work we show that step-wise surface modification of gold nanorods (GNRs) by simple aqueous solution-based assembly and acid-base chemistry can result in the formation of a solventless, GNR-based plasmonic fluid. These fluidic materials show dynamic and reversible color changes upon mechanical shearing, which are visible to the naked eye in ambient lighting. The color changes and their time scale can be tuned by changing the aspect ratio of the GNRs as well as via plasmon coupling by controlling interparticle interactions and arrangements in the fluid matrix. NIMS samples consisting of GNR assemblies in the matrix exhibit a reversible plasmonic response under mechanical actuation, whereas samples consisting of isolated GNRs exhibit no changes. These observations were confirmed by a combination of transmission electron microscopy, time-resolved small-angle X-ray scattering, and time-resolved spectrophotometry. NIMS-based fluids appear to be a new class of plasmonic materials based on one-dimensional metal nanostructures.


10:15 AM *N11.6
Silver Nanoparticles with Broad Multi-Band Linear Optical Absorption. Osman Bakr and Francesco Stellacci; Materials Science and Engineering, MIT, Cambridge, Massachusetts.

The optical and electronic properties of metal nanoparticles (NPs) depend on the particles’ size, shape, surrounding medium, and aggregation state. Silver clusters (up to ~8 atoms) have molecule-like optical transitions with absorption bands that depend on the number of atoms that compose the cluster and with a bright fluorescence emission. Here we will show that it is possible to create aryl thiol coated silver NPs that have less than 100 atoms in their ligand shell is a reproducible way. These particles show intense and broad non-plasmonic optical properties, with eight distinct absorption bands covering the entire visible spectrum with extinction cross-sections as high as 2.59 x105 M-1 cm-1. They have a predominant size (d~1.3 nm ) and a highly defined structure in their optical spectra. We call these particles Intensely and Broadly Absorbing Nanoparticles (IBANs) because they have a larger cross-section than conventional organic dyes and inorganic QDs, and cover a broader range of wavelengths, their absorption bands covering the range from 380-850 nm. These properties make IBANs ideal candidates for light harvesting applications.


10:45 AM *N11.7
Light-controlled Nanomaterials. Bartosz A. Grzybowski, Chemistry and Chemical Engineering, Northwestern University, Evanston, Illinois.

The talk will discuss nanoparticle-based materials whose properties and controlled by light. One example will deal with systems in which local irradiation drives dynamic self-assembly of metal nanoparticles (NPs) embedded in an organogel matrix. Formation of metastable aggregates over regions exposed to light causes color change therein and provides the basis for writing color images into the material. In the absence of light, however, the aggregates spontaneously disassemble and the images self-erase with time constant that can be regulated by NP functionalization. In the secon class of systems I will discuss, nanoparticle-based materials change their conductivity upon irradiation with light corresponding to the NPs' plasmon band. Remarkably, depending on the chemical functionalization of the NPs, the materials can exhibit both positive and previously unknown inverse photoconductance. These effects are due to the formation of polaron states in the material, and can be rationalized by a charge-trapping model. Background literature: 1. Angew. Chem. Int. Ed. 2009. DOI: 10.1002/anie.200901119 2. Nature, 460, 371-375, 2009.


11:15 AM N11.8
Novel Gemini Surfactant-assisted Self-assemblies of Gold Nanorods. Andres Guerrero-Martinez and Luis M. Liz-Marzan; Departamento de Química Física, Universidade de Vigo, Vigo, Spain.

The self-assembly strategy applied to nanoparticle building blocks is a potential starting point for the amplification of the individual components properties and/or the generation of new characteristics unique to the ensemble1. In recent years, substantial well deserved progress has been made in the fabrication of ordered metal nanocrystal assemblies on surfaces, as their nanoscale organization can be manipulated to produce a diverse range of topologies with interesting optical and electrical properties2. Usually, gold nanoparticles employed for these ensembles are spherical and lack a geometrical preference toward self-assembly because of the ease of processing limiting their potential applications to form directional lattices. By contrast, controlled self-assembly of non-spherical gold nanoparticles, such as gold nanorods (NRs), enables these arrays to undergo defined 1D, 2D, or 3D structures with a vectorial dependence of the desired properties. One area where gold NRs are used to build macroscopic devices is in plasmonics because they can be synthesized with tailored optical properties and assembled into optoelectronic devices to manipulate the transfer of light on the nanoscale3. Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research4. In particular, seeded growth method in aqueous solution, based on the use of surfactants as shape inducing agents, provides sufficient flexibility to synthesize NRs with diverse sizes, shapes and functions. We will show herein that the directional self-assembly strategy of NRs can be controlled by using non-conventional surfactants made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, so-called dimeric or gemini surfactants5, with high tendencies to adsorb hydrophilic surfaces mimicking the bilayer formation of lipids. Thus, we take advantage of all these amphiphilic properties, together with the ability of nanocrystals to register oriented liquid-crystalline phases6, to synthesize highly monodisperse NRs that build up novel gemini-assisted standing 2D and 3D superlattices with anisotropic optical properties. 1. E.V. Shevchenko, D.V. Talapin, N.A. Kotov, S. O Brien, C.B. Murray, Nature 439, 55 (2006). 2. M.-C. Daniel, D. Astruc, Chem. Rev. 104, 293 (2004). 3. R. Zia, J.A. Schuller, A. Chandran, M.L. Brongersma, Mater. Today 9, 20 (2006). 4. J. Pérez-Juste, I. Pastoriza-Santos, L.-M. Liz-Marzán, P. Mulvaney, Coord. Chem. Rev. 249, 1870 (2005). 5. R. Zana, Y. Talmon, Nature 362, 228 (1993). 6. T. Ming, X. Kou, H. Chen, T. Wang, H.-L. Tam, K.-W. Cheah, J.-Y. Chen., J. Wang, Angew. Chem. Int. Ed. 47, 9685 (2008).


11:30 AM N11.9
Corona Dynamics in Nanoparticle Liquids. Michael L. Jespersen1,2, Peter A. Mirau1, Richard A. Vaia1, Robert Rodriguez3 and Emmanuel P. Giannelis3; 1Nanostructured and Biological Materials Branch, Air Force Research Laboratories, Wright-Patterson AFB, Ohio; 2National Research Council, Washington, District of Columbia; 3Department of Materials Science and Engineering, Cornell University, Ithaca, New York.

Nanoparticle liquids (NPLs) consist of a core inorganic nanostructure and a covalently-grafted ionic liquid corona. These materials exhibit liquid-like behavior at room temperature while preserving the unique optical and electronic properties of the core nanostructure. The combination of liquid behavior and tunable structural parameters of core nanostructures (including size, shape, and corona chemistry) allow NPLs to be tailored for a wide range of specific applications. For example, in our laboratory, gold and platinum NPLs have been used as conductive surface lubricants in MEMS contacts, significantly improving the durability of those surfaces. In addition, these materials are promising candidates for components in nanoparticle-based optoelectronic devices due to their processability. Still, corona dynamics in nanoparticle liquids remain poorly understood, hindering the development of design rules for tuning NPL properties. In recent work, we have used pulse field gradient NMR to investigate the diffusion behavior of silica nanoparticle liquids. The results from these studies have shown that the presence of the core nanostructure does not greatly affect corona dynamics. The relaxation and diffusion behavior of the bound ionic liquid is very similar to that of the free ionic liquid. The bulky ionic canopy of the NPL exists in a liquid-like state, undergoing rapid exchange between nanoparticles. This is similar to the case of ionic liquids, where a single ion pair exists only as a transient species.


11:45 AM N11.10
Silver/polymer Nanocomposite Microspheres for Ultra Molecular Detection via Surface Enhanced Raman Scattering (SERS). Abdiaziz Farah1,2, Sheng Dai2, Hicham Fenniri1,2, Juan pablo Bravo-Vasquez2 and Jae-young Cho2; 1Chemistry, University of alberta, Edmonton, Alberta, Canada; 2National Institute for Nanotechnology, Edmonton, Alberta, Canada.

Polymeric microspheres containing nanometer size metal particles are promising, since they can exhibit unique optical,electronic, catalytic and magnetic properties. In particular, spherical nanocomposites with thin metal coatings are especially interesting, in the view of their unusual optical properties. For example, the plasmon resonance of the metal-coated nanocomposites can be fine-tuned to different wavelength of interest by choosing the core size, layer thickness and coating material, due to the coupling of surface plasmons at the interfaces of metal shells. Various methods have also been reported for the preparation of metal nanostructured substrates, especially silver for Surface-Enhanced Raman Scattering (SERS) applications. However, the development of a solution polymerization process that could provide hybrid nanocomposites with discrete particles uniformly dispersed throughout the host polymer and with the desired surface characteristics has long been elusive. Herein we report an effective method that describes the preparation of tailored microspheres in which reactive epoxide functionalities are exclusively at accessible sites, allowing a vast number of transformation reactions, less reaction time and non tedious purification steps. We also envisaged a facile and rapid approach for fabricating ultrasensitive Surface-Enhanced Raman Scattering (SERS) substrate by in situ depositing silver NP aggregates onto the surface of these solid polymeric supports. The microspheres were used solely as a support because of their large surface area and mechanical stability to provide maximum metal deposition, and hence the creation of nanostructured metallic surfaces, essential for SERS phenomena. Ultra molecular detection capability of these SERS active nanocomposite microspheres were tested and thoroughly characterized by combination of (x-ray photoelectron spectroscopy (XPS), X-ray dispersion spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Raman and Surface Enhance Raman Spectroscopy (SERS) in details to elucidate their morphology, particle size, composition, thermal stability and their optical and electronic properties.


 

SESSION N12: New Methods in Nanoparticles and Devices
Chair: Jonathan Steckel
Friday Afternoon, December 4, 2009
Constitution A (Sheraton)


1:30 PM *N12.1
Rationally Designed Nanostructures Fabricated by On-Wire Lithography. Chad A. Mirkin, Kyle Osberg and Matthew Banholzer; Northwestern University, Evanston, Illinois.

On-Wire Lithography (OWL) is a powerful tool in the synthesis of one-dimensional colloidal nanostructures with positive and negative architectural features along the lengths of the structures. By taking advantage of selective etch chemistries, OWL enables one to rationally introduce lithographic features into multisegmented nanowires to create novel nanostructures with metal segments separated by gaps that are predefined by the lengths of the etched sacrificial layers. Taking advantage of the precise control afforded by OWL with respect to gap lengths (2 nm to many micrometers), metal segment lengths (50 nm to many micrometers), and composition (Au, Ag, Pt, etc.), many interesting functional materials can be imagined. Of the materials and applications already realized, molecular electronics and materials based on surface-enhanced Raman scattering (SERS) are the two most impacted areas of research. For molecular electronics, precise and reproducible control of gap size down to 2 nm has led to the development of OWL as a promising methodology in the fabrication of molecular transport junctions (MTJs) for the fundamental study of the electrical properties of molecules. While other methods of fabricating molecular transport junctions are known, OWL is unique in its versability and combination of reproducibility and high-throughput. In addition to MTJs, closely spaced noble metal disk arrays can also be fabricated, leading to strongly enhancing SERS “hotspots” can be fabricated by OWL. Fundamentally, the precision and control of the distance between metal components inherent to OWL has led to new insights into the interactions between plasmonic nanostructures over various distances. These hotspots have led to several interesting applications, such as a covert encoding scheme based on the geometry and chemistry of hotspot arrays, spectroscopic tracking of the formation of a molecular wire across a nanogap, and novel detection schemes for DNA and other bioactive molecules.


2:00 PM *N12.2
Memory and Memristor Devices Based on Zinc Oxide Nanoparticles. Jianpu Wang and Neil C. Greenham; Department of Physics, University of Cambridge, Cambridge, United Kingdom.

We will describe two types of device based on thin films of ZnO nanoparticles sandwiched between electrodes. The first uses simple films of ligand-coated ZnO nanoparticles between ITO and Al electrodes. These devices show strong electrical hysteresis, history-dependent conductance, and sweep-rate-dependent current-voltage curves, characteristic of memristive behavior. The resistance can be modified between ~1 and ~104Ω cm2. We suggest that the memristive switching in our devices is associated with a change of the interfacial injection barrier caused by the movement of oxygen vacancies combined with the adsorption of molecular oxygen. The second type of device uses ZnO nanoparticle films deposited on a PEDOT:PSS electrode. The devices initially show high conductance, but switch permanently to a low-conductance state when driven beyond a certain point in the current-voltage curve. The ZnO layer suppresses hole currents in the devices, and the conductance change is associated with injection of electrons into the PEDOT:PSS, giving permanent dedoping. The power required to “write” a device is less than 0.1 W cm-2, much lower than previous write-once read-many-times memory (WORM) devices based on PEDOT:PSS. The devices are therefore suitable for memory in RFID applications.


2:30 PM N12.3
Colloidal Crystal of ZnO Quantum Dots. Xi Zhang, Dazhi Sun and Hung Jue Sue; Materials Science and Engineering, Texas A&M University, College Station, Texas.

Monodisperse ZnO quantum dots (QDs) of a particle size of about 5 nm have been synthesized by adding zinc acetate methanol solution into KOH/methanol solution. Isopropanol together with hexane were utilized to precipitate ZnO nanoparticles in the solution to a condensed phase. The morphology of the condensed ZnO precipitate ranges from white flocculation, gel-like fluid to transparent solid under different ion concentration. The polycrystalline structure in the transparent solid phase of ZnO QDs was characterized by UV-vis spectrum, small-angled X-ray scattering (SAXS), field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). The results suggest that interparticle potential can be adjusted by tuning the ionic strength. Crystallization occurs when the attractive potential is comparable to thermal energy. The procedure of ZnO QDs getting close-packed was monitored via UV-vis spectrum. Bandgap energy of ZnO increased when QDs get closer. The 373 nm absorption onset of the dried crystal is the same as that of bulk ZnO. The colloidal crystal is transparent in the range of visible light and absorbs UV light efficiently. Both TEM and SEM provide the evidence that the ZnO QDs are close-packed in the transparent solid. SAXS pattern shows multiple diffraction peaks, indicating that the material is polycrystalline. To our best knowledge, this is the first demonstration of the ordering process of nanoparticles without the existence of surfactants. The present work is of particular significance for the studying and modeling of the formation of colloidal crystal of hard spheres in nanometer scale. Application of this novel material, such as solar cell manufacturing and UV-shielding, will be discussed in the lecture.


3:15 PM N12.4
Hybrid Organic/Inorganic Infrared Photodetectors. Kaushik Roy Choudhury, Galileo Sarasqueta and Franky So; Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida.

Colloidal inorganic nanocrystal quantum dots are emerging as a viable materials platform for solution-processed optoelectronic applications, especially those requiring detection and/or emission of light in the infrared (IR) part of the electromagnetic spectrum, owing to their broad and tunable spectral response and the paucity of IR-active organic materials. There have been two main approaches to fabricate devices for photo-detection in the IR using semiconductor quantum dots: dispersing them in organic semiconducting matrices (typically polymers) and using a neat film of quantum dots as the photoconductive layers. In the former approach the quantum dots act as photosensitizers while charge transport proceeds primarily via the host matrix. In this case, the inferior transport properties of polymers strongly limit device performance. On the other hand, the photoconductive quantum dot devices suffer from slow response and exceedingly high dark currents due to the narrow bandgap of the materials employed to achieve photoactivity at longer wavelengths. In this work, we fabricate near-IR photodetectors exhibiting broad spectral sensitivity from visible to near-IR wavelengths up to 1600 nm. The solution-processed active layer consists of PbSe quantum dots dispersed in the electron accepting organic semiconductor {6,6}-phenyl-C61 butyric acid methyl ester (PC61BM), sandwiched between ITO and aluminum electrodes. The IR photodiodes exhibit significantly low dark currents and excellent rectification ratios over the whole range of operation. The photodetection efficiency at IR wavelengths was optimized as a function of quantum dot concentration in the PCBM matrix. The hybrid devices yield responsivities of 100 and 250 mAW-1 at 1300 nm and 850 nm, respectively. Subsequent to photoexcitation, the band offset in the device promotes ultrafast electron transfer from the conduction band (-4.22 eV) of the PbSe quantum dots to the LUMO of PCBM (-4.2 eV) leaving holes trapped in the quantum dots. This process of electron transfer is sensitive to the properties of the capping ligand on the surface of the quantum dot. Therefore, in order to optimize the performance of the devices, the capping ligand on the PbSe quantum dots was changed from the original bulky oleic acid to shorter-chain octylamine and pyridine. This modification, while maintaining the same order of dark current, led to a greater than three-fold improvement in device responsivity at 1300 nm, reaching a value of 325 mAW-1.


3:30 PM N12.5
Colloidal Synthesis and Characterization of SnSe Nanocrystals. William Baumgardner1,2, Joshua Choi1,3, Rene Claus1,4 and Tobias Hanrath1; 1Chemical and Biomolecular Engineering, Cornell University, Ithaca, California; 2Chemistry and Chemical Biology, Cornell University, Ithaca, California; 3Applied and Engineering Physics, Cornell University, Ithaca, California; 4Electrical and Computer Engineering, University of California, San Diego, La Jolla, California.

William Baumgardner, Joshua Choi, Rene Claus, Tobias Hanrath IV-VI semiconductors have recently been receiving a significant amount of attention for their possible uses in a variety of optoelectronic applications, most prominently as building blocks for the next generation of high-efficiency, low-cost solar cells. Though excitement surrounding these materials is growing, little progress has been made on SnSe nanoparticles. SnSe is a p-type semiconductor with many qualities desirable for photovoltaics, including a bulk band gap of 0.9 eV, large absorption coefficients, and high carrier mobility. Importantly, unlike many of the IV-VI semiconductors, SnSe is an environmentally benign and non-toxic compound. Taken together, these properties provide a strong motivation for the development of methods for the colloidal synthesis of high-quality SnSe nanocrystals. We present a method for colloidal synthesis of size-tuned, monodisperse, high-quality SnSe nanocrystals. We present a comprehensive comparison of synthetic parameters including the concentration and nature of the precursor, solvent effects, synthesis-time and -temperature, as well as cation exchange reactions with pre-fabricated lead chalcogenide nanocrystals. We measured absorbance and photoluminescence of colloidal suspensions and nanocrystal films to study size-dependent optical properties. Crystallographic properties of the nanocrystals were probed through a combination of x-ray diffraction and transmission electron microscopy and revealed that the nanocrystals are of the orthorhombic D2h space group. Lastly, we investigated the influence of SnSe nanocrystal surface chemistry on charge transport in thin nanocrystal films.


3:45 PM N12.6
Controlled Synthesis of III-V Quantum Dots in Microfluidic Reactors. Adrian M. Nightingale and John C. de Mello; Chemistry, Imperial College London, London, United Kingdom.

Colloidal quantum dots are of scientific and commercial importance due to their tuneable physicochemical properties and their wide-ranging applications in optoelectronics and bioanalysis. The physical properties of quantum dots are determined primarily by spatial confinement effects, requiring the use of well-controlled size- and shape-selective synthesis methods to obtain particles with tightly defined properties. The majority of quantum dot research to date has focused on II-VI systems such as CdSe due to their comparative ease of synthesis. III-V materials, whilst potentially less toxic, have historically presented a greater synthetic challenge[1]. The recent introduction of new synthesis routes, however, based on non-coordinating solvents (which decouple the roles of solvent and stabiliser and enable the precursor reactivity to be tuned via the ligand concentration) have led to a step-change in III-V synthesis; making it practicable to synthesise high quality materials with quantum yields comparable to II-VIs using simple single-step one-pot procedures that can be completed on a time-scale of minutes rather than days[2-4]. In parallel with the above developments, microfluidic reactors have recently emerged as near-ideal systems for the growth of high quality colloidal nanoparticles due to the unprecedented control they offer over reaction conditions (e.g. reagent composition, mixing kinetics, and reaction temperatures)[5]. Initially used to synthesise nanocrystalline CdS, they have since been successfully applied to a diverse range of metal, metal-oxide and compound semiconductor nanoparticles, including Au, Co, Cu, TiO2, Si, Pd and CdSe[5]. In virtually all cases, microfluidic procedures were found to offer clear advantages over bulk synthesis methods, most notably in the ability to fine-tune the physical properties of the final product. Surprisingly there have as yet been no successful reports of III-V quantum dot synthesis in a microfluidic environment, which is perhaps due to the poor quality of particles obtained from many of the early synthesis routes. In this work, we modify a recent non-coordinating solvent based procedure to enable easy controlled synthesis of high quality InP quantum dots in microfluidic devices. The devices we use here, whilst simple in structure, provide a remarkable level of control over the particle properties, allowing the spectral characteristics to be tuned with ease. The successful adaptation of microfluidic reactors to III-V materials synthesis opens up significant opportunities in optical engineering, offering a well controlled and fully automated route to the on-demand production of high quality quantum dots. 1. Green, M. Curr. Opin. Solid St., 2002. 6(4): p. 355. 2. Battaglia, D. and X.G. Peng. Nano Lett., 2002. 2(9): p. 1027. 3. Xie, R., et al. J. Am. Chem. Soc., 2007. 129(50): p. 15432. 4. Xu, S., et al. J. Mater. Chem., 2008. 18(23): p. 2653. 5. Song, Y.J., et al. Small, 2008. 4(6): p. 698.


4:00 PM N12.7
Colloidal Synthesis and Surface Chemistry of Infrared-Emitting Germanium Nanocrystals. Doh C. Lee, Istvan Robel, Jeffrey M. Pietryga, Richard D. Schaller, Donald J. Werder and Victor I. Klimov; Los Alamos National Laboratory, Los Alamos, New Mexico.

The potential transformation from indirect-to-direct gap behavior in nanocrystals of Group IV semiconductors [silicon or germanium (Ge)] makes such nanostructures attractive for their use in optoelectronic devices. Previous syntheses of these nanocrystals have largely relied on physical methods (e.g., molecular beam epitaxy, laser ablation, or plasma synthesis) or high-temperature route (e.g., supercritical fluid synthesis), and photoluminescence from the resulting particles, when measurable, has shown widely varying size dependence. A number of such studies attributed visible emission from Ge nanostructures to band edge states under strong quantum confinement, even though, as a whole, the data collated from different groups demonstrate very weak size dependence. Because of the potential for emission from oxidized surface states, it is suggested that the surface of Ge must be effectively protected from oxidation to observe the band edge photoluminescence in Ge nanocrystals. Colloidal synthesis of Ge nanocrystals is a promising approach to good surface protection with the use of the right capping ligands. In this work, we synthesized Ge nanocrystals that showed infrared photoluminescence with quantum yields as high as 8%. We also observe a strong correlation between resistance to oxidation and favorable luminescence properties, which indicates the importance of surface protection. The synthesized nanocrystals also demonstrate size-dependent band gaps; infrared emission peaks and absorption onsets from the nanocrystals showed a size trend close to that of theoretical predictions made by other groups. Spectroscopic analysis of visible and infrared photoluminescence will be discussed in the context of emerging understanding of carrier recombination pathways in Ge nanocrystals.


4:15 PM N12.8
Abstract Withdrawn

 

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