Quantum dots (QDs) in photonic crystal nanocavities provide a scalable, on-chip, semiconductor platform for the study of cavity QED, but also enable very large dipole-field interaction strengths, as a result of the field localization inside of sub-cubic wavelength volumes (vacuum Rabi frequency is in the range of 10's of GHz). Moreover, this platform can be employed to build devices for quantum information processing, such as ultrafast quantum gates, nonclassical light sources, and spin-photon interfaces. We have demonstrated controlled amplitude and phase modulation between two optical beams at the single photon level (power less than a photon per cavity photon lifetime) interacting via a strongly coupled quantum dot - photonic crystal cavity system, with the switching speed of 25GHz. We have also studied the effects of the photon blockade and photon induced tunneling which result from the anharmonicity of the ladder of dressed states in a strongly coupled QD-nanocavity system. These effects lead to dramatic changes in the transmitted photon statistics, which can be varied from sub-Poissonian to super-Poissonian, and can be employed to generate nonclassical states of light (such as Fock or NOON states) with high efficiency. Beside quantum information systems, many classical information processing devices greatly benefit from the enhanced light matter interaction in such structures; examples include all-optical switches, electro-optic modulators controlled with sub-fJ energy and operating at GHz speed, and lasers with threshold currents of 100nA.

Recent advances in the growth and fabrication of III-V nitrides have allowed for the realization of optically efficient light emitters in the blue spectral range. However, in spite of such achievements the chemical inertness, high defect densities, and internal fields in these materials have hindered progress towards the next generation of devices such as low threshold lasers, single photon sources, and room temperature Bose-Einstein condensates. Here we report on the optimization of material design and fabrication techniques of gallium nitride microdisk cavities which has allowed us to realize structures with record high quality factors (~10,000) and low threshold, room temperature lasing. We will present data on the influence of the disk membrane thickness as well the number of layers of quantum dots (1 vs 3) on the performance of the devices. We will discuss optimization of lithography, masking and etching techniques to provide smooth sidewalls and effective undercut isolation through photoelectrochemical etching. Such considerations are critical in achieving enhanced future performance of optical devices in this material system.

Electrical chip-to-chip connections have not been improved a lot in speed during recent years.Hence, the demand for optical connections gets more and more important. Therefore, the integration of optically active semiconductor structures on silicon is necessary. During the last years, the direct integration of GaP on Si has been much improved by solving the anti-phase domain problem between at the interface between them. Now, an optically active layer within GaP is necessary to drive lasers integrated on Si devices. In this contribution we studied the structural properties of InGaAs quantum dots within GaP, which is potentially an optically active system for the direct integration on Si devices. From an analysis of the spatial structure at the atomic scale by cross-sectional scanning tunneling microscopy we can conclude on the one hand how the growth works in detail, as well as on the other hand we can provide data for the simulation of the optical parameters. The InGaAs quantum dot structures was grown using metal-organic chemical vapor deposition on GaP(001) substrates. For planarization first a GaP buffer was grown, on which the active layer was deposited. Afterwards, they were covered again by GaP, as necessary for a device structure. The InGaAs/GaP quantum dot are found to have a truncated pyramidal shape, with a distinct (001) bottom and top interface, very similar to InAs/GaAs quantum dots. The indium concentration shows a reversed cone stoichiometric profile, as typically also found upon co-deposition of In and Ga during quantum dot growth on GaAs. Due to the higher strain in this system, the InGaAs/GaP quantum dots a slightly smaller as compared with In(Ga)As/GaAs ones. Nevertheless, they are optically active based on 0-deminsionally confined electron states. Hence, this new quantum system is promising candidate for the direct integration of III-V based lasers on Si substrate devices.

ZnO with extremely large oscillator strength and binding energy of 60 meV is an attractive candidate for strongly coupled polariton-based devices operating at room temperature. Here we report room temperature strong-coupling effects in a dielectric microcavity embedded with ZnO nanoparticles. The top and bottom distributed Bragg reflectors (DBR) are fabricated using plasma enhanced chemical vapor deposition and commercially available ZnO nanoparticles in ethanol are sandwiched between the DBRs via spin casting. Results of angle resolved reflectivity and photoluminescence show a Rabi splitting of ~90meV at room temperature. Only the lower polariton branch is observed in the room temperature measurements due to the large Rabi splitting which pushes the upper branch into the scattering and continuum states of ZnO excitons. This agrees well with theoretical coupled oscillator model that takes into account the presence of higher lying scattering and continuum states. It should however be noted that at low temperature (10K), both the polariton branches are clearly observed due to the decrease in homogenous broadening which helps resolve the polariton branches within the scattering and continuum states.

We study optical nonlinearities and exciton-phonon coupling in colloidal 0D to 2D zinc-blende-type (ZB) and wurtzite-type (WZ) semiconductor nanocrystals, i.e. spheres, rods and platelets [1]. Single-particle spectroscopy, temperature-dependent emission and ultrafast recombination dynamics have been measured along with numerical simulations of energy states in 2D-nanosheets which exhibit much narrower ensemble absorption and emission linewidths as compared to colloidal CdSe nanocrystallites ensembles. The observed 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening. We present numerical calculations of the 2D-nanoplatelets quantum well exciton energies including Coulomb interaction and compare the obtained energies with experiments. Due to a large surface to volume ratio, the exciton energies show a strong impact of dielectric confinement. Coulomb interaction corrected numerical simulations reproduce this effect. The geometry-dependence of nonlinear optical coefficients (e.g. two-photon absorption coefficient, nonlinear refractive index) is investigated in Z-scan technique and two-photon luminescence excitation (TPLE) measurements. A pronounced change of volume normalized TPA absorption cross sections at the transition from dots to elongated rods has been found. While the bulk TPA coefficient of CdS is 17GM/nm3, it increases in colloidal CdS nanocrystals by more than an order of magnitude depending on shape and size. The contributions of spatial confinement and local field effects of the dielectric environment are evaluated separately to get a deeper insight in the pure confinement effect on TPA. The results from the two different methods, Z-scan and two-photon luminescence excitation (TPLE), show good agreement and confirm the obtained values and size and geometry dependence of the TPA coefficients of dots and rods. The electronic structure of strongly confined semiconductor materials with high nonlinear coefficients makes them ideally suited for two-photon absorption (TPA) based effects, e.g. for 3D optical data storage elements or biological cell imaging. [1] A.W. Achtstein, A. Schliwa, A. Prudnikau, M. Hardzei, M.V. Artemyev, C. Thomsen, U. Woggon Electronic Structure and Excitonminus;Phonon Interaction in Two-Dimensional Colloidal CdSe Nanosheets, Nano Lett. 12, published online May 24 (2012), DOI: 10.1021/nl301071n

The ability to create ensembles of inorganic nanomaterials with a high degree of control is of great interest in the development of new materials for applications in various fields including catalysis, optoelectronics, high-density data storage, and biological sensing. Nanocrystal superlattices often exhibit unique electronic, optical, and magnetic properties that are distinct from both the corresponding individual particles and the bulk solid as a result of the interactions between the excitons, surface plasmons, or magnetic moments of assembled particles. In order to study the emergent properties of inorganic nanoparticle assemblies and correlate them to the precise structure of the ensemble, materials-general assembly techniques that allow for tunable lattice parameters are preferred. Consequently, we have explored two methods for synthesizing periodic assemblies of noble metal nanoparticles and have probed their emergent optical properties. Entropic depletion forces, arising from the presence of surfactant micelles, have been utilized to assemble anisotropic nanoparticles into one-, two-, and three-dimensional superlattices in solution with anomalously large lattice parameters. The effects of surfactant concentration, temperature, ionic strength, and colloid size on interparticle spacing have been investigated. In a complementary approach, DNA has been used as a programmable linker to assemble gold and silver spherical nanoparticles into superlattices with a variety of crystallographic symmetries ranging from face-centered cubic (FCC) and body-centered cubic (BCC) to more complex arrangements such as AB2, AB3, and AB6. The lattice parameters of these superlattices can be adjusted with nanometer precision, as shown by synchrotron small angle X-ray scattering (SAXS). The surface plasmon resonance (SPR) of the nanoparticle superlattices can be tuned across the visible range by controlling the interparticle spacing and crystallographic arrangement. Furthermore, the assemblies of anisotropic nanoparticles exhibit blue-shifted plasmon resonances compared to the red-shift that occurs for assembled spherical nanoparticles.

The property of induced chiroptical activity was studied in type II-VI semiconductor quantum dots [1]. Different mechanisms for induction were evaluated in light of experimental nanoparticle size and material dependence laws derived in this work. Circular dichroism (CD) and fluorescence detected circular dichroism (FDCD) are used to study the electronic level structure of quantum dots revealing information which is not detected by conventional absorption spectroscopy. It is shown that induction mechanisms working in achiral nanocrystals only lead to very weak effects. A new concept is suggested to enhance the optical activity of nanoscale chiroptically active materials, relying on chiral crystallization of inorganic nanocrystals. This demonstration connects the fundamental work of Pastuer dating as early as the 19th century to nanoscale science for the first time. Huge chiroptical effects were measured in enantiopure samples of nanocrystals of the chiral phase of mercury sulfide (cinnabar). These were obtained by a new synthetic approach which enables control of chirality of the nanocrystals which are formed in a colloidal synthesis. Unique properties are revealed in this newly introduced system such as intriguing dependence of chiroptical activity on shape and crystallinity. This approach is expected to set the ground for a new field of materials science with a variety of applications. Finally, the non linear optical properties of these nanocrystals have been studied, revealing very strong second harmonic generation activity. [1] Ben Moshe, A.; Szwarcman, D.; Markovich, G. "Size Dependence of Chiroptical Activity in Colloidal Quantum Dots" ACS Nano, 2011, 5 (11), 9034-9043.

We present the first semiconductor nanocrystal films of nanoscale dimensions that are electrically conductive and crack-free. These films make it possible to study the electrical properties intrinsic to the nanocrystals unimpeded by defects such as cracking and clustering that typically exist in larger-scale films. Our technique for forming the nanoscale films is based on electronbeam lithography and a lift-off process. The patterns have dimensions as small as 30 nm and are positioned on a surface with 30 nm precision. We achieve unprecedented versatility in integrating semiconductor nanocrystal films into device structures both for studying the intrinsic electrical properties of the nanocrystals and for nanoscale optoelectronic applications. We find that the electrical conductivity of the nanoscale films is 180 times higher than that of drop-cast, microscopic films made of the same type of nanocrystal. In the nanoscopic patterns, we find additional noise in the current that is thermally activated. This noise is unusual in that it is of a comparable order of magnitude to the average current, and we find switching behavior where the average current changes by an order of magnitude in time. This noise cannot be explained by commonly known origins of current noise, and thus we believe that we are observing a novel effect in the nanocrystals.

Among other uses, plasmonics is currently being investigated to improve the efficiency of solid-state light emitters. Coupling to surface plasmons gives rise to a sharp increase in the density of radiative e-h states near plasmonic resonance (so-called Purcell factor). This increase leads to strong enhancement of spontaneous emission, as has been previously observed in photoluminescence (PL) experiments on various hybrid metal/semiconductor (M/SC) systems. Plasmonically enhanced light emitting devices, however, comprise only a subset of all possible hybrid M/SC systems. We demonstrate here that PL enhancement can be achieved in non-resonant hybrid M/SC structures, that the enhancement is strongly localized around the metal nanoparticles, and that it is accompanied by experimental features which clearly differentiate it from resonant (plasmonic) enhancement. Results are presented for two hybrid M/SC systems: An InGaN/GaN MQW with embedded Au NPs, and a GaAs/AlGaAs QW with Ga nanodroplets deposited on the surface. Despite having differing geometries and emission wavelengths, we demonstrate that both systems exhibit localized enhancement in the vicinity of the NPs, and that this occurs in the absence of resonant plasmonic interaction. In addition, the systems share two additional important experimental features: An increase in the PL decay time and power-dependent intensity saturation. Using a simple, but comprehensive, model which takes into account the electrostatic effects of the metal NPs on the electron-hole kinetics within the QW, we are able to explain the origin of the localized enhancement and duplicate the observed experimental features. This demonstrates that the enhancement arises due to electrostatic interaction with the NPs. This method for emission enhancement should be easily applicable to nano-scale light emitters over a wide-range of emission wavelengths due to its non-resonant nature.

Colloidal quantum dots (QDs) are promising materials for various optoelectronic device applications by virtue of their size- and shape-tunable optical and electronic properties. There is great interest in the development of low-temperature solution processing of QD devices, for next-generation photovoltaics, photodetectors and light emitting diodes (LEDs). The central issue in the physics of optoelectronic devices based on nanostructured materials is the control of charge-carrier dynamics. However, manipulation of the excitons in these materials remains a significant challenge, and consequently device performance has been limited. In our research, we demonstrated that the electronic coupling between the adjacent QDs can be dramatically varied by tuning the spacing between them. A variation of a few angstroms in the spacing results in a few orders of magnitude change in exciton dynamics - recombination and dissociation. The LEDs based on PbS QDs have reached record-brightness. The radiance is an order of magnitude higher than in previous QD LEDs. The maximum external quantum efficiency of the LEDs is about 2%. These solution-processed LEDs can be fabricated to emit over the range 800 ~ 1850 nm and compete with the performance of state-of-the-art infrared LEDs fabricated by planar epitaxial technology over the range 900 ~ 1300 nm [Nature Nanotechnology 7, 369-373 (2012)].

We describe spectroscopic measurements of the electroluminescence (EL) and photoluminescence (PL) of quantum dots (QDs) in biased high performance quantum dot light-emitting diodes (QD-LEDs), which allow us to deduce that the origin of the efficiency roll-off commonly observed at high voltages is a reversible electric-field-induced quenching of QD PL quantum yield (QY). To do so, we first devised a method for the simultaneous and non-perturbative measurement of QD PL QY and QD-LED external quantum efficiency (EQE) during active device operation. The PL QY of the QDs is seen to rapidly decrease above 4V and to track the roll-off (‘drooping&’) of the EQE. To our knowledge, this is the first direct demonstration that the QD layer is responsible for the efficiency ‘droop&’ in QD-LEDs, and that this is a result of the quenching of QD PL at high biases. Red-shifting of EL spectra, which we assign to the quantum-confined Start effect (QCSE), suggests that this PL quenching may be a result of electric-field-induced polarization of excitons generated within the QD film. To test this hypothesis, we isolated the impact of electric-field from other potential culprits, such as charge-induced Auger recombination, by monitoring the red-shifting and quenching of QD PL in a reverse biased QD-LED. Using the QCSE as a signature of local electric-field, the bias dependence of the EQE was predicted and found to be in excellent agreement with the efficiency ‘drooping&’ observed in our forward biased devices. This allows us to conclude that electric-field alone is indeed responsible for QD PL quenching, and, in turn, EQE ‘drooping&’ in our QD-LEDs. Further more, electroabsorption measurements of these QD-LEDs, which also exhibit the QCSE and suggest negligible QD charging, corroborate our findings. The ‘hybrid&’ QD-LED investigated here, comprising organic and inorganic charge transport layers, represents the latest generation of device architecture. This study therefore informs the design of QD-LEDs with improved efficiencies at the high current densities required for high-brightness devices.

Single-particle studies of phosphorescent lanthanide-doped upconverting nanoparticles (UCNPs) have shown that they exhibit nearly ideal properties as single molecule imaging probes. UCNPs absorb two photons in the near infrared and emit one at shorter wavelengths in the visible or nIR, an unusual characteristic that distinguishes them from all luminescent molecules in the cell, and one that suggests background-free cellular imaging. We have shown that UCNPs do not blink on and off as most other probes do, and that they posses remarkable photostability, resisting photobleaching under continuous irradiation long after organic dyes, proteins, and even quantum dots are extinguished. The promise of these nanoparticles has been tempered by difficulties in controlling nanoparticle size, emissive color, and in fully characterizing their optical properties. We have recently developed synthetic methods for control of UCNP size, down to GFP-sized nanoparticles, and methods for characterizing quantum yields, lifetimes, and single molecule emissions. A combinatorial lanthanide scan has allowed us to tune emission and excitation wavelengths for extended, multicolor, single- or dual-particle tracking. Finally, we are developing simple, protein-based coatings to passivate the nanocrystals and permit straightforward methods for nanocrystal immunotargeting.

Anisotropic metal nanoparticles have been an attractive research target because of their remarkable spectroscopic properties. We have reported Au-Ag core-shell nanorods of which spectroscopic properties corresponded to those of anisotropic silver nanoparticles [1-4]. Our core-shell nanorods were uniform in their shapes and showed four peaks in their extinction spectra. We evaluated extinction spectra of the Au-Ag core-shell nanorods on a plate in changing the incident angles of the monitor light. The spectral changes depending on the incident angles were helpful to discuss the origins of the bands in the extinction spectra. Deconvolution of the spectra indicated that six bands made an extinction spectrum. The peak locating in the longest wavelength regions (650 nm) was assigned to aggregates of the nanorods. The second longest peak (500 nm) could be assigned to the longitudinal SP band of the nanorods. The changes of the other four peaks were complicated, but two of them were assigned to be parallel (420 nm) and perpendicular (380 nm) transitions against the plate surface. Chemical and electrochemical oxidation can dissolve the silver shells on a plate. The oxidation of silver shells induced dramatic spectroscopic changes. The spectroscopic changes indicated how the silver shells were oxidized. In the case of the electrochemical oxidation, the core-shell nanorods were deposited on an ITO plate, and in-situ spectroscopy was performed during cyclic voltammogram measurements. The in-situ observation showed the characteristics of the oxidation and reduction processes of silver shells. [1] Y. Tsuru, N. Nakashima, Y. Niidome, Optics Commun. (in press). [2] M.R. Cortie, F. Liu, M.D. Arnold, Y. Niidome, Langmuir (in press). [3] L. Wang, A. Kiya, Y. Okuno, Y. Niidome, N. Tamai, J. Chem. Phys. 134 (2011) 054501. [4] Y. Okuno, K. Nishioka, A. Kiya, N. Nakashima, A. Ishibashi, Y. Niidome, Nanoscale 2 (2010) 1489.

As the size of semiconductor nanocrystals has been pushed to their lower limits to fully exploit quantum confinement, new properties have emerged. Nanocrystals that display size-tunable monochromatic emission when small, emit across a broader range of energies when their size is reduced into the ultrasmall sub-2nm range [1]. White-light emission from ultrasmall CdSe nanocrystals is a particularly interesting case of such broad emission because of its potential for solid-state lighting [2,3]. The white light consists of a continuum of emission energies spanning the visual range. Experiments have ruled out a broad distribution of sizes as the cause, and have shown that individual ultrasmall CdSe nanoparticles emit white light. We have investigated small to ultrasmall CdSe nanocrystals using a combination of state-of-the-art scanning transmission electron microscopy and finite-temperature quantum molecular dynamics simulations. Our findings indicate that following excitation, partial thermalization sets the ultrasmall nanocrystals into a disordered fluxional state. These dynamic fluctuations cause the band gaps of the ultrasmall nanoclusters to vary continuously across the visual range on a femtosecond time scale. When averaged over time, transitions across all these different band gaps fill the visual spectrum and produce the white light. Furthermore, although the larger monochromatic emitting nanocrystals we have observed possess stable crystal cores, their surfaces are fluxional. Dynamic fluxionality should be taken into consideration when optimizing nanocrystals for applications. Research at Vanderbilt was supported in part by the U.S. Department of Energy Grant DE-FG02-09ER46554 (TJP, STP) and the McMinn Endowment (STP). Research at Oak Ridge National Laboratory was sponsored by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division (SJP). Computations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. [1] Landes, C. F., Braun, M. & El-Sayed, M.A. On the nanoparticle to molecular size transition: fluorescence quenching studies. J. Phys. Chem. B 105, 10554-10558 (2001). [2] Bowers II, M.J., McBride, J.R. & Rosenthal, S.J. White-light emission from magic-sized cadmium selenide nanocrystals. J. Am. Chem. Soc 127, 15378-15379 (2005). [3] McBride, J.R., Dukes III, A.D., Schreuder, M.A. & Rosenthal, S.J. On ultrasmall nanocrystals. Chem. Phys. Lett. 498, 1-9 (2010).

In recent years, periodic nanostructures at the substrate/air interface have attracted considerable attention in the field of optoelectronic devices because of their good performance in improving light extraction. To date, the direct formation of periodic nanostructures onto glass is achieved by nanoimprint lithography (NIL). However, it is difficult to obtain large-area periodic nanostructures on the surface of glass by NIL. To fabricate inorganic periodic nanostructures onto glass, in this work, we report a novel method of pattern transfer by using nanoimprint films. A nanoimprint film having a hole pattern is employed as a template, which achieves a periodic arrangement of inorganic nanostructures. After filling silica beads into the hole pattern or applying a sol-gel solution to the surface of film, the hole pattern side of film is then placed face downward in contact with the glass substrate and held in place by a weight. After a pattern transfer process, it is found that the periodic inorganic nanostructures were successfully transferred from the nanoimprint film onto the surface of the glass substrate. Various arrangements of inorganic nanostructures can be easily fabricated by using hole patterns with different intervals and arrangements. Our findings indicate that this pattern-transfer method is a promising candidate for the inexpensive fabrication of large-area periodic nanostructures onto glass. We consider that this method of pattern transfer has potential applications in lighting equipment and optoelectronic devices.

Tremendous efforts have been made on the synthesis of metal chalcogenide nanocrystals (NCs) in the last 30 years due to their size-tunable spectroscopic properties. The pioneering work on monodisperse metal sulfide colloidal NCs synthesis by the Bawendi group in 1993 used elemental sulfur as a sulfide precursor. This elemental sulfur precursor has subsequently been widely applied to the synthesis of metal sulfide NCs. In general, the elemental sulfur (S⁰) has to be reduced to S^2- before bonding to metal cation. The molecular mechanisms involved in this process, however, remain complicated and unclear. The lack of a rational synthetic method has caused many complications in metal sulfide colloidal nanocrystal synthesis, such as unreproducible products and low conversion yield. The stoichiometric reaction between (NH₄)₂S and metal salts affords a rational synthetic method to metal sulfides. Due to its poor solubility in nonpolar solvents, (NH₄)₂S has been mainly used as a reagent in aqueous phase nanocrystal synthesis, which generally yields low quality particles with wide size distributions. Here we describe the first use of (NH₄)₂S as a generic sulfide precursor in organic nonpolar-phase nanocrystals synthesis. Our novel method has produced a variety of monodisperse metal sulfide colloidal nanocrystals, including CdS, Ag₂S, Bi₂S₃, SnS, Cu₂S, ZnS, and MnS. The stoichiometric reaction between (NH₄)₂S and metal salts can produce metal sulfide nanocrystals with high conversion yield, and more than 30g monodisperse nanocrystals can be synthesized in a single reaction. (NH₄)₂S exhibits very high reactivity at relatively low temperatures, which provides new opportunities to synthesize small size quantum dots that are difficult to be obtained by the conventional high temperature methods. This work was supported in part by the National Science Foundation under agreement DMR 1120296.

One-dimensional inorganic nanostructures have attracted huge interest because of their promising properties for electronics, plasmonics and sensing applications. One approach for the synthesis of these systems is solution-based, template-directed growth using surfactants, block-co-polymers etc. It was shown recently that silver nanowires can be grown within tubular J-aggregates of amphiphilic cyanine dye molecules1. These inorganic nanowires are grown by reduction of silver salt in aqueous solution and show a diameter around 7 nm with a length up to several micrometers. A detailed structural characterization of the hybrid inorganic-organic system requires complete analysis from the atomic scale to the micrometer scale. Transmission electron microscopy (TEM) provides various tools for structural and chemical characterization, such as HRTEM/STEM, electron diffraction and analytical methods for the inorganic part of the hybrid system, and cryogenic TEM (cryoTEM) as an outstanding method for imaging the organic part. CryoTEM allows in-situ imaging of frozen hydrated samples in their native state and was successfully applied to biological samples but also to hybrid materials in solution.2 To maintain their native state, the samples are plunge-frozen into a suitable cryogen (here liquid ethane at ca. -183 °C), resulting in the material becoming embedded in a thin layer of vitreous ice. The TEM imaging under these cryo-conditions avoids damages of the soft organic material under high vacuum conditions. In the case of the hybrid materials from J-aggregates and inorganic nanowires, cryoTEM was applied to follow the growth of the silver nanowires at different time steps. From the images information about the nucleation and growth mechanism of the nanowires within the self-assembled aggregates is deduced. In combination with results from TEM a first model for the growth of the nanowires within the J-aggregates can be presented here. References: 1 Eisele D. M. et al., J. Am. Chem. Soc. 2010, 132, 2104. 2 Nudelmann et al., Soft Matter 2011, 7, 17.

Many fluorophores, including organic dyes, fluorescent proteins, and nanocrystal quantum dots (NQDs), exhibit fluorescence intermittency, or blinking.(1) Recent experiments demonstrate that judicious core/shell heterostructuring of NQDs yields significantly suppressed blinking through either interfacial alloying or thick shelling.(2,3) Concurrent suppression of non-radiative Auger recombination (AR) has also been established with important implications for low-threshold multi-color lasing and efficient multiexciton emission.(2,4) Expanding beyond limited CdSe-based examples of core/shell engineering, we explore thick-shell heterostructuring of InP NQDs. The effect of shell composition and thickness on NQD photophysical properties is explored by applying thick shells of four different materials (ZnS, ZnSe, CdS, and CdSe) to InP cores. Of the four distinct heterostructures synthesized, InP/CdS and InP/CdSe exhibit a type-II bandgap structure, which affords complete spatial separation of excited-state carriers and red-shifts excitonic emission into the near-infrared, while InP/ZnS and InP/ZnSe exhibit type-I and some limited quasi-type-II behavior, maintaining emission in the visible wavelength range. Among other results, we found that InP/CdS NQDs exhibit suppressed blinking and AR (indicated by long biexciton lifetimes), similarly to their thick-shell CdSe/CdS (“giant” NQD, g-NQD) counterparts; unlike the earlier system, however, blinking suppression is not strictly confined to thick shells. Significantly, room-temperature blinking behavior for a fully type-II system has not been previously demonstrated as attempts have been thwarted by extreme sensitivity to photobleaching and low signal-to-noise at the single-dot level, making our result the first example of both a non-blinking NQD emitting in the NIR and a non-blinking type-II NQD.(5) The long biexciton lifetime of the thick-shelled InP/CdS NQDs shows promise for amplified spontaneous emission (ASE), while core/shell/shell materials that sequester cadmium (i.e., InP/CdS/ZnS) have applications in tissue imaging. As the synthesis of these four heterostructures is optimized, more information is gathered about the interplay between electronic structure, core and shell dimensions, and photophysical properties, including the emission wavelength, quantum yield, fluorescent lifetime, biexciton lifetime, blinking, and photostability. This body of knowledge enables us to design NQD heterostructures for a range of applications from lasing to photonics to biomedical imaging. 1. Chemphyschem 2007, 8, 823; Nature 1997, 388, 355; J Chem Phys 2000, 112, 3117; Nano Lett 2001, 1, 557. 2. Nature 2009, 459, 686. 3. JACS 2008, 130, 5026; J Biophotonics 2010, 3, 706; Nat Mater 2008, 7, 659; Phys Rev Lett 2009, 102, 136801. 4. Nano Lett 2009, 9, 3482; Nano Lett 2010, 10, 2401. 5. J Phys Chem C 2011, 115, 436.

Recent advances in the synthesis of nanoscale structures have extended to the formation of 3D structures of higher complexity than nanowires or quantum dots. For example, the highly symmetric branched structure and nanoscale geometry of the ‘tetrapod&’ is known to yield unique optical and electronic properties enabling the exploitation of these structures for applications including photovoltaics, scanning probe microscopy, and bio-labeling. Tetrapods have been formed in a variety of materials, primarily II-VI semiconductors such as ZnO, CdTe and ZnS. In this work, we report of the formation of silicon carbide tetrapod-shaped nanostructures synthesized via microwave-assisted plasma chemical vapor deposition. While the possibility of SiC tetrapods has been theoretically predicted, to the best of our knowledge, this is the first report of the synthesis of SiC tetrapods. The growth of the tetrapods is seeded from adamantane embedded in a silica sol-gel matrix. Our adamantane-seeded CVD growth of SiC is a facile approach for the formation of tetrapods, requiring no sophisticated chemical syntheses. The tetrapods were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and microphotoluminescence spectroscopy. The average size of the tetrapods can be controlled from between ~10 nm to ~150 nm arm to arm by tuning the growth conditions. Remarkably, however, the size distribution is uniform for an individual set of conditions. TEM confirms that the tetrapod nanoparticles synthesized in this work are SiC, having a zinc-blende (3C) core and wurtzite (4H) arms, in accord with the suggested model for SiC tetrapods. The tetrapods exhibit room temperature photoluminescence (PL). The emission wavelength of the PL is sub-band gap for SiC. The emission maximum varies for different tetrapods, appearing between 550 nm and 800 nm with a FWHM of ~5 nm. The mechanical strength and chemical stability of SiC coupled with the unique nanoscale structure of the tetrapod and their room temperature photoluminescence enables potential applications in photonics, biolabeling and sensing.

The anatase TiO2 is generally synthesized with crystal facets dominated by thermodynamically stable (101) surface rather than chemically reactive (001) surface. Recently, synthesis of anatase TiO2 with a large percentage of (001) surface is of great research interests because it has great potential in applications such as high performance catalyst, fuel cell, and chemical sensors. In this study, we report a simple hydrothermal approach for the synthesis of anatase TiO2 nanoparticles having (001) surface. A lowly toxic ammonium hexafluorotitanate (IV) is used as the Ti precursor instead of the highly hazardous hydrofluoric acid. The as-prepared TiO2 nanoparticles have a square shape and contribute to a large percentage of reactive (001) surface. The unique structures and the optical properties were characterized by SEM, TEM, XRD, Raman Spectroscopy and UV-Vis absorption spectroscopy.

Titanium oxides are intensively exploited in the field of photo-catalysis or photovoltaic energy conversion, due to their relevant physical properties, their non-toxicity and their relatively cheap synthesis. In particular, dye-sensitized solar cells (DSSC) based on nanocrystalline TiO2 electrodes demonstrate high power conversion efficiencies over 12%, which allowed the emergence of commercial products developed at low costs, as alternative to inorganic solar cells. However, although many research efforts have been devoted to identify alternative metal oxides or alternative electrode architectures, anatase TiO2 processed from colloidal pastes remain the material of choice to achieve the best performance up to now. Further development of DSSC will require the demonstration of novel device concept that may exploit additional charge generation mechanisms. In this context, several attempts are reported towards the doping of anatase TiO2 with transition elements in order to tune its electronic and optical properties. In particular, nitrogen-doping was used as a relevant strategy to reduce the optical band gap of the metal oxide, while improving the charge kinetics in DSSC (reduced recombination, improved charge transport). Although improved device performance is observed, only partial interpretations of the underlying mechanisms are given. In this work, we investigate the electronic properties of TiO2 and N-doped TiO2 nanocrystals synthesized by laser pyrolysis and used as building blocks for solid-state dye-sensitized solar cells. By exploiting both electron paramagnetic resonance analysis and numerical simulations, of the titanium oxide structure, we discuss the influence N-doping on the electronic and optical properties of the nanopowders, and we emphasize the role of ambient oxygen and light on their features. The photovoltaic performance of the corresponding devices is also discussed with regard to these elements.

Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. Here we report a novel method for the fabrication of metal selenide nanoparticles in organic semiconductor films that is compatible with solution processable large area device manufacturing. Our approach is based upon the controlled in situ thermal decomposition of cadmium selenide precursor complex in a film of the electron transporting material 1,3,5-tris(N-phenyl-benzimidazol-2-yl)-benzene (TPBI). Specifically we show that the photoluminescence quantum yield (PLQY) of the thermally converted CdSe quantum dots (QDs) in the TPBI film is up to 15%. The PLQY depends on the concentration of the solution, blend ratio of the precursor and the annealing temperature. Time-resolved photoluminescence studies show the fast energy transfer from the organic host to the emissive QDs. The results emphasize the importance of the alignment of energy levels between the host and guest dopants. We also show that laser irradiation can form the QDs from the precursor. This is an important result as it enables direct laser patterning (DLP) of the QDs. The DLP is performed on these blends with a picosecond laser at 266 nm wavelength at various irradiation doses. The confocal microscopy reveals the formation of the emissive QDs after laser processing. The optical and structural properties of the QDs are also analysed by means of UV-Vis, PL spectroscopy and transmission electron microscopy (TEM). The results show that the QDs are well distributed across the film and their emission can be tuned over a wide range by varying the temperature or laser power used on the blend films. Our findings provide a route to the low cost patterning of hybrid electroluminescent devices.

Three dimensional (3D) poly(3,4-ethylenedioxythiophene) (PEDOT) can be useful for infrared (IR) electrochromic devices which exhibits reversible changes of the refractive index upon doping in mid to long-infrared regime (3-5 µm, 8-12 µm) at low operational voltages (< 3 V). The difference in optical properties between the doped and undoped states of PEDOT can be enhanced and engineered by structuring PEDOT in the form a 3D photonic crystal. However the realization of 3D PEDOT has not yet been demonstrated. Here we present successful infiltration of PEDOT into 3D silica colloidal crystal templates by electropolymerization. Preliminary optical results show that 3D PEDOT improves the absorbance by 150% relative to that of a planar film for IR wavelengths. The contrast in IR transmission between doped and neutral states of 3D PEDOT is further enhanced. Moreover, the larger surface area from 3D structures compared to that of a planar film significantly reduces the switching time to less than 1 second by increasing the efficiency of electrochromic processes.

Many polymers degrade in-service due to exposure to near ultraviolet light. Such degradation can involve photo-oxidation reactions that proceed via free radical mechanisms causing polymer chain scission and branching. In this work, zinc oxide (ZnO) nanoparticles were synthesised by flame spray pyrolysis. The nanoparticles were then fired in a reducing atmosphere to produce luminescent zinc oxide (ZnO:Zn) particles. Both the as-prepared ZnO and the reduced ZnO:Zn nanoparticles were well characterised by means of X-ray diffraction, dynamic light scattering and laser Raman spectroscopy. A transparent polystyrene composite sheet incorporating reduced ZnO:Zn nanoparticles was produced using a solvent casting method. The composite sheet had comparable transmission to a virgin polystyrene film. This was achieved by uniformly dispersing the ZnO:Zn nanoparticles into the polystyrene, as is made evident by SEM images and optical micrographs. The photoluminescent characteristics of the ZnO:Zn, both as a pure powder and embedded in a polystyrene matrix, are reported. The ZnO:Zn pure powder can absorb broadly across nearly the entire near ultraviolet range and emit bright green light. The polystyrene host does not inhibit either the absorption or the emission of the ZnO:Zn. The uniformity of the photoluminescence of the composite sheet under near ultraviolet excitation is reported. The luminescent ZnO:Zn nanoparticles therefore have applications for use not only as an inhibitor of the ultraviolet degradation of polymers, but also for providing polymers with light emitting functionality.

The narrow emission spectra, tunabilitiy, and high quantum yields of colloidal quantum dots (QDs) have made them quite appealing candidates for various applications, including fluorescence probes for biomolecular and in vivo analytical applications, light emitting diodes, solar cells, etc. Currently significant research efforts are being devoted towards utilizing the plasmonic properties of metallic nanoparticles (MNPs) to improve the optical properties of QDs and envision new applications, such as active nanoparticle systems, nanothermometers, biological and chemical sensors, etc. A main feature of colloidal QDs, however, is that when they are irradiated, their fluorescence and physical structures can change significantly. This includes photo-induced processes which increase their emission efficiencies (photoinduced fluorescence enhancement) and photo-oxidation which can suppress their emission wavelengths and intensities. In this contribution we discuss the impact of such photoinduced processes (or light irradiation) on the plasmonic enhancement of emission of close packed CdSe/ZnS colloidal quantum dots when they are in the vicinity of gold metallic nanoparticles. Since in our samples QDs can interact with each other, our results reveal the interplay between QD interdot interaction and plasmonic effects. In this investigation, we vary the MNP sizes and laser intensities to examine how such an interplay is influenced by the heat generated by MNPs and their plasmonic field strengths. Our results outline the strong dependency of plasmonic emission enhancement of QD solids on the exciting light intensity. Therefore, these results are important for the diverse on-going research involving colloidal QDs and plasmonic effects.

Magnetic nanoparticles have potential applications in magnetic separation, tissue imaging, drug delivery, and information storage. Therefore their synthesis has scientific and technological interest. Recently, incorporation of metallic gold onto magnetic nanoparticles makes the core/shell composite nanoparticles extremely interesting for magnetic, optical, and biomedical applications due to their plasmonic properties, stabilizing effect toward corrosive biological conditions and well known Au-S chemistry which permits the easy functionalization of the surface. In this work, we have fabricated silica coated magnetic cobalt nanospheres decorated with gold nanoparticles (Co-SiO2-Au ). These magnetic and optical bifunctional nanoparticles take advantage of the strong resonance absorption for SERS studies and easy separation via external magnetic field. The performance of the prepared gold nanoparticles attached magnetic silica spheres as SERS substrate was evaluated using brilliant cresyl blue (BCB), rhodamine 6G (R6G) as model compound. The SERS detection of 4-mercapto benzoic acid (4-MBA) from aqueous solutions using bifunctinal Co-SiO2-Au nanoparticles were also investigated.

We present a facile method for the synthesis of uniform Au rhombic dodecahedra via seeded growth with well-controlled sizes and optical properties. We could reproducibly obtain Au rhombic dodecahedra with a narrow size distribution (<5% in standard deviation) and in high percentage (>90%). In order to synthesize Au rhombic dodecahedra with uniform in shape, controllable sizes, and in high percentages, N,N-dimethylformamide (DMF), which can stabilize {110} facets of Au nanocrystals, was adopted both as a solvent and a reducing agent to a seed-mediated growth method, which could offer great flexibility in controlling both the shape and the size of the nanocrystals, using single-crystal Au nanocrystals with uniform in shape and size as seeds. Meanwhile, since only poly(vinyl pyrrolidone), a common stabilizer for nanocrystals, in the growth system using DMF often resulted in the formation of Au nanocrystals enclosed by {111} facets, trisodium citrate was additionally introduced in the present system as a stabilizer for the formation of Au rhombic dodecahedra. Moreover, size of uniform Au rhombic dodecahedra can be systematically controlled over a wide range and each size of Au rhombic dodecahedra was selectively synthesized in high percentage. The edge lengths of these Au rhombic dodecahedra could be readily controlled from 19 to 67 nm in a controllable fashion by varying the amount of seeds or concentration of HAuCl4, or both. The corresponding localized surface plasmon resonance peak positions of the Au rhombic dodecahedra could be continuously shifted from 532 to 655 nm depending on their sizes. The uniform shape and size of the Au nanocrystals allowed us to gain better understanding of the effects of various reaction parameters on the evolution of nanocrystals, including water contents, concentration of trisodium citrate, and reaction temperture.

Semiconductor-metal heterostructures form an important class of materials as they are useful in photocatalytic, labeling and other optical applications. For enhanced property there is a need for cleaner surface as it promotes facile electron transport. Physical deposition methods are useful in creating such clean surfaces but controlling particle size and uniformity is difficult to achieve. However, by wet-chemical methods control of particle size is good and uniform, but the use of surfactant makes it difficult to clean and often a monolayer persists. Also it is not desirable to use such hazardous surfactant molecules as most of them are not very environment friendly. Therefore we propose a surfactant-less wet-chemical route which combine the advantages of both the methods. Surfactant-less synthesis is already known for 2D Au structures in the form of Au triangles and hexagons but their sizes are in microns. Here we demonstrate the use of support to control particle size of Au to few nanometers by rapid microwave method. CdS-Au has been chosen as the model system to demonstrate the principle in detail. The modification of surface states of CdS-Au hybrid has been discussed along with its effect on optical properties with loading.

The thermodynamic stability of ZnTe/ZnSe and ZnSe/ZnTe core/shell quantum dots (QDs) was studied experimentally by monitoring the evolution of their near-surface elemental composition using X-ray photoelectron spectroscopy (XPS). Core/shell QDs were synthesized using established protocols involving injection of precursors into a hot mixture of coordinating solvents consisting of hexadecylamine (HDA) and trioctylphosphine (TOP). The precursors used in this study were diethylzinc, selenium powder dispersed in TOP, and tellurium powder dispersed in TOP. QD cores with average size of 2.3 nm were grown and capped with thin shells. Transmission electron microscopy (TEM) was used to measure the core size and shell thickness. Samples were extracted during shell growth, performed at 235 degrees Celsius, and the near-surface elemental composition of the QDs was measured using XPS. For ZnSe/ZnTe core/shell QDs, the near-surface elemental composition rapidly evolved during shell growth and reached a plateau corresponding to a Te-rich surface after 10 minutes of processing. For ZnTe/ZnSe core/shell QDs, the near-surface elemental composition of the QDs evolved more slowly and did not become Se-rich even after one hour of processing. Thermal annealing studies of core/shell QDs were also performed. QDs with 2.3 nm cores and 0.2 nm thick shells were purified and injected into a fresh mixture of HDA and TOP that was kept at 235 degrees Celsius. Samples were extracted at specific time intervals over a period of one hour and the near-surface elemental composition of the QDs was measured using XPS. In both types of QDs, Te segregation towards the surface was observed. The surface segregation of Te was much more pronounced in ZnTe/ZnSe core/shell QDs in comparison to ZnSe/ZnTe ones, indicating higher stability of the latter. The experimental observations are consistent with theoretical predictions of Te surface segregation in ZnSe(1-x)Te(x) ternary QDs [S. C. Pandey et al., Appl. Phys. Lett., 2010, v. 96, 201910].

It is known that the source of photo-enhancement for single quantum dots (SQDs) is surface passivation by photo-induced charge carriers and the formation of additional quantum states in the quantum dots when the core is neutral and the shell charged. This effect has been studied through observation of the blinking behavior over long time scales for colloidal single quantum dots (SQDs) at low excitation intensities. This research explores the blinking behavior of ensembles of single quantum dots over time scales much greater than the blinking of the individual SQDs and how blinking and photo-enhancement are influenced by photo-oxidation. A laser source focused through a high numerical aperture (NA) microscope objective is used to excite an ensemble of SQDs and collect the emissions. Samples are generated to provide a typical separation distance between the quantum dots such that the emissions from SQDs are resolvable (approximately 0.7 microns). The emissions are photographed and analyzed to determine the impact of excitation source intensity on the photoluminescent blinking behavior of both single SQDs and their ensemble. Analysis of the emission photographs shows the presence of large time scale individual and ensemble blinking behavior. At low excitation intensity, photo-enhancement dominates and the total intensity of the ensemble shows a net increase while the emission intensity of the ensemble members/SQDs fluctuates. At high excitation intensity, photo-oxidation dominates and the total intensity of the ensemble shows a net decrease. The emission intensity fluctuations of ensemble members are also observed during photo-oxidation. The fluctuations observed in the total emission intensity are additionally studied using off-resonant and resonant photoluminescence spectroscopy; corroborating the behavior of the ensemble over large time scales.

Quantum-confined semiconductor nanostructures have size-dependent optical properties that opened up possibilities for revolutionary advances in semiconductor-based devices, such as light-emitting diodes, lasers, and solar cells. However, application of the nanostructures to real-world devices has been strongly curtailed by the enhancement of dissipative Auger processes that undergird all aspects of carrier relaxation and recombination. Fortunately, according to recent experiments, nanostructures with graded confining potentials may present a route to substantially suppress the rates of the Auger processes [1]. Here we provide a reliable theoretical description of the Auger processes in nanostructures with graded (as opposed to abrupt) confining potentials. We investigate the Auger recombination processes in III-V alloyed heterostructures with gradually varying composition, where the range of carrier densities chosen is similar to the typical operating densities in quantum well lasers and light emitting diodes. The calculations are performed in the framework of the eight-band effective mass approach, accounting for the realistic electronic structure and capturing the major properties of the materials in question. The present study is a generalization of the previous endeavor by G. Cragg and Al. L. Efros [2], where a simplified one-dimensional case was studied. Thus, a fully three-dimensional model has been developed, based on an analytical solution of the effective Schrodinger equation, describing III-V semiconductor heterostructures. The calculations demonstrate that significant quenching of the non-radiative Auger recombination could be reached by varying the degree of smoothness, shape, depth, and width of the confining potential. [1] X. Wang, X., et al, Nature 459, 686 (2009). [2] G. Cragg and Al. L. Efros, Nano Lett.10, 313 (2010)

Size-tunable optical properties of colloidal nanocrystals (NCs) makes them promising for a variety of applications. Growing attention to CdSe/CdS core/thick shell NCs (“giant NCs”) is stimulated by their superior optical properties over any other NC heterostructures prepared up to now. The photoluminescence is never completely quenched in these NCs and the blinking is almost completely suppressed. At low temperatures these structures demonstrate suppression of non-radiative Auger recombination and almost 100% photoluminescence quantum yield. These outstanding optical properties are associated with a suppression of the nonradiative Auger recombination of charged excitons and biexcitons. To understand the unusual and potentially useful properties of the giant NCs we have conducted theoretical analyses of the fine structure of the band edge excitons, trions and biexcitons. The theory takes into account the multiband structure of the valence band, complex structure of the inter-particle Coulomb interaction as well as temperature dependence of the conduction band offset, which transfers the quasi-type II CdSe/CdS structure into type I at low temperatures. The calculations allow us to explain temperature dependence of the radiative decay time and the suppression of the nonradiative Auger recombination observed in the giant CdSe/CdS core shell NCs.

Functional nanocomposites consisted of quantum dots (QDs) and silica spheres have been widely studied to take advantage of unique optical properties of QDs and high stability and well-known surface chemistry of silica materials. Recently, we have reported the enhanced photoluminescence (PL, 2~3 times of the starting QDs) of QDs self-assembled and encapsulated in the silica submicrospheres, where the order of PL enhancement according to the composite diameter was experimentally 300 nm > 900 nm > 70 nm. However, we reasoned that the PL enhancement of 70 nm sized composite could be improved further, since the light scattering effect could be reduced if we can control the aggregation of nanocomposites. In this study, we have improved the PL enhancement far more (up to ~10 times of the starting QD-MPA) by synthesizing the nanocomposites with reduced degree of aggregation and have shown that the nanocomposite could be integrated for white LED packaging with better color rendering index (CRI). The current nanocomposites are consisted of a silica core (~60 nm) with aminopropyl moieties, a self-assembled QDs layer, and a silica shell. For a typical synthesis, the silica cores were adjusted to pH ~4 for a positive charge. The surface of CdSe/CdS core/shell QDs were modified with mercaptopropionic acid (MPA) and adjusted to pH ~10 for water-solubility and a negative charge. Slow addition of silica cores into QD solution developed self-assembled QDs on the silica core, which were then encapsulated with silica to produce a stable core/shell/shell composite structure with enhance PL intensity. The fabrication of white LEDs using the current nanocomposites (1%), together with blue LED and yellow phosphor (YAG:Ce, 10%), showed efficacy = 61.9 lm/W and CRI = 71.3, which was compared with the case without nanocomposites showing 86.6 lm/W and CRI = 65.4. In summary, we have shown the synthesis of silica nanospheres encapsulating QDs with far more enhanced PL intensity and their integration into white LED packaging with improved CRI. Further collaborative study is under way to reveal the theoretical reason for the enhanced PL intensity.

Organic fluorescent dyes due to their interesting opto-electronic properties have found potential applications in the area of sensing and organic electronics. These dye molecules display high fluorescence quantum yields in dilute solutions, since the interaction between two fluorophores is minimal. However, for concentrated solutions and in solid state the fluorescence is highly quenched. The quenching of fluorescence in aggregated state has been attributed to a variety of non-radiative relaxation pathways the exciton can follow. Recently a class of fluorescent conjugated molecules was reported that displays the opposite effect. In these molecules the quantum yield of fluorescence increased upon aggregation and the effect was named aggregation induced enhanced emission (AIEE). AIEE has been reported in donor - acceptor dyes containing strong electron donating and withdrawing groups. However, the effect of aggregate size on the PL intensity has not received much attention. Here we present detailed photo-physical investigation of the AIEE characteristics of a “donor-acceptor” triphenylamine with weak donor and acceptor moieties. We also correlate the size of the aggregates to the fluorescence properties. We observed that the properties of the surrounding medium can have a significant impact on the AIEE properties. Concomitantly, the size of the formed particles influences the photo-physical properties of the aggregates. We observed that a critical aggregate size is required for obtaining the enhancement in fluorescence. The dependence of the critical aggregate size on the nature of the utilized solvent - non-solvent mixture will also be discussed.

It is well known that metallic nanostructures are optical resonators that support plasmonic resonances due to excitation of collective free electron oscillations. In recent years, it is shown that high index semiconductor nanostructures (e.g. Si, Ge nanowires) are also optical resonators that support Mie resonances. In this work, we will discuss strategies of combining optical resonances in metallic and semiconductor nanostructures in hopes of achieving novel optical responses from such hybrid nanostructures due to interactions of optical modes of different nature. We will show examples of structures that combine gold and silicon which exhibit optical cloaking, enhanced optical response and tailored polarization response, all of which can be rationally designed and engineered. We will also demonstrate optoelectronic devices that take advantage of these novel optical response from hybrid semiconductor/metal nanostructures, such as invisible photodetector, highly efficient and compact photodetector, and other potential applications for sensing, imaging and photovoltaics.

Recently, de Boer et al reported a remarkable redshift of excited PL band in Si nanocrystals embedded in a SiO2 matrix as reducing the size. This band starts from about 3 eV at diameter 5.5 nm and reaches at diameter 2.5 nm 1.98 eV. This emission was assigned by the authors to Γ- Γ direct band gap transitions despite the common expectation that quantum confinement should lead to blue shift as size is reduced. De Boer et al using effective mass models by assuming a negative effective mass of Γ-electron supported this assignment. Here we analyze the absorption spectra of Si nanocrystals both via theoretical calculations using state-of-the-art atomistic pseudopotential method and by experimental measurements of two different types of samples. To test the hypothesis of de Boer about the intrinsic (non surface defect) origin of the effect, we have removed in our atomistic simulation surface states by embedding the Si nanocrystals in a widegap matrix, so the results must reflect intrinsic physics. Although surface defect related transitions were frequently observed in photoluminescent (PL) spectrum of Si nanocrystals, in absorption spectrum its relative weak signal shouldn&’t mask the strong signal of direct bandgap transitions, which is reflected in the bulk crystalline Si spectrum as a sharp jump. However, in both our atomistic simulation and experimental measurements, no redshift of this sharp jump is observed in absorption spectra as reducing the Si nanocrystal size. To further distinguish real direct Γ- Γ transitions from quasidirect Γ-X transitions in absorption spectra, we perform a projection of our atomistic calculated electron states of Si nanocrystals to bulk Bloch states including Γ, X, and L. We indeed find that as nanocrystal size is reduced the Γ-derived states are blue shifted, which explains why there is no redshift of direct band transitions observed in our Si nanocrystal absorption spectra. Instead, the Γ-X coupling is significantly increased, especially for states close to bulk Γ-electron in energy, upon reducing the size. We conclude that the observed redshift hot PL peak is due to a surface-state radiative channel rather than quantum confinement induced channel. We do clearly observe enhanced light absorption in Si nanocrystals in comparison to in bulk Silicon below the bulk Γ- Γ direct bandgap transitions (3.2 eV). We assign this enhanced absorption to interface induced Γ-X coupling of electron states in Si nanocrystals.

A quantum dot (QD) has recently attracted much attention in material science and industrial applications. This is because many distinct optical properties appear by reducing the particle size to an order of nanometer. Namely, the band gap energy, luminescence wavelength, and transition probability are tuned by changing the QD size. In particular, Multi-Exciton Generation (MEG) of QD has been investigated extensively in various QD systems, e.g. PbSe, PbS, PbTe and so on. The reason for the extensive researches on MEG is an excellent property for developing new-generation photovoltaic. Since such a MEG effect has been discovered in silicon (Si) QD, the absorption process of Si-QD has been further crucial topic. Recently, the Si-QDs have been synthesized by various methods, e.g., chemical synthesis, decomposition of silane gas via plasma process, electrochemical etching of Si wafer, and laser ablation. We have fabricated Si-QDs by pulsed laser ablation of Si crystal in liquids. This method has several excellent properties. i) easy process such as 1 step and 1 pot synthesis, ii) QDs are dispersed in solution, iii) QD size of an order of nanometer is easily obtained in a short time, iv) easy surface passivation of QD by solvent molecules. Here we show the Si-QD synthesis by pulsed laser ablation in various organic liquids. The obtained Si-QDs were investigated by dynamic light scattering, UV-Vis-near IR absorption spectrum, FT-IR spectrum, and ICP-OES. As a result, it was ensured that the average size of Si-QDs is 1.1 nm. The absorption coefficient is 10-100 times larger than those of amorphous and crystal Si. Note that the absorption coefficient at the wavelength of 520 nm, in which the maximum of the sun power spectrum exits, is 40 fold greater than that of bulk crystal silicon. In addition, we ensured that the molar extinction coefficient of generated Si-QD at 350 nm is 3 times larger than the published those of all Si-QDs that have the similar size to the current QDs. According to the FT-IR measurements, the surface of Si-QDs passivated by oxygen and carbon atoms was revealed from the observation of Si-C and Si-O vibrational modes. In conclusion, it was considered that very small size (1.1nm) and the surface passivation accomplishes the giant absorption and molar extinction coefficients of current Si-QDs. We will report the solvent dependences of extinction coefficient, size, concentration, spectral shape, and band gap energy of Si-QDs.

It is a well-known fact that the quantum confinement modifies the electronic structure of nanoparticles when their radius is comparable to or smaller than the exciton Bohr radius. On the other hand, the nanoparticles doping leads to phenomena not found in the bulk because their electronic states are confined to a small volume. The most common strategy for doping is to include in the synthesis a precursor containing the impurity. CdS:Li nanoparticles were synthesized using cadmium chloride (CdCl2), thiourea (H2NCSNH2) and lithium chloride (LiCl) dissolved in a stabilizing agent or surfactant (oleylamine). In this case, the nanoparticles growth and the passivity of the dangling bonds are controlled by the surfactant in the solution. The main objective of this work was to study the effect on the optical and structural properties of the CdS nanoparticles produced by the Li incorporation. The study was carried out by means of the optical transmission, photoluminescence, X-ray diffraction and HR-SEM techniques. From transmission measurements the optical energy band gap was estimated and values from 3.6 to 4 eV (Eg=2.42 eV in bulk) were obtained. The values depend upon the nominal lithium percentage in the nanoparticles. The photoluminescence spectra present a line placed at 455 nm (512 nm for the material in bulk). No shift in the peak position was observed in the spectra, which suggests that the electronic levels generated by the doping process are very close to the conduction band. However, a significant change in the peak intensity is exhibit, which varies with the amount of lithium. The X-ray patterns show that there is no displacement of the peaks with respect to their position in bulk material, indicating that no significant changes in their lattice parameters. A particle size of about 5 nm was estimated using the Scherrer equation, which is in agreement with high resolution scanning electron microscopy measurements. This work was partially supported by VIEP (HETM-ING10-I, GRJJ-EXC10-G) and SEP (BUAP-CA-190). The author have a fellowship of CONACYT No 211081.

Color changing materials to the external stimuli, such as electric, chemical or magnetic signal, have received a great deal of attention for sensors as well as fabric, paint and many other designing materials. Here, we developed a new color changing microstructure capable of being a pixel of color changing surface. This micostructure contains one dimensionally assembled magnetic nanoparticles which play roles of both one dimensional bragg reflector and magnetic axes. The structural color is generated by this bragg scatterer and changed by tilting the microstructure using the magnetic axes. This strategy offers very simple fabrication and operation method for color changing surface with high resolution. The magnetic nanocomposite material for color changing microstructure is based on a combination of photocurable polymer, PEGDA 258 with 10% photoinitiator, and superparamagnetic nanoparticles. The superparamagnetic nanoparticle consists of several single domain magnetites and this core is capped with negatively charged material and silica shell. The nanoparticles are randomly dispersed in resin without the applied external magnetic field. However, when the magnetic field is applied, they aligns along the magnetic field line forming chain-like nanostructures. If the magnetic field line direction is changed, the aligned nanostructures rotate following the changed field line to minimize the magnetic dipole interaction energy of the system. This self-assembling behavior is exploited both to generate the structural color and the rotational property and to drive microstructure to change the color. To fabricate these microstructures, first, the polymer containing nanoparticles is injected on the partially coated glass substrate. By the application of the external magnetic field, the nanoparticles are self-assembled and the entire area shows a specific color by this regular bragg reflector. And, we photopolymerize a microstructure confining the aligned nanoparticle. After the remaining resin is exchanged to the appropriate environment, the colored microstructure is actuated to change the color. The reflected color is changed by tilting the microstructure because the actuation of the microstructure changes the reflected angle of incident light and the angle dependence of the color is fundamental property of the structural color. The resulting microactuator can obtain all red, green and blue colors originally. In the case of red colored microactuator, by the application of the magnetic field, the microactuator bends changing its color from red to green and blue. As the deflection angle increases, the reflected color is blue-shifted. The color spectrum of the microactuator is directly related to the deflection angle, and the deflection angle can be analyzed using a simple analytical model. Acknowledgement: This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (2011-0030269).

We report on the behavior of electron and hole transport and resulting distributions of carriers in III-nitride (III-N)-based light-emitting diodes (LEDs) in relation to the effect of hole injection layers and electron blocking layers (EBLs). In order to effectively trace the influence of the transport of carriers and resulting distributions for radiative recombination, we employed a triple-wavelength (TW)-emitting multiple-quantum-well (MQW) active region which has different In content in each InxGa1-xN QW. In addition, Si doping was introduced in selected quantum-well barriers (QWBs) to intentionally control the carrier transport. LED epitaxial structures with various hole-injection layers and with and without EBLs were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition in a Thomas Swan 6×2Prime; reactor system. The electro-optical properties of TW-LED structures were characterized from both fabricated LEDs and as-grown LED structures. In the case of the LEDs with higher indium mole fraction in p-InxGa1-xN, emission from QW1 becomes stronger. This gradually increased EL intensity of QW1 compared to QW2 and QW3 for the LEDs with increasing indium mole fraction in p-InxGa1-xN layer indicates that more holes can be transported to the lower QW by hole injection layers. The EL spectrum of the TW-LED without an EBL showed the highest emission peak at QW3, one closest to the p-type layer, and gradually decreased in QWs with increasing distance from the p-GaN layer. For the TW-LED with an InAlN EBL, QW2 has the highest emission peak intensity among the three QWs even at low injection current and the emission from QW1 is also much higher than that of the TW-LED without an EBL. To further investigate the carrier transport, we utilized Si doping in a selected QWB. The Si-doped QWB acts as a hole blocking layer so that the hole transport into lower QWs is hindered and correspondingly, most holes are confined at QW3. We will compare electroluminescence characteristics of TW-LEDs employing InAlN and AlGaN EBLs in comparison to one without EBLs and employing various p-InxGa1-xN layer in order to distinguish the carrier transport and distribution in the active region. This different carrier dynamics and related efficiency droop behavior will be further discussed.

Nanoparticles (NPs) with sizes less than 30 nm have strong size-dependent Raman spectra and photoluminescence (PL) spectra due to the phonon confinement and quantum confinement effect, respectively [1-2]. Both the phonon confinement and the quantum confinement effect provide a convenient way to characterize the size of nanoparticles by simply using Raman and PL spectroscopy. We explored the phonon confinement effect in anatase TiO2 NPs and the quantum confinement effect in ZnO NPs, together with fast grain growth kinetics in these NPs as thermosensor materials. When TiO2 and ZnO NPs are heated up, their size will grow as a function of temperature and time. The temperature- and time-dependent of grain growth is monitored by measuring the size-dependent Raman and PL spectra. This allows us to forensically retain the complete thermal history (temperature and time) of an event that they went through. Our study showed that both temperature and time can be determined simultaneously by using these nano-thermosensors in the range of 400-800C and 0.3-60 s, assuming that the temperature is constant (a step-function approximation to a thermal spike) during a thermal event. These nano-thermosensors can be loaded into the one-dimensional cylindrical pores (5-30 nm) of mesoporous silica particles (SBA-15), to be used in hostile environments. SBA-15 particles serve as the carriers and protectors for the nano-thermosensors encapsulated inside, which record the thermal history through grain growth during the thermal event. By spatially distributing these bare or encapsulated thermosensors, a spatially and temporally non-uniform thermal environment can be determined by a direct read off their Raman/PL spectra at various locations. 1. V. Swamy, A. Kuznetsov, L.S. Dubrovinsky, R.A. Caruso, D.G. Shchukin, and B.C. Muddle, Finite-size and pressure effects on the Raman spectrum nanocrystalline anatse TiO2, Physical Review B, 71, 184302 (2005). 2. K. Lin, H. Cheng, H. Hsu, and W. Hsieh, Band gap engineering and spatial confinement of optical phonon in ZnO quantum dots, Applied Physics Letters 88, 263117 (2006).

A solvent tunable polymer film synthesized from a bifunctional monomer BAHB, 4,4&’-bis(6-(acryloyloxy)-hexyloxy)biphenyl with a photonic structure as a new photonic band gap (PBG) material has been developed by imprinting the helical structures on polymer matrices through multiple photocrosslinking in an induced chiral nematic mesophase. With increasing the times from one time to seven times of photocrosslinking/diffusion procedure, the density of the polymer structure in UV irradiated area was increased due to the diffusion of more amounts of monomers. Here, the polymer matrices themselves served as a chiral template, which exhibited Bragg reflections in the absence of both a chiral dopant and anisotropic materials due to the memory effects of the polymer network. Tuning of colors from blue to red was achieved by making a refractive index contrast in the two periodic media of imprinted solid helical structure and the isotropic liquids that fill it. Upon incorporation of various isotropic liquids, such as methanol, acetone, chloroform, THF and toluene, in the imprinted matrices, a sharp peak in the reflection spectrum shifted drastically from 452 to 658 nm, which indicated that the wavelength shifts strongly depended on the sort of liquids that filled the matrices. The effects of temperature on the imprinted polymer template feeding the various liquids were studied through the reflectance spectra. The fabricated sample cell exhibits a significant reflection band even though the cell temperature is higher than the clearing temperature 118oC. This result suggests that filling the cell with isotropic materials could also reflect the incident light, revealing Bragg reflection. As far as we know, this is the first report regarding the fabrication of solvent tunable photonic films using imprinting methods.

Electronically interfacing light responsive macromolecules or small molecules with multi-walled or single-walled carbon nanotubes (SWNTs) opens up possibilities for new types of photodetectors, light-gated transistors, and energy storage devices. Here, the orientation and photoisomerization kinetics of a monolayer of azobenzene chromophore on SWNTs was probed using optical second harmonic generation (SHG). The monolayer of chromophore was created by non-covalently anchoring a pyrene functionalized Disperse red 1 (DR1P) onto the SWNTs. With a coverage of 3 chromophores per 100 carbon atoms on SWNTs, the p-polarized SHG is sufficiently above the noise to measure the SHG angular dependence for both s and p polarized fundamental, enabling the measurement of an average chromophore tilt angle of 40 ± 3 degrees. Reversible switching between the trans and the cis form was achieved by cycling illumination of a 495 nm LED. Chromophores in the cis state form a dense layer on the nanotube sidewalls, with steric interactions that would be expected to alter the first order cis-trans back-isomerization kinetics commonly observed in solution. The evolution of the SHG signal presented biexponential time constants for 495 nm illumination representing cis-trans back-isomerization kinetics which were T1 = 7.6 s and T2 = 75.3 s. The biexponential kinetics observed here have also been observed for DR1 in a polymer host, in which the cis-isomers were trapped in a strained conformation, leading to an anomalously fast component of the relaxation; in these polymer systems the fast constants were reported to be asymp; 0.3 ~ 0.5 s-1, with the slow constant an order of magnitude larger. As a control, identical illumination experiments with 710 nm light resulted in a very different behavior with no detectable changes in SHG signal since 710 nm is well beyond the lambda;max of DR1P and illumination at this wavelength should dramatically slow the rate of trans-cis isomerization. Upon applying an electric field, chromophore orientation was significantly enhanced and the cis-trans back isomerization time constants were effectively controlled. The electric field dependence suggests that the gate field of hybrid SWNT-chromophore transistors can act analogous to a poling field, controlling both the chromophore orientation and dynamics. These results are consistent with previous studies of chromophore/SWNT hybrids configured as transistors and provide evidence that trans-cis photoisomerization is responsible for light-induced changes in SWNT-chromophore transistors. Our use of SHG to probe chromophore orientation in hybrid nanotubes should be applicable to a wide range of current research in these systems, including nanotube-hybrids as tunable photodetectors, functionalization of multi-walled nanotubes, and azo-benzene containing polymers wrapped on nanotubes.

When noble metal nanostructure is optically excited, localized surface plasmon is generated, which produces large electric field at the nanostructure surface. When a molecule near the surface is excited by such large localized electric field, Raman and fluorescence intensities increase dramatically. Thus, various research groups have reported fluorescence-intensity enhancement effect as metal-enhanced fluorescence (MEF). According to recent review articles on MEF, almost researches of MEF have been conducted using gold or silver nanoparticles, and a typical value of fluorescence-intensity enhancement factor (EF) has been 20. On the other hand, large enhancement factors were reported using a bow-tie-shaped gold nanoantenna (EF=1340) and a Au/Ag bimetallic nanostructure (EF=4000). These values are very large as EFs of MEF, but are significantly smaller than EF of surface enhanced Raman scattering (SERS), e.g. SERS EF ranging from million to billion. To realize the high EF for MEF, the crucial issue is how to reduce fluorescence-intensity quenching via energy transfer from excited molecules to the substrate metal. We investigated whether the fluorescence intensity can be enhanced using silicon (Si) particle. Si is a typical semiconductor material and has the following characteristics: (1) It is an indirect-transition semiconductor that can reduce fluorescence-intensity quenching due to forbidden transition. That is, energy transfer from a fluorescent molecule to an enhancement substrate may be suppressed. (2) it is nontoxic and exists in great abundance, and (3) it is inexpensive, because high purity Si is not required for an enhancement substrate. Accordingly, if the enhancement effect is observed with Si, the development of economical enhancement substrates formed of a nontoxic quench-free material would be possible. That is, the number of practical applications will increase, such as those involving high-sensitivity biosensors and LED-intensity enhancement. Here we show the fluorescence intensity of crystal violet (CV) solution using Si fine particles of submicron size. The fluorescence-intensity enhancement was measured under the condition of a single Si particle measurement using fluorescence microscope spectrometer. That is, Si fine particles with the diameter of 500 nm gave the enhancement factor of dye molecules up to 500, whose value is 20 times larger than a typical fluorescence-intensity EF using a noble metal nanoparticle. By measuring the EFs and scattering spectra as functions of particle size, we revealed that the localized electric field at the Si fine particle causes the fluorescence-intensity enhancement of crystal violet.

Photochromism is the reversible conversion of a chemical species that describes a change of color in the presence of ultraviolet (UV), visible light. Photochromism is great of interest for some applications, such as rewritable color copy paper, multiwavelength optical memory, holographic data storage and smart glasses [1]. TiO2 is one of the most popular photochromic materials being used due to its adjustable semiconductor property; however it needs to be doped with novel transition metal silver to obtain and enhance this property. Upon exposure to visible light, electrons of silver nanoparticles migrate to the conduction band of TiO2, which activates the oxidation of Ag0 into Ag+ species. This reversible transformation can be observed from the initial brownish-gray color change as a result of its interaction with visible light and the following color change back to the initial starting color with UV light [2]. Accordingly, in the current study, the potential of using Engelhard Titanosilicate (ETS-10) for photochromic applications was investigated that can be of interest due to the distinctive nature of ETS-10, which contains uniquely arranged -Ti-O-Ti-O-Ti- chains that are regarded as a 1-D quantum confined form of titania with a band gap energy of 4.03 eV. For this purpose, Ag+ loaded ETS-10 microcrystals were obtained and their thin films were fabricated successfully. Ag0 nanoparticles were formed on ETS-10 nanocrytals by thermal treatment of ETS-10 at 200, 300, 400, and 500 °C. It was observed that 10-30 nm Ag0 nanoparticles were successfully formed at 500 °C and their distribution on the crytals was very homogeneous. Then, they were subjected to visible light to investigate the reversible transformation into Ag+ in order to achieve the photochromic behaviour. The yellow color of Ag loaded ETS-10 film observed after thermal reduction had disappeared and the films had become semi-transparent with Visible laser exposure. The extent of transformation was investigated as a function of Visible Laser exposure parameters, such as 600 mm/s of rate, 15 kHz of frequency, 20-24 mA of current. The consistent change from Ag0 to Ag+ as a function of the harshness of such parameters was observed by UV-Vis Spectrophotometer, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscopy (SEM) measurements. Accordingly, the photochromic behavior observed for the first time can be ascribed to the -Ti-O-Ti-O-Ti- quantum wire. References: [1] Crespo-Monteriro, N., Destouches, N., Nadar, L., Reynaud, S., Vocanson, F., Michalon, J.Y., Irradiance Influence on the multicolor photochromism of mesoporous TiO2 films loaded with silvernanoparticles, Applied Physics Letters, 99, 173106, 2011 [2] Naoi, K., Ohko, Y., Tatsuma, T., TiO2 films loaded with silver nanoparticles: Control of Multicolor Photochromiic Behavior, Journal of Chemical Society, 2004, 126, 3664-3668

Considerable research in the past decade on improving output power and reducing cost of GaN-based light emitting diodes (LEDs) has led to their successful commercialization in areas such as solid-state lighting, large-panel display and back lighting for liquid crystal displays (LCD), with the advantages of high brightness, low power consumption and long lifetime. On the other hand, specialized LEDs such as LEDs with polarized light emission are also highly desirable for their potentials to make the imaging and display systems more compact and robust. In this work, we demonstrate a novel way of producing polarized light emission directly from InGaN/GaN multiple quantum well (MQW) LEDs by integrating subwavelength metallic gratings with the LED chip. Subwavelength aluminium gratings are fabricated on top of the p-contact layer of InGaN LEDs grown on sapphire substrate by metalorganic chemical vapour deposition (MOCVD). To explore the grating period dependent polarization performances, gratings with different periods ranging from 150 nm to 300 nm in step of 50 nm are fabricated. Both blue (center wavelength 450 nm) and green (center wavelength 520 nm) LED chips are used for comparison. It is shown that the smaller period grating renders better polarization performance and the subwavelength gratings are more effective for longer wavelength lights. Two-dimensional finite-difference time-domain (FDTD) analysis is performed to fully examine the polarization behavior of subwavelength metallic gratings. The trends predicted by simulation agree well with the experimental data.

Solar selective absorbers, which maximize solar absorption and minimize thermal emittance losses, are important components for concentrated solar power (CSP) systems in converting optical power into thermal energy. The development of solar selective absorbers operating at high temperatures >500 C is of particular interest since high temperature operation offers higher steam turbine efficiency. The main requirements for high-temperature solar selective absorbers are high solar absorptance, low thermal emittance in the mid-infrared regime, and good thermal stability at temperatures >500 C. In our previous work, we demonstrated solution-derived Ni nanochain-Al2O3 cermet with >90% solar absorptance, <10% thermal emittance, and thermal stability up to 400 C. This performance is comparable to conventional cermets fabricated by more costly vacuum deposition technology. It is also found that the metal nanostructures have enhanced solar absorption efficiency due to the surface plasma polariton (SPP) mechanism. In this work, we extend our investigations to a new NiSi nanostructure-SiO2 cermet solar selective absorber with thermal stability up to 700C. NiSi exhibits metallic behavior in electrical and optical properties, while its thermal stability and resistance to oxidation are far superior to metal materials. Due to this reason, it has been widely used in integrated circuits for high thermal stability, low resistivity electrical contacts. However, NiSi-based high-temperature solar selective absorbers have never been investigated in literature. Our optical simulations show that the absorption cross-section of plasmonic NiSi nanostructures is 3~4 times larger than Ni nanostructures of the same feature size in ultraviolet and visible regime, and 7~10 times larger in the near-infrared regime of the solar spectrum. NiSi nanostructures embedded in SiO2 matrix are fabricated by spin-coating a suspension of Ni nanochains dispersed in hydrogen silsesquioxane (HSQ) solution, followed by annealing in N2/H2 forming gas. Upon annealing, HSQ undergoes phase separation into Si nanoclusters embedded in SiO2, which then react with Ni nanochains to form NiSi nanostructures. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to characterize the nanostructured NiSi formation process and optimize the NiSi fabrication parameters. To increase the load of NiSi nanostructures in SiO2 matrix, we also investigate the spin-coating and reaction process of HSQ/Ni nanochain multilayers with different layer thicknesses. The solar absorptance and thermal emittance are evaluated using ultraviolet-visible-near infrared spectroscopy and Fourier transform infrared spectroscopy (FTIR), respectively. The preliminary results indicate that the solution-derived NiSi nanostructure-SiO2 cermet is a promising candidate for high-performance, low-cost, and thermally stable solar selective absorbers up to 700C.

Colloidal photonic crystals, the periodic structures of monodisperse colloids, have attracted great attention due to their selective light reflection properties and potential applications. Recently, many researchers have tried to fabricate responsive colloidal photonic crystals to tune the optical properties via external field. For example, magnetic or electric anisotropy was added to the colloidal photonic crystals by using iron oxide or carbon black particles. Due to the net magnetic or dipole moment, structural colors from photonic crystals could be controlled under external magnetic or electric field. However, colloidal photonic crystals in previous researches have limits to the tunable range of photonic bandgaps, which could only display the on-off control of single bandgaps or the limited change of double photonic bandgaps. In this study, we reported microfluidic synthesis of colloidal photonic crystals including multiple and magnetically tunable photonic bandgaps by using different sizes of colloidal nanoparticles and magnetic particles. Cylindrical microfluidic chip with four inlets were prepared by bonding two hemicylindrical channels with two inlets, respectively. The hemicylindrical microfluidic channels were fabricated by conventional photolithography and soft lithographic procedures with poly(dimethylsiloxane) (PDMS). Due to the small Reynolds numbers inside microfluidic channels, four laminar flows could be formed by inserting different fluids through four different inlets. When introducing photocurable solution composed of ethoxylated trimethylolpropane triacrylate (ETPTA) with 5 wt% photoinitiator, cylindrical microparticles, each with four compartments, could be fabricated by photo-polymerization under UV light. We also could synthesize colloidal photonic crystals with multiple photonic bandgaps using photocurable silica suspension including different sizes of silica nanoparticles. Each quarter of cylindrical photonic crystals display diverse structural colors, which could be tuned by adding iron oxide nanoparticles to the photocurable silica suspension and exposing UV light under the magnetic field. Due to the net magnetic moment from aligned magnetic particles, microparticles could be rotated under the external magnetic field. Consequently, multiple photonic bandgaps could be manipulated by application of external magnetic field.

Marking of biomolecules by fluorophores is one of the indispensable techniques for investigation of biological activities and identification of biomolecules. Fluorescence microscopes are generally used for the observation. Combination of optical waveguides and microfluidic channels develops optical devices for the fluorescence detection on chips. The downsizing of the detection devices decreases sample consumption, footprint and cost. The key points of the integration are fluorescence collection efficiency of the waveguides and geometric arrangement between the waveguides and the microfluidic channels. In the present study, we develop microfabricated optical devices on Si chips that microfluidic channels cross TiO2 waveguides, and investigate transmission properties of light at the microfluidic channel. In addition, we demonstrate detection of fluorescence from quantum dots in the fluidic channels. The devices were fabricated on Si substrates using microfabrication processes. First, we prepared TiO2 cores of channel waveguides. We formed Cr etching masks on TiO2 film (600 nm) deposited on oxidized Si(100) wafers using photolithography and lift-off process. We obtained linear TiO2 cores by dry etching for the TiO2 layers using CF4/Ar gas mixture. Next, we deposited SiO2 claddings by chemical vapor deposition with Tetraethyl orthosilicate. After making Cr etching masks on the claddings by electron beam lithography, we formed microfluidic channels perpendicular to the waveguides by dry etching. The devices were sealed by Polydimethylsiloxane in order to flow quantum dot solution in the microfluidic channel. Incidence of laser into the waveguides was carried out using a single-mode optical fiber. Transmitted laser was emitted from the edges of the waveguides and was collected by a multi-mode optical fiber. We used a diode-type photodetector for measurement of the photointensities. In the case of measuring of fluorescence, the emitted laser was operated by optical filters. Prior to the detection of fluorescence, we have investigated the variation of photointensity of propagating laser at the microfluidic channels in the waveguides. The photointensities of the transmitted laser decrease as the width of the microfluidic channels becomes wider. The amount of decrease is within the estimation by a classical etalon model. We have found that the drop of photointensities at the fluidic channel of 1 mu;m width is small, which means that the laser at the channels have enough intensities to excite fluorophores and that emitted fluorescence are efficiently collected by the waveguides. Comparing the kinds of liquid fulfilling the microfluidic channels, we have discovered the photointensity difference of the filtered lights between 8 nM quantum dot solution and pure water. We conclude that the device can detect several hundred of quantum dots, which is estimated by the number of quantum dots in the detection volume.

A conventional white phosphor-converted light-emitting diode (pc-LED) is composed of a blue LED chip and a yellow [Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce)] powder phosphor packed with epoxy resin to obtain white emission. However, this white pc-LED has limited conversion efficiency by high scattering and reflection loss of the emission from the powder phosphor layer. To reduce this loss, a transparent polycrystalline YAG:Ce ceramic plate phosphor (CPP) has been studied. However, the low light extraction efficiency of the CPP by the total internal reflection (TIR) and waveguide effect is the most significant drawback for application in white LED. Therefore, in order to enhance a low extraction efficiency of YAG:Ce CPP, a TiO₂ nanostructure as photonic crystal layer (PCL) were coated on YAG:Ce CPP by a combination of polystyrene (PS) nanosphere lithography (NSL), atomic layer deposition (ALD) and reactive ion etching (RIE) processes. First, the PS monolayers with various diameters (350, 580, 960nm) were scooped by the YAG:Ce CPP using a scooping transfer technique based on a water-air self-assembly process. Subsequently, a TiO₂ film layer was coated over the PS-assisted YAG:Ce CPP by ALD process. To obtain the TiO₂ nanostructure, the TiO₂ layer-coated YAG:Ce CPP with PS spheres was etched by RIE process. Finally, the etched YAG:Ce CPP was annealed at 450 °C. The effects of the TiO₂ nanostructures with various lattice constants (350, 580, 960nm) were investigated on the extraction efficiency of YAG:Ce CPP. The structural, morphological and optical properties of 2D TiO₂ nanostructure PCL-assisted YAG:Ce CPPs on top of a blue LED cup were investigated by performing scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements.

Yellow light-emitting AgIn₅S₈/ZnS-alloyed nanocrystals (NCs) were prepared via a facile hot injection method. AgIn₅S₈ cores were synthesized by injection of sulfur source into a mixture of silver and indium precursors in the presence of 1-dodecanethiol. The photoluminescence (PL) quantum yield (QY) of AgIn₅S₈ NCs was significantly enhanced via solid-solution with ZnS and their PL emission was shifted toward shorter wavelengths (~30nm). The wafer-scale AgIn₅S₈/ZnS-alloyed NCs-polymer composite films were fabricated by spray coating process. Because spray coating process is simple and scalable method, this process is apt to fabricate the wafer-scale luminescent films. To fabricate the free-standing films, the AgIn₅S₈/ZnS NCs-polymer composite films were deposited onto sacrificial ionic substrates (NaCl substrates). The obtained luminescent films were used as a color-converting material in white light-emitting diodes (LEDs). The luminous efficacy, color rendering index (CRI) and correlated-color temperature (CCT) of AgIn₅S₈/ZnS NCs-based LEDs were measured as a function of the applied current. The development of wafer-scale AgIn₅S₈/ZnS-alloyed NCs-polymer composite free-standing films by spry coating process in this study provides the many potential applications in the fields of photovoltaic cells, light-emitting diodes and flexible devices.

The Talbot effect is a self-imaging effect of periodic structures under coherent illumination with applications in lithography, interferometry, optical trapping, and array illumination. The Talbot effect, however, is only valid in the paraxial limit where the periodicity, a0, is significantly larger than the illumination wavelength lambda;. To understand the Talbot effect beyond the paraxial limit, we investigated the imaging property of a periodic Au hole array film with a0 comparable to lambda; both theoretically and experimentally. We found, depending on the ratio of a0/ lambda;, the self-images of the hole array were not necessarily periodic in the direction perpendicular to the film, and the self-image distances deviated from the paraxial Talbot distances. These differences from the classical Talbot effect can be explained by the deviation from the phase matching conditions beyond the paraxial limit. Interestingly, defects within the hole array film or above the film were healed in the self-images as the light propagated from the surface. The healing effect can be potentially applied to nano-lithography and imaging where defect-free patterns can be generated from a defective mask.

Transition metal doped II-VI semiconductors are promising for tunable middle infrared (mid-IR) laser sources operating in the 2-5 mu;m spectral range. Cr²⁺ ions incorporated in ZnSe crystals in particular have allowed room temperature continuous wave (CW) laser operation over a tunable range in excess of 1000 nm in the 2-3 mu;m region. Lasers based on Cr²⁺:ZnSe may enable highly specific detection of molecular compounds with unique absorption lines in this spectral region. Mid-IR CW operation of laser sources using bulk crystals of this material has been demonstrated at room temperature with slope efficiency and output power greater than 60% and 10 W, respectively. Lasing in Cr²⁺:ZnSe waveguiding thin-film structures has also been achieved under optical excitation, although with significantly lower efficiencies. In this work we explore the fabrication of nanostructured multilayered Cr²⁺:ZnSe-based thin films with potential for lasing under electrical excitation and enhanced efficiency of energy transfer from charge carriers in the II-VI host to the Cr²⁺ optical centers. Thin films of alternating layers of Cr²⁺:Zn_1-xCd_xSe (active layer) and ZnSe are created by pulsed laser deposition (PLD) on ITO-coated quartz substrates. The choice of x = 0.2 in the active layer composition leads to an energy gap of 2.5 eV (compared to Eg = 2.7 eV of ZnSe) enabling confinement of optically and electrically injected carriers in the region of the mid-IR active impurities. Nominal thickness of individual active layers is varied between 1 and 10 nm based on deposition rate calibration. Thickness of the intervening ZnSe layers and total number of alternating layers are kept fixed at 20 nm and 30, respectively, leading to overall thickness of the structure in the 100-1000 nm range. The chosen thicknesses of the active layer should enhance energy transfer from excitons to the Cr²⁺ ions due to their close proximity in the confined region and the increased oscillator strength of excitonic recombination due to quantum confinement. PLD is carried out using solid ZnSe and Cr-doped Zn_0.8Cd_0.2Se ceramic targets. Targets are prepared by mixing powder precursors, compressing them into solid pellets and annealing. Cr-doped pellets are annealed in sealed quartz ampoules at 1000°C for 10 days to ensure diffusion of Cr impurities. Targets are ablated by a KrF excimer laser at 2 J/cm² at pressures below 1 × 10^-6 Torr with substrate temperature kept at 550°C. Deposited thin film structures are analyzed by atomic force microscopy (AFM) for surface morphology and by X-ray diffraction (XRD) and Raman spectroscopy to characterize the crystalline quality of the films. The optical absorption and emission characteristics of the multilayered films are used to verify the incorporation of Cr²⁺ ions into the II-VI host and evaluate the effect of structure characteristics (layer thickness and dopant concentration) in the potential of the nanostructures for lasing operation.

A variety of strategies have been adopted for the fabrication of color-tunable OLEDs, including emissive layer doping, use of exterior color tuning layers, and use of interior complex layers. Although these methods can tune the emission color, they have several limitations, such as degradation of electroluminescent properties, complex processing techniques, and high-cost fabrication procedures. In this work, we demonstrate a novel way of emission color tuning of an OLED by embedding Ag nano-dots between the anode and organic materials of the OLED. We observed the increases in the magnitude of absorbance and the emission spectra shift toward longer wavelengths (red shift) according to the increase of Ag nano-dot size and spacing. These results suggest that Ag nano-dots can effectively generate LSPs resonance at the ITO/Ag interface and that their size and spacing determined the emission color of the OLED. Using the LSPs resonance, we demonstrated color-tunable OLEDs of various emission colors from cyan to yellow-green. This report presents a useful and simple fabrication route that is widely applicable to optoelectric devices.

Noble metal nanomaterials with interesting physical and chemical properties are ideal building blocks for engineering and tailoring nanoscale structures for specific technology application, such as analytical sensors or fuel cell catalysts. Among them, Ag nanomaterials are of particular interest because Ag nanostructure with different size and shape show a wide range of colors corresponding to their localized surface plasmon resonance. Most importantly, Ag has been used as excellent surface-enhanced Raman scattering substrates because it exhibits the best SERS effects compared with other metals. Meanwhile, Pt is a well-known catalyst that has high catalytic activity, especially for methanol electro-oxidation and oxygen electro-reduction reactions. By building on the well-known properties of this precious metal, platinum has been combined with numerous other metals, such as Pd, Au, Ag, etc. to increase electro-catalysis and attempt to limit the poisoning of the Pt surface by strongly adsorbed intermediates (e.g CO). Therefore, Pt-Ag system has received much attention from researchers and scientists for not only electro-catalytic activity but also SERS. However, there are still many obstacles in the synthesis of core@shell structures. The galvanic replacement reaction poses a challenge to synthesizing uniform Ag@Pt core@shell structures, while successful formation of Pt@Ag core@shell NPs is hampered by lattice mismatch. This presentation focuses on our recent results in the study of Pt@Ag core@shell NPs with controllable size and shell thickness. In addition, the plasmonic properties and unique electronic structure of this system are intriguing and suggest a tunable nature that can be used in catalytic, SERS and other applications. The results are discussed in terms of UV-Vis, XRD, TEM, HR-TEM, EDS, XPS, and HAADF-STEM.

Energy transport on the nanoscale via non-radiative energy transfer (NRET) is extensively studied for applications in areas such as light-harvesting, colour conversion, colour tuning and sensors. Quantum dots (QDs) are of particular interest for many applications due to their size tunable emission, broad absorption and good photostability. However, in QD solid structures the non-radiative energy transfer distance is typically less than 10 nm, placing a severe restriction on potential device designs and architectures. It has been shown that non-radiative energy transfer can be enhanced through interaction of the energy donor and energy acceptor with localized surface plasmons. CdTe QD sandwich structures, incorporating a monolayer of gold spheres, have been used to study surface plasmon modified NRET. The concentrations and separations of the constituent QD and gold nanoparticle monolayers can be independently controlled. The monolayer of 5.5 nm diameter gold nanoparticles has a localized surface plasmon peak overlapping the donor emission and acceptor absorption of the energy transfer pair. Strong enhancement of the rate and distance over which NRET occurs is observed. A surface plasmon enhanced NRET efficiency of 21% was measured in a sandwich structure with a donor to acceptor separation of ~24 nm. In the absence of the gold nanoparticle layer a NRET efficiency of 0.14% is expected over this distance. Further investigation shows a strong dependence of the surface plasmon NRET mechanism on the gold nanoparticle concentration and different regimes are identified. At the lowest gold nanoparticle concentrations investigated an overall enhancement of the acceptor emission can be observed. At higher gold nanoparticle concentrations the energy transfer rates are further increased, however, this reduces acceptor QD emission due to quenching effects.

The transfer of quantum information between various systems is desirable for most applications of quantum resources. One realization of quantum information transfer uses a composite system of metallic nanoparticles (MNP) and semiconductor quantum dots (QD), with plasmons in MNPs moving qubits from QD to QD. An accurate description of this process requires an understanding of the coupling and hybridization between states of the QD and the MNP plasmons. Ultimately a quantum description of an entire composite system, potentially including multiple MNPs, and multiple QDs, is needed. To this end, we apply real-time, real-space Time-Dependent Density Functional Theory to systems of MNPs and QDs. Here we examine MNP dimers (Au jellium nanospheres) and systems of one MNP and one QD. The quantum dots are modeled as spherically symmetric finite square wells with two bound states, one filled and one empty. In previous work, we showed that the optical response of single small MNPs consists of “quantum core plasmons”, charge density oscillations primarily localized near the center of the MNP, and “classical surface plamons”, charge density oscillations throughout the particle [1]. We showed that both of these are collective oscillations, and that as the size of the MNP increases, there is a transition to classical behavior. Widely separated MNP dimers behave similarly to isolated MNPs. However as they are brought more closely together, the resonances split and shift as a result of hybridzation. We examine the spatial character of the resonances, comparing them to the quantum core plasmons and classical surface plasmons seen in the single MNP to determine how size quantization modifies the plasmon hybridization. Results for the QD/MNP composite system will be discussed to show how hybridization with a discrete excitation differs from plasmon hybridization in this regime. [1] E.Townsend, G.W. Bryant, Nano Lett. 12(1), 429 (2012).

We have recently proposed the plasmoelectric effect as a new physical mechanism for conversion of optical power into DC electrical power using an all-metal circuit [1,2]. When a plasmonic nanostructure is free to exchange charge density through an electrical connection, heat from optical absorption near the plasmon resonance frequency can be converted into an electrochemical potential, i.e. a plasmoelectric potential, that drives current through a circuit load. Unlike the more familiar thermoelectric or photovoltaic effects, the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and the incident radiation. Radiation at higher frequencies induces an increase of electron density in the nanostructure that blue-shifts the plasmon resonance. This response is thermodynamically favored due to the increased entropic heat that accompanies the increased absorption. Similarly, radiation at lower frequencies decreases electron density in the nanostructure to induce a red-shift of the absorption maximum. We experimentally test these unique predictions by characterizing the electrical response of colloids of monodisperse Au or Ag nanoparticles spin-cast on ITO films. Kelvin probe force microscopy (KPFM) provides nanoscale resolution of the surface potential of the device structure while varying the frequency of incident radiation near the plasmon resonance. Further, the ensemble optoelectronic response of samples can be determined by photoelectrochemical measurements in which the plasmoelectric structure is immersed in an electrolyte and the sign and magnitude of the photovoltage is measured with respect to a counter electrode while varying incident wavelength. We observe clear evidence for the size-dependent and frequency-dependent trends in electrochemical potential consistent with our theoretical framework for the plasmoelectric effect. Our results provide deeper insight into the efficiency of optical power conversion via the plasmoelectric effect, practical considerations, and further applications of this new class of optoelectronic phenomenon. [1] “The plasmoelectric effect: optically induced electrochemical potentials in resonant metallic structures” Matthew T. Sheldon, Harry A. Atwater, 2012, in submission, arXiv:1202.0301 [2] “The plasmoelectric effect: conversion of optical power into DC electrical power by plasmonic nanostructures in all-metal circuits”, Matthew T. Sheldon, Harry A. Atwater, oral presentation, Symposium KK, MRS Spring Meeting, 2012

Heterostructures consisting of different materials within one nanoparticle are interesting for investigating the coupling of electronic excitations at the nanoscale. The model nanosystems investigated so far mainly consisted of two semiconductors, as they offer the possibility of designing their relative band alignment across the interface, resulting in type I and type II heterostructures. However, there is an increasing interest in focusing on the electronic coupling between semiconductor and metal nanoparticles and their fundamental excitations, namely exciton-plasmon interactions.[1] Such studies have up to now been limited to hybrid structures, where the metal is attached to the semiconductor by chemical means. Recently, copper chalcogenide (Cu_2-xTe, E=S, Se, Te) nanocrystals (NCs) have been shown to offer the unique property of holding excitons and highly tunable plasmon resonances in one material.[2] As such, they are envisaged as an appealing material for the investigation of exciton-plasmon interactions at different levels of coupling. However, an excess of free charge carriers, an inherent property of this material class, adds additional pathways for the exciton to recombine, for example via Auger recombination. In this contribution we report on the synthesis of heterostructured nanorods consisting of CdTe and Cu_2-xTe via partial ion-exchange. In this type of heterostructure the exciton of CdTe interacts with the plasmon resonance of Cu_2-xTe as they overlap in energy. A unique property of this material is the possibility to control (enhance or suppress) the plasmon resonance in Cu_2-xTe, as demonstrated previously.[2] This creates a material system in which the exciton within the same nanostructure can be investigated with and without exciton-plasmon interactions. This allows to directly address the influence of the plasmon resonance on the excitonic properties. We demonstrate that the suppression of the plasmon resonance in Cu_2-xTe leads to a recovery and red-shift of the CdTe fluorescence arising from the radiative recombination of the exciton. We further show that the exciton dynamics is altered in the presence of the plasmon resonance, and support our results by theoretical calculations. In particular, we calculate the effect of exciton-plasmon Coulomb interaction on the absorption spectra of CdTe/Cu_2-xTe nanocrystals using the microscopic model. [1] Lee, J.; Hernández, P.; Lee, J.; Govorov, A. O.; Kotov, N. A., Exciton-plasmon interactions in molecular spring assemblies of nanowires and wavelength-based protein detection. Nat Mater 2007, 6 (4), 291-295. [2] Kriegel, I.; Jiang, C.; Rodríguez-Fernández, J.; Schaller, R. D.; Talapin, D. V.; da Como, E.; Feldmann, J., Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals. Journal of the American Chemical Society 2012, 134 (3), 1583-1590.

Coulomb and electromagnetic interactions between excitons and plasmons in hybrid nanostructures lead to several interesting effects: Energy transfer between nanoparticles, plasmon enhancement, exciton energy shifts, Fano interference, and new mechanisms of optical chirality [1-6]. An interaction between a discrete state of exciton and a continuum of plasmonic states gives rise to interference effects (Fano-like asymmetric resonances and anti-resonances) [2,4]. These interference effects can strongly enhance a visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticles and molecules. If a system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to alter and enhance the circular dichroism (CD) of chiral components [5-8]. In particular, the exciton-plasmon interaction may create new chiral plasmonic lines in CD spectra of a biomolecule-nanocrystal complex [5,7]. Strong CD signals may also appear in purely plasmonic systems with a chiral geometry and a strong particle-particle interaction [6,8]. Recent experiments on the protein-nanocrystal and multi-nanocrystal complexes showed the appearance of strong plasmonic signals in CD spectra [7,8]. Potential applications of dynamic hybrid nanostructures include sensors and new optical and plasmonic materials. [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters 6, 984 (2006). [2] W. Zhang, A. O. Govorov, G. W. Bryant, Phys. Rev. Lett. 97, 146804 (2006). [3] J. Lee, P. Hernandez, J. Lee, A.O. Govorov, and N. A. Kotov, Nature Materials 6, (2007). [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008). [5] A.O. Govorov, Z. Fan, P. Hernandez, J.M. Slocik, R.R. Naik, Nano Letters 10, 1374 (2010). [6] Z. Fan, A.O. Govorov, Nano Letters 10, 2580 (2010). [7] J.M. Slocik, A.O. Govorov, and R.R. Naik, Nano Letters 11, 701 (2011). [8] A. Kuzyk, R. Schreiber, Z. Fan, G. Pardatscher, E.-M. Roller, A. Högele, F.C. Simmel, A. O. Govorov, T. Liedl, Nature, 483, 311 (2012).

The energy gap between valence and conduction levels in colloidal semiconductor quantum dots can be tuned via the nanoparticle diameter when this is comparable to or less than the Bohr radius. In materials such as cadmium mercury telluride which readily forms a single phase ternary alloy this quantum confinement tuning can also be augmented by compositional tuning, which brings a further degree of freedom in the bandgap engineering. Here we show how compositional control of 2.3nm diameter CdxHg(1-x)Te nanocrystals by exchange of Cd2+ and Hg2+ ions can be used to change their optical properties across a technologically useful range, from 500nm to almost 1200nm. We provide data on composition-dependent changes in the optical properties: bandgap; extinction coefficient; emission energy and spectral shape; Stokes shift; quantum efficiency; and radiative lifetimes as the exchange process occurs. These data are highly relevant for those seeking to optimise the materials and device designs. Our data allow for important insights into the cation exchange process in ternary nanocrystal alloys, which are consistent with a fast initial uptake of Hg2+ at the surface of the CdTe quantum dots, followed by a gradual interdiffusion of the two cations.

We report the fabrication of structured, nanoparticles with total sizes < 100 nm using the thin-film physical vapor deposition technique known as glancing angle deposition, or GLAD. This process has previously been shown to allow for morphologies with features on the order of hundreds of nanometers up to a few microns with various geometries, but to the best of our knowledge, it has not been used as a step on the way to monodispersed suspensions of colloidal nanoparticles. Here we show that with the ease of fabrication combined with a large set of possible evaporation materials and nanostructure morphology control, GLAD can compete with and out-perform some of the best nanoparticle fabrication techniques in the synthesis of asymmetric and non-centrosymmetric nanoparticles with complex morphologies or compositions. We will also report on the well-known disadvantages of the technique which can be especially pronounced when growing nanoscale-featured particles. Examples include competitive growth of columns, column merging, and effects of adatom mobility; the latter directly counteracts the shadowing effect that we seek to exploit in order to fabricate nanoparticles with small feature sizes. We demonstrate ways to minimize these effects. To illustrate the effectiveness of the technique, we will outline a specific example of wafer-scale fabrication of helical oxide nanoparticles that in solution exhibit polarization-sensitive light scattering depending upon the enantiomorphic sense of the helix. The nanoparticles are fabricated on a substrate with GLAD, removed via sonication, and suspended with the help of a surfactant agent into a colloidal solution. We believe these are the smallest complex chiral colloids that have been grown to date. The fidelity of the grown structure to the design is verified by SEM and TEM analysis.

The promise of colloidal quantum dots (CQDs) as a technological material, including light emitting diodes, gain devices, photovoltaic cells, spintronics, quantum information, sensors and biological platforms, demands high quality quantum dots. Ultimately, it may depend on tailoring their behavior through doping. Doping nanocrystals, and especially magnetic doping, leads to phenomena not found in the bulk as a result of confinement of carriers in a small volume. The strong sp-d exchange interactions between the charge carriers and the magnetic ion may result in a giant Zeeman splitting of the valence and conduction band states , a large Faraday rotation, or the formation of Magnetic Polarons (MPs), depending on the host nanocrystal . Exploration of all of these issues depends on having reliable methods to incorporate impurities. The most used strategy for doping is to include a precursor containing the impurity in the high temperature colloidal synthesis, which yields particles of high crystal quality . Nevertheless, it is often difficult to bring dopant into the bulk interior of nanoparticles by these methods, and the doping level is very low (typically an order of magnitude less than hoped). A colloidal synthesis strategy for doping nanocrystal is presented here, adapted for the synthesis of Mn-doped CdTe/CdSe core/shell CQDs. This strategy is based on a construction layer by layer of CQDs, allowing a more precise control of dopant location: inside the core, the shell, at the interface or on the surface of the NCs, concentration and properties, and can also be adapted to all variety of doping configurations, host material, size, shape, impurity location or amount. We gave evidence of successful doping, and shown an example in which optical properties of Mn-doped nanocrystals depends on extremely precise Mn position inside the nanoparticles. This includes large blue-shift in photluminescence spectra, differences between emission of polarized and non polarized photons or existence of two at least mechanisms for exciton recombination. Single-dot measurements, with or without polarization dependence offered us the opportunity to explore with a high resolution the giant magnetization and sp-d coupling in Mn-doped CQDs.

Indium phosphide (InP) quantum dots (QDs) are of interest for applications that require cadmium-free, visible-emitting quantum dots, while indium arsenide (InAs) QDs are of interest for applications involving near infrared emission. However, InP and InAs QDs are challenging to synthesize with the tight size distributions readily achieved for cadmium selenide and lead selenide QDs. Rapid precursor conversion rates for the group-V precursors used in InP and InAs QD synthesis are believed to prevent the formation of a highly monodisperse QD ensemble. We have attempted to address this problem by seeking out less reactive precursor chemistries for these materials in hopes of improving QD size distributions. Tris(trimethylsilyl)arsine and tris(trimethylsilyl)phosphine have historically been used in InP and InAs QD synthesis, however these molecules are pyrophoric, and challenging to synthesize and manipulate. By eliminating the silicon-group V bond, we have identified new phosphorous and arsenic precursors that are less reactive than existing precursor chemistries and produce QDs with superior size distributions. To compare the effect of precursor chemistry on QD ensemble size distribution, we have synthesized QDs under identical conditions and varied the precursor. Ensemble size distribution was inferred from the shape of the absorption spectra. We characterized the reactivity of the precursors using UV-Vis absorption spectroscopy to quantitatively observe the rate of formation of QDs. The molecular precursor conversion was monitored using NMR spectroscopy. Our results show that the development of less reactive group-V precursors is a way forward toward the improvement of III-V QD synthesis.

Metal-decorated colloidal nanocrystals are receiving great interest for photocatalytic hydrogen generation. [1-3] Here we report how controlling the catalyst cluster influences the hydrogen generation efficiency. First, we show that catalysts based on different metals show drastically different hydrogen generation quantum efficiencies. Second, we control the catalyst particle size and coverage by size selected cluster deposition under UHV conditions and show that the hydrogen generation efficiency depends on both these parameters. Finally, we demonstrate that the efficiency and stability of the nanocrystals can be tuned via the redox potential of the hole scavenger [4]. [1] M. Berr, A. Vaneski, A. S. Susha, J. Rodríguez-Fernández, M. Döblinger, F. Jäckel, A. L. Rogach, J Feldmann Appl. Phys. Lett. 97, 093108 (2010). [2] A. Vaneski, A.S. Susha, J. Rodriguez-Fernandez, M. Berr, F. Jäckel, J. Feldmann, A.L. Rogach, Adv. Funct. Mater. 21(9), 1547-1556 (2011). [3] M.J. Berr, A. Vaneski, C. Mauser, S. Fischbach, A.S. Susha, A.L. Rogach, F. Jäckel, J. Feldmann, Small 8(2), 291-297 (2012) . [4] M.J. Berr, P. Wagner, S. Fischbach, A. Vaneski, J. Schneider, A.S. Susha, A.L. Rogach, F. Jäckel, J. Feldmann Appl. Phys. Lett. 100, 223903 (2012).

Recently it has become possible to move past the electrical and optical exploration of an ensemble of dopants and to identify the effects of a solitary dopant as well as to study locally the fundamental properties of a solitary dopant atom in a semiconductor. This allows opening the new field of solotronics (solitary dopant optoelectronics). A Scanning Tunneling Microscope (STM) is an excellent tool to probe and manipulate a single impurity either in the surface layer or just a few mono-layers below the clean semiconductor surface. We have recently shown that a silicon atom in the outermost layer of GaAs has a bi-stable character much alike the well-known DX-center in Al(x)Ga(1-x)As. In the ground state the Si atom is negatively charged and in the excited metastable state the Si impurity is positively charged. These two charge states are related to a modification of the bond configuration of the silicon atom in the GaAs surface layer. By a proper electric switching sequence we can bring the silicon atom in either of the two states while probing it with an STM tip. We have successfully used these procedures to create a memory element that is based on a single impurity atom. Next to this electrical manipulation of single impurity atoms, our setup allows to illuminate the tunneling area or to collect tunneling induced photons from the area below the STM tip. We will show our recent results in detecting local STM induced luminescence on doped GaAs crystals. The same setup was used to explore Au surfaces where we are able to observe atomically resolved plasmon excitation. In the study of the bi-stable silicon impurity we have investigated the optically manipulation of the bond configuration and corresponding charge state of a single silicon impurity atom as a function of the excitation wavelength. This allowed us to unravel different pathways for the excitation and relaxation processes that are involved in this manipulation process. In conclusion we show that the electrical and optical characterization and manipulation of a single impurity in a (spin-polarized) STM setup is possible.

The influence of energy migration and energy-transfer upconversion (ETU) among neighboring erbium ions on luminescence decay and steady-state population densities in amorphous aluminum oxide microstructures is investigated by means of photoluminescence decay measurements under quasi-CW excitation. The experimental results are analyzed by several models. As expected from the basic physical assumptions made by these models, only Zubenko&’s microscopic model provides good agreement with the experimental data, while other donor-acceptor treatments found in the literature are unsuccessful and the macroscopic rate-equation approach provides meaningful results only when misinterpreting the intrinsic lifetime as a free fit parameter. Furthermore, a fast quenching process induced by, e.g., active ion pairs and clusters, undesired impurities, or host material defects such as voids, that is not revealed by any particular signature in the luminescence decay curves because of negligible emission by the quenched ions under quasi-CW excitation, is verified by pump-absorption experiments. This quenching process strongly affects device performance as an amplifier. Since Zubenko&’s microscopic model treats all ions equally, it is unable to describe a second, spectroscopically distinct class of ions involving a fast quenching process. The model is extended to take into account the fraction of quenched ions. This approach finally leads to excellent agreement between the luminescence-decay, pump-absorption, and gain experiments within the frame of a single theoretical description [1]. Via luminescence decay measurements, absorption and emission spectra, and a Judd-Ofelt analysis we determine luminescence lifetimes, radiative and non-radiative decay-rate constants, and branching ratios of the erbium inter-manifold transitions. With a continuous-wave pump-probe technique the excited-state absorption (ESA) spectrum is recorded between 900 and 1800 nm and the cross-sections of ESA transitions from the first and second excited state are determined. The microparameters and efficiencies of resonant and phonon-assisted energy-migration and ETU processes among erbium ions occurring from the first and second excited states are evaluated. From the ratio of the green and red luminescence intensities as a function of erbium concentration we prove the existence and quantify the macroscopic ETU coefficient of an additional two-phonon-assisted ETU process [2]. 1. L. Agazzi, K. Wörhoff, M. Pollnau, "Energy-transfer-upconversion models, their applicability and breakdown in the presence of spectroscopically distinct ion classes: Investigations on the example of amorphous Al2O3:Er3+", submitted. 2. L. Agazzi, K. Wörhoff, A. Kahn, M. Fechner, G. Huber, M. Pollnau, "Spectroscopy of upper energy levels in an Er3+-doped oxide", submitted.

Many solid-state electronic devices utilize a pn-junction, traditionally formed by random doping of donor and acceptor impurity atoms. Here we present a new type of pn-junction not formed by impurity-doping, but rather by polarization-induced p and n conducting regions within compositionally graded non-centrosymmetric semiconductor nanowires.* Linearly grading AlGaN nanowires from GaN to AlN forms the polarization-induced n-type region, while grading back from AlN to GaN forms the p-type region. A quantum disk at the center of the junction serves as the active region for a light emitting diode. Since electrons and holes are injected from AlN barriers into quantum disk active regions, graded nanowires allow deep ultraviolet light emitting diodes across the AlGaN band gap range. While previous work in graded III-nitride planar structures showed that polarization-induced p-type doping was only possible with supplemental acceptor doping, the graded nanowires in this study show that p-type material is possible without the use of supplemental doping. This demonstrates an advantage for polarization engineering in nanowires compared with planar films. Polarization-doping in nanowires provides a strategy for improving conductivity in wide band gap semiconductors. As an added benefit, because polarization charge is uniform within each unit cell, polarization-induced conductivity without the use of impurity-doping provides a solution to the problem of conductivity uniformity in nanowires and nanoelectronics, and opens a new field of polarization engineering in nanostructures that may be applied to other polar semiconductors. The nanowire devices presented here are grown on n-Si(111) substrates by plasma-assisted molecular beam epitaxy. Scanning transmission electron microscopy images and energy dispersive x-ray spectroscopy show that each nanowire is linearly graded from GaN to AlN and back to GaN, as designed. Electroluminescence and current-voltage characteristics are provided for a variety of devices, showing tunable emission from 360 nm (3.44 eV) to 266 nm (4.66 eV). A minimal change in device performance at cryogenic temperatures shows that carriers are not thermally-ionized, but are rather field-ionized by bound polarization charge. A comparison of samples with and without the use of impurity doping is provided to show the effectiveness of polarization-induced doping in III-nitride nanowires. This work is supported by the Office of Naval Research (N00014-09-1-1153) and by the National Science Foundation CAREER award (DMR-1055164). S.D. Carnevale acknowledges support from the National Science Foundation Graduate Research Fellowship Program (2011101708). * S.D. Carnevale, T.F. Kent, P.J. Phillips, M.J. Mills, S. Rajan, R.C. Myers. Nano Lett. 12, 2, 915-920, 2012.

The epitaxial integration of rare earth pnictides (RE-Pn), such as ErAs and ErSb, in III-As semiconductors has attracted attention for applications in novel high speed photodetectors and photoconductive switches as well as enhanced tunnel junctions. Here we report on integration of the RE-Pn GdN as discrete particles in a GaN matrix by plasma assisted molecular beam epitaxy. It is hypothesized that the growth of GdN particles proceeds in a similar fashion to that of RE-Pn in III-As zincblende systems, with growth proceeding by nucleation of RE-Pn islands followed by epitaxial lateral overgrowth of the surrounding uncovered III-V matrix. Periodic structures of GdN nano-island layers spaced between GaN regions were prepared and subsequently characterized by a variety of methods. High resolution X-ray diffractometery shows that the cubic, rocksalt structure, GdN islands are epitaxially oriented to the hexagonal wurtzite GaN matrix with the relationship GdN [111]||GaN[0001]. The XRD result also indicates the precise layer thickness control due to the presence of superlattice interference fringes. Cross-sectional scanning transmission electron microscopy (STEM) combined with in-situ reflection high-energy electron diffraction (RHEED) allows for the study of island formation dynamics, which occurs after 1.2 monolayers of GdN coverage. Superconducting quantum interference device (SQUID) magnetometry reveals multiple ferromagnetic phases, with one attributable to the GdN particles with Curie temperature of 70K and an anomalous phase with ferromagnetism persistent to room temperature as well as a paramagnetic background, which is proposed to be due to contamination of the GaN matrix with Gd during growth. The room temperature ferromagnetic phase is strongly anisotropic, with out of plane magnetization nearly 300% larger than in-plane at fields less than 1T. In addition, electroluminescent devices were fabricated by embedding GdN nanoparticles in an aluminum nitride matrix. At low device biases of 15V, clear, narrow emission at 318nm is observed, corresponding to impact excitation of Gd^3+ intra-f-shell atomic transitions.

From ensembles of quantum nanostructures to their individuals, the self-assembly approach has its advantage on keeping their optically active functionality. In this talk, I&’m going to review recent developments not only from the traditional strain-driven self-assembly but also from the droplet epitaxy and their hybrid approach, to cover a rich spectrum of nanostructured morphologies such as quantum rings, quantum-dot molecules, and nanoholes.

The incorporation of self-assembled quantum dots in systematically scalable quantum devices requires a method of nucleating dots with nanometer-scale spatial accuracy while preserving their narrow optical line width. We have developed a technique combining e-beam lithography, wet etching, and molecular beam epitaxial (MBE) growth to deterministically position InGaAs quantum dots with spectrometer limited photoluminescence line widths. Previous demonstration of deterministic alignment between self-assembled quantum dots and subsequently fabricated devices employed a two-step process of first locating a randomly positioned dot, then fabricating a device around it [1]. A drawback of this approach is that it does not lend itself to the scaling up of individual devices into systems. Although nucleation sites can be prescribed by patterning the substrate surface before growth [2], preserving narrow optical line width in these site-controlled dots remains a challenge because patterning the surface invariably introduces defects and impurities that broaden nearby quantum dots&’ optical line width. One solution would be to grow a thick buffer of pure GaAs on the patterned surface before nucleating dots; however, the anisotropy in the growth of GaAs tends to fill in or elongate the pattern. Our technique actually takes advantage of this growth anisotropy to evolve an etched pattern of holes and lines into faceted structures in which single dots nucleate. Using this technique, we were able to grow a GaAs buffer layer of up to 90 nm thickness between the processed surface and the dot nucleation surface. Additionally, by varying the spacing between lines in the original pattern, we can change the number of dots nucleating per site from single to a chain of several. Our dots are grown in a Schottky diode structure. Their PL spectrum shows discrete charging transitions, as well as narrow lines on the order of the spectrometer&’s resolution limit of 20 micro eV. [1] A. Badolato et al. Deterministic Coupling of Single Quantum Dots to Single Nanocavity Modes. Science 308, 1158 (2005). [2] P. Atkinson, et al. Formation and ordering of epitaxial quantum dots. Comp. Rend. Physique 9, 788 (2008).

Intersubband transitions (ISBTs) within the conduction band of semiconductor quantum wells (QWs) have been widely explored, which eventually led to their application in QW infrared photo-detectors and quantum cascade lasers. Compared to electron ISBT there has been considerably less research on the hole ISBT in the valence band (VB), mainly because the VB has a complex structure that makes it more difficult to model. However ISBTs in the VB are interesting because of the optical activity for both in-plane and out of plane light polarization, opening up the possibility of surface-perpendicular emission or absorption. Hole ISBT between different kinds of hole states, i.e. between heavy and light hole states are not subject to the same selection rules as electron ISBT (light emission and absorption restricted to the plane of the semiconductor), therefore allowing light emission and absorption in the direction normal to the semiconductor surface. This may eventually lead to novel optoelectronic devices like vertical cavity surface emitting lasers operating in the mid-infrared range. We report heavy to light (H-L) hole intersubband absorption in the VB of GaAs QWs with AlAs barriers in the mid-infrared range. The samples were grown with atomic layer precision by molecular beam epitaxy. For the p-type doping a high-purity solid carbon source was used. The p-type doping was 1.2×1012 cm-2 for our samples. Previously we reported broad features in the s-polarization and attributed them to H-L hole ISBT. Good agreement was found between the experiment and theory for the mid-infrared heavy to heavy (H-H) hole absorption while the H-L hole absorption experimental values had a poor agreement with the theory. Now we report both strong H-L hole and H-H hole transitions measured with in-plane (s-polarized light) and out of plane (p-polarized light) respectively, for three different QW widths of 42 Å, 51 Å, 59 Å. XRD and TEM scans ensure that the QW widths are within 0.5 Å of the designed values. A multi-pass waveguide geometry was used for the mid-infrared absorption measurements. All the measurements were taken at 77K. For the H-L hole transition in s-polarization the absorption peak varied from 255meV to 175meV for well width variation of 42 Å to 59 Å. In the p-polarization the absorption peak varied from 258 meV to 171 meV over the same well width range. For the H-H transition (p-polarization only) the absorption peak changed from 163 meV to 103 meV over the same width-range. The experimental results were compared with theoretical simulations using the 6x6 k.p model. The effect of inter-diffusion was incorporated in the simulations. The best agreement between theory and experiment was found for an inter-diffusion range of 6 to 10 Å. The comparative study between experimental values and theory provides us with valuable information and insight for future novel devices based on hole ISBTs.

Epitaxial Stranski-Krastanov type III-V semiconductor quantum dots (QDs) have found application and research directed toward a number of optoelectronic technologies, including light emitters, photodetectors, and solar cells. The inclusion of such nanostructures provides the ability to effectively tailor the energy of photon emission and/or absorption within device structures. This sort of band gap adjustability can also be achieved via metamorphic (lattice-engineered) materials, which have also recently experienced significant research and application toward the same optoelectronic technologies. As such, the combination of these two approaches is an obvious step on the path to ultimate semiconductor materials tunability. The vast majority of III-V QD work to date has utilized standard substrate materials (e.g. GaAs, InP) as the host, thereby somewhat limiting their applicability, while lattice-engineered materials have seen comparatively little attention, save for a few reports concerning InAs QDs on In_xGa_1-xAs. However, recent developments in the growth of GaP on Si, and subsequent GaAs_yP_1-y metamorphic buffers, have helped make accessible the wide range of band gaps with photovoltaic and optoelectronic applicability available between the Si and GaAs lattice constants, with the added advantage of direct, monolithic integration with Si substrates and devices. We will present here work focused on MBE-based growth and characterization of QD nanostructures within metamorphic GaAs_yP_1-y matrix materials. The objective of this effort is the understanding of how pre-existing dislocations and characteristic surface cross-hatch in metamorphic materials affects the nucleation and growth of epitaxial QDs, as well as strain-compensation of encapsulated QD arrays. Initial exploratory experiments on the nucleation of InGaAs QDs on photovoltaic-relevant GaAs_0.90P_0.10 host materials, grown on both GaAs and Si substrates, confirmed, by AFM, the formation of dense arrays of self-assembled QDs, even in the presence of a pre-existing threading dislocation density and surface cross-hatch. 20-layer periodic encapsulated QD structures exhibited strong low-temperature PL emission 400 meV blue-shifted from the bulk emission wavelength due to the quantum confinement effect. TEM imaging shows that the QDs are vertically aligned, although tilted away from surface normal, indicating that the dislocation network does not substantially interrupt the strain-induced alignment observed in non-metamorphic QD systems. Further work, currently in progress, will be presented regarding the further refinement of the In_xGa_1-xAs/GaAs_0.90P_0.10 based QD/matrix system, as well exploration of other potentially useful materials combinations, including GaAs_yP_1-y and In_zGa_1-zP QD materials capable of visible light emission, including the elusive green wavelengths.

Semiconductor InAsSb/InGaAs/InP nanostructures (quantum dots and quantum wires) are promising candidate materials for fabricating mid-infrared (2-3 um) emitters, which have a wide range of applications in military, telecommunications, molecular spectroscopy, biomedical surgery, environmental protection and manufacturing industry, etc. Some work has been devoted to growing self-assembled InAsSb quantum dots and extending their emission wavelength. However, it still presents a big challenge to achieve high quality InAsSb nanostructures with controlled morphology and optical properties due to the large lattice mismatch between InAsSb and InP, surfactant effect of Sb atoms, and low growth temperature requirement for Sb compounds. In this work, two novel growth techniques (Sb exposure and graded growth) are applied to realize the control over the morphology and optical properties of InAsSb nanostructures. As for the technique of Sb exposure, the surface of InGaAs buffer layer is exposed to trimethylantimony precursor flow before the growth of InAsSb nanostructures. As a result, the surface/interface energy in the system is reduced, while the strain energy in the system is enhanced, which leads to a shape transition from dot to dash, and to wire for the InAsSb nanostructures with increasing the Sb exposure time. Consequently the linear polarization degree of the photoluminescence emission from the InAsSb nanostructures increases as a result of the shape change of the nanostructures. In addition to Sb exposure, graded growth of InAsSb nanostructures is also explored to control their morphology and optical properties. The nominal composition of As / Sb was graded linearly from a starting value to a ending value during the deposition of InAsSb while keeping the total As and Sb constant. It is found that the actual Sb composition in InAsSb nanostructures increases with increasing the initial Sb composition during the graded growth, which can be explained by the different sticking coefficients of As and Sb atoms. As a consequence of the change of actual Sb composition, the shape of the InAsSb nanostructures evolves from circular dots to elongated dots, to dashes and then to wires with increasing initial Sb composition during the graded growth. Accompanying this shape change is a narrower height distribution due to the decrease in their average height. Compared with InAsSb nanostructures obtained with lower initial Sb compositions, the InAsSb nanostructures fabricated with higher initial Sb composition present a larger linear polarization degree and smaller red-shift rate with temperature in their photoluminescence emission. This research demonstrates two technologically important techniques, Sb exposure and graded growth, to engineer the morphology and physical properties of InAsSb nanostructures, the principle of which can also be applied to other material systems.

Quantum Dot molecules are created when coherent tunneling of electrons or holes creates molecular orbitals delocalized over multiple quantum dots. The delocalized molecular states can have a variety of unique new optical, electronic and spin properties of interest for future device applications. These properties can be engineered with molecular structure by changing the spatial extent and material compositions over which the molecular states are delocalized. We will present recent results on the experimental observation of delocalized molecular states and charge interactions in lateral quantum dot molecules. We will compare and contrast these results with the more firmly established understanding of charge and spin interactions in vertical quantum dot molecules, emphasizing the ways in which different quantum dot geometries create different molecular interactions and properties.

Semiconductor nanoparticles, quantum dots (QDs), present unique physical properties due to quantum confinement effect when the size of particles is less than exciton Bohr radius. Recently the semiconductor materials with non-toxic elements attract great interest for the application to the industry, such as light emitting diodes, bio imaging and photovoltaic devices. InP semiconductor QDs are one of non-toxic materials and attract attention due to its strong luminescence and appropriate bandgap for visible light emission. Herein, we present the synthesis and characterization of blue emitting InP core /ZnS shell QDs, which were hard to obtain by conventional methods. The synthesis of small QDs is necessary in order to acquire InP QDs with higher bandgap. We dropped the temperature drastically after mixing of the In and P precursor to synthesize small QDs. Not only did we minimize the initial size of InP core, we could further control the size of InP core through etching by residual acetic acid at three different growth temperatures. Thereby, we synthesized the InP core QDs, whose band edge absorptions are 425 nm, 438 nm, and 456 nm with the increasing growth temperature. Luminescent intensity and stability can be increased if InP cores are encapsulated by ZnS shell material. It is important that the size of InP cores remain unchanged during formation of the ZnS shell. By reducing shell synthesis temperature, we could successfully attain blue (475 nm), greenish-blue (485 nm), and bluish-green (497 nm) emitting InP core /ZnS shell QDs. Three colors of InP core /ZnS shell QDs emitting at 475 ~ 497 nm pointed on the CIE 1931 chromaticity diagram will be presented, together with the TEM, XRD, XPS, and spectroscopic analysis.

1D array of ZnO nanorods (NRs) have been attracted much attention for nanoscale sensors, detectors, and optoelectronics devices. Electron beam lithography (EBL) is one of the most versatile nanofabrication tools of making patterns on seed layer that has enabled the growth of periodic ZnO nanostructures. In case of EBL, the primary electron beam has either forward or back scatter as approached towards the resist coated substrates. The total electron yield (σ) which is the sum of the above depend upon the accelerating voltage that is being used. Generally, higher accelerating voltage (σ <1) induces negative charges and the lower one (σ >1) makes it positively charged. As ZnO has negative charged carriers in majority, surface attachment supposed to be hindered at the higher voltage and conversely enhanced at the lower one. Here we have prepared Al: doped ZnO (AZO) seed layers of ~280 nm thick on the glass substrate using RF magnetron sputter at an ambient of 3500C. The AZO layer on glass was spin coated with ~50 nm of PMMA (Mol. wt. 950K) and post baked at 1800C for 2 mins. The samples were then patterned using EBL at different beam energies of 2, 5, 10, and 20 keV. After the patterning, the resist was undergone development in Methyl-isobutyl-ketone and isopropyl alcohol (MiBK+IPA) at a ratio of (1:3) for 30 sec, rinse in IPA for 15 sec, and then dried N2. Patterned samples were processed for ZnO nanorods growth using hydrothermal technique in a solution of Zn (NO3)2 and hexamethylenetetramine (HMT) at 900C for 4 hrs. After successive NRs growth, the unexposed PMMA were lifted-off using N-methyl-2-pyrrolidinone (NMP) for 10 mins at 600 C. Then samples were rinsed in DI and dried in nitrogen, and ready for FESEM imaging. In order to make 1D nano-array, the solid attachment of ZnO NRs on to the patterned surface is essential. Especially, at higher beam energy (~ 20 keV), the incident electron beam decelerates due to the accumulated negative charges that already built on the surface in due course of irradiation. This causes a negative shielding potential close to the surface at micron level, and completely unfavorable for the ZnO attachment. However, at a comparatively lower beam energy (~ 5 keV or less), the secondary electrons (SE) are responsible for the pattern with the irradiation zone centered by the local positive field. Therefore, the negatively charged ZnO NRs can be controlled at lower voltages easily and also put site-selective at increasing the dose. This work is supported by the DoD (CEAND) Grant Number W911NF-11-1-0209 and W911NF-11-1-0133 (US Army Research Office), NSF-CREST (CNBMD) Grant number HRD 1036494 and NSF-RISE Grant number HRD-0931373.

Semiconductor quantum dots (QDs) are of great interests for their unique photophysical properties including tunable and broad spectral responses (visible to near IR), high luminescent quantum yields, long exciton lifetimes and ultrafast interfacial charge separation dynamics. These properties make QDs an attractive material with numerous potential applications in solar energy conversion. The photoinduced charge separation and energy transfer dynamics in QDs are among the key processes that control the efficiency of harvesting and converting sunlight into electricity (or fuels) using QDs. This presentation/poster introduces the fundamental studies about interfacial charge separation and energy transfer dynamics from QDs. These studies target important scientific questions such as the role of surface defects in charge separation, the ultrafast dynamics in highly reductive QDs, and strategies of utilizing QDs for more effective sunlight harvesting, paving the road for practical applications of QDs in solar energy conversion.

Light emission from semiconductors depends not only on the efficiency of carrier injection and recombination but also extraction efficiency. For ultraviolet emission from high band gap materials such as ZnO, nanowires have higher extraction efficiencies than thin films, but conventional approaches for creating a pn diode result in low efficiency. We exploited the noncentral symmetric nature of n-type ZnO nanowire/p-type GaN substrate to create a piezoelectric potential within the nanowire by applying stress. Because of the polarization of ions in a crystal that has noncentral symmetry, a piezoelectric potential (piezopotential) is created in the crystal under stress. The piezopotential acts as a “gate” voltage to tune the charge transport and enhance carrier injection, which is called the piezo-phototronic effect. We propose that band modification traps free carriers at the interface region in a channel created by the local piezoelectric charges. The emission intensity and injection current at a fixed applied voltage have been enhanced by a factor of 17 and 4, respectively, after applying a 0.093% compressive strain and improved conversion efficiency by a factor of 4.25. This huge enhanced performance is suggested arising from an effective increase in the local “biased voltage” as a result of the band modification caused by piezopotential and the trapping of holes at the interface region in a channel created by the local piezoelectric charges near the interface. Our study can be extended from ultraviolet range to visible range for a variety of optoelectronic devices that are important for today&’s safe, green, and renewable energy technology. Reference: Qing Yang,Wenhui Wang,Sheng Xu,and Zhong Lin Wang, Nano. Lett., 11 (2011) 4012

Cholesteric liquid crystals (CLCs) are attractive photonic materials for their ease of fabrication and their dynamic properties. Typically CLCs with a positive dielectric anisotropy are used in dynamic photonic switching applications, where an AC field across a cell causes a change from a colored reflective state to a clear state. On the other hand, negative dielectric CLCs find very little use due to the fact that applied fields cause no rearrangement of the anisotropic molecules in a typical planar aligned liquid crystal cell. Here we present unique dynamic coloration effects caused by broadening and tuning of the selective reflection in polymer-stabilized CLC&’s with a negative dielectric ansitropy under DC electric fields. Large scale wavelength changes of several hundred nm can be induced at low field strengths. The mechanism of this previously unreported approach to dynamic color change are explored in this talk.

Plasmonic nanoparticles embedded into a dielectric matrix create an interesting optical medium, where the features related to the plasmon resonance can be shifted to visible or even infrared range. Generally, the optical properties of such medium can be tailored by variation of size and spatial distribution of the nanoparticles as well as by the chemical composition of the metal inclusions and of the dielectric host. We report results of our study of the optical properties of metamaterials consisting of a crystalline AlGaAs matrix, where metallic nanoparticles were created by the process of self-organization. The driving force for the self-organization was supersaturation of the matrix by group-V elements, which was achieved by special growth conditions in molecular beam epitaxy. In undoped AlGaAs the plasmonic nanoparticles were pure As with hexagonal structure and almost spherical form. Doping the matrix with Sb allowed us to produce nanoparticles of AsSb alloy. In both cases the spatial distribution of the nanoparticles was random. Patterning of the spatial distribution of the AsSb nanoparticles was achieved when we utilized local delta-doping with Sb instead of the uniform doping. Our study shows a weak influence of the random system of the As nanoparticles on the optical properties of the AlGaAs films, since the plasmon resonance in such particles does not match the transparency window of the AlGaAs matrix. Alloying of As with Sb shifts the plasmon resonance toward lower energies. As a result the AlGaAs medium with a random system of the AsSb nanoparticles becomes opaque. This process is governed the Mie scattering and absorption. The spatially ordering of system of the AsSb nanoparticles drastically changes the optical properties of the medium, giving rise to a Bragg resonance. The magnitude of the Bragg resonance appears to be as strong as 40% of reflectivity with the total volume fraction of the metallic nanoinclusions being well below 1% and with 12 periods in the Bragg sequence.

The development of the effective optoelectronics devices for the quantum information technology requires creation of a new type of light sources. Such sources have to emit carefully controlled number of photons with minimal timing jitter. Self-assembled InAs quantum dots (QDs) offer the high potential for the realizing single photon and entangled photon sources. The most common technique for growing self-assembled QDs is the Stranski-Krastanov (SK) method. However, a high density of InAs QDs in case of SK growth mode limits the study of optical properties of single QD by measurements of an ensemble of QDs. In this work we report on the formation of QDs structures of low density and appropriate size. The QDs structures were grown by molecular beam epitaxy in subcritical regime, as the InAs layer thickness (1.4 ML - 1.6 ML) was below critical thickness for SK mode. For additional spatial separation of QDs we introduced the growth on misoriented (vicinal) GaAs substrates. For the optical characterization we applied a near-field spectroscopy of such low density arrays of InAs QDs required for addressing single quantum dots. Low-temperature near-field scanning optical microscope (NSOM) operating at a temperature of 10 K inside a high vacuum chamber was used (spatial resolution < 0.2 mu;m). NSOM allows us to get the information about the spatial position of QDs and record corresponding optical spectra. With the NSOM technique appropriate QDs were accurately selected for detailed study an ensemble of sharp lines, originating from the emission of different excitonic states. Based on excitation power dependencies of these sharp lines intensity exitonic states were specified.

Bismuth is a low toxicity heavy element with a low melting point, presenting peculiar physical properties. In the bulk state, it is classified as a semi-metal and presents a particularly high electron mean free path, low effective mass and long De Broglie wavelength. These exciting features have stimulated research that evidences confinement effects on the electronic response of nano-bismuth structures, such as thin films, nanowires or nanoparticles. In contrast, the optical response of nano-bismuth under the form of nanoparticles remains unexplored so far, despite of its demonstrated potential for thermo-optical sensors and switches [1]. In this work, we provide experimental evidence of the strong sensitivity of the optical response (from the near UV to the near IR) of bismuth nanoparticles to their size, shape and organization. At such aim, tailor-made 2D assemblies of bismuth nanoparticles presenting different characteristic nanoparticles size, shape and organization, and sandwiched between thin amorphous aluminium oxide layers, were prepared by pulsed laser deposition. This structure constitutes a robust nanocomposite system for reliable structural studies and for experimental assesment of the bismuth nanoparticles optical response. Furthermore, from static and dynamic optical calculations, we have investigated the electronic mechanisms (plasmon excitation and damping mechanisms) driving the topology-sensitive optical response of the bismuth nanoparticles. Finally, we propose a roadmap to the elaboration of nano-bismuth-based thermo-optical sensors and switches with maximized optical contrast. [1] E. Haro-Poniatowski, R. Serna, M. Jiménez de Castro, A. Suárez-García, C. N. Afonso and I. Vickridge, Nanotechnology 19, 485708 (2008) Acknowledgements: This work has been supported by the EU under project FP7-NMP-2010-Eu-Mexico Grant Agreement no. 263878: Functionalities of Bismuth based Nanostructures (BisNano), by the Spanish CICYT under project MAT2009-14369-C02-02, and by CSIC- CONACYT-2008MX0050 collaborative action. J. T. acknowledges a Juan de la Cierva Grant JCI-2009 - 05098.

Dedicated research on self-assembled InAs/GaAs quantum dots (QDs) has been triggered due to their inherent capability to extend emission of GaAs-based optical devices to long wavelengths such as 1.3 or 1.55 µm. Bilayer QD heterostructures are an apt solution to the major obstacle of large PL linewidth. But increasing QD density and simultaneously maintaining uniformity of QD heterostructures is a big problem. By stacking multilayer of QDs by Stranski-Krastanov growth we can solve these problems but the strain built-up and consequent relaxation of strain by dislocation formation is uncalled for. Thus, keeping all aspects in mind we would like an extension of bilayer QD structure into multilayer coupled structure which is the primary focus of this study. A comparison study of the bilayer sample a with the multilayer samples (b and c) is made by varying the barrier width between the stacked QD layers in samples b (11 nm) and c (12.5 nm) and maintaining other parameters same. Large QDs were grown at a very low growth rate to achieve the 1.3um emission target. Due to larger dots, strain is created in the spacer layer resulting in intrinsic dislocations. The thin GaAs spacer induces electronic and strain coupling creating non radiative recombination centers and broadening the linewidth. It was observed that sample c has much higher IPL and activation energy than that of sample b indicating lesser dislocations due to a relatively thicker spacer layer. Moreover the FWHM of sample c was lesser than that of sample b indicating better homogeneity in the dots. The peak intensity and line width of all the samples were significantly improved by annealing which in turn increases the modal gain of lasers and absorption coefficient of photodetectors. There is clear evidence that the amount of In-Ga intermixing induced by annealing is dependent on the QD size. The larger dot compared to the smaller dot will have lesser hydrostatic strain which induces greater In-Ga intermixing in the vertical direction resulting in more blue shifting. This decreases the energy difference between the shifted dots, making the linewidth narrower. The integrated PL intensity of sample c annealed at 750°C is much better than that of sample a and the FWHM is also nearly the same which serves the purpose of our study to extend the bilayer into the multilayer. The temperature dependent PL spectra showed that the peak intensity of the larger dots increased upon increasing the temperature till 140K while that of the smaller dots decreased. This is attributed to tunneling of the electrons from the smaller QDs to the larger QDs on increasing the temperature due to the lower confinement energy of the larger QDs. It is clear from this study that strain patterning is necessary for the development of multilayer QD heterostructures with the same homogeneity as that of bilayers. Annealing can be effective in improving the homogeneity and reducing intrinsic defects. Acknowledgement: DST, India

GaSb/GaAs type-II quantum dots have attracted much attention for new optoelectronic devices at long wavelengths, where only holes are confined in dots. However, it is an issue that it is easy to produce the Sb segregation and As-Sb compositional intermixing during the hetero-epitaxial growth, because the bonding strength of Ga-Sb is weak compared with that of Ga-As. In the case of MOMBE using TDMAAs and TDMASb precursors, the etching of GaSb dots was caused by the supply of these precursors at high temperature. Then the blue shift of wavelength arises by the reduction in dot size or Ga-As mixing. To realize long-wavelength emission of GaSb/GaAs dots, it will be important to study the growth process of GaAs cap-layer to bury GaSb dots. In this paper, we examine the hetero-growth processes of GaSb/GaAs dot system using MOMBE. We propose 2-step growth of GaAs cap-layer and demonstrate the longer wavelength emission in GaSb/GaAs dots. GaSb/GaAs dots were grown on semi-insulating GaAs(001) substrates using MOMBE. MO precursors used were TEGa, TDMAAs, and TDMASb. GaAs surfaces were cleaned at 570^oC with the simultaneous supply of TDMAAs and then a 100-nm-thick GaAs buffer layer was grown at 540^oC. After cooling down to the growth temperature of GaSb, TDMASb was supplied to the (2x4)-GaAs surface. Then GaSb dots were grown on the Sb-terminated (1x3) surface in the Stranski-Krastanow growth mode. The dot formation was identified by the change of RHEED patterns. After the growth of GaSb dots, a 30-nm-thick GaAs cap-layer was grown at the temperature of 400~510^oC. The hetero-growth processes of GaAs cap-layers were evaluated by RHEED and AFM observations. Photoluminescence (PL) properties of the GaSb/GaAs dots were measured at 20 K. Typical GaSb dot size grown at 480^oC was 8 nm in height and 70 nm in width. By the irradiation of TDMAAs, the etching of dots was clearly observed at >430^oC. Although the flattened out of the growing surface was observed during GaAs cap growth above the temperature of 430^oC, PL peak wavelength of the GaSb dots was blue-shifted by the increase of the GaAs growth temperature. Since the disappearance of the GaSb dots was not observed by the TDMAAs supply at <430^oC, GaAs cap-layer was grown at 400^oC. At the initial growth of GaAs layer, dot structures were formed on the terrace region between GaSb dots. Red-shift of PL peak was observed, but luminescence efficiency of dots was decreased due to the degradation of crystalline quality of the cap-layer. To improve the luminescence efficiency, 2-step growth of GaAs cap-layer was done. After GaAs dots were grown on the terrace between GaSb dots at 400^oC, the GaAs cap-layer was grown at 450^oC to bury the GaSb and GaAs dot structure. The red shift of the luminescence peak up to 1.2 µm and the improvement of luminescent efficiency of GaSb dots were observed.

The future of lighting will likely be tied to solid-state light emitting diodes (LEDs) because they are more efficient, environmentally friendly, and cost-effective compared to traditional light sources. InGaN compounds are of particular interest for solid state lighting because of their potential to emit light over the entire visible spectrum. During the past decade, attempts have been made to understand the carrier dynamics in InGaN multiple quantum wells utilized as the active light-emitting material in LEDs; given the small dimensionality of the quantum wells, transmission electron microscopy (TEM) has been extensively used to study their structural and chemical uniformity. Several TEM studies showed that strain induced in InGaN due to lattice mismatch and the miscibility gap between InN and GaN cause formation of In-rich clusters- the presence of which has been argued, by some, to be crucial for the superior performance of these materials [1]. In contrast, separate studies have suggested that these In-rich clusters are an artifact of TEM imaging caused by electron irradiation [2] and have no effect on a device&’s performance. Towards this end, we employ aberration-corrected (Cs) TEM to image a series of InGaN samples including those from reference 2 with atomic resolution at energies and doses below the knock-on threshold. We have systematically shown that operating Cs-TEM at 120 kV inhibits knock-on damage without compromising the atomic resolution. Low-loss electron energy-loss spectroscopy and Cs-corrected scanning TEM (STEM) data complement our TEM findings. Additionally, a unique cathodoluminescence-STEM setup has enabled us to directly correlate structural and optical properties of our samples. This work furthers understanding of structural features that underpin the carrier dynamics in InGaN quantum wells and will help towards further optimization of these structures. [1] J. R. Jinschek et al., Solid State Comm., 137, 230-234 (2006) [2] T.M. Smeeton, C.J. Humphreys, J.S. Barnard, et al., J. Mater. Sci. 41 2729 (2006)

Harvesting energy directly from sunlight is increasingly recognized as an essential component of future global energy production. In this presentation, I will demonstrate the applications of surface plasmon in the conversion from solar energy to electricity and chemical energy along two different mechanisms. In our work, the surface plasmonic features of metallic nanocrystals are tuned by tailoring their size, shape, structure and composition in solution-phase synthesis. In turn, these tunable plasmonic nanostructures are implemented in solar energy conversion in two example systems. In the first system, we demonstrate that the photothermal effect of silver nanostructures can provide a heat source for thermoelectric devices, which serves as an important supplement to plasmonics-enhanced photovoltaic devices. In the second case, we have explored the role of sunlight in organic reactions that are catalyzed by a class of newly-designed bimetallic nanocrystals. It is expected that the present work will enable us to expand the applications of surface plasmonics and open the door to new concepts of solar energy conversion.

The optical rectenna consists of two main parts: a nanoantenna that absorbs the incoming photons, converting them to electrical current and a rectifying diode. To harvest the solar energy efficiently the key parameter is the absorption efficiency of the nanoantenna at optical frequencies. In this work, gold and silver metallic nanoparticles were used to fabricate the antenna nanostructures because their absorption wavelength, corresponding to their localised surface plasmon resonance, can be effectively tuned in the visible and infrared region. These structures also play a critical role in the metal-insulator-metal (MIM) diode structure. Various structures of optical rectenna have been fabricated using a combination of nanosphere lithography, oxygen plasma treatment and argon ion-milling process. In the first approach a mono-layer of polystyrene nano-spheres was deposited onto the top of a silicon substrate that had been pre-coated with a layer of metal and an ultra-thin oxide layer. This was achieved by dipping the substrate into a solution containing nanospheres with diameter of 300 nm or 620 nm, and withdrawn either vertically or at an angle. This layer of polystyrene then served as a mask for metal deposition, thus creating nanotriangle-type nanostructures as the top MIM contact. In the second approach a polystyrene mask was deposited on top of fully-formed MIM layers, before defining etched structures using an Argon ion-milling process. By varying the energy and the angle of incidence of the Argon ions during the milling process, aligned nanorods, nanodiscs, and nanopencils were produced. To form smaller diameter structures, the polystyrene sphere size was reduced post deposition using an oxygen plasma. Integrating sphere, UV-Vis and Fourier transform infrared spectroscopy (FTIR) were used to measure the optical properties of these nanoantenna. The electrical properties were tested with conductive atomic force microscopy (c-AFM). The results will be presented and discussed in the context of optimised solar absorption.

Metal nanostructures can act as plasmonic nanoantennas (PNAs) due to their unique ability to concentrate light over sub-wavelength spatial regions. However engineering the optimum PNA in terms of a given quality factor or objective function has been a challenge due to the large number of geometrical parameters involved. We propose a novel design strategy of PNAs by coupling a genetic algorithm (GA) to the analytical multi-particle Mie theory. The positions and radii of a metallic nanosphere clustes are found by requiring maximum electric field enhancement at a given focus point. Within the optimization process we introduced several constraints in order to guarantee the physical realizability of the tailored nanostructure with electron-beam lithography (EBL) Aiming at modeling a realistic device based on a bow-tie antenna and on an underlying substrate, we used the GA approach discussed above to obtain an initial solution, which was then refined by using a full vector three-dimensional finite-difference time-domain (3D FDTD) simulator. Our GA optimization results unveil the central role of the radiative coupling in the design of PNA and open new exciting pathways for the engineering of metal nanostructures. Samples are fabricated using Electron Beam Lithography and surface-enhancement Raman scattering measurements were performed in order to validate the theoretical predictions.

Surface plasma waves are typically quantized by direct analogy to electromagnetic waves in free space or in dielectric media. As a result, the quantum theory of these waves predicts that their quanta—surface plasmons—should exhibit the same quantum phenomena that photons do. Experiments that test this analogy have begun only relatively recently, however, and it remains to be seen just how faithfully surface plasmons reproduce the quantum behaviors of their free-space counterparts. In this work, we study the plasmonic analog of two-photon quantum interference (TPQI), also known as the Hong-Ou-Mandel effect. In free-space TPQI experiments, photons that arrive simultaneously at adjacent inputs of a 50-50 beam splitter are always observed to exit the splitter together in one output port or the other, but never in separate outputs. Quantum theory explains this result as a consequence of destructive interference between the probability amplitudes of two processes: one in which both photons are transmitted and the other in which both are reflected. Importantly, TPQI requires that the two photons be as nearly indistinguishable as possible; any distinguishing information tends to reduce the visibility of the effect. We investigate TPQI in waveguides that involve plasmonic elements. We generate pairs of single photons at 810 nm by spontaneous parametric down-conversion of light from a 405 nm diode laser, collect them into optical fibers, and couple them into sub-micron silicon nitride waveguides, which deliver them to and collect them from various configurations of dielectric-loaded surface plasmon polariton waveguides and couplers. In particular, we consider a structure in which a plasmonic directional coupler plays the role of the 50-50 beam splitter in traditional TPQI measurements and similar structures in which plasmonic waveguides are inserted before one or both inputs of a dielectric 50-50 directional coupler. We study the effects of these plasmonic elements on the visibility of TPQI and comment on the roles of dispersion and loss.

The decay of surface plasmon resonances into hot electron-hole pairs has recently been utilized in Schottky barrier optical detectors and energy conversion devices, in contrast to many applications where damping is to be ardently avoided. Proposed devices include resonantly-enhanced infrared sensors, sub-bandgap light absorption in photoelectrochemical cells, and metal-absorber power conversion. However, such devices are subject to the physics governing internal photoemission and thermionic emission, and for practical applications the quantum efficiency is an important parameter to consider. Here, we couple optical finite-element simulations of such devices to a model for hot-carrier collection efficiency and compare calculations to experiment for plasmonic antenna/Schottky barrier geometries. The main system considered consists of Au electric dipole optical antennas on Si as a Schottky barrier infrared detector, as previously demonstrated. We first calculate the spatial profile of light absorption in the antenna to estimate the spatially-resolved hot-electron generation rate, and then calculate the emission of plasmonically-generated hot electrons over the Au/Si Schottky barrier, considering electron-electron and electron-phonon scattering, the energy distribution of hot electrons, reflections at interfaces, classical emission over the energy barrier, and the limited escape cone arising from the critical normal momentum requirement. In general the efficiency is low due to the small escape cone for energies near the barrier energy. The presence of a Ti adhesion and damping layer is examined in detail as well as possible dielectric layers to enhance scattering. If we assume that carrier excitation is nondirect so as to decorrelate the electron momentum distribution with the original plasmonic electric field, the maximum external quantum efficiency is limited to values around 1% on resonance. Experimental results will also be presented comparing various geometries to simulation with the goal of optimizing efficiency. The possibility of solar energy conversion devices is also analyzed. In this case the diode must be operated in forward bias, and the thermionic emission current is a major detriment. Considering the power conversion efficiency as a function of barrier energy, the best-case values (total solar absorption in a 10 nm thick metal film) are limited to around 0.3% at room temperature. Increasing the maximum efficiency of such devices requires correlation of the plasmon electric field with the hot carrier electron momentum to relax the escape cone constraints to carrier injection efficiency. Models for correlated and uncorrelated electron distributions will be compared to experimental measurements for Au/Si hot carrier injection quantum efficiency as a function of electric field wavevector and polarization.

Plasmonic antennas operating over a wide spectrum with unidirectional patterns are essential for emerging plasmonic-photovoltaic applications. While promising, spectral engineering of close-packed honeycomb plasmonic antennas with unidirectional absorption is challenging due to the large number of morphological parameters within a Wigner-Seitz unit cell, particle coupling, and interaction between neighboring unit cells. In this study, we introduce morphological parameters within the Wigner-Seitz unit cell to propose a generalized close-packed honeycomb array. The proposed morphological parameters provide additional flexibility for manipulating the spectrum by relaxing geometrical restrictions due to a strong correlation between unit-cell and nanoparticle morphology. In addition, we achieve spectral broadening by breaking the symmetry within a Wigner-Seitz unit cell on a hexagonal grid, rather than breaking the symmetry of the hexagonal grid itself. We demonstrate the advantages of close-packed arrays in terms of spectral response, field enhancement, directionality, and absorption over large surfaces.

Asymmetric nanostructures can result in interesting optical phenomena. Fabricating these nanostructures over large areas presents significant challenges. Template stripping is a technique that relies on the weak adhesion between coinage metals (Au, Ag, Cu) and SiO2 (e.g. native oxide covered silicon) to provide ultrasmooth surfaces. We report on the fabrication of ultrasmooth asymmetric nanostructured films from both metal and oxide materials over cm² areas on both flexible and rigid substrates using template stripping. We show how these films can be incorporated as the back reflector in various solar cell devices resulting in enhanced light trapping effects. Both theoretical and experimental results will be presented and comparisons will be made with symmetric nanostructures.

Photobleaching, an irreversible light-induced chemical reaction, changes the structure of a fluorescent molecule and permanently destroys the molecule&’s fluorescence capability. It imposes a fundamental limit of the total number of photons that one can harvest from the molecule. Considerable effort has been devoted to suppress the photobleaching, which culminates primarily in chemical-based strategies: making the local environment more chemically inert to fluorophores, or modifying the structure and energy landscape of the fluorophores to be more resistant to photobleaching. Here, we demonstrate that the strong Purcell effect of a plasmonic nanostructure could effectively steer a fluorophore away from photobleaching and prolong a fluorophore&’s lifespan, offering a “physical” approach to manipulate the photochemical reactions of bleaching. The underlying mechanism lies in the fact that the two channels—the relaxation and the bleaching— compete with each other during fluorescence process. As a result, we can reduce the probability of the molecule entering the bleaching channel by enhancing the relaxation process with plasmonic structures. Experimentally, up to 1,000 folds more photons have been harvested from a single fluorescent molecule located in a plasmonic cavity, in good agreement with theoretical predictions. Such a giant suppression of photobleaching is well beyond conventional chemical schemes. Our findings will not only accelerate single-molecule fluorescence applications, but also open up a new avenue to electromagnetic control of chemical reactions.

Plasmonic nanocavities enable significant light confinement in a subwavelength-scale volume. Although many plasmonic structures were fabricated through the deposition or patterning of metal films, the top-down approaches can cause high loss because deposited polycrystalline metal films introduce undesirable defects of scattering centers. In order to address these issues, we propose new plasmonic nanocavity that consists of silver nanowire and dielectric plasmonic bandgap structures. A chemically synthesized silver nanowire with single crystalline, defect-free surface is placed on a patterned dielectric substrate. Surface-plasmon-polaritons (SPPs) are then excited at the interface of nanowire/substrate. Numerical simulation shows that plasmonic bandgap can be opened in the silver nanowire on a high-index dielectric substrate with grating structure, and the bandgap frequency is determined by the grating periodicity. In addition, by missing a few gratings in the bottom dielectric substrate, a plasmonic nanocavity to confine SPPs in a desired region can be formed. To find an optimal design of the nanocavity, we changed several structural parameters such as cavity lengths, widths, and depths of air grating slots. We also investigated the dependence of nanowire cross-sections on the optical properties. The simulation results showed that Q factors of plasmonic modes excited by a nanowire with a square cross-section are higher than those of plasmonic modes by a nanowire with circular ones. To further enhance the confinement of plasmonic mode, thin air gap between silver nanowire and substrate can be introduced only in the cavity region. Hybrid plasmonic modes are then excited in a nanocavity with an air gap of 5 nm, and can show increased Q factors. We believe that this promising and novel plasmonic nanocavity developed using bottom-up synthesized silver nanowire will be useful for an ultimate light source in an ultracompact integrated circuit.

We generate power by illuminating arrays of plasmonic nanoantennas, consisting of electron beam-patterned Ag crossbar lines with “teeth” 70 - 300 nm long lying atop and parallel to ultrathin (less than 20 nm) tunneling barriers (NiO and NbOx (native)/Nb2O5). These oxide barrier layers, sandwiched between the Ag antennas and bottom metal (Nb or Ni) ground plane, form metal-insulator-metal (MIM) diodes [1,2]. These MIM diodes can be extremely fast, if their capacitance is low enough that the electron tunneling transit time dominates the response time, and their thickness is less than ~ 20 nm. The tunneling barriers rectify alternating currents, generated in the top metal layer by the incident visible/near-infrared (vis/nir) laser beams. When illuminated with a 532 nm nanosecond laser beam (pulse width ~ 10 nsec, peak power density 4*10^6 W/cm^2), both the nanorectenna arrays and the tunneling diodes themselves produce a small amount of power (~ 1 mV open circuit voltage, 20 uA short circuit current for Pt-Nb2O5-NbOx(native)/Nb diodes) [3]. Illuminating Ag-Nb2O5-Nb diodes with a continuous wave visible laser beam (wavelength 475 - 585 nm, power density 80 W/cm^2) produces an alternating voltage that tracks the diode&’s current-voltage characteristic, consistent with rectification of the THz voltages generated across the MIM diode, although the signals are larger than what is predicted by classical rectification theory and our quantum mechanical mode of MIM diode conduction [4]. We measure the enhancement of these effects in the presence of the plasmonic Ag nanoantenna array with broad visible and narrower near-infrared resonances. In order to better understand the power conversion efficiency, we vary the diode material, top metal, laser beam angle of incidence, wavelength, power, and polarization. We interpret our results in terms of nanorectenna rectification, and/or other effects like unequal heating of carriers on either side of junction, photon-assisted tunneling, etc. [1] P. Periasamy, et. al., Adv. Materials 23 (2011) 3080. [2] M. L. Hoey, et. al., Appl. Phys. Letts. 97 (2010) 153104. [3] R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011). [4] R. M. Osgood III, submitted to J. Appl. Phys. (2012).

Gold nanorods were fixed on an ITO plate and used for the spectroscopic sensing and Surface-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (SALDI-MS) of oligopeptides (angiotensin I). The longitudinal surface plasmon bands of the gold nanorods responded to the 10 fM angiotensin solution that was cast on the ITO plate. The SALDI-MS measurements had an ultra-high sensitivity to the angiotensin on the ITO plate. A very small surface density (5 × 10-19 mol/cm2) of angiotensin could be detected at m/z = 1297 with a good signal/noise ratio (S/N = 11). The ITO plate, which was modified with gold nanorods, was found to be effective in collecting angiotensin molecules adjacent to the gold nanorods, and the SALDI processes that were induced by the photoabsorption of the gold nanorods efficiently contributed the desorption and ionization of the angiotensin.

Nanocomposite films of Yttria stabilized Zirconia (YSZ) encapsulated Au nanoparticles have been fabricated by Physical Vapor Deposition (PVD). Extremely stable and repeatable sensing results of these films when exposed to gases such as CO and H₂ at high temperatures (500°C) in an air carrier gas make them potential candidates for applications such as emissions monitoring and fuel cells. Well defined Au nanoparticles were formed by depositing and annealing a thin Au film on a YSZ surface. A thin capping layer of YSZ was then deposited to stabilize the Au nanoparticles at high temperature. A sensing response from the optical excitation of surface plasmons on the gold nanoparticles can be brought about by a change in the dielectric constant of the surrounding medium or from a change in the free electron density of Au nanoparticles, both of which can be affected by the oxidizing or reducing nature of the analyte gases. With YSZ playing the crucial role of a high-temperature oxygen ion conductor, films with varying thickness of the YSZ capping layer have been investigated for differences in the sensitivity and response time. Results from sensing tests will be shown, and future directions as to the selective sensing of gases among CO, NO₂ and H₂ will be discussed. Selective sensing could be achieved by either employing materials such as Pd, for example, which is sensitive to hydrogen, by changing the film composition or a combination of both. Prospective methods of achieving a single selective and sensitive nanocomposite film will also be presented and described.

Improving the photostability of chlorophylls is one of the main challenges to facilitate their industrial and biotechnological use. In this regard, we have employed gold nanoparticles (AuNPs) to photoprotect chlorophyll-a (Chla). Light absorption by Chla results in the formation of its excited singlets (1Chla*). If not readily used photochemically, they get converted into triplet excited states (3Chla*). Chlorophyll triplet states are long-lived energetic species which can easily react with the triplet ground state of oxygen (3O2) to produce strongly photooxidative reactive oxygen species (ROS). These ROS attack the nitrogens of the porphyrin macrocycle of Chla and cause its photodegradation. Therefore, if these nitrogen sites could be shielded from the attack of ROS by some harmless agent, Chla will be well protected. In the present work, we have demonstrated that AuNPs can greatly increase the photostability of Chla. The UV-Visible, fluorescence and X-ray photoelectron spectroscopic studies have been carried out to assess the effect of AuNPs on the photostability of Chla. The results show that with an appropriate concentration of AuNPs, as much as 80% photoprotection can be provided to Chla in solution. It is further seen that under in-vitro conditions, AuNPs are much better photoprotective agents of Chla than conventional protective agents, i.e., β-carotene or quinones, which are known to be very effective in natural living conditions (plants). The protecting ability of Chla by AuNPs is the result of their efficient binding with Chla at its nitrogen sites in dark, thus, inhibiting the reaction of ROS with Chla. The same property of AuNPs, i.e., to bind with Chla in dark, renders them better photoprotectant than carotene or quinones. The use of gold nanoparticles as efficient photoprotector of Chla, thus, offers us new possibilities to increase the photostability of various porphyrins used in a wide range of industrial (optoelectonic devices such as OLEDs and photovoltaics) and medical (photodynamic therapy) applications.

It has been recently shown that statistical ray optical light trapping theory does not transfer to the ultrathin film solar cell regime and optically resonant metallic and dielectric structures can be used to enable significant absorption enhancements that can potentially exceed the 4n^2 statistical ray optics limit. Here we compare and contrast dielectric and plasmonic nanostructures for their potential to minimize surface reflectance and increase broadband light trapping for thin films of GaAs, the material used in record efficiency (n = 28.8%) single-absorber photovoltaic cells. To understand absorption enhancement mechanisms of dielectric and plasmonic nanostructures, we investigated 100 nm - 1 um GaAs thin films on both silica and metal substrates with periodic arrays of surface-applied silicon nitride and Ag nanostructures. We find that the active layer thickness determines the dominant enhancement mechanism: i) for ultra-thin (100 - 200 nm) films, plasmonic and dielectric Mie resonances that couple into waveguide modes are most significant enhancement mechanism; ii) for thicker films (500 nm - 1 um) both particle resonances and film thickness-related Fabry-Perot resonances determine cell performance. Using optimized structures, we obtain a 5-20% increase in the 1 Sun AM 1.5G photocurrent for 100 nm - 1 um thick GaAs cells, relative to the best conventional dielectric two-layer antireflection coatings. We find that plasmonic structures offer a large absorption enhancement for ultra-thin films while they provide limited access to resonance-driven light trapping for thicker films due to destructive Fano interference at the front interface. Dielectric structures on the other hand can be engineered to act as efficient light-trapping Mie scatterers and can serve as a broadband AR coating for both thin and thick films. To experimentally verify our predictions, we fabricated thin-film GaAs cells using epitaxial lift-off techniques. Nanoscale light trapping surface structures were fabricated using electron beam lithography, soft conformal nanoimprint lithography, and PECVD growth techniques. Results of GaAs cell spectral response measurements will be discussed and compared with model predictions.

We unconsciously have had a damaging effect on environment with producing a variety of coloring materials for the development of humankind. As a result we had serious environmental problems caused by pollutions. Considering the rise of an environmental awareness, we need to produce environmentally friendly color materials. Structural colored materials must be the strong candidates because a variety of colored materials can be prepared just by changing the microstructures of the materials. Here, we describe the preparation of new structural colored materials by mixing white particles (SiO2) and black particles (Fe3O4) using a microflow system. Structural colors are caused by interference of light, scattering, and diffraction effects, and the color tint are influenced by refractive index and structural form of substances. [1]One of the most widely anticipated structural colored materials is a colloidal crystal film with a periodic optical nanostructure, composed of mono-dispersed submicron sized particles. Spherical assemblies (SAs) of the mono-dispersed colloidal particles are particular interest because of the possibilities for dispersed structural colored materials. In addition, such color materials can have the application potential for the biosensor. To use the SAs for such application, we need to obtain mono-dispersed SAs. To this end, we apply micro flow focusing device (MFFD) for preparing the mono-dispersed SAs of colloidal particles. We prepared MFFD made of poly(methyl methacrylate), which has a flow focusing geometry integrated into a planar flow channel of 300 mm diameter to form aqueous liquid drops containing particles in a continuous and immiscible oil phase (Span 80 surfactant is dissolved in hexadecane at 0.2 wt%). After drying the aqueous liquid drops in the oil phase by the application of heat, mono-dispersed SAs of silica particles were obtained. The size of the SAs can be controlled by changing the concentration of particles in the aqueous solutions. We made a series of the SA revealing different colorations with or without adding the magnetite particle (Fe3O4) and phenyl methyl ammonium chloride working as a salt. The hexagonal sharp peaks in the power spectra obtained by two-dimensional (2D) fast Fourier transforms (FFTs) of the scanning electron microscopy (SEM) images for the surface and inside structures of the brilliant colored SAs confirm the presence of short and long-range order. On the other hand, symmetrical and circular patterns around origin in 2D FFTs of SEM images for both structures of matte colored SAs confirm the presence of short-range order. The reflection spectra from these SAs also indicate these optical properties. In conclusion, we easily obtained a variety types of the spherical structural colored pigments composed of white and black particles by using of MFFD. References 1. Takeoka, Y. Angew. Chem. Int. Ed., 50, 4012-4015

Silver nanowires are attractive as an alternative to ITO transparent conductors because of their high optical transmission and low sheet resistance. However, nanostructures inherently scatter light in comparison to thin continuous films, and this haze feature can be useful or detrimental depending on the application. In this paper, we will present a semi-empirical model that describes the nanowire dimension-property relationship. The model was developed based on the Mie theory of light scattering and percolation theory of random resistive 2D networks. Transparent conductor films were made with roll to roll continuous coating and spin coating using nanowires with various diameters and lengths. Based on the experiments and model, we will discuss the effect of diameter and length on scattering haze, and the critical exponents for sheet resistance near the percolation threshold. We will also present an optical and electrical model for micron size wide metallic periodic grids as transparent conductors. These results for random nanowires and periodic grids will be compared with literature values of other transparent conductors including ITO, CNT, PEDOT, and graphene.

Today's semiconductor photovoltaic energy converters are dominated by photon absorption and photo-carrier generation of electron-hole pairs across a semiconductor band gap. The 'Shockley-Queisser' limit of photovoltaic efficiency dictates that thermalization of hot carriers limits the energy that can be extracted from photons whose energies are above the band gap energy, whereas photons of less than band gap energy are not absorbed. Is it possible that plasmonic mechanisms could enable us to exceed the Shockley-Queisser limit by recovering the energy lost via either hot carrier relaxation or subgap photon non-absorption? We discuss two approaches to this concept: i) plasmonically-generated hot carrier collection mechanisms and ii) ‘plasmoelectric&’ mechanisms for power conversion. Hot carriers created by the decay of plasmons excited in resonant optical nanoantennas can be injected across a Schottky barrier to produce a hot carrier photocurrent. We articulate the design approach for plasmonic hot carrier nanoantennas, and discuss the energy conversion efficiency limits of this approach. The plasmoelectric effect is a mechanism for conversion of optical power into DC electrical power using resonant absorption in plasmonic nanostructures. In this effect, an optically-induced electrochemical potential results from the dependence of optically generated heat on the change the plasmon resonance frequency that occur with changes of electron density. We model an all-metal plasmoelectric structure capable of characteristic conversion efficiency of >10% energy conversion under 1 kW m-2 intensity, single-frequency radiation. We discuss strategies for enhanced efficiency, broadband power conversion, and further applications of plasmoelectric power generation.

Interest in plasmon-exciton interactions is increasing owing to potential impact in light harvesting and optical signal manipulation. Recently a new mechanism of plasmon induced current generation was observed in porphyrin-Au nanoparticle hybrid nanostructures.[1] The plasmons associated with the gold nanoparticles enhanced photo conduction by many factors even an order of magnitude. To understand this phenomena we have first developed an approach to the analysis of temperature dependent transport measurements that can lead to an unambiguous determination of mechanism in complex systems. [2] Then the temperature and wavelength dependent transport is examined as a function of nanoparticle size and distribution and molecule optical properties. [3] The Au-porphyrin combination is designed to distinguish potential mechanisms for plasmon induced current. We will show new evidence for a mechanism involving 'hot electron' generation. This has the potential to vastly increase efficiency of energy harvesting devices. [1] Banerjee et al ACS Nano 4 (2010) 1019-1025 [2] Conklin et al NanoLetters 12 (2012) doi 10.1021/nl300400a [3] Conklin et al Advanced Materials 21 (2011) 4712-4718.

We demonstrate 3D surface plasmon photoresponse in nanopillar arrays resulting in enhanced responsivity due to both Surface Plasmon Polariton Bloch Waves (SPP-BWs) and Localized Surface Plasmon Resonances (LSPRs). Angular photoresponse measurements show that SPP-BWs can be spectrally coincident with LSPRs to result in x2 enhancement in responsivity at 1045 nm. Full-wave Finite Difference Time Domain (FDTD) simulations substantiate the coupling of the SPP-BW and LSPR for enhanced absorption and the nature of the LSPR. The LSPR is due to a nanopatch optical antenna formed by the gold cap partially coating the nanopillar and the gold ground plane. Avalanche multiplication is also investigated in core-shell GaAs nanopillar arrays resulting multiplication factors ~20 at -12V. Enhanced electric fields in the nanopillar allows a more deterministic multiplication of carriers and breakdown at lower voltages. 3D control of surface plasmon absorption and enhanced carrier multiplication are essential steps towards the realization of plasmonically enhanced nanopillar single photon detectors.

The typically short exciton diffusion length is an important factor in limiting the performance of organic photovoltaic (OPV) devices, and presents a trade-off with the need to maximize light absorption. While the bulk heterojunction architecture is one route to breaking this trade-off, the BHJ approach presents a number of other undesirable shortcomings (e.g. potential long-term morphological instability, the need to simulaneously tune solubility and electronic properties, etc.). Another approach is to maximize light absorption for relatively thin, planar structures. Plasmonic structures potentially allow the optical density of states to be increased inside the active organic layers, thereby increasing absorption, but nearly always with parasitic effects (e.g. dissipation as heat) dominating the improved absorption. Here we study the wavelength-dependent enhancement of photocurrent through surface plasmon-exciton (plexciton) coupling in ultrathin, smooth, metal-dielectric-metal (MDM) OPVs. The use of metallic cavities produces a strong optical confinement of the internal waveguided modes and efficient coupling to molecular dipoles throughout the device. We observe a very high, 60% plasmon-to-photocurrent conversion efficiency and an absorption enhancement of 76% above what is predicted from direct absorption through free space-coupled modes. This absorption enhancement follows the trend of the calculated photon density of states in the waveguide and the radiative recombination rate of the donor material, implying that exciton coupling to waveguided plasmon modes through photon recycling further enhances the absorption in subwavelength-scale MDM OPVs. These structures are potentially compatible with simple encapsulation layers and do not require in-plane nanopatterning or external sensitizer layers.

A laser is a device primarily consisting of a resonant cavity and a gain material that emits monochromatic, coherent electromagnetic radiation and is thus an invaluable tool in various scientific fields. However, the size of a laser is constrained by the diffraction limit, meaning that the light beam it produces cannot be spatially confined to less than a few hundred nanometers, or half the wavelength of the light being emitted in free space. In the past decade, a deeply sub-wavelength analogue to the laser, using surface plasmons instead of photons, has been proposed that exhibits surface plasmon amplification by stimulated emission of radiation, i.e., the SPASER. These devices consist of a resonant metallic nanoparticle, which can support surface plasmons, that is in intimate contact with a high-gain material. SPASERs could enable ultra-small optical amplifiers, higher-resolution microscopes and densely integrated optical communications to be further developed. In recent years, a few experimental demonstrations of SPASER devices have been realized using dye-doped-silica-coated spherical metal nanoshells and semiconductor nanowire/metal film configurations. However, the development of SPASER devices is still in its infancy and further exploration is required to identify SPASER designs that could achieve low thresholds for surface plasmon amplification by stimulated emission, high spatial coherence and both color purity and tuneability. Here, we propose the use of high-gain conjugated polymer semiconductors as the active materials through which surface plasmons are amplified in SPASER devices. The advantages of these materials are: (1) unlike dye molecules, they do not undergo concentration quenching in the solid-state and so a very high chromophore density can be packed into the near-field of the resonant metal nanoparticle. (2) Conjugated polymer gain media exhibit large gain cross-sections (up to 10^-15 cm² for polyfluorene derivatives). (3) The polymer material is far more stable than small organic laser dye molecules. (4) The oscillator strength and transition dipole orientation of the conjugate polymer gain material can be manipulated by controlling polymer chain orientation. In addition to employing high-gain conjugated polymer materials, the use of anisotropic metal nanoparticles instead of spherical metal nanoparticles is expected to yield stronger (i.e., lower loss) surface plasmon resonances and, hence, lower threshold SPASER operation. We will present our recent work on the fabrication of conjugated-polymer-coated gold nanorods using miniemulsion and grafting techniques, and describe the resonant optical interactions between gold nanorods and the conjugated polymer gain material as a function of excitation pump energy.

In this talk, we will present some of our most recent theoretical results of our work on exploring several novel features of nonreciprocal metamaterials and magnetoactive metamaterials. These metamaterials and metastructures involve scenarios in which magnetoactive materials (materials with time-reversal symmetry breaking due to magnetic field) are combined with plasmonic nanostructures or extreme-parameter metamaterials such as epsilon-near-zero (ENZ) structures. We have been investigating, theoretically and numerically, several topics including: magnetic ENZ metamaterials, nonreciprocal plasmonic circuits and interconnects (such as one-way waveguides), notion of one-way hot spots, plasmonic boosting of magneto-optical response, plasmonic antenna with broken time-reversal symmetry, just to name a few. We will discuss some of novel phenomena stemming from such magnetoactive nonreciprocal metamaterials, and will present physical insights behind our findings.

Perfect absorbers have so far involved relatively complex structures consisting of either wavelength-scale asymmetric Fabry-Perot cavities or plasmonic metamaterials. It is commonly assumed that plasmonic elements are necessary for the creation of perfect absorbers with thicknesses smaller than a quarter of the operating wavelength. In this report, we show that perfect absorption can be achieved with a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at the interfaces between lossy media. This design is implemented with an ultra-thin (~lambda;/65) vanadium oxide (VO2) layer on a sapphire substrate, with the VO2 temperature tuned in the vicinity of its insulator-to-metal (IMT) phase transition, leading to 99.75% light absorption at lambda; = 11.6 µm. The operation wavelength can be tailored by modifying the thickness of the VO2 film. The effect can be explained using the language of critical coupling to a resonator, which is formed by the VO2 layer and the substrate. The absence of lithography in device fabrication, its structural simplicity, and its immunity to small variations in material composition enable various large area applications. Furthermore, the large tuning capabilities (from ~80% to 0.25% in reflectivity) enabled by the large variation of optical properties in the vicinity of the IMT are promising for infrared devices such as thermal emitters, modulators, and bolometers.

In recent years, a variety of nano-optical components have been demonstrated including plasmonic amplifiers, lasers and LEDs. However, due to inefficient injection and coupling of surface plasmons, the integration of plasmonic components at the nanoscale remains a major challenge. In this work, we report an electrically driven nano-scale semiconductor surface plasmon source fabricated one-dimensional nanowires and two-dimensional nanoplates by chemically assisted dry etching of an epitaxially grown and a doped AlGaAs/GaAs semiconductor wafer. The device structures are characterized by a vertically-oriented p-i-n junction with an intrinsic region containing AlGaInP/InGaP multi-quantum wells that define the emission range from 600 - 700 nm. We successfully realized electrically driven semiconductor nanowire and nanopalte LEDs with moderate current injections of few mu;A. In addition, an optimized subwavelength-scale metallic waveguide design demonstrates efficient delivery of electromagnetic energy with high confinement and long propagation lengths. Electroluminescence spectroscopy measurements and CCD image acquisitions confirm that the electrically driven source can efficiently launch and propagate surface plasmons into the optimally designed subwavelength-scale metallic waveguides. To further enhance optical functionality, we introduced adiabatically tapered metallic structures at the end of the waveguides and observed concentration of light down 50 nm and a significant enhancement of electromagnetic field density at the end of the tapered gap.

In the last few years, the Second Harmonic Generation (SHG) from planar arrays of metal nanoparticles (NPs) has been investigated for a variety of NPs shapes, sizes, and under different excitation-collection and polarization conditions. However, the role of the planar array geometry on the SHG from metallic NPs is not yet fully understood. Using deterministic aperiodic nanostructures (DANS), we investigate the role of the array geometry on the SHG from planar arrays of Au nano-cylinders for progressively increasing structural complexity. Arrays of nano-cylinders are fabricated by electron beam lithography on silica substrates, their plasmonic response is characterized by dark field scatting, and SHG is produced by excitation with 120fs laser pulses at 780 nm. Through polarization-resolved measurements we demonstrate multipolar generation that is largely tunable by the array geometry. We consider the components of the collected signal that are parallel and orthogonal to the scattering plane, defined by the directions of excitation and collection. We measure the SHG intensities of the two polarization components as a function of the input polarization angle of the pump beam. Quadrupolar SHG behavior is clearly displayed by the orthogonal components in quasi-periodic DANS. On the contrary, the photonic interactions among NPs radically modify the SHG radiated from periodic and more disordered DANS. For these array geometries, the polarization patterns of the orthogonal components are different from a pure quadrupole, and clear high order multipolar SHG emission is observed. These results are important for the development of novel optical elements for nonlinear nanophotonics applications, such as switchers, frequency converters and nonlinear optical sensors on a planar chip.

We present a plasmonic coupler that unidirectionally converts circularly polarized incident light of opposite handedness (spin) into counter-propagating surface plasmon-polariton (SPP) modes. In addition to the spin-sorting properties, our design enables the conversion of light of arbitrary linear polarizations into SPPs, in contrast to conventional grating couplers. The concept relies on achiral arrays of polarization sensitive subwavelength plasmonic aperture antennas. Its small unit cell footprint and rectangular symmetry enable straight-forward modification to match a broad range of wavelengths and applications. We experimentally demonstrate switchable, unidirectional launching, polarization insensitive coupling and an improved plasmonic lens. The fabricated structures are based on apertures in a flat gold film that are optimized to operate at 632 nm. The designs are optimized with the aid of finite-difference time-domain (FDTD) simulations and fabricated by focused ion beam (FIB) milling into a gold film. The SPP waves are measured by a near-field scanning optical microscope (NSOM) under back-illumination of the structures by a HeNe laser. The device introduced in this report could be useful when precisely controlled coupling between free space light and propagating SPPs is required. The insensitivity to the polarization of the incident light can be used to overcome some of the coupling limitations frequently encountered in plasmonics. This property also has the potential of fully encoding information contained in both the intensity and polarization of light in SPPs, which could have profound impact on future developments in optical information processing.

Using biochemical-targeting methods, one can conjugate the plasmonic nanoparticles to many parts of the cell, healthy or sick. Since the nanoparticles have comparable size to many parts of the cell, binding plasmonic (or nonplasmonic) nanoparticles to parts of the cell could change their properties including curing, or, most likely, killing sick cells. Using plasmonic nanoparticles has the advantage of using their enhanced scattering to image the response of the cells (including their death) to the effect of binding the nanoparticles to the selected part of the cell. Not only one can image the response of the cells directly bound to the nanoparticles but also the reaction of the community of the surrounding nanoparticle-free cells. References: (1) Kang, B.; Mackey, M. A.; El-Sayed, M. A. J. Am. Chem. Soc Comm. 2010, 132, 1517 (2) Austin, L.; Kang, B.; Yen, C.-W.; El-Sayed, M. A. J. Am. Chem. Soc. 2011, 133, 17594. (3) Austin, L. A.; Kang, B.; Yen, C.-W.; El-Sayed, M. A. Bioconjugate Chem. 2011, 22, 2324. (4) Kang,B, Austin, L.A., M. A..El-Sayed, Nano-letters, Submitted.

Diatoms are single-celled algae which produce nanostructured silica (SiO2) "frustules" through biomineralization processes. These frustules present special optical properties induced by their intricate 3D morphology, making them very interesting for light harvesting purposes. Their optical behavior can be described as a photonic band structure, making these frustules equivalent to photonic crystals. A key challenge is to both improve and control the optical properties of the frustules. The aim of the present work is to demonstrate that such a challenge can be solved by plasmonic enhancement. Some metallic nanostructures, including gold and silver nanoparticles, are well known for generating Local Surface Plasmon Resonance (LSPR) in the visible spectral range. Combining the light channeling properties of photonic crystals with plasmonic nanoparticles (NPs) offers a unique opportunity to turn frustules into active optical enhancers, with great potential for photovoltaics, sensors and enhanced Raman scattering applications. Another aspect lies in the high specific surface conveyed by the nanoporous structure of the frustules. These objects allow a high density of deposited NPs and can be considered as 3D substrates. Indeed, a layer of plasmonic NPs-decorated frustules features a much higher surface density of NPs than most standard plasmon enhanced substrates. The present work first focuses on the elaboration of plasmonic bio-synthesized nanostructures by decorating frustules with plasma sputtered gold NPs. Although the plasma sputtering method does not provide any control over the shape of the gold NPs, it is simple and fast. Because gold spreads and forms a continuous layer when directly deposited on the silica surface of frustules, two different methods are compared to obtain individual gold NPs instead. One known method is thermal dewetting of a plasma sputtered gold layer on silica frustules. By dewetting at 700 °C a previously sputtered 5 nm-thick gold layer, gold NPs of 20 nm are easily obtained on the frustules surface. Moreover, the thickness of the sputtered layer appears to be a good control parameter to obtain different NP sizes and size distributions. The second method consists of pre-coating the frustules with a layer of polyethylene glycol (PEG) before sputtering gold on top of it. The PEG layer shows a remarkable stabilizing effect also leading to gold NPs of around 20 nm on the frustules surface. Characterizing the optical properties of the modified frustules is the second focus of this work. The plasmon absorption induced by the gold NPs is highlighted by UV-visible absorption analysis of the gold-decorated frustules. Additionally, diffraction from the frustules can be observed by spectrophotogoniometry. This is a very good example of how to obtain combined optical properties by slightly modifying complex bio-synthesized nanostructures.

Biological photonic structures that are at the origin of structural color have been found in a variety of land-, air- and seaborne animals. By contrast, only a few plants are known to involve photonic structures in their interaction with light. To this end we present the investigation of the photonic system found in the shiny blue seeds of a tropical plant. The strong nearly isotropic color results from the interaction of light with a hierarchical structure found inside individual tissue cells, consisting of concentrically arranged regular layers. This unique photonic architecture inspired the creation of a novel artificial photonic fibre system, exploiting the main features of the plant&’s photonic structure: nanoscale regularity superposed by a microscale curvature. A variety of fiber designs have been realized using a scalable technique that can be applied to make fibers from a wide range of materials with different optical and mechanical properties. We present a detailed experimental and simulation-based characterization of the fibers. Special attention is given to fibers made from elastic materials that show a large tuning range of over 200nm in their spectral band-gap position when a longitudinal mechanical strain is applied.

State of the art localized surface plasmon resonance biosensors rely on plasmonic nanostructures on rigid solid substrates as spectrally homogenous transduction platform. While such substrates are sufficient for vapor and liquid phase sensing, they are not amenable for trace detection on solid surfaces. Here, we demonstrate that a common filter paper uniformly adsorbed with biofunctionalized plasmonic nanostructures can serve as a flexible substrate for collection and detection of trace amounts of bioanalytes (few µg) on real-world surfaces. Compared to conventional rigid substrates, bioplasmonic paper offers numerous advantages such as high specific surface area (resulting in large dynamic range), excellent wicking properties (naturally microfluidic), mechanical flexibility, compatibility with conventional printing approaches (enabling multiplexed detection and multi-marker biochips), and significant reduction in cost.

Optical scatterers such as nano-holes, grooves or slits etched in a metal film are efficient sources of propagating surface plasmon polaritons (SPPs). Interference between SPP waves excited by a scatterer array can lead to unprecedented control of light at nanoscale, e.g. higher-efficiency solar cells, surface enhanced Raman scattering, and compact biochemical sensors. Plasmonic interferometry has the potential to bring the advantages of conventional optical interferometry to micro- and nano-scale, promising for high-throughput, real-time biochemical monitoring. Here we propose the detection of light transmission through a dense array of plasmonic interferometers as a novel optical technique to measure the dispersion of any dielectric deposited on the metal surface, over small area (<10 µm2) and volume (as low as femtoliters). A typical plasmonic interferometer consists of a groove-slit pair etched in a metal film. A broadband beam incident on the groove excites SPPs propagating toward the slit, where interference with the incident beam occurs. Light intensity transmitted through the slit of each interferometer carries information about the effective SPP excitation efficiency β due to diffractive scattering by the groove, and the propagating phase dependent on refractive index of the dielectric. Measuring and stacking light intensity spectra by increasing groove-slit distance in the range of 0.25 - 10 µm, a color map, unique for the particular metal/dielectric combination, can be generated. Color maps for different interfaces, i.e. Ag/air, Ag/water and Ag/40wt% glucose in an aqueous solution were obtained experimentally. A “cut” in the color map at a wavelength can reveal an intensity profile at this specific wavelength. The difference between intensity maxima and minima is proportional to β. The relationship between effective excitation efficiency and wavelength at Ag/air interface was determined as: β = 128.2 /lambda; (lambda; in nm). In principle, this technique can be employed to determine β at any wavelength, for any groove, or any metal/dielectric combination. The dispersion of the refractive index for Ag, water and glucose in an aqueous solution were derived from analysis of the color maps. A database for the dispersion of different materials can be established in a similar fashion, leading to label-free biosensing applications. We recently published a paper describing a proof-of-concept sensing capability of plasmonic interferometry for glucose sensing. In conclusion, SPP excitation mechanism is illustrated by studying the optical performance of a plasmonic interferometer array. The effects of varying different key parameters on the optical property of plasmonic interferometers and a comparison between experimental and FDTD simulation results will be discussed. Our findings show that plasmonic interferometry is an alternative optical technique for accurately analyze thin dielectric films at multi-wavelength over a small area.

TBD

With the ultimate goal of developing improved fluorescence resonance energy transfer (FRET) networks for characterization and control at the nanoscale, we have been utilizing DNA to build optically active nanostructures that are easy to design, fabricate, and reconfigure. This work utilizes the powerful techniques of structural DNA technology as developed by Seeman and others together with well-established ligation chemistries to prescriptively space dyes with a resolution approaching the limit set by the spacing between nucleotides. The nanostructures to be reported on range from simple linear wires to complex 3-dimensional structures, all of which are constructed from a common set of components and tested under identical conditions with the goal of understanding the factors that determine the end-to-end transfer efficiency. To investigate transfer efficiency in FRET networks we developed a series of DNA structures that probe either single FRET transfers from Cy3 donors to a Cy5 acceptor or 4-step FRET cascades involving Cy3, Cy 3.5, Cy5 and Cy5.5. We have studied linear and bidirectional arrangements as well as 4-way and 8-way junctions in order to look at the effect multiple donors have on increasing the output of the final acceptor. Within each of these structures we have varied the distance between the dyes to determine the optimal spacing enhanced FRET efficiency. We have also designed a series of DNA dendrimers to create a true funneling effect. Results from the pair wise data indicate that the 4 way structures show the greatest acceptor output with equal or slightly less from the 8-way structure. The dendrimer structure, having the same number of initial donors as the 8 way cascade, appears to mitigate this limitation by optimizing the enhancement at each step of the cascade. Overall this study looks to exploit the power of structural DNA technology to explore the space of FRET networks and frame design rules for achieving optimal transfer efficiencies.

Owing to virtually unlimited multiplexing ability and excellent photostability, Raman scattering based bioimaging is gaining immense attention. Design and synthesis of Raman probes, which exhibit large and uniform Raman signature is paramount in advancing this novel bioimaging modality. Surface enhanced Raman scattering probes in which a Raman marker is trapped at the interstices of closely spaced metal nanostructures is a powerful class of Raman probes. Core-satellite plasmonic nanoassemblies are ideally suited for SERS probes considering the isotropic and large SERS signals from such structures. Recently, we have demonstrated non-covalent assembly of shape-controlled plasmonic nanostructures into highly Raman-active core-satellite structures. However, these structures are not ideal for bioimaging due to their poor stability in physiological conditions. To overcome the aforementioned issue, we demonstrate covalently bonded isotropic core-satellite nanostructures using “click” approach. Although click chemistry is has been widely used as a synthetic technique, it has not been explored in the context of metal nanostructure assemblies. Here, to the best of our knowledge, for the first time, we demonstrate core-satellite nanoparticles using click chemistry. To obtain these structures, the individual cores and satellites are modified with two custom made click molecules with inbuilt Raman reporter. Subsequently, the surface modified nanoparticles are catalyzed by CuSO4.5H2O and sodium ascorbate to form a five membered azide ring through Huisgen, 1,3-dipolar cycloaddition of alkyne and azide. Furthermore, these covalently assembled nanostructures are protected with SH-PEG-NH2 to increase the serum stability for both in vitro and in vivo applications. Finally, we demonstrate in vitro Raman imaging of SKBR3 cells using ERBB2 antibody functionalized core-satellites. Our studies lay the path forward for achieving highly stable, efficient, and cost-effective Raman probes for sub-cellular probing, bio-sensing and in vivo imaging.

Graphene holds great promise for ultra-compact and electronically controlled plasmonics [1,2]. Recently, resonant coupling of propagating THz waves to plasmons in micro-ribbons has been demonstrated [3], while IR near-field microscopy has been applied to observe the coupling of graphene plasmons to phonons [4]. In our work [5] we use (similar to ref. [6]) scattering-type scanning near-field optical microscopy (s-SNOM) to visualize propagating and localized infrared plasmon modes in graphene nanostructures in real space. By spectroscopic imaging we measure the graphene plasmon wavelength lambda;p as a function of excitation wavelength, which confirms the theoretically predicted plasmon dispersion. We observe that the plasmon wavelength lambda;p=lambda;0/40 is remarkably reduced compared to the illumination wavelength lambda;0, which can directly be attributed to the two-dimensionality and unique conductance properties of graphene. Furthermore, we demonstrate tunability of the plasmon wavelength by gating graphene nanoribbons on a SiO2 substrate. The possibility to tune plasmons of extreme subwavelength electronically opens up a new paradigm in optical and opto-electronic telecommunications and information processing. [1] A. Vakil, N. Engheta, Science 332, 1291-1294 (2011) [2] F.H.L. Koppens, D.E. Chang, J. Garcia de Abajo, Nano lett. 11, 3370 (2011) [3] L. JU, et al., Nat. Nanotech. 6, 630 (2011) [4] Z. Fei, et al., Nano Lett. 11, 4701 (2011) [5] J. Chen, et al., , Nature doi:10.1038, 11254 (2012) [6] Z. Fei, et al., , Nature doi:10.1038, 11253 (2012)

Graphene, a two-dimensional atomic layer of carbon atoms, is particularly promising for plasmonics for three main reasons: i) the charge density, and hence the plasmon frequency is widely tunable by doping and electrostatic gating from 1 to 100 THz, ii) plasmon losses can be extremely small in graphene due to its high electron mobility, and iii) at large carrier densities, the plasmon oscillator strength in graphene is larger than any other known material due to its unique band structure and this means that plasmons in graphene are robust against decay. Thus, compared to the more common metal plasmonic materials, graphene offers frequency tunability, lower losses, and terahertz frequency operation, each of which broadens the application space for plasmonics [1-3]. In this talk we will present experimental results on some basic graphene terahertz plasmonic devices. Microfabricated graphene microstructures, such as strips, discs, rings, etc., confine plasmon modes and make it possible to excite these modes by free-space radiation. Graphene used in our work was grown by chemical vapor deposition (CVD) on centimeter-scale copper foils and then transferred onto Silicon and quartz substrates. Photolithography and oxygen etching was used to define graphene microstructures. Patterned graphene was doped by exposing to Nitric acid in order to adjust the carrier density. Electrostatic doping was also employed. Measurements were performed using a far-IR FTIR with a silicon bolometer detector. Measurements on graphene strips show that when the incident radiation is polarized perpendicular to the strip the confined plasmon resonance is excited and appears as a strong narrow peak in the absorption spectrum. We have developed an electromagnetic FDTD model that captures the frequency-dependent graphene conductivity to simulate the fabricated plasmonic structures. Fabricated micron scale wide graphene strips exhibit plasmon resonances in the 4-5 THz range for carrier densities in the 5E12-8E12 /sq-cm range. Comparison of simulations and measurements enables us to extract the carrier scattering time, the carrier density, and the carrier mobility values from measurements of plasmon resonance spectra. Scattering times and mobility values of 50-75 fs and 1700-2800 sq-cm/V-s were extracted at room temperature using this method and these values agree well with the typical reported values for CVD graphene. Much higher plasmon frequencies are obtainable in smaller microstructures. Simulations show that plasmon resonances in closely located microstructures interact strongly and therefore the measured frequency splittings can be used to study the strength of these interactions and realize novel plasmonic structures. In this talk, we will present experimental results for various graphene based plasmonic microstructures at different temperatures. [1] Nature Nanotech., 6, 630-634 (2011); [2] Nature Nanotech., 7, 330-334 (2012); [3] Nature Nanotech., 6, 611-612(2011)

This talk will discuss how the shape of a Ag nanocrystal can be controlled to maximize its capability to enhance local electric field. Specifically, I will focus on seed-mediated growth of Ag nanocrystals with defferent morphologies by using Ag nanocubes as the seeds. I will discuss how the growth habit of Ag cubic seeds can be controlled to maneuver the shape of Ag nanocrystals. By controlling the reaction parameters, including the ratio of Ag precursor to the seed, the capping agent, the reductant, and the foreign ions, we have successfully prepared a series of Ag nanocrystals with well-controlled shapes and sizes. The products include cubes, cuboctahedrons, octahedrons, octapods, trisoctahedrons, concave cuboctahedrons, and concave octahedrons. In addition to the synthetic protocols and mechanisms, I will also discuss the use of these nanocrystals in field enhancement applications in the contect of LSPR and SERS.

In addition to being ideally suited for single-molecule surface-enhanced Raman (SERS) spectroscopy [1,2], rough silver films grown with the Tollens reaction show intrinsic nonlinear emission from discrete hot spots comprised of second-harmonic generation that blinks [3] and continuum emission that can be used for simple, but high resolution transmission microscopy [4]. By carefully controlling the silver film growth process, a wide range of structural morphologies of complex silver plasmonic nanosystems can be grown [5] that range from a dense packing of discrete silver nanoparticles that couple in the near and far-field radiation zones to semi-continuous rough silver films with optical responses that are defined by near-field coupling of their constituent nanoparticles. Single nonlinear hot spot microscopy of the different plasmonic film morphologies reveals different regimes of electromagnetic coupling that leads to the hot spot formation. In particular, nonlinear excitation spectroscopy and polarization anisotropy where the nonlinear emission intensity is recorded as a function of excitation energy and polarization, respectively, reveal broad resonances in the films of discrete nanoparticles that are reminiscent of localized surface plasmon modes, while the nonlinear hot spots in the semi-continuous metal films have multiple, surprisingly narrow resonances that reflect delocalized excitation of the hot spots mediated through propagating surface plasmons. The intermediate morphologies that bridge discrete nanoparticles and semi-continuous films show a smooth transition between the two extremes reflecting a possible hybridization of delocalized and localized plasmonic resonances. By combining the excitation spectroscopy of single nonlinear hot spots with both electron microscopy and localization microscopy with ~10 nm spatial resolution, structural profiles of the unique hot spots in all of the regimes are obtained. This approach establishes the foundation to build an extensive library of nanoparticle arrangements with documented excitation resonances and paves the way for careful engineering of well-defined plasmonic systems derived from the random metal films which give rise to some of the largest field enhancements necessary for applications such as single-molecule SERS [1,2]. [1] M. J. Walter et al., Phys. Rev. Lett. 98, 137401 (2007). [2] M. J. Walter et al., J. Am. Chem. Soc. 130, 16830 (2008). [3] N. J. Borys et al., Phys. Rev. B 80, 161407(R) (2009). [4] D. Chaudhuri et al., Nano Lett. 9, 952 (2009). [5] N. J. Borys et al., J. Phys. Chem. C 115, 13645 (2011).

In contrast to electronics, which features elements in the nanometer regime, the size of optical components has been inherently limited by the wavelength of light. Recent demonstrations of metallic nanostructures that support plasmons have pushed over this boundary. Plasmons are collective oscillations of free electrons driven by electromagnetic waves. When light is coupled to isolated metallic nanostructures, localized surface plasmon resonances(LSPR) are excited which create unique near- and far-field properties. Plasmonic nanoparticle assemblies can serve as a platform in which optical resonances, and hence the near-field intensity distributions, can be precisely tailored by the number, position, and shape of nanoparticles in a cluster. These metallic nanostructures have traditionally been fabricated with lithographic techniques whose planar nature limits the type of structures that can be achieved. One alternate technique that can overcome some of the limitations of lithography is templated self-assembly (TSA), which is a bottom up method that allows for the construction of large arrays of particle clusters based on a pre-designed template. In this report, we show that a broad range of plasmonic nanoshell clusters can be assembled onto lithographically-defined elastomeric substrates with relatively high yields using TSA. We assemble and measure the optical properties of three cluster types: heptamers which demonstrate Fanoresonance dips, linear chains that work as linear antennas, and rings of nanoparticles which exhibit strong magnetic properties. The assembly of plasmonic nanoclusters on an elastomer paves the way for new classes of reconfigurable plasmonic devices and optical metamaterials that can be transfer-printed onto various substrate mediums.

We studied how the present of a substrate in close contact to silver nanocube might break the symmetry of the local surface plasmon resonance (LSPR) modes. The resonant coupling could be controlled by distance between nanocube and refractive properties substrate, by different dielectric properties of environments, and by the shape of the nanocubes. In this work, we considered silver nanocubes to investigate the resonance changes caused by symmetry breaking in close contact with glass surface, high index substrate such as alumina and rounded alumina substrat. We present experimental measurements and numerical simulations based on the FDTD method which consider LSPR variations as caused by interaction from primitive dipole mode with higher multipolar nanocube modes. Instead of using the primitive modes for sensing application in this work we focus on higher multipolar cube modes as potential mode for higher sensitivity. In fact, our calculations show that the higher nanocube LSPR modes show much higher sensitivity compare to sensing with the primitive mode and about factor 4 higher figure of merit.

A substrate-based templated assembly route has been devised which offers large-area, high-throughput capabilities for the fabrication of periodic arrays of sub-micrometer and nanometer-scale structures. The approach overcomes a significant technological barrier to the widespread use of substrate-based templated assembly by eliminating the need for periodic templates having nanoscale features. Instead, it relies upon the use of a dynamic template with dimensions that evolve in time from easily fabricated micrometer dimensions to those on the nanoscale as the assembly process proceeds. The route is based on the discovery that the dewetting characteristics of ultrathin films can be dramatically altered when the film is placed not on the bare substrate, but on a sacrificial layer which simultaneously evaporates or sublimates. This alteration can result in orders of magnitude enhancements to the areal extent over which the film agglomerates. Directing this assembly process through the use of shadow masks which define an array of pedestals of the sacrificial layer upon which the agglomerating material is deposited leads to the formation of nanostructured arrays. Moreover, the same shadow mask can be used to fabricate nanostructures of different sizes by merely adjusting the amount of material placed on the pedestal. Demonstrations of the technique have yielded large-area gold nanoparticle arrays having nanostructure dimensions as small as 18 nm. The route also has broad applicability, having already produced arrays of gold, silver, copper, platinum, nickel, cobalt, germanium and Au-Ag alloys on substrates as diverse as silicon, sapphire, silicon-carbide and glass.

Coupling of the localized surface plasmon resonances within small aggregates of plasmonic nanoparticles is known to induce intense and localized electromagnetic hot-sites that are chiefly responsible for a family of surface enhanced spectroscopies. Experimental measurements and theoretical calculations have shown the intricate dependency of LSPR on the geometrical arrangement of nanoparticle structures but few have the relative electronic field strength at the various electromagnetic hot-sites. In this paper we investigate electric field hot sites in nanoparticle aggregates both by spectroscopic measurement and electromagnetic calculations. It is well recognized that the electromagnetic hot-sites in NP aggregates are of paramount importance in inducing strong SERS activity. This makes SERS an ideal technique for probing these electromagnetic hot-sites. Since the enhancement response in SERS is proportional to the fourth power of the local electric field, the molecules situated at the EM hot-sites will easily dominate the Raman spectrum of an adsorbate on NP aggregate. Observed variability of SERS strength between NP aggregates has often been portrayed as evidence of the irreproducibility of SERS on a NP substrate. However, this variation in SERS strength is actually a manifestation of the variability of electromagnetic field strength at the hot-sites in the various nanoaggregates assemblies. We have recently reported an experimental quantification of this SERS variability by correlating measured SERS signals of specific small NP aggregates with their geometric arrangements. Experimental measurements of LSPR and SERS were obtained through dark-field microscopy and Raman spectroscopy while structural geometry of the NP aggregates was elucidated with scanning electron microscopy (SEM). We also carried out comprehensive electromagnetic calculations using the discrete dipole approximation (DDA) to provide insights and guide our interpretations of experimental results.

Chemically synthesized nanocrystals (NCs) are considered as promising materials for integrating them into the optoelectronic devices due to their bandgap tunability, easily depositing via spray coating, dip-coat and spin- coat at reduced costs over large area where lattice mismatch does not arise. Large area (48 cm2) and semi-transparent light sensitive nanocrystal skin (LS-NS) is demonstrated via spray-coating nanocrystals on top of polyelectrolyte-polymers based on photogenerated potential buildup, which is highly sensitive to UV and visible light. These LS-NS devices are operated on the basis of photogenerated potential buildup readout, as opposed to conventional charge collection. High sensitivity in the absence of external bias is due to the close interaction of NCs with the top contact while the other part is isolated using high dielectric spacing layer. Furthermore, monolayer of NCs, make the device semi-transparent with sufficient absorption, reduce noise generation and dark current. However, NCs show small absorption at long wavelength region, which limited the device performance. To enhance the sensitivity and operational range extension, embedding silver nanoisland into LS-NS is demonstrated. Using metallic Ag nanoisland, we optimized the localized plasmonic enhancement by tuning the plasmonic resonance and spacing between the plasmonic structure and semiconductor materials to prevent quenching. Various bilayers of polyelectrolyte polymers (PDDA and PSS) deposited on 1 nm Al2O3 as interparticle distance between metal nanoparticle and NCs to facilitate the effects of distance dependent behavior on localized plasmonic enhancement. Consequently, we found localized plasmon enhancement is possible for spacing about 8-15 nm in our device. We observed, 10 nm spacing to be optimal in which 153 % sensitivity enhancement observed compared to the bare case. Hence device operation was extended to further 100 nm wavelength region. Consequently, presence of metal nanoparticle result in enhanced absorption of the CdTe NCs film to generate more electrons and holes. As a result, more voltage buildup can be made to enhance the sensitivity of the device.

Self-assembly of multicomponent colloidal nanocrystals (NCs) into large area ordered arrays provides a bottom-up approach for the fabrication of plasmonic metamaterials that might exhibit intriguing optical properties such as negative index of refraction, emerging optical magnetism and macroscopic visible-frequency invisibility. In this talk, we will present experimental studies on the plasmonic resonance of self-assembled noble metal-nonmetallic binary nanocrystal superlattices (BNSLs). An interfacial assembly method is used to organize these NCs into superlattices over centimeter-scale areas, which were then transferred onto optically-transparent substrates for microspectrophotometric measurements on individual superlattice domains (grains). By changing the NC composition and size ratio between the large and small NCs, we demonstrate that the plasmonic resonance of BNSLs is strongly dependent upon the lattice constants and symmetry and is broadly tunable over the entire visible frequency. We will also discuss some progress towards self-assembled reconfigurable metamaterials through controlled transformation of BNSLs.

The limitation for fabricating molecular plasmonic substrates is due to the drastic requirement of controlling, on large areas in a reproducible way, a well-defined spacing between metallic nanostructures and molecules: their mutual interaction is indeed governed by the local topography of the electromagnetic field. A strategy to design and fabricate hybrid metallic-dielectric substrates for optical spectroscopy and imaging is proposed, based on an original technique, low energy ion implantation beam synthesis (LE-IBS) for the wafer-scale fabrication of Ag nanocrystals (Ag-NCs) planar arrays embedded in silica and silicon nitride layers on a silicon substrate [1]. By coupling this technique to micro fabricated stencils used as templates, we precisely control the size, density, and location of silver nanoparticles in the dielectric matrix. By coupling ballistic simulations and TEM observations we showed that sputtering and diffusion effects are the limiting phenomena for the control of the size, position and volume amount of NCs. Concerning NC stability, we demonstrate that post-annealing process strongly limits silver oxidation, which otherwise excludes the use of Ag NCs on free surfaces [2]. Different architectures consisting of three dimensional patterns of metallic nanoparticles embedded in dielectric layers are hence conceived to simultaneously exploit the optical interference phenomenon in stratified media and localized surface plasmon resonances on metal nanoparticles. These structures are based on a simultaneous control of opto-electronic properties at 3 scales (3S) (~ 2 / 20 / 200 nm) and along 3 directions (3D). Elastic (Rayleigh) and inelastic (Raman) scattering imaging assisted by simulations were used to analyze the optical response of these “3S-3D” patterned layers. The reflectance contrast is strongly enhanced when resonance conditions between the stationary electromagnetic field in the dielectric matrix and the localized plasmon resonance in the silver nanoparticles are realized [3]. These novel kinds of plasmonic-photonic architectures are reproducible and stable, they preserve flat and chemically uniform surfaces, offering opportunities for the development of efficient and reusable substrates for optical spectroscopy enhancement (like SERS) and high contrast imaging. [1] R. Carles, C. Farcau, C. Bonafos, G. Benassayag, B. Pécassou and A. Zwick Nanotechnology 20, 355305 (2009). [2] P. Benzo, L. Cattaneo, C. Farcau, A. Andreozzi, M. Perego, G. Benassayag, B. Pécassou, R. Carles and C. Bonafos J. Appl. Phys. 109, 103524 (2011). [3] R. Carles, C. Farcau, C. Bonafos, G. Benassayag, M. Bayle, P. Benzo, J. Groenen, and A. Zwick ACS Nano 5, 8774 (2011).

An optical sensing technique based on the surface plasmon resonance of Au nanoparticles embedded within a metal oxide thin film is being developed for harsh environment gas sensing applications where typical sensing devices readily fail. Previous work has shown stable and reliable sensing response to H₂, CO, and NO₂ at a temperature of 500°C for hundreds of hours. However, gas selectivity is still a challenge that needs to be addressed. From a materials approach, modification of the metal oxide chemistry or the Au nanoparticles may be advantageous in terms of influencing catalytic reactions or sensing response. The current work focusses on control over the Au geometry by using electron beam lithography to pattern arrays of Au nanorods which are then encapsulated in yttria stabilized zirconia (YSZ). The nanorod geometry is unique in that two localized surface plasmon resonant (LSPR) peaks result from electron oscillation along the transverse and longitudinal directions. In addition, the longitudinal LSPR peak position is tunable simply by changing the aspect ratio of the Au rods. This could potentially affect selective response based on an aspect ratio dependence to changes in the dielectric environment. Preliminary sensing results will be shown for several different aspect ratios along with some of the developmental issues in regards to patterning features on a quartz substrate and stabilizing the rod geometry at high temperature.

The combination of noble metal nanostructures and three dimensional metallic networks has led to the development of an effective SERS-active vapor detection platform. The adsorption behavior of silver nanocubes (AgNC) on the pore walls of porous alumina membranes (PAM) and their application toward vapor detection of important compounds found in many explosives has been investigated. The adsorption behavior of AgNCs coated with polyvinylpyrrolidone (PVP) was found to vary depending on the polyelectrolyte used to coat the PAM pore walls, allowing for a heterodispersed pattern of nanocube aggregates or a more evenly spaced pattern of individual nanocubes by using polyallylamine hydrochloride or polyethylenimine, respectively. Oxygen plasma etching was implemented before infiltration to allow for multiple AgNC loading cycles with minimal surface pore clogging. The high particle density over a large surface area lends itself to an increase in the overall SERS sensitivity for trace level detection through the wide-spread presence of accessible SERS hotspots. Therefore, trace vapor detection of the explosive stabilizing compound n-methyl-4-nitroaniline (MNA) was conducted and parts per billion concentrations were detected through the identification of three characteristic Raman peaks, demonstrating the effectiveness of this SERS platform for trace level vapor detection.

Surface enhanced Raman spectroscopy (SERS), an analytical technique with high sensitivity and high chemical differentiation power, relies on the strong localization of electric fields by surface plasmons. The electric field distribution around nanostructures depends, in turn, on the nanoscale geometry of the surface and the spatial variation of the dielectric function. Small changes in this local electric field result in large changes in Raman scattering signals. Due to process variations in the mass fabrication of plasmonic nanostructures, SERS repeatability is non-trivial. Synthesis of monodisperse silver nanocubes and templated self-assembly of the nanoparticles into small clusters were used to generate hundreds of microscopic SERS substrates with useful attributes to demonstrate the relationship between structure and plasmonic properties. A strong link between SERS enhancement, number of nanoparticles in a cluster, cluster orientation and relative alignment between particles in a cluster is established. Lower variability in SERS enhancements was achieved by controlling only a few of the geometric parameters of the clusters, within the tolerances of conventional nano/micro- fabrication processes. In clusters of silver nanocubes, while previous work predicted a higher SERS enhancement with nanocubes aligned face-to-face, our experiments repeatedly show that edge-to-face junctions produce higher substrate enhancements. Furthermore, through the analysis of experimental and computational results from a large set of cluster geometries, a strong case was made for the central role surface plasmon resonances have in mediating between the nanoparticle geometry and the far-field spectroscopy; i.e. matching the laser frequency and the localized plasmon resonance frequency is a key step in optimizing SERS.

We study the photoluminescence enhancement of dyes using plasmonic resonant cavities composed of vertical silver nano-wires. Plasmon resonance occurs when the length of the cavity is an odd multiple of quarter plasmon wavelength. Photoluminescence is enhanced by increasing both absorption and emission of the dyes by designing a cavity that has two resonances that are aligned with dye extinction and emission. Absorbance of the dye inside the cavity is increased due to the high localized electromagnetic energy density accounting for more efficient excitation that is proportional to the square of the electric field (E2). Emission is also increased by an E2 factor due to more efficient coupling of dipole emission into the radiative modes. Photoluminescence spectrum and decay lifetime of the dyes within the 50-100nm wide cavities are measured as the plasmon resonance frequency is tuned near absorption and/or emission. We discuss how plasmon resonances correlate with reduction in lifetime, increase in emission intensity and change in the photoluminescence spectrum. Our experimental results will be related to numerical simulations which predict a localized electric field enhancement of ~12 suggesting more than four orders of magnitude photoluminescence enhancement limit. We conclude with pointers for increasing the field enhancement and potential realization of a sub-wavelength plasmon laser. Prepared by LLNL under Contract DE-AC52-07NA27344.

Titania, has a bandgap of ~3.2 eV and therefore absorbs strongly only in the UV. However, devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film can be fabricated which when illuminated at 600 nm produced over 1000-fold increase in photoconductance over what TiO2 films devoid of AuNPs produce. The overall current resulting from illumination with visible light is ~50% of the device current measured with UV illumination. We ascribe this to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor. The electronic properties of the device measured alternately in the dark and when illuminated support this picture.

Gold nanobelts are chemically synthesized plasmonic nanowires with sub-100 nm rectangular cross sections and lengths greater than 10 microns. They exhibit a strong scattering plasmonic mode that is tunable with cross-sectional aspect ratio, yielding nanobelts of different colors in dark field microscopy. Several nanobelt structures and potential applications will be discussed. Uniformly tapered gold nanobelts have a spatially varying plasmon resonance that has been used to image the spectral plasmonic enhancement of nearby quantum dots. Nanobelts occasionally split during their growth, leaving two nanobelts with a uniform sub 10 nm gap that may be an ideal substrate for analytical surface enhanced Raman spectroscopy. Plasmon propagation has been detected in gold nanobelts at visible and near infrared wavelengths. The specific modes that support propagation have been identified and studied. Finally, individual gold nanobelts have been attached to probe tips to evaluate their mechanical properties and performance as atomic force microscopy tips. The nanobelts exhibit elastic buckling behavior and are sufficiently robust for tapping mode imaging. Their sharp scattering plasmon resonance and gold surface chemistry may make them highly versatile near field probe tips.

Layered plasmonic/dielectric systems exhibit interaction of collective charge density oscillations in the plasmonic layer (typically metallic nanostructures) with the dielectric overlayer. The nature of the interaction affects the intensity and energy of the localized surface plasmon resonance (LSPR), the latter seen as an extinction band in the UV-to-NIR range. The transducer response to a dielectric overlayer is based on the interaction of the layer with the plasmon evanescent electric field, extending from the nanostructures into the medium; the response decays exponentially with the distance between the metal nanostructures and an adsorbed layer.[1] Recently, several groups reported response to dielectric layer adsorption at distances far greater (hundreds of nanometers) than previously seen, beyond the known plasmon decay length (typically few to tens of nanometers). This was termed “long-range LSPR response”, and its use in sensing applications was suggested. Several attempts were made to explain the effect, though none was satisfactory. Using random Au nano-island arrays of several average particle sizes, fabricated by means of vapor deposition on glass slides followed by high-temperature annealing, we found the oscillations to be independent of the particle size.[2] The oscillations were reproduced using computational models, and were found to be the result of interference effects, modulating the energy distribution between absorption, transmission and scattering modes. Two-dimensional data analysis allowed us to extract the phase shift at the nanoparticle interface. [1] Kedem, O.; Tesler, A. B.; Vaskevich, A.; Rubinstein, I. Sensitivity and Optimization of Localized Surface Plasmon Resonance Transducers, ACS Nano 2011, 5, 748-760. [2] Kedem, O.; Sannomiya, T.; Vaskevich, A.; Rubinstein, I. The Origin of Oscillations in the Long-Range Response of Localized Surface Plasmon Resonance Transducers, submitted.

Significant progress has been achieved in the field of plasmonics of complex nanostructures, but observation of localized surface plasmon resonances (LSPR) at a length scale of a few nanometers is still a challenge. In the case of contact limit between two nanoparticles, the influence of particle shape is rarely discussed. Two challenges still remain: The ability to precisely fabricate large numbers of monodisperse aggregates and the precise characterization of the local enhanced field. Template-assisted fabrication techniques present one route to address this challenge allowing for high-throughput synthesis of nanorod arrays with precise control of rod length and spacing between auxiliary aligned rods for high locally enhanced surface plasmon resonances. However, template-assisted technique typically yields nanorods with rough non-flat ends (usually a concave or convex crescent shape) that can dramatically alter the optical properties. In this work, we conduct FDTD simulation of the influence of the tip face shape of gold nanorod monomers/dimers and compare it with single particle LSPR spectra obtained from dark field optical microscopy. The inclusion of specific tip shapes provides understanding for plasmonic properties of non-trivial face ends. Understanding the close convex-concave contact between nanorods will be important to improve the sensitivity of LSPR sensors.

We have shown the implementation of novel hybrid plasmonic-photonic integrated devices for efficient on-chip sensing and spectroscopy. Plasmonic nanoresonators have localized surface plasmon resonances (LSPR) that exhibit ultra-high field enhancements. They have been used for LSPR sensing with very large sensitivities, and also surface enhanced Raman spectroscopy (SERS). Photonic waveguides and whispering gallery resonators can guide and trap light on the chip with ultra low losses. We have shown that if plasmonic nanoresonators are integrated with waveguides and on-chip whispering gallery resonators, both advantages of integrated photonics and plasmonic structures can be combined in a unified platform enabling new potentials for on-chip sensing and spectroscopy. The proposed hybrid plasmonic-photonic structures consist of waveguide-based devices for LSPR sensing, and resonator-based devices for on-chip SERS. In the proposed waveguide-based structures, the LSPR mode of individual gold nanorods can be excited with efficiencies exceeding 10% in a broad bandwidth covering the entire resonance linewidth of the gold nanorods. We have demonstrated large sensitivities (~200 nm/RIU) for gold nanorods integrated on Silicon Nitride (SiN) waveguides. We have also shown that by integrating a single gold nanoparticle with a SiN microring resonator, the field enhancement in the microring whispering gallery resonance modes can be combined with the field enhancement in the plasmonic nanoresonators to achieve ultra-high SERS enhancements. Using such a double resonator structure with a 15um-radius microring resonator, it is possible to couple about 80% of the input light to the localized modes of 100nm×40nm×30nm gold nanorods, and therefore an additional 900 fold Raman enhancement (~3 orders of magnitude improvement) is possible. In this paper, we demonstrate our latest sensing results using the proposed hybrid plasmonic-photonic structures. We will experimentally show the integration of microfluidic channels with the hybrid substrate as a fully functional sensing platform. The proposed unconventional substrates are alignment-insensitive, since the plasmonic nanoparticles are lithographically fabricated at their optimal locations, and once light is coupled to the on-chip bus waveguides, the LSPR modes of plasmonic nanoparticles are excited. We will discuss the design as well as the fabrication and characterization of these structures. The fabrication involves two steps of electron beam lithography (EBL) to define photonic and plasmonic structures. The photonic guided-wave structures are realized by inductive-coupled-plasma (ICP) etching of the SiN film, and the plasmonic nanoparticles are realized by metal evaporation and lift-off. The characterization is done using a supercontinuum broadband laser source covering a wavelength range of 500-1800nm. We will demonstrate sensing of different analytes at different concentrations using the proposed structures.

Silver is the ideal material for plasmonics because of its low loss at optical frequencies, but is often replaced by a more lossy metal, gold. This is because of silver&’s tendency to tarnish and roughen, forming Ag₂S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. This occurs because of the chemical reaction of the silver surface to hydrogen sulfide (H₂S) and carbonyl sulfide (OCS) in ambient air. Graphene has been found to be an impervious membrane, unable to be penetrated by atoms as small as helium. By passivating the surface of silver nanostructures with monolayer graphene, these atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag₂S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days, graphene-passivated silver nanoantennas exhibit 27-times higher sensitivity over that of bare Ag nanoantennas, and two orders of magnitude improvement in peak width endurance. We compare these experimental results with finite-difference time-domain (FDTD) simulations as well as calculations using quasistatic Mie theory. By employing graphene in this manner, the excellent optical properties of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.