The controlled realization of one-dimensional (1D) nanostructures, e.g. silicon nanowires (SiNWs) has opened up new options for device applications in electronics, optoelectronics, thermoelectronics, photocatalysis, photovoltaics and sensing. Key issues for all device concepts based on SiNWs in addition to the crystal structure, dopant concentrations and impurity levels are geometric properties such as aspect ratio, pitch, alignment of SiNW with respect to the substrate etc. and the interfacial properties between the SiNW and the substrate as well as between the SiNW core and wrap-around coatings.
Large-area aligned SiNW arrays are fabricated on Si wafers and multi-crystalline layers on glass substrates via metal-catalyzed wet chemical etching (WCE) or dry etching processes with or without the use of densely packed well organized polystyrene (PS) spheres as a mask. The diameter, length, packing density, and even the shape of SiNWs could precisely be controlled and tuned by adjusting either plasma etching duration or chemical etching conditions and the diameter and pitch of the PS spheres. The anti-reflective properties of SiNWs and thus the extremely high absorption in thin SiNW layers are essential for NW based next generation solar cells. Several cell concepts with SiNWs are realized including most interesting ones:
(i) a hybrid organic/inorganic solar cell using SiNWs as absorber and PEDOT:PSS as a conducting polymer.
(ii) a semiconductor-insulator-semiconductor (SIS) cell concept with SiNWs as absorbers and a tunneling barrier for charge carrier separation. The thin tunneling oxide is Al2O3 with a thickness of only a few Å and a transparent conductive oxide (TCO - here: Al:ZnO) are both grown conformally around the SiNWs using atomic layer deposition (ALD).
The first solar cell prototypes of 1-2cm2 area on glass substrates reached (i) open-circuit voltage of 625 mV, a short-circuit current density of 20 mA/cm2 and efficiencies >7% and (ii) an open-circuit voltage of 550 mV, a short-circuit current density of 33 mA/cm2 and efficiencies >10%.

Piezoresistance is the change in electrical resistivity of a solid with an applied mechanical stress. In crystalline semiconductors such as silicon, an applied stress changes the electronic band structure, principally modifying the transport effective mass of the charge carriers. In 2006 a giant piezoresistance, up to 100 times larger than that observed in bulk silicon, was reported in bottom-up grown silicon nanowires [1] with diameters ranging up to 100 nm. It was later noted that the apparently giant effect occurred in nanowires that were partially depleted of charge carriers and an electrostatic, or “piezopinch”, description of the effect was developed [2]. In this model, the applied mechanical stress is presumed to modify the total surface charge via a change in the electronic structure of trap states. Charge neutrality dictates that any such change will also modify the free charge density within the wire and in thin, lightly doped wires where the Debye screening length is large compared to the wire diameter, this results in a change in the wire conductivity. A stress-induced linear shift in the peak surface density of states will result in an exponential change in the wire conductivity [2].
In order to test these notions we investigated the piezoresistance of a large number of top-down fabricated silicon nanowires, nanoribbons and microwires, all of which have cross-section dimensions smaller than or comparable to the Debye length. Of the nominally n-doped, p-doped and unintentionally doped samples studied, without exception the measured piezoresistance is always that of bulk silicon [3]. Moreover when a voltage is applied to these nano- and micro-structures, a stress-independent current drift resulting from the charging and discharging of surface trap states is observed. This drift is much larger than the true piezoresistance and yields signals that strongly resemble those initially claimed as being due to the application of an external mechanical stress [1].
Regarding the veracity of the giant piezoresistance, one of two possible conclusions can be drawn; either the reported giant effect was an artifact of the measurement, or the properties of bottom-up grown nanowires are fundamentally different from top-down wires. One possibly important difference between the two, which we are now investigating, is the presence of significant quantities of gold in bottom-up grown wires, and on their surfaces. I will present our on-going work on bottom-up silicon nanowires, as well as on two-dimensional silicon structures into which gold has been artificially introduced.
[1] R.R. He and P. Yang, Nature Nanotechnology 1, 42 (2006)
[2] A.C.H. Rowe, Nature Nanotechnology 3, 312 (2008)
[3] J.S. Milne, A.C.H. Rowe, S. Arscott and Ch. Renner, Phys. Rev. Lett. 105, 226802 (2010)

A surface is essentially an abrupt termination of the translational symmetry of the crystal and hence, the surface atoms are less coordinated than atoms in the bulk. In a nano-scale material, the influence of atoms at the surface and near surface region begins to dominate all aspects of its behavior. In this study, silicon nano-wires (SiNWs) fabricated top-down from Silicon-on-Insulator wafers are used to explore surface effects on the mechanical, electrical and electro-mechanical properties. Crucial to this study is the development of a versatile lab on-chip nano-mechanical loading technique [1], sufficiently equipped to deform freestanding SiNWs, while simultaneously permitting investigations of the carrier transport under large strains. The Young&’s modulus of Si was observed to unexpectedly decrease from 160 GPa down to 96 GPa, when varying the thickness of silicon from 200 down to 30 nm. The fracture strain increases when decreasing the volume of the test specimen to reach 5% in the smallest samples.
In order to proceed with characterizing its electro-mechanical properties, it is necessary to understand the transport properties of SiNWs. The transport characteristics of SiNWs rarely conform to expectations from geometry and dopant density, exhibiting significant variations as a function of different surface terminations/conditions. The association of these mechanisms with surface states and their exact influence on practical SiNW devices still remain largely unclear. Herein, we show that the SiNW conductance can vary by up to ~ six orders of magnitude, while comparing its characteristics for the three most probable surface conditions: native oxide, thermal oxide and HF induced H-terminations. These results emphasize the necessity to interpret the transport characteristics of SiNWs with respect to its surface condition, during investigations pertaining to the physical properties of SiNWs, like piezo-resistance [2]. The piezo-resistance of SiNWs have been investigated under large uniaxial tension up to fracture. Reduction of resistance up to a factor of 5.8, higher than the theoretical prediction of 4.5 [3] is reported, without any sign of saturation. The influence of impurity concentration, surface depletion and inversion layers, on the SiNW piezo-resistance is also presented.
[1] U. Bhaskar, V. Passi, S. Houri, E. Escobedo-Cousin, S. H. Olsen, T. Pardoen, J.-P. Raskin, Journal of Materials Research 2011, 27, 571-579.
[2] R. He, P. Yang, Nature Nanotechnology 2006, 1, 42-46.
[3] Y. Sun, S. E. Thompson, T. Nishida, Journal of Applied Physics 2007, 101, 104503.

Porous aggregates of nanoparticles have emerged as one of the most important architectures for solar cells, fuel cells, batteries, and water-splitting devices. The complexity of these materials, as exemplified by the huge number of unique interfaces within a single nanoparticle aggregate, has frustrated attempts to identify the relationships between structure and electronic properties, a knowledge of which are crucial for improving device performance. In the present work, we examine nanoparticle-based iron oxide (hematite) electrodes, which are of particular interest for solar water splitting. We address the challenge imposed by material complexity by combining a novel transmission electron microscopy (TEM) method with conducting atomic force microscopy (C-AFM). The TEM approach determines the relative orientation of adjacent nanocrystals within a large number of nanoparticle aggregates. Meanwhile, C-AFM provides information about the size, shape, and charge transport characteristics of distinct regions within each aggregate. By correlating TEM and C-AFM analyses, we deduce how nanocrystalline structure influences charge transport across grain boundaries and how and whether electronic percolation networks form. While such correlations have been made previously on crystals that are hundreds of nanometers to microns in size, the real advance in our approach is that we can obtain meaningful information from particles as small as 5 nm within complex nanoparticle aggregates. This breakthrough in characterization allows the influence of grain boundaries on electrical transport in nanoparticle aggregates to be identified. With this approach, we successfully identify the structural characteristics of nanoparticle aggregates that provide the highest photocurrent for water splitting. Indeed, when we build electrodes from these “champion” nanostructures, the electrodes achieve the highest photocurrent for water splitting of any metal oxide photoanode yet reported.

A. J. Standingdagger;, L. Gaodagger;, A. Assalidagger;,yen;, J. E. M. Haverkortdagger;, E. P.A.M. Bakkersdagger;,yen;,
dagger; Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
yen; Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
Nanowires are a promising platform for the realization of efficient solar cells1. In addition, due to the large surface area of a nanowire array, the system may be ideal for the photoelectrochemical production of biofuels, like hydrogen2. Here, Gallium phosphide (GaP) nanowires were studied for the purpose of photoelectrochemical water splitting. Vertically aligned p-doped nanowire arrays have exhibited >10% external quantum efficiency for the production of molecular hydrogen, for more than 20 hours. The photocurrent at positive potentials was higher than that observed for planar p-GaP, due to reduced carrier recombination in the nanowires. It is important to optimize the carrier concentration and the surface chemistry of the nanowires, to enhance the performance of these devices.
To maximize the photoelectrochemical current, and therefore the molecular hydrogen production, several factors were studied. The molar fraction dopant material during the growth was varied, to change the dopant density and therefore manipulate the mobility of charge carriers3. The growth time was changed in order to study the effect of nanowire length and possibly decrease series resistance4. These factors were investigated using cyclic voltammetry, allowing the photocurrent, as well as open circuit potential and fill factor to be studied.
The surface of the nanowires was modified after growth by several methods. Etching, to remove any layer growth that may have occurred, was performed. Catalysts were electrochemically deposited on the surface of the nanowires, in an attempt to boost efficiency. And finally the gold growth catalyst was removed after growth, as the gold catalyst has been shown to reduce the optical quality of wires5, so may also cause an effect on the electrochemistry. These experiments were studied by XPS and SEM, as well as cyclic voltammetry to study more deeply the effect on the surface of the nanowires.
References:
1. Garnett, E. C.; Brongersma, M. L.; Cui, Y.; McGehee, M. D.; Annual Review of Materials Research
Vol. 41, (2011), p269
2. Oh, I.; Kye, J.; Hwang, S.; Nano Lett., vol 12 (1), (2012), p298
3. Kaiblinger-Grujin, G.; Kosina, H.; Kopf, Ch.; Selberherr, S.; Materials Science Forum Vols. 258-263 (1997), p939.
4. Baek, R. H.; Baek, C. K.; Jung, S. W.; Yeoh, Y. Y.; Kim, D. W.; Lee, J. S.; Kim, D. M.; Jeong, Y. H.; IEEE Transactions on Nanotechnology, vol. 9, No. 2, March 2010, p212.
5. Ramin, Y.; Rasat, M. M.; Journal of Solid State Chemistry, Volume 183, Issue 7, p1733.

Nanocrystalline solids have become the subject of intense study due to their unique optical properties and their capacity to form self-assembled superlattices. These properties make them suitable for use in a variety of applications such as solar cells, light-emitting devices, transistors, etc. Chalcogenide semiconductor nanocrystals are invariably "capped" by organic ligand molecules which are believed to drive the self-assembly and stabilize the final superlattice. Despite the importance of ligand-ligand interactions, there is very little fundamental understanding of their role in structure-direction.
In this study, we investigate the role of shape and size of nanocrystals on the electronic energy states, which are critically important to their performance as photovoltaic solar cells. While shape is believed to play a role in defining the properties of nanocrystals, the definition of this role is essentially unexplored. Using ab initio simulation approaches, we show how surface ligands play an important role in modifying the electronic properties of small nanocrystals. We also provide the electronic properties of nanocrystals coupled through organic linker molecules; understanding these properties is an important milestone for helping experimentalists to optimize the choice of ligand or linker molecule.
We begin with a study of the surface reconstructions on the nanocrystal and the binding of ligands to the surface of the reconstructed nanocrystal. This foundation leads to a study of the electronic coupling between two nanocrystals and the determination of electron and hole transfer rates between them. We calculate the charge coupling and transfer rates for bare nanocrystals of different shapes and also for ligand-clad nanocrystals to study the effect of ligands on the charge transport. We also calculate the electron and hole mobility in FCC and BCC lattices to compare charge transport in different superlattice symmetries.

Extremely long electron and nuclear spin coherence times have recently been demonstrated in isotopically pure 28Si [1,2] making silicon one of the most promising semiconductor materials for spin based quantum information. The two level spin state of single electrons bound to shallow phosphorus donors in silicon in particular provide well defined, reproducible qubits [3] and represent a promising system for a scalable quantum computer in silicon. An important challenge in these systems is the realisation of an architecture, where we can position donors within a crystalline environment with ~20-50nm separation, individually address each donor, manipulate the electron spins using ESR techniques and read-out their spin states.
We have developed a unique fabrication strategy for a scalable quantum computer in silicon using scanning tunneling microscope hydrogen lithography to precisely position individual P donors in a Si crystal [4] aligned with nanoscale precision to local control gates [5] necessary to initialize, manipulate, and read-out the spin states [6]. During this talk I will focus on demonstrating electronic transport characteristics and single-shot spin read-out of precisely-positioned P donors in Si. Additionally I will report on our recent progress in performing single spin rotations by locally applying oscillating magnetic fields and initial characterization of transport devices with two and three single donors. The challenges of scaling up to practical 2D architectures will also be discussed.
[1] M. Steger et al., Science 336, 1280 (2012).
[2] A.M. Tyryshkin et al., Nature Materials 11, 143 (2012).
[3] B.E. Kane, Nature 393, 133 (1998).
[4] M. Fuechsle et al., Nature Nanotechnology (2012).
[5] B. Weber et al., Science (2012).
[6] S. Mahapatra et al., Nano Letters (2011).

The ability to form well controlled doping profiles at the nanoscale is especially appealing in the context of photovoltaic and photocatalysis. Developing a process which allows the formation of well-controlled, heterogeneous, and registered doping profiles in nanostructures without the application of high resolution lithographic methods pose a significant challenge to-date. This is because the combined requirements of symmetry breaking, control of chemical composition and registry at the nanoscale pose a significant challenge. In my talk I will present recent results obtained by a new method we term Monolayer Contact Doping (MLCD) and surface chemistry approaches developed for introducing dopant-containing molecular films at oxide interfaces that were recently developed in my group.

The successful synthesis of silicon nanocrystals with >50% photoluminescence quantum yields has been reported. However, the source of the highly efficient emission is contentious. While numerous reports suggest that the radiative process originates from the particle surface with an energy (and presumably an efficiency) that can be tuned through surface chemistry, other reports indicate that the emission arises from quantum-confined states associated with the particle core. Here, we describe our experimental efforts aimed at understanding the emission origin. Specifically, we investigate the physical and optical properties of multiple samples of colloidally prepared, alkane-terminated silicon nanocrystals as a function of hydrostatic pressure. We determine the diamond-phase bulk modulus, observe multiple phase transitions, and importantly find a systematic photoluminescence red-shift that matches the lowest-energy, indirect transition of bulk crystalline silicon. These results, reinforced by density functional theory calculations, suggest that the efficient photoluminescence, frequently attributed to defects, arises instead from core-states that remain highly indirect despite significant quantum-confinement.

The surface chemistry of colloidal quantum dots can dictate both photophysical phenomena and transport. We have investigated lead selenide and lead sulfide quantum dots with modified synthetic procedures and post-treatments in an attempt to control doping and manipulate trap states. Pholuminescence as a function of temperature reveals trapping dynamics and energetics that can be correlated with analysis of quantum dot composition and surface ligand coverage. Studies of photoluminescence of quantum dot arrays under applied potential provide further evidence of the role of trap states in photovoltaic device performance.

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in field-effect transistors, solar cells, photodetectors, and light emitting diodes. As all of these prospective applications rely on p-n junction architectures, they are greatly hindered by the difficulties in achieving true electronic doping the NCs, that is, incorporation of charges into the intrinsic, quantum-confined states within the NC core. Reported approaches to NC-doping have utilized incorporation of metal ions, chemical treatments, or electrochemical charge injection, but the persistent introduction of charge carriers into the quantum-confined band-edge states of lead chalcogenide NCs, and infrared NCs in general, remains an unmet challenge. Here, we report n-doping of PbSe- and PbS-based NCs using ground-state electron transfer from mild chemical reducing agents. The use of inorganic passivation allows us to maintain the high doping level of eight electrons per NC for at least a month even at room temperature. Doping within the band-edge states are confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, conductivity measurements and fabrication of a proof-of-principle p-n diode demonstrate the promise of this approach in real device applications.

In modern semiconductor industry, ion-assisted etching techniques are widely used to manufacture integrated circuits for microelectronic devices. The etching of a substrate or thin film, known as dry-etching, plays a crucial step to transfer circuit patterns to the wafer. In this study, a theoretical model was developed to investigate the surface morphology and profile evolution for ion etching process. Integrating the kinetics of ion sputtering, redeposition and diffusion process, the characters of the surface profile evolution during ion etching were well illustrated through this theoretical model. Various surface morphologies can be developed as expected by controlling a proper condition of ion irradiation, such as ion flux, ion energy and temperature. For high ion flux or ion energy, a sputtered and roughened feature profile was created with a high etching rate. With an increased temperature, the diffusion mechanism became a dominant factor to smoothen the surface profile with a long time ion irradiation. These simulation results were fully consistent with many experimental observations. This theoretical model provides an efficient numerical approach to fully understand the mechanism for the formation of irradiated surface profile, and direct experiments with appropriate parameters to form specific nanostructures during ion etching process.

Due to the extraordinary large surface-to-volume ratio, surface effects on semiconductor nanowires have been extensively investigated in recent years for various technological applications. Here, we present a facile interface trapping approach to alter electronic transport properties of GaAs nanowires as a function of diameter utilizing the acceptor-like defect states located between the intrinsic nanowire and its amorphous native oxide shell. Using a nanowire field-effect transistor (FET) device structure, p- to n-channel switching behaviors have been achieved with increasing NW diameters. Interestingly, this oxide interface is shown to induce a space-charge layer penetrating deep into the thin nanowire (<40 nm) to deplete all electrons, leading to inversion and thus p-type conduction as compared to the thick (>70 nm) and intrinsically n-type GaAs NWs. More generally, all of these might also be applicable to other nanowire material systems with similar interface trapping effects; therefore, careful device design considerations are required for achieving the optimal nanowire device performances.
References
N. Han, F. Y. Wang, J. J. Hou, F. Xiu, S. P. Yip, A. T. Hui, T. F. Hung, J. C. Ho, “Controllable p-n Switching Behaviors of GaAs Nanowires via an Interface Effect,” ACS Nano, 6, 4428 (2012).

The presence of trap states in nanocrystal solids is one of the significant factors that limit the efficiency of the optoelectronic devices. The present work describes the nature of trap states and the effect of the charge transfer in the closed pack system such as an array of PbS encapsulated into a CdS matrix by spectroscopy measurements. The effect of the different types of passivation and the tuning of the distance between the nanoparticles on light-emitting properties of nanocrystal solids is investigated. Preliminary spectroscopic data and theoretical conclusions will be presented.

Shape and surface of nanostructures play critical roles in determining their physical and chemical properties. In our work, it was found that optical absorption of nanostructures can be largely tuned by engineering shape of nanostructures. Specifically, we have developed a self-organized approach to fabricate an assortment of three-dimensional nanostructures, including arrays of nanowires, nanopillars, nanotowers and nanocones with precisely defined shape, i. e. height, diameter, aspect ratio. Systematic investigations of their optical properties reveal that their light harvesting capability highly depends on the aforementioned geometric factors for a given material. Particularly for nanowires, it was found that long wavelength light absorption largely depends on their diameter due to waveguide mode in material. Meanwhile for nanocones, their broadband light absorption outperforms that of other nanostructures in general, due to gradual change of diameter. These results can shed light on rational design of novel optoelectronic devices such as photodetectors and solar cells.

In order to perform the synthesis of semiconductor colloidal nanocrystals, typically long chain organic acids or amines are used which adsorb at the semiconductor surface. While offering excellent size and shape control during their synthesis, these molecules are less interesting towards device integration of these inorganic building blocks. In this respect, attempts have been made to replace the synthesis ligand by short chain organics or, more recently, inorganic metal chalcogenide or chalcogenide ligands. These provide a huge improvement in electronic coupling between different nanocrystals in a closely packed thin-film. Another application of semiconductor colloidal nanocrystals is their use as precursor inks to form a dense semiconductor layer after a thermal treatment. In this work, we combine the approach of stabilizing nanocrystals by chalcogenides and their use as wet precursor ink, since this raises no issue of contamination by carbon in the final semiconductor layer.
We explored the annealing behavior of CuInS₂ nanocrystals capped by (NH₄)₂S ligands. Using in-situ x-ray diffraction, in-situ transmission electron microscopy and thermal gravimetric analysis combined with mass spectrometry (TGA-MS), we reveal the link between the surface chemistry and the sintering behavior of CuInS₂ nanocrystals. More specifically, we show that the sintering of CuInS₂ nanocrystals can both be inhibited and promoted by a thermal treatment using inert and forming gas, respectively. A similar effect is not observed with the original, oleylamine capped CuInS₂ nanocrystals. Moreover, the inhibition of nanocrystal sintering under inert atmosphere is observed with a variety of other semiconductor nanocrystals stabilized by (NH₄)₂S ligands, including CdSe and CZTSe. Based on a TGA-MS study, we link this inhibition or promotion of sintering to the chemistry of the sulfur rich nanocrystal surface.

Covalent organic frameworks (COFs) are porous materials that offer considerable potential for applications in gas storage, catalysis, and optoelectronics. They rival metal-organic frameworks and zeolites on properties of specific surface area, porosity, and thermal stability. Predictable assembly of COFs via covalent bond-forming reactions from simple building blocks has led to the discovery of many structurally diverse 2D and 3D networks, but the underlying nucleation and growth mechanisms are poorly understood. Although equilibrium stacking properties of many 2D COF sheets have been experimentally and computationally measured, the effect of solvent, temperature, and surface effects on the thermodynamic processes leading to the growth of thin-film and crystallite COFs has not been studied. COF-forming reactions are reversible in solution and thus lead to the formation of the most thermodynamically stable products. Our computational approach involves calculating the relative free energies of boronate ester-linked Pc-PBBA moieties of increasing octahydroxyphthalocyanine and 1,4-phenylenebis(boronic acid) count in a mixture of mesitylene and dioxane solvents. This approach is used to determine growth rates in the lateral and perpendicular crystallographic directions. We follow reversible pathways from individual building blocks to COF fragments with multiple formula units to obtain free energy differences between intermediate products. From these data, we were able to determine the active growth species. Free energy calculations of COF moieties provide a route to resolve mechanisms of growth too rapid and species too small to be observed experimentally.

In all solar cells, and especially in extremely thin absorber (ETA) solar cells, proper energy band alignment is crucial for efficient photovoltaic conversion. Portraying a correct picture is essential for understanding underlying mechanisms and to point out possible routes for improving the solar cells. However, available tabulated data usually do not agree with actual results. In addition, ETA cells suffer from a very low Voc/Egap ratio, which could be an outcome of an unfavorable energy band alignment. To investigate this we chose as a model system ZnO-CdS-CuSCN cells. These cells usually achieve a Voc in the range of 600-700 mV, while the band gap of the absorber is 2.4eV and should therefore result in higher Voc.
Here we investigate limiting factors of ZnO-CdS-CuSCN ETA cells, applying XPS, CREM, Kelvin probe and I-V characterization. We show that electric fields are gradually developed in the cell upon increased absorber thickness, and believe them to be beneficial for increasing the open circuit voltage. We also find that an unfavorable accumulation layer forms at the oxide-absorber interface, for which an effective chemical treatment is demonstrated. We further apply this knowledge to increase the Voc of cells by chemically modifying the absorber and achieve a Voc for this type of cell of 860 mV.

Extremely Thin Absorber (ETA) solar cells were studied using electrodeposited CdSe as absorber, nanoporous TiO2 as electron conductor and CuSCN (CTC) as hole conductor. Electrodeposition of CdSe forms nanoparticles of about 4 nm, which aggregate into ca. 100 nm clusters, sporadically distributed throughout the mesoporous film thickness. When annealed the nanocrystals merge and grow in size as a function of temperature and time. By adjusting the annealing temperature and environment, the sensitized mesoporous films implemented into solar cells, delivered unusually high open circuit voltages (VOC) for CdSe, up to 0.84 V under one sun illumination (0.90 V with mildly concentrated illumination).
Various characterization methods were used to correlate the variations in the cell parameters to changes in energy band structure, surface chemical composition, or morphology of the CdSe. The increase in the VOC was finally attributed to an increase in the cadmium to selenium ratio in the absorber surface, as well as to the formation of, as expected, oxidized species at the surface and grain boundaries of the CdSe during annealing. It is shown that the improvement in the cell&’s performance is attributed not to changes in band alignment but rather from kinetic effects. More specifically, using transient absorption measurements it is shown that the oxides species at the absorber surface, either form a buffer layer that prevents electrons (holes) from recombining with holes (electrons) in the CuSCN (TiO2) or create a passivation of the grain boundaries in the CdSe clusters which reduce recombination in the CdSe itself.

Recent advances of colloidal chemistry provide a novel method to generate both metal nanoparticles with precisely controlled size and shape, and mesoporous materials with a high surface area and ordered pore structure. Since heterogeneous catalysts are prepared by nanoparticles deposited on oxide supports, oxide-metal interfaces have attracted much attention as an important catalytic site. Herein, uniformly synthesized Pt nanoparticles were incorporated into mesoporous oxides for the preparation of nanoparticle catalysts. Poly (vinylpyrrolidone) (PVP) capped Pt nanoparticles in the size range of 1 - 5 nm were synthesized by polyol reduction methods. There are two different strategies to prepare ordered mesoporous oxides: soft-templating (cooperative assembly) and hard-templating (nanocasting) approaches.
High ordered mesoporous oxides including Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 were prepared in the presence of P123 triblock copolymer through the soft-templating approach and used as a catalyst support. The Pt nanoparticles supported on mesoporous oxides of Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 were evaluated in the hydrogenation reaction of furfural to study the effect of catalyst supports on selectivity. While Pt nanoparticles with the size ranges of 1.5 - 7.1 exhibited strong structure-dependent selectivity, various supports loaded with only 1.9 nm Pt nanoparticles produced dominantly furan as a major product. Compared to the inert silica support, TiO2 and Nb2O5 facilitated an increase in the production of furfuryl alcohol via carbonyl group hydrogenation as a result of a charge transfer interaction between the Pt and the acidic surface of the oxides.
The supported Pt/aluminas were compared with Pt/silicas under CO oxidation and hydrogenative reforming of n-hexane to investigate particle size-dependent catalytic properties of Pt and support effects of alumina and silica. Although Pt nanoparticles were proven to be structure insensitive under CO oxidation, turnover frequencies (TOFs) of Pt/aluminas and product distributions were varied by types of alumina supports due to their different acidity.
Mesoporous oxides of Co3O4, β-MnO2, NiO, α-Fe2O3, and CeO2 are synthesized through the hard-templating approach. PVP-capped Pt nanoparticles with 2.5 nm size were incorporated into the mesoporous oxides through a capillary inclusion method by simple sonication. CO oxidation was conducted under oxidizing and reducing conditions over PVP-capped Pt nanoparticles supported on mesoporous oxides. While the mesoporous oxides of Co3O4, MnO2, NiO, Fe2O3, and CeO2 were catalytically active for CO oxidation, the reaction rate was dramatically changed on Pt nanoparticles loaded oxides under either oxidizing or reducing environments of CO oxidation. From the reaction studies, collective interactions between a metal and supports were proved to change the catalytic activity and selectivity.

Self-ordered semiconductive oxide nanostructures, such as nanotubes or pore arrays, have attracted wide scientific and technological interest in the last few decades. The combination of unique geometry with nanoscale properties has made the 1D structures promising, for example, as electrodes in energy conversion and storage applications, as well as related fields. Many of these oxide structures can be formed by optimized electrochemical anodization of a metal substrate.
Very recently, we reported on the formation of various TiO2 even more advanced nanostructures such as mesosponge or fishbone structures that can be achieved by anodization of Ti in glycerol
electrolytes containing K2HPO4 at high temperatures. We demonstrate also the in the case growth of highly ordered metal oxide stacks.
In this presentation, we introduce the synthesis of a self-ordered oxide nanotube/nanoporous superlattice structure alternating heterojunctions of TiO2/Ta2O5 and TiO2/Nb2O5 self-assembled with nm precision. The growth of such an ordered nanotubular/porous structure achieved from bi-metallic multilayer substrates. We investigate the photoelectrochemical and electrochemical properties of these superlattice structures and show the main potential application

We reported that silicon dissolves oxidatively in HF solution containing oxidant at sites where catalytic metals such as Pt, Au, and Ag are in contact. As a result of the site-selective dissolution (etching), depending on the morphology of the particles, straight holes [1] or helical holes [2] with a diameter of 50 nm - 1 mu;m are formed in Si. Hydrogen peroxide is used as the oxidant for the reaction. An important point of the process is that the sites in direct contact with the metals are etched but the sites unloaded with the metals are hardly etched. Here, we report new methods for making fine Cu wires in Si by using the above-mentioned metal-catalyzed etching method.
1. Formation of Cu wires in Si by filling the holes formed in Si with Cu by electroplating.
By immersing an n-type Si wafer, on which aggregates of Au particles (about 1 mu;m each), in 2.6 M HF solution containing 8.1 M H2O2 for 60 min, holes with a depth of about 60 mu;m were formed. Then, the Si wafer was electrochemically biased at - 0.275 V vs Ag/AgCl in a solution containing 0.1 M Cu2SO4. Since the Au particles remaining at the bottom of the holes acted as seeds for electroplating, the holes were filled with Cu at a speed of about 5 mu;m/h. Although it took time, the holes were filled with Cu and straight Cu wires were formed in Si.
2. Formation of buried fine Cu wires on surface of a Si Wafer by filling grooves formed by in Si.
Stripes of thin Au layers with a width of 0.15 mu;m were formed on Si substrate using EB lithography. Grooves were formed in Si by the etching in a solution containing HF and hydrogen peroxide using the Au layers as the catalyst. Then, the grooves were filled with Cu by electroless plating. By polishing the surface, we obtained Cu wires horizontally embedded in Si.
It was rather difficult to make grooves with a width larger than 2 mu;m because supply of etching solution to the Au/Si interfacial region becomes difficult with the increase in the width. However, by using an Ag layer deposited on Si by electroless plating, grooves with a width wider than 2 mu;m were formed. In this case, etching solution was supplied to the Ag/Si interfacial region through the gap of Ag grains that composes the Ag layer. After the etching process, many Si nano-wires are formed in the groove since the Si at the gap sites is not etched. These Si nano-wires were easily removed by sonication in pure water.
[1] K. Tsujino and M. Matsumura, Adv. Mater., 17 (2005) 1045.
[2] K. Tsujino and M. Matsumura, Electrochem. Solid-State Lett., 8 (2005) C193-C195.

Among multicomponent nanostructures, hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. In this study, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibited a higher turnover frequency by a factor of two during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles didn&’t depend on light irradiation. We also found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. We discuss the possible mechanism for how hot carrier flows generated during light irradiation affect the catalytic activity of CO oxidation.

II-VI semiconductor ZnO, famous for its unique optoelectronic property, has made huge progress in light emitting, conductivity, and related areas. These advance benefit from the identification and control for impurities and intrinsic defects, which dramatically affect subsequent application of ZnO material. Taking ZnO-based diluted magnetic semiconductors (DMSs) for example, the insertion of 3d transition metal into lattice is sensitively linked to the growth methodology and processing condition. Under a limit of solubility, a large chunk of the dopants could be localized elsewhere as clusters or defect pairs, especially near intrinsic defects. The presence of complex defect center becomes a primary hurdle to obtain robust ferromagnetic (FM) response due to their tender nature susceptible to outside influences like light, heat, etc. However, energy level position, stability, and impact on a certain performance of these defect pairs still remain unknown and extensive investigations need to be pursued to determine their identities.
Here we focus on results of an overall study of point defects and defect pairs present in ZnO:Cu nanocrystalline film, which could be bottom-up fabricated using colloidal ZnO:Cu quantum dots (QDs). Oxygen vacancies (VO) and interstitial zinc (Zni) related Cu defect complexes have been resolved by temperature-dependent electron paramagnetic resonance (EPR), Raman, UV spectra, and photoluminescence (PL) technique. Through the optimization of equilibrium annealing parameters, strong p-d hybridization and clear magnetic domain could be observed in our samples by magnetic force microscopy (MFM), indicating a robust FM behavior. Light-sensitive experiments confirmed the instability of defect pairs and the origin of FM quenching. Based on the above, dual-donor (VO and Zni) mediated FM mechanism was proposed. Different from single defect mediated case, our conclusion provides an additional evidence for various defects induced FM alignment of ZnO:Cu. This work will further open the new ways to manipulate complex FM behavior by defect engineering.

Non-planar architectures such as FinFETs, with better electrostatic control and Ion/Ioff ratio, have created new pathways for miniaturization in CMOS technology. Nanowire transistors remain potential candidates for future CMOS applications but their integration into fabrication still remains a challenging problem. Nevertheless, individual Si nanowire transistors down to a 2nm diameter have been successfully fabricated. Controlling the position of the dopants remains an unsolved problem in small nanostructures. For large devices, the electrical potential around a single dopant and the exact position of the dopant, play a negligible role in the current-voltage characteristic. However, for devices with nanometer dimensions, a small amount of dopants can be found in the active region of the device and the current voltage characteristic are very sensitive to the number and the spatial configuration of dopants in the device. Just one or two dopants can be found in the channel of sub-10 nm devices.
The impact of a single dopant in the channel of a nanowire transistor was investigated assuming a ballistic regime. In this paper, we consider the effect of temperature (100K-400K) and phonon scattering in a nanowire FET, with a single donor close to the source. Elastic and inelastic phonon scatterings have been considered. The nanowire cross section is 2.2x2.2 nm2 and the channel length is 10nm. The n-type source/drain doping is 1e20 cm-3 and the channel is undoped. A donor type impurity has been located 3nm from the beginning of the source/channel interface. We have calculated the current reduction due to scattering for a drain bias of 10mV/400mV and for a range of temperature from 100K to 400K. Note that the lowering of the temperature is a way to identify the dopant resonances by measuring the device conductance.
This work shows that the donor creates a resonant level, which makes the effect of the phonon scattering bias dependent. We calculate the current reduction percentage as 100x(1-Is/Ib), where Ib and Is are the ballistic and dissipative on-current, respectively. The current reduction vs gate bias curve has a minimum around the threshold voltage. The minimum depends on the temperature and the drain bias, but it is independent of the gate bias. This minimum is smaller (7%) at a low drain bias than the corresponding value at a high drain (20%). The current reduction is usually large at high gate bias reaching 70%. The behavior of the current reduction vs. gate bias is related to the coupling of the resonant level with the phonon absorption/emission process. We have found that the off-current and sub-threshold slope decrease with decreasing temperature as observed in experiments [1]. At high temperatures and after threshold, the effect of phonon scattering is large and consequently the current reduction.
[1] M. Pierre et al Nature Nanotech. 5, 134 (2010).

Iron pyrite (FeS2) has long been considered an attractive semiconductor for photovoltaic devices. However, its development has been hampered by lower than expected photovoltages. The chemical cause of this remains unclear but surface effects are often implicated. In this study, we perform surface treatments on pyrite (100) nanocubes and report the resulting electronic effects. These effects are engineered in an attempt to create particles amenable to the preparation of band-gap photovoltage thin films.

Recent investigations of polymer/metal oxide nanocomposites indicate many new promising properties with a wide range of potential applications in optoelectronics, protective coating, sensors, etc. Due to the nanoparticles&’ high surface-to-volume ratio, the surface of the filler is often the dominating factor responsible for the unique properties of a nanocomposite. Surface modification of the grains, by coating or embedding the particles in polymers, may significantly alter the optical and electronic properties of the nanoparticles. In our work, as-received BaTiO₃ (BTO) nanopowders with an average grain size of ~50 nm were blended with polymethyl methacrylate (PMMA) in toluene solution and spin coated on Si substrates to form thin film nanocomposites. The room temperature photoluminescence spectrum of the pure powders was composed of a band-gap emission at 3.0 eV and defect level emission at 2.5 eV. The relative intensity of the defect emission was found to be larger than the band-gap emission at room temperature. Embedding the BTO nanoparticles in PMMA significantly enhanced the room temperature band-gap luminescence and increased the relative emission intensity of the BTO particles at low temperature as compared to the pure powders. The mechanism responsible for the increase in relative intensity of the band-gap luminescence is hypothesized to be the passivation of surface states of the BTO particles by the polymer.

In a recent study [1], thin diamond films grown by chemical vapor deposition were investigated by surface photovoltage (SPV) and photoluminescence (PL) spectroscopy. It was suggested that the transition at ~3.1 eV observed in both SPV and PL spectra originates from the grain boundaries rich in sp² carbon. Testing of this hypothesis could be accomplished by employing nanoscale diamond specimens with variable and well-controlled content of the surface graphitic carbon. Henceforth, SPV and PL experiments were performed on nanodiamond powder samples produced by detonation with a controlled sp³/sp² composition to elucidate their electronic structure. The studied specimens were produced by processing commercial nanodiamond. Control of the graphitic content was achieved by air oxidation of nanodiamond at 425°C followed by boiling in aqueous HCl and HNO₃. It was found that both the sp³/sp² ratio and the experimental conditions (ambient, temperature) significantly affect the spectral shapes. In particular, the increase of the sp³/sp² results in a higher overall PL intensity. The relative intensity of the ~3.1 eV PL transition changes proportionally to the sp³/sp² ratio. It was also observed that the relative intensity of the ~2.9 eV PL band in purer diamond samples increases and that of the ~2.4 eV PL band decreases. At low temperatures (~8 K) the overall PL intensity drops by an order of magnitude accompanied by changes in the relative intensities of spectral features. In addition, the PL intensity is significantly higher when measured in vacuum compared to that measured in air. Such sensitivity of the electronic structure in the studied materials to the surface conditions was further confirmed by the SPV experiments (performed in different environments) the results of which are also reported.
[1]. Nemashkalo et al., J. Appl. Phys. 111, 023704 (2012)

Sulfide semiconductor nanomaterials having layered crystal structures are very attractive because they have strong intralayer covalent bonding and interlayer weak Vander Waals interaction, which lead to attractive photovoltaic and optoelectronic applications. Considerable progress has been made in the past few years for the fabrication of different layer-structured III-VI semiconductors in various nanoforms via chemical and physical processing techniques in controlled manner, among which gallium sulfide (GaS) and indium sulfide (In2S3) are most important materials with promises of applications in photoelectric devices, electrical sensors, and nonlinear optical technologies. In this work, we report for the first time the fabrication of various high-quality one-dimensional GaS nanostructures (thin nanowires, angular nanobelts, nanohorns, nanotubes) and In2S3 nanostructures (nanobelts and zigzag nanowires) by catalyst assisted thermal evaporation process. The morphology and structures of the products were controlled by temperature and position of the substrates with respect to the source material. The morphologies of GaS and In2S3 nanostructures were examined by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), and energy dispersive X-ray analysis (EDAX). Mainly, two processes of growth, vapor-liquid-solid and vapor-solid were active in controlling the shapes and structural varieties of the nanostructures. Interplay of geometry and surface free energy of the nanostructures are found to be the guiding forces behind the formation of the selected morphologies. We studied for the first time field-emission behavior of GaS nanostructures, which revealed nonlinear steady behavior over the entire range of applied field for a long period of time. The emission current stability measurements indicate pre-conditioning of the emitter. The results suggest the use of GaS nanostructures as a promising electron source for applications in field emission based devices. On the other hand In2S3 nanostructures show an enhanced luminescence and stable electrical conductivity similar to a rectifying type, which is a novel observation in In2S3. These unusual group of nanoscale structures attribute to tune the functional properties for the applications in nanoscale optical and electronic devices, which are discussed.

Iron pyrite (cubic β-FeS2), known as an earth abundant and nontoxic semiconductor, has been attracting resurgent attention as a promising candidate for solar energy conversion thanks to its suitable band gap (0.95 eV indirect, 1.03 eV direct), high absorption coefficient (~6×10^5 cm^-1), and high mobility (> 300 cm^2/V s). However, the application of pyrite for solar cells has been hindered by its low open circuit voltage (up to 200 mV) and thus low efficiency (~3%), which has been attributed to rich band gap states located at pyrite/electrolyte and pyrite/metal interfaces. To improve and enhance the performance of pyrite solar cell, the understanding and effective passivation of pyrite surface is urgently desired. Using the phase-pure pyrite nanorods (NRs) as the model system, we developed a method to passivate pyrite NR surfaces. Employing scanning probe microscopy (SPM) and e-beam induced current (EBIC) techniques, we investigate the surface potential dominated by surface defects and show the enhancement of minority carrier diffusion length and other semiconductor characteristics after the passivation treatment. These results can be important for improving pyrite single crystal and thin film structures for solar energy applications.

Chemical functionalization is required to adapt graphene's properties to many applications, such as transparent electrodes, biochemical sensor arrays, and permeation membranes. However, most covalent functionalization schemes are spontaneous or defect driven, and are not suitable for applications requiring directed assembly of molecules on graphene substrates. Here, electrochemically-driven covalent functionalization of graphene with phenylene-based compounds is described. First, trifluoromethylphenylene (CF₃Ph) was covalently attached to epitaxial graphene by electrochemical activation. Without an electrochemical bias, only molecular physisorption was observed. Submonolayer and full monolayer chemisorption was demonstrated by varying the duration of the electrochemical driving potential. Chemical, electronic, and defect states of CF₃Ph-graphene were studied by x-ray and ultraviolet photoemission spectroscopy, and spatially resolved Raman spectroscopy. Covalent attachment rehybridizes some delocalized graphene sp² orbitals to localized sp³ states. A 0.5 eV increase in graphene's work function was observed, and is consistent with oriented dipolar molecular chemisorption onto graphene. Atomic force micrographs demonstrate that a self-limiting monolayer forms in the case of CF₃Ph-graphene, with the ability for selective-area patterning. The surface wettability (surface energy), work function, and sheet conductance of CF₃Ph-graphene will be compared to other phenelyene-based graphene functionalizations, including nitrophenylene, carboxyphenylene, and methoxyphenylene. The application of these tailored graphene substrates as transparent electrodes in organic photovoltaic cells will also be compared. [Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.]

Carrier trapping is one of the main sources of performance degradation in nanocrystal-based devices. Yet very little is known about the specific mechanisms that govern this phenomenon. Motivated by recent experimental investigations [1] showing that, in the case of a band edge exciton, surface trapping occurs only for the hole but not for the electron, we present a comprehensive investigation into the efficiency of hole transfer to a variety of trap sites located on the surface of the core or the shell or at the core/shell interface, in CdSe nanocrystals (NCs) with both organic and inorganic passivation. Using LDA-quality wave functions within the semiempirical pseudopotential approach, we calculate the rates of an Auger-mediated mechanism where the energy of the transition from the VBM to the trap state is transferred non-radiatively to the CBM s electron, exciting it into one of the p states.
We find highly dispersed hole trapping times ranging from hundreds of femtoseconds to tens of nanoseconds, even for a single trap, in remarkable agreement with recent experimental observations [1,2]. Indeed our results provide the first quantitative theoretical confirmation of the validity of one of the most successful models explaining blinking to date [3,4], which relays heavily on such an assumed (but as yet undemonstrated) large variation of the trapping times as a function of the trap energy. Most importantly we show that trapping can be extremely efficient in these systems, orders of magnitude faster than radiative recombination.
The results of our investigation on trapping at the core/shell interface prompt us to suggest that, contrarily to common beliefs, the increase in PL quantum yield usually observed following shell growth, and commonly associated with improved passivation, could be due to an increase in trapping time above radiative recombination instead. This is no trivial point, as the presence of interface trap states in inorganically capped NCs could affect their charge transport properties. Indeed hole trapping has been recently identified as the cause for hysteretic response and low switching speed in field effect transistors based on solution-processed arrays of inorganically capped CdSe NCs [5].
References
1) P. Kambhampati, J. Phys. Chem. C 115, 22089 (2011).
2) K. E. Knowles, E. A. McArthur, E. A. Weiss, ACS Nano, 5, 2026 (2011).
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5) D.S. Chung, J.-S. Lee, J. Huang, A. Nag, S. Ithurria, D.V. Talapin, Nano Lett. 12, 1813 (2012).

Nanoscale, quantum-confined colloidal structures are the object of great scientific and technological interest. The size-tunability of their electronic structure is useful for solar-energy conversion, both for matching absorption to the solar spectrum and for tuning redox potentials. Meanwhile, the solution processing of these materials offers the hope for inexpensive manufacture of devices. Colloidal nanocrystal quantum dots such as CdSe and PbSe have been an incredibly fruitful platform for scientific investigation of strongly-quantum confined systems and applications. From a long-term perspective, though, systems based on more abundant materials such as carbon are desirable.
We will present results of recent optical studies of graphene quantum dots (GQDs) with a particular focus on the characteristics of biexcitons in these structures. The GQDs of interest here are of the order of 100 sp² hybridized carbon atoms (~2 nm across) with HOMO-LUMO transitions of 1.6-2.0 eV. Because of the two-dimensional nature of the GQD lattice, screening of Coulombic interactions is weak compared to the case of traditional three-dimensional nanocrystal quantum dots. It is important to understand carrier interactions and dynamics in GQDs, as they have a fundamental role in our understanding of the photophysics of single-excitons. It is also important to understand the biexciton for its fundamental role in processes such as multiple exciton generation by absorption of a single photon, a potential route to enhanced photocurrents in photovoltaic devices. GQDs are cousins of carbon nanotubes, and nanotubes of sub-micrometer lengths display non-radiative biexciton Auger recombination (AR) times of ~1 ps. Extension of the linear scaling of biexciton lifetime with nanotube length down to the length scale of GQDs would predict a biexciton AR time of ~5 fs, which if true would place tremendous constraints on, for example, charge transfer across GQD/acceptor interfaces.
We have studied the optical response of ensembles of GQDs of 132 and 168 C atoms dissolved in toluene by transient absorption (TA) and time-resolved photoluminescence. From measurements of ultrafast (~100 fs) TA, we observe carrier-induced Stark shifts of 100-200 meV reflecting the strong carrier interactions expected for the two-dimensional carbon lattice. TA and photoluminescence upconversion measurements indicate that multiexcitons decay nonradiatively on timescales of ~0.3 ps, i.e., almost two orders of magnitude slower than the naive expectations from extrapolation of biexciton AR lifetimes in nanotubes, while single excitons show dynamics from carrier cooling on the ~1 ps timescale and singlet-to-triplet intersystem crossing on timescales of nanoseconds. We will discuss these observations in the context of potential applications of GQDs as sensitizers for solar energy conversion.

Freestanding silicon nanocrystals (Si-NCs) are under intense study because they combine excellent physical features (e.g. wavelength tunable emission [1,2], multiple exciton generation [3], and the possibility of doping [4]) with the low-cost device fabrication associated with the nanoparticle processing [5]. From the available synthesis methods, plasma decomposition of SiH4 is the one already scaled to industrial level to produce high quality, size-controlled, and doped Si-NCs [1,2,4,5], which have recently been applied in the demonstration of light emitting devices [6]. Due to the small dimensions, the surface shell/termination, formed by surface-attached foreign elements, is very important [1,7]. Thus, interface and surface phenomena eventually dominate the optical properties of Si-NCs. However, little has been established about core-shell interactions in Si-NCs. Here, we compare the light emission of freestanding Si-NCs with different forms of surface shell, such as Si-H bonds, native oxide, and organic capping, using temperature-dependent steady-state and time-resolved photoluminescence (PL). Under UV/blue excitation, Si-NCs with a native oxide shell display a broad (fwhm of asymp;130 nm) emission with two components centered at 805 nm (E1) and 690 nm (E2). In contrast, the emission spectra of H-terminated Si-NCs reveal only the E2 component. The existence of distinct components is revealed both by analysis of excitation spectra monitored along the emission band and by time-resolved PL, which shows two spectral components with distinct lifetimes (0.189±0.001 and 0.424±0.003 ms for E1 and E2, respectively). From comparison between the steady-state and time-resolved emission we conclude that E2 and E1 are likely due to a mechanism involving donor-acceptor pairs and to an excitonic process, respectively. We also establish that, along with the specifics of the surface shell, the external environment plays also a central role in the PL features. For example, the absolute emission quantum yield of surface-oxidized Si-NCs in powder form is 9±1%, which increases to 43±3% (close to values observed for solutions of organic-capped Si-NCs [1]) when the NCs are dispersed in a liquid. Our experimental findings are discussed based on interfacial effects, in particular energy/charge transfer, in closely-spaced Si-NC ensembles.
1) R. J. Anthony, D. J. Rowe, M. Stein, J. Yang, U. Kortshagen, Adv. Funct. Mater. 21, 4042, 2011
2) A. Gupta, M. T. Swihart, H. Wiggers, Adv. Funct. Mater. 19, 696, 2009
3) M. C. Beard, K. P. Knutsen, P. Yu, J. M. Luther, Q. Song, W. K. Metzger, R. J. Ellingson, A. J. Nozik, Nano Lett. 7, 2506, 2007
4) R. N. Pereira, A. J. Almeida, A. R. Stegner, M. S. Brandt, H. Wiggers, Phys. Rev. Lett. 108, 126806, 2012
5) L. Mangolini, U. Kortshagen, Adv. Mater. 19, 2513, 2007
6) K.-Y. Cheng, R. Anthony, U. R. Kortshagen, R. J. Holmes, Nano Lett. 11, 1952, 2011
7) R. N. Pereira, D. J. Rowe, R. J. Anthony, U. Kortshagen, Phys. Rev. B 86, 085449, 2012

Over the last 5 years, various studies have addressed the chemistry of the organic/inorganic interface between the nanocrystal core and the ligand shell, where especially in the case of CdSe, PbSe, PbS and InP, very detailed studies have been published. This has led to a general picture where anionic moieties such as carboxylates or phosphonates bind as X-type ligands to the nanocrystal, which is itself usually cation rich. Next to these studies on the ligand binding, various authors have shown that ligands also affect the physical properties of nanocrystals. In particular, the photoluminescence quantum yield (PLQY) of colloidal quantum dots (QDs) is proved to be sensitive to the ligand shell composition, where both PL enhancement and PL quenching have been reported. In this respect, the quenching of the photoluminescence of CdSe QDs synthesized using trioctylphosphine oxide as the coordinating solvent by short chain alcohols such as methanol stands out since these are typically used as non-solvents during the purification of colloidal QD dispersions.
Here, we take these observations as a starting point to analyze the interplay between typical non-solvents - short chain alcohols and acetonitrile - and the ligand shell composition and the PLQY for colloidal CdSe and PbSe QDs stabilized by X-type carboxylate ligands only. Such QDs typically result from hot injection syntheses that use non-coordinating solvents in combination with long carboxylic acids to dissolve metal cation precursors and stabilize the resulting QDs. For both materials, we use solution 1H NMR to monitor the changes in the ligand shell composition during the titration of well purified dispersion with a non-solvent and after successive purification steps starting from a crude reaction product.
Making use of the unique capability of solution NMR to distinguish bound from free ligands, both approaches show that short-chain alcohols induce the release of carboxylate ligands, while acetonitrile leaves the ligand shell untouched. Concomitantly, only the addition of alcohols eventually quenches the photoluminescence. We show that the different impact of short chain alcohols and acetonitrile on the ligand shell is linked to the possible transfer of protons to the carboxylate ligands. Hence, this work indicates that the use of aprotic non-solvents like acetonitrile is strongly recommended during nanocrystal purification or processing when the ligand shell and the PLQY are to be preserved.

Inorganic nanocrystals (NCs) are of great interest for their tunable electric, optical, and magnetic properties. Traditional colloidal synthesis produces NCs capped with long hydrocarbon chains which complicate their applications in electronic and optoelectronic devices such as LEDs, solar cells and photodetectors. Our group has developed the concept of inorganic ligands that include molecular metal chalcogenide (MCC) ions and metal-free chalcogenide ligands. These small inorganic molecules displace inorganic ligands and provide stabilization of NCs in colloidal solutions while not blocking the electronic communication in the layers of close-packed NCs. We will discuss the applications of these inorganic surface ligands to various classes of nanomaterials including metals, oxides and semiconductors. Our recent developments include the synthesis of novel functional MCC ligands and inorganically-capped III-V semiconductor NCs as well as the exploration of catalytically active polyoxometalates (POMs) as functional inorganic ligands.
We also used inorganic surface ligands as active elements for design of novel functional materials and composites. For example, infrared-emitting PbS/CdS NCs can be integrated into solution-cast IR-transparent amorphous As₂S₃ chalcogenide matrix. The combinations of colloidal NCs with MCC ligands can be used as the soluble precursors for synthesis of CuInSe₂ (CIS) and Cu₂ZnSn(S,Se)₄ (CZTS) films for thin-film photovoltaics. Finally, we will show the utility of inorganic ligands in colloidal synthesis and demonstrate new opportunities for synthesis of nano-heterostructures with complex compositions and topologies. This new surface chemistry enabled colloidal version of the atomic layer deposition process that could be used for true layer-by-layer design of complex nano-heterostructures

The dynamics of 1D excitons in semiconductor quantum wires (QWs) are under investigation using a number of cw and time-resolved microscopy techniques. Due to the large surface-area to volume ratio for the QWs, the properties of the surfaces of the QWs dictate the dynamics of 1D excitons. Irregularities of the surface give rise to potential minima, or trap sites, that can ruin the 1D nature of excitons in the QWs. We show that it is possible to fill a majority of the trap sites with excitons that are generated using wide-field illumination. Photoluminescence (PL) imaging data indicate that additional excitons that are generated at a diffraction-limited spot on the QW can diffuse along the entire length of the QW. We have also been able to increase the PL quantum yields (QYs) of 1D excitons to as high as 25% by synthesizing CdTe QWs with a monolayer CdS shell. The PL QYs do exhibit a dependence on excitation energy, and we have characterized this dependence for a number of different nanoparticle morphologies.

This talk will describe the relationship between emission anisotropy and aspect ratio in CdSe nanorods. As the aspect ratio of these nanocrystals is varied from 1:1 to 10:1, there exists an intermediate size regime whose optical properties are distinct from dots and nanowires. This transition was probed with steady-state photoluminescence excitation (PLE) studies that yielded the dependence of the emission anisotropy on the excitation wavelength. The emission anisotropy of these ellipsoidal nanocrystals as a function of excitation wavelength tracks closely with the absorption spectrum of the sample. This result indicates that the electronic structure of the nanocrystals has been sufficiently perturbed such that transition moments are linearly polarized, but that their density of states is low enough that distinct transitions can be observed. The surface chemistry of the nanorods plays an important role in the magnitude of their emission anisotropy. As the CdSe nanorods sit in a wet solution, their emission anisotropy steadily decreases due to aggregation in the presence of small amounts of polar solvent.

We investigate the structural and optical properties of single crystal phase and non-tapered wurtzite (WZ) and zincblende twinning superlattice (ZB TSL) InP nanowires (NWs). The NWs are grown by using vapor-liquid-solid (VLS) mechanism and in-situ etching with HCl at high growth temperature. Our stacking fault-free WZ and ZB TSL NWs allow access to the fundamental properties of both NW crystal structures, whose optical and electronic behaviors are often screened by polytypism or incorporated impurities. The WZ NWs only emit light at the free-exciton (1.491 eV) and the donor-bound exciton transition (1.4855 eV). They do not show acceptor related emission, implying that the VLS-grown NW is almost carbon-free due to sidewall removal by HCl. The ZB NWs exhibit a PL spectrum being unaffected by the twinning planes. Surprisingly, the acceptor related emission in the ZB NWs can be almost completely removed by etching away the carbon contaminated sidewall grown via a vapor-solid mechanism.

The origins of the commonly observed green emission (GE) from ZnO nanostructures remain highly controversial despite extensive studies over the past few decades. Herein, through a comprehensive optical spectroscopy study using time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS), new insights into its origin and the charge trapping dynamics at the GE-centers in ZnO nanowires are gained. TAS revealed a small Stokes shift of ~180 meV between the GE-centers (located at ~0.7 eV above the valence band) and the GE peak - yielding the first experimental evidence of the GE originating from charge transitions of the V_ZnO di-vacancies proposed in recent density functional studies. Importantly, TAS also uncovered an ultrafast Auger-type hole-trapping process to V_ZnO that occurs in a sub-ps timescale - shedding new light on this highly effective mechanism that competes with the ZnO bandedge emission. A deep understanding of the origins of the GE and the carrier dynamics in light of the competing radiative and non-radiative pathways is key to optimizing the optoelectronic properties of these ZnO nanostructures.

Nanowires (NWs) have shown great promises in recent years due to the ability to control their dimensions, their doping and to form novel heterostrucures. The ability to create these novel axial and radial heterostructures stems from their special 3-D geometry that is able to accommodate misfit strain effectively between the two heterostructured layers.
Although bulk semiconductors grow in specific and thoroughly researched crystal structures, the same semiconductor materials grown as NWs exhibit exotic crystal structures like hexagonal wurtzite. Silicon, which in bulk grows in the diamond structure, is able to grow in the wurtzite phase in NWs¹.
Here we propose a study of pure, defect-free Wurtzite Silicon based on a NW core/shell heterostructure. Optical studies and advanced structural characterization is performed while the influence of the growth conditions is presented.
¹“Crystal Structure Transfer in Core/Shell Nanowires” R. E. Algra, M. Hocevar, M. A. Verheijen, I. Zardo, G. G. W. Immink, W. J. P. van Enckevort, G. Abstreiter, L. P. Kouwenhoven, E. P. A. M. Bakkers, 2011 Nanoletters 11, 1690-1694, DOI: 10.1021/nl200208q

Because the surfaces of nanoscale structures can dominate their properties, implementing functional nanoscale materials depends to a large extent upon developing control of their surfaces. One way to control the properties of a solid surface at the nanoscale is through functionalization by either organic or inorganic layers. To this end, we can use many of the lessons learned from functionalization of planar surfaces to develop strategies for functionalizing nanomaterials. We will first discuss molecular attachment chemistries that form monolayers at semiconductor surfaces. Bifunctional molecules such as diamines and diols are of particular interest because of the possibility to have one of the functionalities available for further modification. Our results will show that electronic effects as well as basic geometric factors play a role in determining the product distributions of organic molecules at these surfaces. Beyond single monolayers, layer-by-layer functionalization is another powerful way to change the properties and introduce new functionality to solid surfaces. Both atomic layer deposition (ALD) and molecular layer deposition (MLD) allow for addition of surface coatings onto nanomaterials with sub-nanometer thickness control. We will discuss strategies for introducing new functionality to surfaces through ALD and MLD surface coatings. An example in which surface functionalization is used to improve the efficiency of quantum dot solar cells will be presented.

The interest in colloidal nanocrystals of I-III-VI semiconductor compounds has steadily increased over the past years. As compared to the more widely used Cd-based II-VI and Pb-based IV-VI, they are less toxic, while having similar opto-electronic characteristics. Moreover, band gap engineering can be realized both by altering the composition of the material and the use of quantum confinement for nanocrystal sizes below the Bohr exciton radius.
Typical synthesis procedures for 3-4 nm CuInS₂ quantum dots involve the thermal decomposition of thiol-based precursors while larger CuInS₂ nanocrystals are made in a heating up or hot injection procedure with oleylamine as sole ligand. Especially these larger nanocrystals have been used to form dense CIGS absorber layers for thin film photovoltaics by solution-based processing. In this process, both the formulation of nanocrystal inks and the transformation of a nanocrystal film into a dense thin film strongly depend on the nanocrystal surface chemistry. Nevertheless, little is known about the surface chemistry of these CuInS₂ nanocrystals and possible ligand exchange procedures.
Here, we use ¹H solution nuclear magnetic resonance spectroscopy (NMR) to investigate the surface chemistry of CuInS₂ nanocrystals. We show that oleylamine is surprisingly strongly bound to the nanocrystal surface, in contrast with the dynamic stabilization of most Cd- and Pb based chalcogenide nanocrystals by amines. We present procedures to exchange oleylamine for carboxylic acids or thiols and show that this exchange is completely reversible. This indicates that CuInS₂ nanocrystals are stabilized by molecular ligands, i.e amines, (protonated) acids and thiols, a conclusion we further confirm using elemental analysis.
The results we obtained demonstrate a unique and unexplored surface chemistry for CuInS₂ nanocrystals, which is important to further fine-tune I-III-VI colloidal nanocrystals towards applications.

The electronic and optoelectronic properties of semiconductor nanocrystals are determined by the presence and chemical nature of ligands at their surfaces, which motivates a molecular-level understanding of these interactions. To develop this framework, we will describe the use of heterometals as sensitive and diagnostic spectroscopic probes of electronic structure and bonding. Heterometallic complexes were installed as passivating ligands for various semiconductor nanocrystal surfaces using reactive ligand exchange chemistries developed by our group. These ligands were confirmed as surface constituents using energy-filtered transmission electron spectroscopy (EFTEM). The surface composition was determined quantitatively using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Finally, we will show that these ligands provide the ability to study ligand-to-nanocrystal bonding directly using core level spectroscopy (XAS, XES, nanoAuger, etc.), which provides a more comprehensive view of this important interaction. Collectively, this approach may provide new opportunities to manipulate the physical properties of nanocrystals and their assemblies for energy applications.

Most semiconductor quantum dot ligands have little effect on the electronic structure of the inorganic core and serve only as highly insulating tunneling barriers. However, some ligands are able to couple strongly to the core and influence exciton energies and dynamics. Ligands that exhibit strong electronic coupling to QDs provide an avenue for post-synthetic modification of QD properties and help mitigate deleterious effects of highly insulating ligands on electron transport in QD films. Thus far, the most active ligand we have investigated is phenyldithiocarbamate (PTC), which selectively delocalizes the excitonic hole in CdSe, CdS, and PbS, and results in a stabilization of the exciton by up to nearly 1eV in CdS. Understanding the mechanism of PTC hole delocalization will enable us to create new hierarchical materials with selective control over the transport properties of both carriers. To that end, we have modeled the surfaces of chalcogenide nanocrystals with cation enriched, fully passivated clusters. We have found that, in contrast to previous DFT studies which only deal with stoichiometric clusters, we can accurately predict the effect of ligand exchanges on QD properties and gain detailed insight into the mechanism by which those properties change.

The integration of inorganic nanocrystals as building units into mesoscale architectures yields materials wherein the components and their interfaces are both essential in defining electronic functionality. I will describe the assembly of nanocrystals and heterogeneous building blocks, including inorganic clusters and block copolymers, into functional inorganic nanocomposite and hierarchically porous films. In each case, we leverage our understanding and control over nanocrystal surface chemistry to bring these building blocks into intimate contact, thereby controlling the assembly process. This involves reactive stripping of organic ligands that natively passivate nanocrystal surfaces and coordination of the resulting naked surfaces by the inorganic clusters or block copolymers. The resulting mesostructured materials exhibit large surface or embedded interfacial area, which is tunable by adjusting architectural metrics such as nanocrystal size, pore size, and nanocrystal volume fraction. Reconstruction at these interfaces strongly perturbs and can enhance electronic and ionic transport. I will illustrate the potential for practical impact of such interface reconstruction with the case of nanocomposite electrochromic materials. These all-inorganic nanocomposites uniquely exhibit dual-spectral band switching and their optical contrast is enhanced more than 5 times by interface reconstruction. Meanwhile, the large surface area and tunable metrics of mesoporous architectures have led us to discover design rules for lithium ion battery electrodes that can sustain a thousand cycles or more without significant degradation. The potential for additional applications of these approaches is expansive since conducting, electrochemically active, magnetic, and catalytic materials can all be crafted into mesostructured films.

Surface structure and surface coordination chemistry dominate the properties of colloidal semiconductor nanocrystals and is central to their application in optoelectronics and fluorescence labeling. I will describe our picture of nanocrystal surface structure as well as a novel ligand exchange technique that allows us to fabricate semiconductor films using solution techniques. In particular, we have exchanged carboxylate ligands for chloride ligands in the presence of alkylphosphines and prepared thin film transistors with unusually high electron mobilities (8 cm^2/V/sec). Using 31P NMR spectroscopy we monitor the binding and exchange of tri-n-alkylphosphines and can estimate the relative binding affinities of amines, phosphines and phosphine oxides. In addition, I will discuss the important role that ligands play in passivating surfaces traps and strategies to passivate these defects.

Atomic layer deposition (ALD) is a thin film fabrication process capable of conformally coating high-aspect ratio structures with sub-nm precision in material thickness. While self-limiting growth is one of the main advantages of ALD, it limits the available deposition temperatures to a specific range. In some cases it may be desirable to increase the ALD deposition temperature, as this may result in improved material properties due to effects such as film crystallization, increased grain size, and evolution of the size and shape of island, or quantum dot (QD) structures. However, the upper limit to the ALD temperature window limits the ability to increase the deposition temperature past a certain point in order to maintain self-limiting growth.
This work [1] establishes a modified ALD method, adding a rapid high temperature annealing step during every cycle of deposition without adding significant time to the overall reaction. To achieve the ultra-fast heating requirement, an in situ flash lamp annealing (FLA) apparatus was designed and integrated into the lid of an ALD chamber. The FLA ALD technique was applied to modify the morphology and crystallinity of as-deposited PbS quantum dot (QD) structures and thin TiO2 films. The ability to control the PbS QD size, shape and standard deviation in size as a function of FLA conditions was observed. Furthermore, the FLA technique enabled the direct ALD growth of crystalline TiO2 thin films, which were amorphous as deposited. This technique provides a new opportunity to facilitate diffusion and crystallization throughout the ALD growth process, while maintaining the low-temperatures required during the self-limiting surface reactions.
[1]. M. C. Langston et al., "In Situ Cycle-by-Cycle Flash Annealing of Atomic Layer Deposited Materials” In Press (2012).

Semiconductor nanocrystals are emerging as essential materials in the development of next-generation electronic and optoelectronic devices. While exquisite control over colloidal nanocrystal assemblies has been developed, the electronic conductivity in nanocrystal films is limited by the presence of insulating organic ligands. We recently developed a technique to increase the electronic communication between nanocrystals via post-assembly ligand exchange of organic ligands with chalcogenidometallate clusters (ChaMs). Previous investigations of these materials have demonstrated enhanced ionic and electronic transport properties over typical nanocrystal films as a result of this novel surface chemistry. However, little is known about transport mechanisms and an understanding of their structure-function relationship is lacking. We used PbSe-GeS₂ nanocmposites as a model system to characterize the molecular structure at the nanocrystal interface and to determine the influence it has on the optoelectronic properties of nanocrystal composites. X-Ray photoelectron (XPS) and Near Edge X-Ray absorption (NEXAFS) spectroscopies reveal an ill-defined interface at the nanocrystal surface where intimate mixing of the semiconductors could enable surface charge transfer. Furthermore, unique bonding motifs arise in the inorganic matrix phase upon incorporation into the nanocomposite suggesting that nanocrystal surfaces play a role in confining the molecular geometry of amorphous GeS₂. These results indicate a diverse chemical environment at the nanocrystal interface that has the potential to be tuned for targeted electronic applications.

The utilization of fluorescent and non-cytotoxic nanodiamonds as an active biolabeling agent to replace organic fluorophores and semiconductor nanocrystals is garnering extensive attention. Nanodiamonds, being almost wholly carbon, and containing photostable fluorescent defect centers (Nitrogen Vacancy Centers (NVCs)) have the advantage of being heavy-metal free, surface functionalizable, and with the world&’s most sensitive electric and magnetic field center in NVCs. Experimentally the literature has concentrated on detonation nanodiamonds, and we have found some stark contrasts in our investigation of high-pressure, high-temperature (HPHT) nanodiamonds. In this work with HPHT nanodiamonds we have several new insights concerning their crystallographic and electronic structure quality, impurity identification, and surface species identification after oxidative purification leading to aggregate free nanodiamonds with readily active NVCs. Namely, we identify their superior crystallographic and electronic structure via Raman spectroscopy in conjunction with NEXAFS and HRTEM. The major impurity in the HPHT nanodiamond was found to be amorphous carbon and not graphene nor graphite. Raman spectroscopy also revealed that dangling carbon atoms on the HPHT surface reconstruct to form localized sp2 networks referred to as Pandey chains. Also, we identify that with increasing oxidative temperature the percentage of alcohol surface groups increases quickly in comparison to ketones. Using DRIFTS, XPS and NEXAFS we conclude that the surface of oxidized HPHT NDs does not contain detectable concentrations of carboxylic acids, but instead is largely alcohols and ketones. This is in stark contrast to the literature which cites that aerobic oxidation leads to a largely carboxylic acid terminated surface, especially in the case of detonation NDs. We also find that piranha treatment followed by aerobic oxidation at 575°C can readily decrease the presence of ketones altogether, while maintaining an alcohol rich surface. Interestingly this leads to a view that chemical and aerobic oxidative treatments etch unstable carbons with 2 and 3 dangling bonds, leading to a 111 diamond surface largely terminated with alcohols (1 dangling bond). These findings unambiguously show that HPHT NDs share properties similar to bulk diamond, and thus our findings have broad implications for the successful chemical functionalization routes of HPHT NDs, and thereby result in stable NVCs that can be manipulated,optically readout and used for neuron staining.

Surfaces and interfaces shape the properties of semiconductor nanostructures through, e.g., quantum confinement or surface states. But surfaces also control the electrostatics of nanostructures. Indeed, the existence of a dielectric discontinuity profoundly affects the behavior of charged - and even sometimes neutral - nanostructures with respect to bulk materials. These electrostatic corrections typically scale as 1/R, where R is the characteristic dimension of the system, and can therefore be prominent in sub-10 nm nanostructures. Moreover, since Coulomb interaction are long-ranged, the properties of semiconductor nanostructures can be altered by remote materials not bound chemically to the nanostructure (at variance with quantum confinement for example which only depends on the very local environment of the nanostructure). We illustrate this prevalence of the electrostatics in the physics of semiconductor nanostructures with two works dealing with the modeling of their electronic properties :
1) The physics of impurities in semiconductor nanowires, as a paradigm of charged systems : We have shown that the binding energy and excitation spectrum of phosphorous and boron impurities in silicon nanowires are highly dependent on the diameter of the nanowire and on its dielectric environment. The binding energy of dilute dopant impurities increases when the diameter of the nanowire decreases in a "low-k" environment due to the interaction of the carriers with the so-called "image charges" of the impurities, which account for the dielectric discontinuity at the surface. We will discuss how remote materials can induce polaronic corrections in a sub-10 nm silicon nanowire, and discuss the implications of these results. We will highlight the typical effects of surfaces on the electrostatics of nanostructures (image charges, self-energy corrections...) on this example.
2) The physics of the band offsets at the nanoscale, as a paradigm of neutral systems : We have shown that the traditional model of "square wells and barriers" used for the potential in planar heterostructures can break down in heterostructured nanowires and nanocrystals due to the competition between the charge transfers (dipoles) at the interface between materials and the charge transfers with the surfaces of the nanostructure. As a consequence, the band edges can show significant variations far from the interfaces if the nanostructures are not capped with a homogeneous shell. The potential can even be modulated along a homogeneous nanowire for example if the surface termination is not uniform, as a result of the "local gating" by surface dipoles. We will discuss how these results suggest new strategies to engineer the electronic and optical properties of nanostructures through surface manipulation and chemistry.

Solar cells based on lead sulfide colloidal quantum dots (PbS QDs) have made dramatic improvements in efficiency in recent years, owing in large part to novel surface passivation techniques involving organic or inorganic ligands. In spite of these advances, the influence of these ligand treatments on QD charge transport, doping, and energy level structure is still incompletely understood. Different ligand treatments have been shown to alter the valence band and conduction band binding energies in InAs and CdSe QDs by up to 0.3 eV through modification of QD-ligand interface dipoles [1-3], but the importance of similar energy shifts for electronically relevant ligands in PbS QD solids and the effects of these shifts on photovoltaic device performance have hitherto been unexplored. Here, through a combination of ultraviolet photoelectron spectroscopy, Kelvin probe, and carrier transport measurements, we demonstrate that even between structurally and chemically similar ligands such as 1,2-ethanedithiol (EDT), 1,2-benzenedithiol (1,2-BDT), and 1,3-benzenedithiol (1,3-BDT), sizeable changes in the PbS QD valence band energy and work function can dramatically affect photovoltaic device performance. In particular, 1,2-BDT-treated PbS QDs are found to have a ~ 0.2 eV shallower valence band energy and work function than 1,3-BDT-treated PbS QDs; this shift necessitates a re-optimization of the energy levels of the electron- and hole-extracting contacts for solar cells employing these ligands. In addition, 1,2-BDT and 1,3-BDT treatments result in higher power conversion efficiencies than EDT treatment for PbS Schottky junction and ZnO/PbS heterojunction photovoltaics, respectively, despite the fact that BDT-treated films demonstrate a lower charge carrier mobility than EDT-treated films. These results emphasize the importance of optimizing interfacial energy offsets and surface passivation, rather than solely the carrier mobility, in PbS QD photovoltaics.
[1] Soreni-Harari, M. et al. Nano Lett. 2008, 8, 678
[2] Munro, A.M. et al. Appl. Mater. Interfac. 2010, 2, 863
[3] Jasieniak, J. et al. ACS Nano 2011, 5, 5888

Ultrathin colloidal lead selenide (PbSe) nanowires, with continuous charge transport channels and tunable bandgap, provide potential building blocks for solar cells and photodetectors. Here, we demonstrate a room-temperature hole mobility as high as 490 cm2/Vs in field effect transistors incorporating single colloidal PbSe nanowires with diameters of 6minus;15 nm, coated with ammonium thiocyanate and a thin SiO2 layer. A long carrier diffusion length of 4.5 mm is obtained from scanning photocurrent microscopy (SPCM). The mobility is increased further at lower temperature, reaching 740 cm2/Vs at 139 K. We also present resent experimental data on the transport and optical properties of single ultra-thin CdSe nanowire field effect transistors.

The electronic properties of semiconductor nanocrystal thin films are strongly influenced by their surfaces and the molecules attached to them, particularly when those surfaces provide energetic states within the band gap. While many important parameters including effective doping density, transport energy, exciton and carrier diffusion, and mobility are strongly affected by surface trap states, probing the density of those states or carriers trapped in them remains a challenge. We measure the density of trapped carriers in a PbS nanocrystal film by monitoring the intensity dependence of the photocurrent in a lateral geometry. With a reliable way to measure trapped carrier density, we can probe the extent to which the semiconductor surface chemistry affects carrier trapping and correlate that trapping with conduction and photovoltaic properties. For properties that are primarily dictated by the number of trapped carriers, this technique enables direct screening of surface and film treatments without the need to optimize device architecture. We present the results of ligand studies, identify which aspects of conduction and photovoltaic performance are determined by trapped carriers, and discuss the implications of these results for further improvement in semiconductor nanocrystal photovoltaics.

Surfaces and interfaces play a critical role in determining properties and functions of nanomaterials, in many cases simply dominating bulk properties, owing to the large surface- and interface-to-volume ratio. Using Si nanomembranes, a well-controlled two-dimensional single-crystalline semiconductor, as a prototype system, I will discuss how surfaces and interfaces influence electrical transport properties at the nanoscale. We show that electronic conduction in Si nanomembranes is not determined by bulk dopants but by the interplay of surface and interface electronic structures with the “bulk” band structure of the thin Si membrane. Additionally, I will present our recent experimental results on the control of highly ordered molecular structures on Si surfaces, which is of intense interest for the integration of ordered organic thin films in silicon-based electronics. It could also potentially lead to the rational design of Si nanostructures with controlled properties via the regulation of surface chemistry.
This work is funded by the U.S. Department of Energy (DOE) Office of Science Early Career Research Program (DE-SC0006400) through the Office of Basic Energy Sciences.

The lack of intrinsic defect sites on III-V (110) surface may result in a lower Dit in MOSFETs fabricated on (110) surfaces than on (001) surfaces. One limiting factor in III-V based MOSFET performance is defect states at the oxide/III-V interface which prevent effective modulation of the Fermi level. These defects can result from missing As-As dimers on the InGaAs (2x4) surface but these defects are absent from the (110) surface. Furthermore, the (110) orientation is the more likely orientation of FinFET sidewalls on (001) III-V surfaces. Scanning tunneling microscopy/spectroscopy (STM/STS) has been employed to study the atomic and electronic structure of trimethylaluminum(TMA) on the GaAs (110) surfaces with 4x1018 cm3 Si doping. 10 Langmuir of TMA was dosed at 25 °C, and the sample was subsequently annealed to 135°C for 30 min. The clean GaAs (110) surface has rows which are comprised of a zigzag As-Ga chain. The filled dangling bonds of As atoms are directly observed in STM. On the clean n-type surface, the Fermi level was located closer to the conduction band than the valence band consistent with an unpinned polar surface. For partial TMA chemisorbate coverage on the (110) surface, the rows are rotated 40° compared to the clean surface rows and have 17 A between rows. A larger dose of TMA (50 L) results in a complete monolayer of reacted sites. Ordered rows of TMA chemisorbates have clearly formed. The spacing between adjacent TMA chemisorbate rows is 5.9 Angstroms, which is identical to the spacing of clean GaAs rows. A model has been proposed of TMA chemisorption which accounts for the observed 5.9 Å spacing. DFT calculations were performed and it was found the TMA dissociated into dimethylaluminum (DMA), and the DMA passivated surface has a conduction band edge state. It is believed these edge states arise from the Al-Ga bonds. To suppress the conduction band edge states -OH molecules were inserted between the Al-Ga bonds. The calculated density of states shows suppression of the conduction band edge states. The STM data is consistent with DMA molecules bonding between adjacent As and Ga atoms forming a highly ordered complete monolayer with an extremely high nucleation density which will allow for aggressive EOT scaling. STS is being performed to determine the electronic structure of this surface. Similar studies are being performed on InGaAs (110) for comparison. This is the first report of atomic imaging ALD nucleation on III-V (110) surfaces and the nucleation in each unit cell is very promising of EOT scaling on these surfaces while the lack of defects sites is promising for minimizing trap state density.

GaAs nanowires (NWs) are considered to be a prime candidate for future photonic and electronic devices. GaAs NWs have been grown by metal organic chemical vapour deposition (MOCVD) employing vapour-liquid-solid (VLS) growth using Au nanoparticles as the catalyst.[1] Although nearly intrinsic exciton lifetimes were reported from GaAs/AlGaAs core-shell NWs at 10 K,[2] only ~30ps carrier lifetimes were observed from the same NWs at room temperature[3], which indicates significant room for an improved passivation. Here, we present a systematic optimization study of shell growth temperature and time in order to develop an optimised passivation procedure. As a result, nano-second carrier lifetimes have been achieved for the first time in Au-catalyzed GaAs NWs grown by MOCVD. The carrier lifetimes measured at room temperature show a dependence on both the temperature (550 oC - 800 oC) and time (30 sec - 480 sec) of the AlGaAs shell growth. The longest carrier lifetimes of 1.02 ns on average were observed in those NWs whose AlGaAs shell was grown at 750 oC for 3 min. From this, an upper limit of surface recombination of velocity of the structures is calculated to be 1300 cm/s.[4] This is comparable with decent GaAs/AlGaAs planar double heterostructure and with GaAs/AlGaAs core-shell NWs grown without Au.[5] The occurrence of the peak of carrier lifetimes can be attributed to the balance between the AlGaAs shell material quality and the intermixing at the interface during the high temperature shell growth. By demonstrating this, we show that Au catalysts are not inherently detrimental to the carrier lifetimes in GaAs NWs and that this opens up opportunities for future GaAs NW based optoelectronic devices.
References
[1] H. J. Joyce, et al., "Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process," Nano Lett., vol. 7, pp. 921-926, Apr 2007.
[2] S. Perera, et al., "Nearly intrinsic exciton lifetimes in single twin-free GaAs/AlGaAs core-shell nanowire heterostructures - art. no. 053110," Appl. Phys. Lett., vol. 93, pp. 53110-53110, Aug 2008.
[3] P. Parkinson, et al., "Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core-Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy," Nano Lett., vol. 9, pp. 3349-3353, Sep 2009.
[4] N. Jiang, et al., "Long minority carrier lifetime in Au-catalyzed GaAs/AlxGa1 - xAs core-shell nanowires," Appl. Phys. Lett., vol. 101, p. 023111, 2012.
[5] S. Breuer, et al., "Suitability of Au- and Self-Assisted GaAs Nanowires for Optoelectronic Applications," Nano Lett., vol. 11, pp. 1276-1279, Mar 2011.

Geometrical structures of optical, mechanical and electrical devices have significantly decreased to nanoscale regime during the last years. Thus the high surface to volume ratio leads to significant impact of the surface on the material properties. Surface reactions may cause instability and unreliability of corresponding devices which are not acceptable for application purposes. Hence it is of great importance to understand the surface properties and mechanisms responsible for this kind of phenomenon. For example a significant influence of the operation atmosphere on the charge transport and herewith on the stability is known from metal oxide thin film transistors (TFTs) showing nanoscale granular features in their active layer.
In this work 10nm granular zinc oxide layers were deposited by spray pyrolysis as active layer. Thin film transistor structures were used to investigate the influence of the surface properties on the charge transport processes. A strong but reversible increase of shallow and deep trap states was found when the nano-layer was exposed to humid atmosphere. This results in a decrease in mobility, increase in hysteresis and shift of the threshold voltage during gate bias stress due to coulomb interaction of surface and active channel charges. In order to gain information about the active sides at the surface which induce the change of the electronic surface structure oxiranes were selectively bound to hydroxyl groups at the metal oxide surface. The binding of hexafluoropropylene oxide self assembled monolayer (SAM) on the surface caused a significant decrease in hysteresis by a factor of 4. Furthermore under negative gate bias stress no shift of the on-set was observed after oxirane treatment and the mobility remained stable. We conclude that hydroxyl surface groups act as active sides to induce mainly deep trap levels at the surface of ZnO deposited by strap pyrolysis in humid atmosphere.

Colloid synthesis of nanoparticles in the 0.8 - 10 nm range produces monodispersed size distributions and shapes that are stabilized by an organic cap of surfactants or polymers. Removal, or modification of the organic cap, is important to insure the chemical activity of the NPs in catalytic reactions. Inorganic caps produce core-shell NP structures that exhibit high thermal stability. Oxide-metal interfaces that are fabricated show hot electron flow during photon illumination or during exothermic catalytic reactions, which correlate with photon flux or catalytic turnover rates. The surface compositions of bimetallic NPs change with exposures of oxidizing (O2, NO) or reducing (CO, H2) adsorbates as shown by synchrotron based ambient pressure XPS studies. Small metal nanoparticles below 1.5 nm change their oxidation states as their electronic structures change with reduction of NP-size.

This presentation will center on the role of surfaces in nanocrystal photochemistry, specifically on how the nature of surface-capping ligands affects photochemical pathways. The focus will be on light-driven H2 generation by complexes of semiconductor nanocrystals and the enzyme hydrogenase. In these systems, nanocrystals serve as light-harvesting elements, while the enzyme catalyzes H+ reduction utilizing the photogenerated electrons. The binding and the electron transfer between the nanocrystals and the enzyme are mediated by surface-capping ligands. Our current understanding of the electron transfer pathways, their relationship to the photochemical H2 generation, and the role of the ligands in controlling these processes will be described.

Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. Interest in these materials has intensified due to recent reports multiexciton generation in semiconductor nanocrystals and devices. The efficiency of these semiconductor nanocrystal-based devices depends on many processes, including light harvesting, carrier relaxation, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Compared with single component quantum dots (QDs), semiconductor nanoheterostructures, combining two or more materials, offer additional opportunities to control their charge separation properties by tailoring their compositions and dimensions through wavefunction engineering. In a series of recent studies, we show that the efficiency of single and multiple exciton dissoication from semiconductor nanocrystals can be effectively controlled. With (quasi)-type II band alignment, forward reactions (charge separation and hole filling) can be facilitated, while the backward recombination (charge recombination and exciton-exciton annihilation) can be simultaneously retarded. We show near-unity quantum yield of redox mediator (methylviologen radical) generation with asymmetric CdSe/CdS dot/rod nano-heterostructures. When coupled with catalysts (Pt), these nanorods lead to a much higher hydrogen generation efficiency compared to molecular dyes and other nanocrystals.

Due to its unique electronic, mechanical, and optical properties, graphene is at the forefront of the next generation of organic electronics as a two-dimensional zero band-gap semiconductor. For such applications to be successful, it is essential to understand the molecular properties of interfaces formed between metals and graphene, since they play a critical role in dictating charge transport within these devices. Furthermore, characterization of the effects of metal deposition on graphene is expected to have significant implications for the performance and stability of many devices for which graphene is currently being explored, including OPVs, transistors, catalysts, biosensors, hydrogen storage devices, and batteries. Understanding and controlling the formation of metal contacts on graphene may allow tailoring of graphene properties (e.g. work function, electrical transport, increased reactivity) resulting in improved device performance.
In this study, surface Raman spectroscopy in ultrahigh vacuum is used to investigate the interfaces formed by vapor deposited metals (Ag, Ca, Mg, and Al) onto single layer graphene (SLG) grown by chemical vapor deposition (CVD) followed by solution transfer. Deposition of Ag results in spectral enhancement of the SLG modes due to formation of Ag nanoparticles on the surface. This enhancement allows direct spectral identification of defects in the SLG lattice, defects which are likely formed during the CVD growth process. Such defects may have a significant effect on the electron transport properties of graphene, and it is imperative to understand the conditions of their formation. Upon deposition of Ca, Mg, and Al onto SLG, the Raman spectral results substantiate metal-to-graphene electron transfer without any additional changes to the graphene lattice. This electron transfer is reported by shifts of both the G and 2D vibrational bands to higher and lower frequencies, respectively, due to alteration of the graphene Fermi level. The observation that SLG films can withstand electron doping without a breach in chemical integrity, unlike previous studies of organic films, is of particular significance for production of devices with well-defined interfaces. Overall, these results substantiating both metal-to-graphene electron transfer and the spectral identification of SLG defects benefit the development of high quality electronics based on graphene.

Semiconductor quantum dots (QDs) have been called artificial atoms because of their discrete electronic structures. Assembling them into artificial molecules [J. Phys. Chem. B 2012, 136, in press] or on the surface of a bulk semiconductor [PNAS 2011, 108, 965] may greatly expand our capability in controlling charge and energy transfer on the nano scale. We address the dynamic nature of surface capping molecules [J. Am. Chem. Soc. 2021, 134, 7792] and the roles of capping molecules in controlling electronic interactions between QDs [J. Phys. Chem. Lett. 2011, 2, 795]. We discuss the competition between electronic relaxation in QDs [Nano Lett. 2012, 12, 1588] and charge transfer at QD/semiconductor interfaces [Science 2010, 328, 1543] and how such competition is dictated by capping molecules. These examples illustrate the challenges and critical importance in controlling nanoscale semiconductor interfaces.

Recent growth experiments in several laboratories have shown that semiconductor nanowires provide an excellent path to unexpected crystal structures: particular growth conditions can lead to wurtzite or zinc blende in some III-V materials. New devices may therefore become possible through an atomic-level understanding of the factors that control nanowire structure. To examine structure and growth, we have therefore developed techniques to grow nanowires of both group IV and group III-V in situ in the transmission electron microscope by supplying vapour-phase precursors to a heated substrate decorated with metal catalyst particles. Real-time observations enable us to identify the nanowire and catalyst phase during growth, observe the introduction of defects such as twin planes, and directly evaluate the effects of changing conditions. The experiments have shown that nanowire growth proceeds through the flow of steps at the nanowire/catalyst interface, and that the movement of steps is associated with a periodic change in the structure of the trijunction, where nanowire, catalyst and vapour meet. The exciting consequence is that it is possible to observe the moment at which each new layer is added to a nanowire, and relate this “local growth rate” to the conditions and the local structure of the nanowire. We start by discussing the average growth rates of Si and GaP nanowires, obtained by measuring the average interval between layer addition events. The relationship between average growth rate and external conditions such as pressure, temperature and V/III ratio help to clarify the rate-limiting processes under different regimes. But unexpectedly, for GaP nanowires we find that new layers are not added at a uniform rate. Instead, the local growth rate varies, and it is correlated with the formation of defects such as twin planes. This non-uniform growth reflects a change in the energetics of layer nucleation that we believe is relevant to the issue of crystal phase control in III-V nanowires. We will describe a model that can help understand the average and local growth rates in terms of the chemical potentials and pathways of the growth species. Through this unique view of growth kinetics at the local, layer by layer level, we hope to obtain a deeper control of nanowire structure.

Boron carbide nanowires are synthesized by co-pyrolysis of diborane and methane over nickel film-coated SiO2/Si substrates or sapphire substrates at relatively low temperatures. Planar faults such as variable width twins and stacking faults are observed in as-synthesized nanowires. These faults can be further categorized into transverse faults and axial faults, depending on the geometrical relationship between the fault plane and the growth direction of the nanowire. Extensive Transmission Electron Microscopy examination is performed to study the catalytic particle at the tip of each nanowire, targeting to reveal its crystal structure and composition, as well as the interface structure between the particle and the nanowire. The results on the dependence of fault orientations on the composition, shape, defect and crystallography of catalytic particles, and the diffusion mechanisms on how the source atoms are incorporated into the growth front of boron carbide nanowires are revealed and will be presented in this talk. In addition, the effect of planar faults on thermal conductivity will be discussed.

The growth of semiconductor nanowires by the vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) mechanisms takes place when the growth material, supplied from a gas phase species such as a chemical vapor deposition (CVD) precursor, dissolves into catalyst particles (liquid or solid, respectively) and precipitates at the catalyst/substrate interface. In order to gain a detailed understanding of this process at the atomic level, we examined Si nanowire growth in situ, to provide a direct view as growth takes place. Here we discuss observations made in two different instruments, an ultra high vacuum (UHV) TEM and an aberration-corrected environmental TEM (ETEM). The UHV TEM allows us to introduce low pressures (below 10-5 Torr) of a precursor gas such as disilane to a heated Si sample decorated with Au or other metal catalysts. With UHVTEM we can achieve oxide-free surfaces and well-controlled growth kinetics. We find that in the UHV experiments, Si adds to the nanowire by flow of bilayer height steps across the nanowire/catalyst interface. We can understand the step flow kinetics and structure at the trijunction through considerations of the phase and chemical potential of the catalyst. However, in conventional CVD reactors, Si nanowires grow at higher pressures, over 10-3 Torr, rather than under UHV conditions. ETEM allows us to explore Si nanowire growth kinetics at these pressures, and to compare the results with those from UHV TEM. In ETEM we can observe details of the Si/catalyst interface, defects and epitaxy. We will describe the addition of Si bilayers during VLS growth in ETEM, and the addition of bunched steps consisting of three Si bilayers during VSS growth with AuAg catalysts. However, we find that the details of growth dynamics in ETEM are different compared to UHV, in particular, the structure and dynamics at the trijunction. Even though the samples appear clean at the start of growth, we find that an amorphous layer of SiO2 builds up under the beam. We speculate that this layer arises from beam-induced reactions between disilane and residual water vapor in the microscope. This layer pins the liquid at the original trijunction location, changing the local kinetics of the process. ETEM is therefore extremely valuable for understanding growth, but we need to consider effects arising from the combination of non-UHV conditions with the electron beam for a quantitative interpretation of the results.

In this presentation, data are presented that show the direct preparation of crystalline group IV semiconductor nanomaterials through a new electrodeposition process called electrochemical liquid-liquid-solid (ec-LLS) crystal growth. As discussed here, ec-LLS is an unexplored, low temperature route to obtain crystalline semiconductors directly from fully oxidized, dissolved precursors. The ec-LLS strategy for both Ge and Si highlighted here is based upon the use of liquid metals as both working electrodes and crystallization solvents. Results will be presented that detail the synthesis of nanocrystals and nanowires at low (< 100 °C) temperatures in common solvents including water without the use of any chemical or physical templating agent. The crystallinity of the as-prepared Si and Ge materials will be demonstrated through X-ray diffraction, high resolution transmission electron microscopy and selected area electron diffraction data. The influence of temperature, precursor concentration and electrodeposition parameters on the morphology of Si and Ge crystallites through ec-LLS will be discussed. The potential for ec-LLS as a promising non-energy-intensive materials synthetic strategy will be discussed.

The solvothermal method is a common synthesis route for semiconductor nanocrystals. For such solution synthesis, many investigations have considered diffusion-limited growth, in which the diffusion of reactants through the boundary layer limits the nanocrystal growth rate. These studies often model the growth rate with a diffusion boundary layer thickness much larger than the nanocrystal size and with unphysically low diffusion constants on the order of 10^-12 cm²/sec. In this work, we have examined the growth of Ge nanocrystals synthesized by injecting Ge amide precursors into a solution of 1-octadecene, oleylamine, and hexadecylamine. We have previously established this low-temperature, low-pressure synthesis route.^1,2 The resulting Ge growth rate compares well with our model, in which we consider both boundary layer diffusion and surface kinetics of Ge precursors and organic adsorbates. Our modeling results suggest that the nanocrystal growth is limited not by diffusion, but by the surface adsorption and desorption kinetics. The boundary layer thickness in the stirred reaction vessel is calculated to be on the same order of magnitude as the crystal radius; therefore, the surface kinetics cannot be ignored. Furthermore, the synthesis temperature is near 300 °C, where the Ge monomer diffusion coefficient within the growth solution is substantially increased and estimated to be on the order of 10^-4 cm²/sec. These considerations agree well with our experimentally measured growth rate and strongly suggest that the nanocrystal size evolution is controlled primarily by the surface kinetics. We will present the experimental size evolution of Ge nanocrystals as a function of time and discuss our modeling effort in detailing the surface kinetics.
1. H. Gerung, T. J. Boyle, L. J. Tribby, S. D. Bunge, C. J. Brinker, and S. M. Han, "Solution Synthesis of Germanium Nanowires Using a Ge²⁺ Alkoxide Precursor," J. Am. Chem. Soc. 128, 5244-5250 (2006).
2. H. Gerung, S. D. Bunge, T. J. Boyle, C. J. Brinker, and S. M. Han, "Anhydrous Solution Synthesis of High-Quality Ge Nanocrystals from the Germanium (II) Precursor Ge[N(SiMe₃)₂]₂," Chem. Commun. 14, 1914-1916 (2005).

It is well accepted that physical structures, such as steps and defects, can affect surface properties. In some cases, ordered structural patterns can be realized, leading to interesting applications. In contrast, chemical patterning has received much less attention, in part because it has been difficult to control the spatial order of chemical reactions without using physical structures, such as steps.[1] Chemical nanopatterns have not been produced on defect-free and flat surfaces, whether vapor phase or wet chemistry methods are used, until recently.
We have shown that atomically flat, H-terminated Si(111) surfaces could be nanopatterned by immersion in a neat solution of methanol at 65^oC to produce a sqrt(3)xsqrt(3) methoxy pattern.[2] This observation is not consistent with current models of surface reaction, as random chemical attack cannot lead to well-ordered arrangements. In this talk, we show experimentally that nanopatterning can be obtained and demonstrate with theoretical modeling that it is because the grafting process for methanol is dynamic, thanks to the presence of H₂ in solution. We first provide evidence that H₂ is a natural product of the partial decomposition and self-reaction of methanol and show, through isotopic experiments, that the presence of H₂ allows the back reaction, i.e. adsorbed methoxy to react with H₂ to re-form Si-H with the release of methanol. Using first principles methods (DFT) to calculate all reaction barriers and potential energies, we confirm with kinetic Monte Carlo that, once the back reaction is allowed, the surface can attain a much higher degree of order and the overall methoxy coverage is limited. Both the total coverage and the nature of the nanopattern depend on the total H₂ concentration, and therefore on the temperature of reaction.
This finding opens new possibilities for controlling the long-range order of wet-chemically modified surfaces, in addition to expanding the chemistry allowed on H-terminated Si surfaces. By recognizing that and understanding how the grafting process can be dynamic, a number of interesting chemical nanopatterns will be achievable, thus making it possible to tailor the surface for a variety of applications.
[1] Nitrogen interaction with hydrogen-terminated silicon surfaces at the atomic scale, M. Dai, Y. Wang, J. Kwon, M. D. Halls, and Y. J. Chabal, Nature Materials 8 (10), 825 (2009).
[2] Nanopatterning Si(111) surfaces as a selective surface-chemistry route, D. J. Michalak, S. R. Amy, D. Aureau, M. Dai, A. Esteve, and Y. J. Chabal, Nature Materials 9 (3), 266 (2010).

Zinc tin oxide (ZTO), a composite material of ZnO and SnOx, is a low-cost transparent conductive oxide (TCO) with potential applications in several modern technologies such as thin film photovoltaics and displays. The ZTO material properties are determined by the film&’s atomic composition and structure, and hence it is of importance to be able to deposit high-quality ZTO films of the desired stoichiometry, enabling optimization of desirable properties such as high transparency and conductivity. The ZTO composition can be finely tuned by the layer by layer growth of the atomic layer deposition (ALD) method, and we have recently utilized ALD to deposit ZTO thin films from tetrakis(dimethylamino)tin (TDMASn), diethylzinc (DEZn), and water. It is important to understand the fundamental surface chemistry of ZTO ALD for the purpose of optimizing the process for use in applications. We have studied ZTO deposition in detail by systematically varying the SnOx/(SnOx+ZnO) ALD cycle fractions from 0 (ZnO ALD) to 1 (SnOx ALD), and by simulating it by means of DFT calculations carried out using the PBE0 functional. Information from several techniques including the PBE0 calculations, spectroscopic ellipsometry, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-Vis) spectroscopy, and inductively coupled plasma optical emission spectrometry (ICP-OES) is combined to determine the most important factors giving rise to the experimentally observed ZTO growth characteristics and material properties. Generally, a reduced ZTO growth rate as compared with the growth rate expected from the binary ALD processes is observed by spectroscopic ellipsometry, and a strong drop in the growth rate is observed when mixing a small fraction of SnOx cycles with ZnO ALD. DFT-calculated reaction energies provide evidence that the drop originates from reduction in surface hydroxyl density due to treatment of the OH-terminated ZnO surface with TDMASn. The OH density reduction contributes to deposition of less Zn as compared with the expected composition, and this suggestion is supported by XPS and ICP-OES determined atomic compositions. For higher SnOx/(SnOx+ZnO) ALD cycle fractions, the growth rate and film atomic composition converge toward pure SnOx ALD. XRD demonstrates a detrimental effect on crystallinity when mixing SnOx cycles with ZnO ALD, leading to amorphous ZTO films for SnOx/(SnOx+ZnO) ALD cycle fractions of 0.33 and higher. Thus, the observed convergence of the growth rate towards pure SnOx ALD is not only due to the reduced OH* density but has complex contributions from the growth of amorphous material as well. Overall, the main factors giving rise to the observed ZTO growth and material properties have been determined and the results are expected to facilitate rational development and optimization of the ZTO ALD process for use in modern technologies.

In this contribution two application of AFM-based nano-oxidation are presented. AFM nanolithography can be applied to build molecular architectures and in the process to pattern functional materials with nanoscale accuracy. The selective deposition is driven by the electrostatic interactions existing between the molecular materials and nanoscale features. Specifically, by combining a top-down tip-based nanolithography and bottom-up electrostatic interactions it is possible to form regular arrays of protein molecules with an accuracy that matches the protein size (~10 nm).
Silicon nanowires compatible with integrated circuit technologies have great potential for the development of fast and very sensitive chemical and biological sensors. AFM oxidation nanolithography shows its capacity to fabricate single-crystalline silicon nanowires and to position them with great accuracy in a circuit lay-out. AFM oxidation nanolithography is applied to define a very narrow silicon oxide mask on top of a silicon-on-insulator substrate. In a plasma etching, the silicon oxide mask protects the silicon underneath from the etching process while the uncovered part of the Si layer is etched away. This generates a silicon nanowire with a rectangular section. The nanowire can be integrated into device to be the main element of a field effect transistor. A silicon nanowire-based biosensor has been applied for label-free and ultrasensitive detection of the early stage of recombinational DNA repair by RecA protein. We measure relative changes in the resistance of 3.5% which are related to the interaction of 250 RecA single stranded DNA complexes.
Recent References
#9679;M. Chiesa et al. , Nano Letters 12, 1275 -1281 (2012).
#9679;R.V. Martinez, M. Chiesa, R. Garcia, Small 7, 2914-2920 (2011).
#9679;R. V Martinez, J. Martinez and R. Garcia, Nanotechnology 21 245301 (2010).
#9679;R. V. Martinez, J. Martinez, M. Chiesa, R. Garcia, E. Coronado, E. Pinilla-Cienfuegos, and S. Tatay, Advanced Materials 22, 588 (2010).

Oxide epitaxy with abrupt interfaces on Si and Ge (001) surfaces is a promising route towards integrating new functionality such as non-volatile transistors, chemical sensors, and magnetic devices into traditional semiconductor devices. To date, all successful oxide epitaxy on these surfaces has required the initial formation of an alkaline earth template layer. To understand how this layer promotes oxide epitaxy while inhibiting oxidation of the underlying semiconductor, we employ scanning tunneling microscopy (STM), electron diffraction, and other experimental techniques, complemented by density functional theory (DFT), in order to explore the interaction of alkaline-earth metals with the Si and Ge (001) surfaces on the atomic scale. Our results reveal a complex series of phase transitions as the alkaline-earth coverage is varied. Each phase transition is accompanied by significant changes in the surface morphology that can only be explained by mass transfer induced by the formation of alloy surfaces. Through comparison of bias-dependent atomic-resolution STM images with first-principle calculations, we develop atomic structural models of the surface alloy phases. In addition, incorporation of the larger alkaline earth atoms into the Ge surface creates anisotropic strain that is ultimately relieved by the formation of remarkably well-ordered arrays of islands and trenches. With applications in mind, we investigate alkaline-earth deposition onto a Ge substrate lithographically pre-patterned with shapes, designed to direct the self-organization of the alkaline-earth induced surface structures. Sr deposition onto a Ge (001) substrate patterned with cross-shaped nano-templates results in phase segregation within the template boundaries. This self-segregation, along with the formation of atomically abrupt junctions between the different surface alloys, is encouraging in terms of potential applications to device self-assembly.