Surface plasmons live for a few femtoseconds. On dephasing, they produce a shower of energetic electrons and holes that equilibrate adiabatically over 10-100 femtoseconds to form a Fermi distribution with electron temperatures of several thousands of Kelvin. Most of the energy of this hot electron gas is then dissipated through electron-phonon interactions over a few picoseconds. By fabricating appropriately nanostructured devices allowing a reasonable fraction of these hot carriers to be harvested before they thermalize, the hot electrons can be transferred to appropriate catalyst systems and the device can be used to carry out light-enabled redox chemistry. To do this efficiently one needs to design and construct materials and systems with appropriate dielectric properties, structures and architectures, and interfaces of the appropriate relative band properties to irreversibly extract the carriers in the handful of picoseconds during which they are “hot”, and to design a new suite of catalytic materials that collaborate appropriately with the plasmonic system and the electrons (and holes) that are produced by them. Although the problem seems formidable, remarkable progress has recently been achieved by several groups internationally. Our focus has been on using plasmonic materials as absorbers and photocatalysts for artificial photosynthesis, some examples of which will be presented and discussed.

High activation barriers of rate-limiting reaction steps require many conventional heterogeneous catalytic reactions to be operated at relatively high temperatures. This study suggests that the combination of nanoscale catalysts and plasmonic nanostructures can locally overcome the activation barrier under sunlight without requiring high temperatures in the bulk of the reactor material.
To date, the execution of electron-driven photocatalysis has been demonstrated on coinage (Ag, Au, and Cu) metal nanoparticles through the excitation of Localized Surface Plasmon Resonance (LSPR), where LSPR excitation is used to transfer photon energy to nearby semiconductors^, molecular photocatalysts, and metals.
This study demonstrates the first proof of hydrogen generation via alcohol steam reforming at a local temperature above 200°C, achieved by plasmonic heating under moderately concentrated solar irradiation, in an otherwise cold photocatalytic reactor. The plasmonic heating is localized in time and space, allowing the use of a high-temperature catalytic process without excessive heating of the immediate surroundings.

The field of plasmonics has attracted a lot of attention in recent years because of potential applications in various fields such as photovoltaics, energy production and conversion, catalysis and therapeutics. We report, for the first time, highly engineered ‘Omega;&’ (omega) shaped nanowire-dielectric-metal (Si-SiO₂-Au) structures in which cavity effects of core antenna have been used to enhance the localized surface plasmons in metallic films. These plasmonic interactions can be easily controlled by tuning the antenna properties. The degree of enhancement has been measured by cavity heating and temperatures close to 1000K have been successfully measured for an antenna size resonant with excitation wavelength. Photoreforming reactions performed on these cavities showed ~50% enhancement in hydrogen production compared to control samples. Because of the ease of synthesis, nature of enhancement and high temperature stability of these structures, we expect them to have applications in the field of energy conversion and high temperature catalysis. This work also demonstrates successfully, temperature measuring capability of a plasmonic system and presents a technique to study high temperature properties of various materials.

Managing carbon emissions is one of the most pressing issues currently faced by the energy sector. One interesting approach for dealing with these emissions is to catalytically convert CO₂ into liquid fuels, olefins, aromatics, and industrial chemicals that can be sold to offset carbon management costs. This approach requires the development of novel catalysts capable of utilizing carbon-friendly forms of energy to activate CO₂ and drive a chemical reaction. My talk will focus on a very simple Au-ZnO heterostructure where visible-light plasmonic excitation of the Au generates heat to drive chemical reactions on the ZnO substrate. In this system, CO₂ and H₂ can be converted to CH₄, CO, and H₂O through a chemical pathway that experiments and thermodynamic simulations show is entirely thermal. The instantaneous and localized nature of plasmonic heating offers many advantages and challenges for reactor design and heat management in catalytic processes. These aspects of plasmon-based reaction engineering will also be highlighted.

In human body, nature has vividly demonstrated that biological molecular motors in precise assemblies function cooperatively to link the chemical-mechanical energy conversion from molecular-scale to macroscopic world for useful work. Inspired by nature, we design, measure and control cooperative functions in engineered systems in order to better harness light for improved applications. Herein, we report our advances in two types of cooperative light-harvesting systems based on light-driven supramolecules and plasmonic metamaterials. Advanced nanotools are applied to improve the fundamental understanding and control of the nanoscale light-matter interactions in these systems down to the single-molecule and single-nanoparticle levels. We leverage our improved understanding to develop the rational design of the systems where molecules and nanoparticles as building blocks assemble precisely and function cooperatively to control light concentration and enhancement or to convert light into other forms of energy for useful work. Once fully developed, cooperative functions in the engineered systems will pave the way towards enhanced light harvesting for energy and healthcare applications.

Surface enhanced Raman scattering (SERS) involves the dramatic enhancement of Raman scattering from molecules adsorbed on or in close proximity with nanostructured metal surface. SERS is considered to be a highly attractive platform for chemical and biological sensing and molecular bioimaging. Most of the SERS substrates and contrast agents rely on individual or lightly aggregated metal nanostructures that either offer limited enhancement or suffer from poor stability and reproducibility. In this work, we investigate the size-dependent SERS activity of plasmonic nanostructures comprised of Au nanospheres (AuNS) or Au nanorods (AuNR) as cores and porous Au nanocubes and cuboids as shells. The SERS activity of Au nanorattles with spherical core was found to increase with increase in the edge length of the porous shell. On the other hand, the SERS activity of Au nanorattles with AuNR core was found to decrease with increase in the size of the porous cuboid shell due to the decrease in plasmon coupling between AuNR and porous shell. Finite difference time domain electromagnetic simulations show excellent agreement with our experimental results.

Precision manufacturing of metal foils with functional materials is of tremendous research interest due to its potential for spectrum of nanotechnological applications. Laser shock pressure (~GPa range) has been exploited to deform metals with large area patterns comprising of nanoscale features. In this paper, we investigate on laser shock imprinting of metal foil with a functional coating, and mechanical and optical properties of the integrated structure. Gold coated aluminum foils are imprinted into electron beam fabricated silicon molds, resulting in designer micro/nano scale features. The imprinting process exploits formability of aluminum thin foil while gold surface coating provides plasmonic functionality. Gold trench patterns with width starting from several microns to ten nanometers and various depths have successfully been generated by a single laser pulse in nanoseconds. The 3D nanoshapes patterned over large area are reproducibly attained with high precision. Surface gold coating can be replaced by other functional materials and thus the process is material-wise versatile. Numerical simulations based on Finite Element Method (FEM) and Molecular dynamics (MD) are employed to analyze the process. Mechanical properties of imprinted structures are examined by monitoring load-displacement curve in nanoindentation and hints at enhanced interfacial strength between the bilayers. Capability of optical field enhancement of the patterned metal foil has been demonstrated by applying it as SERS substrates for low concentration molecular sensing.

Surface-enhanced Raman scattering (SERS) technique is a powerful non-destructive analytical technique for chemical and biological sensing.^1,2 Noble metals like Au, Ag, and Cu noble metals have been widely studied for SERS and applied in chemicals detection.^3,4 However, the applications of metal substrates are largely limited by their shortcomings such as high-cost, low stability, poor biocompatibility, and no reusability. Recently, photocatalytic self-cleaning SERS substrates have been developed by combining plasmonic metal with semiconductor such as TiO₂ and ZnO which makes SERS substrate recyclable.^5,6 We report a facile synthetic method of a novel Ag:TiO₂ nanocomposite material which showed very high SERS enhancement factor (EF) of around 250 with reference to the silver thin film. Furthermore, the as-prepared Ag:TiO₂ nanorods could be self-regenerated, fully recovered, and recycled after use in SERS analysis because of the self-cleaning property of its TiO₂ core under simple UV light irradiation. Slanted TiO₂ nanorods arrays were deposited on the glass substrate by means of glancing angle deposition (GLAD) and followed by the deposition of Ag on the top of them. Silver decorated TiO₂ nanorods can be used as stable high-sensitive SERS substrate, along with its photocatalytic activity to photo degrade the absorbed species on the surface. Highly sensitivity SERS detection of Rhodamine 6G dye was achieved on Ag:TiO₂ nanorods samples with a capability of recover quickly in UV light illumination.. SERS activity may be attributed to the local electric field enhancement on the silver metal surface and on the Ag:TiO₂ interface.⁷ To monitor the photocatalytic degradation of surface absorbed dye molecules, Raman measurements were carried out at an interval of 30 minutes and found that after 150 minutes the samples were recovered significantly. The Ag:TiO₂ nanorods substrate serve as an easily recyclable and highly efficient SERS substrate.
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Multilayered nanostructures consisting of metal and dielectric layers have great potential for efficient photon to electron conversion leading to cost-efficient photoelectrochemical devices[1], [2], based on the plasmonics enhancement of the optical absorption coupled with hot-carrier injection.
A periodic metallic photonic crystal composed of high aspect ratio nanorods of layered gold, TiO₂ and Al₂O₃ is proposed that has improved absorption in visible and near infrared region which is directly related to higher external quantum efficiency, leading to potential applications in photodetectors, photovoltaics and solar water splitting.
In this work, we study the influence of different nanostructure shapes and dimensions on the optical absorption in the vacuum wavelength range of 400 nm to 1500 nm, allowing us to tune the device performance and optimize our design accordingly.
The parameters studied by electromagnetic simulations based on Finite Difference Time Domain Method (FDTD) are the thicknesses of the Au and TiO₂, aspect ratio, 2D lattice field factor, sidewall angle, and the geometry of the cell element shape, namely different paraboloids and elliptical rods. Higher absorption was calculated for structures based on elliptical nose cones. Furthermore, the total absorption in case of inverted elliptical nose cone is found to be higher than in the case of upright elliptical nose cone.
Deeper nanostructures are also found to be advantageous, but high aspect ratios are limited by achievable fabrication constraints.
The absorption in the visible and near infrared region was seen to decrease with an increase in TiO₂ thickness with a corresponding shift of absorption towards the short wave infrared region. The width of the absorption peak increased as well. Nevertheless, the total absorption of the solar spectrum was increased by increasing the TiO2 layer thickness, due to the added contribution of the infrared radiation.
More than 70% of the total solar spectrum is seen to be absorbed by the structure considered for feasible aspect ratios.
A comparison of our results with other proposed structures[3] based for metal/metal oxide absorption is reported along with relative merits based on fabrication readiness.
Key words: nanostructures, plasmonics, water splitting, FDTD, solar energy
References
[1] S. C. Warren and E. Thimsen, “Plasmonic solar water splitting,” Energy Environ. Sci., vol. 5, no. 1, pp. 5133-5146, Jan. 2012.
[2] C. Clavero, “Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices,” Nat. Photonics, vol. 8, no. 2, pp. 95-103, Feb. 2014.
[3] J. B. Chou, Y. X. Yeng, Y. E. Lee, A. Lenert, V. Rinnerbauer, I. Celanovic, M. Solja#269;icacute;, N. X. Fang, E. N. Wang, and S.-G. Kim, “Enabling Ideal Selective Solar Absorption with 2D Metallic Dielectric Photonic Crystals,” Adv. Mater., vol. 26, no. 47, pp. 8041-8045, Dec. 2014.

The performance of wide bandgap oxide-based (such as TiO₂) photocatalytic cells has been limited by the poor absorption of solar energy. We developed metallic-semicoductor photonic crystals (MSPhC) where broadband solar absorption in the metal occurs[1]. Once the photons are absorbed, excited hot electrons can potentially be used for photoelectrochemistry. Recently, we demonstrated a broadband sub-bandgap photoresponse at 590 nm due to surface plasmon absorption with our wafer-scale Au/TiO₂ metallic-semiconductor photonic crystal[2]. We also showed a photoresponse enhancement factor of 4.5 at 639 nm compared to a flat chip. However, our experimental demonstrations of hot electron current have been attributed to only surface plasmon polariton (SPP) modes at 590 nm.
In order to achieve photocurrent above 700nm spectrum, we coated gold nanorods on the surface of MSPhC to incur localized surface plasmon (LSP) modes absorption in this frequency range. The resonance in the gold nanorods under the action of the incident light is dependent on the rod radius and length. We chose gold nanorods, length 45 nm and radius 15 nm, expecting the LSP resonance frequency along the longitudinal axis to be around 700 nm.
The deposition of nanorods should be as uniform as possible and sparse reasonably not to block the cavities of MSPhC. We used electrophoretic deposition (EPD) method to deposit nanorods on the surface, sidewall, and bottom of the photonic cavities. Two electrodes were immersed in the nanorods suspension: a functioning MSPhC as a working electrode and Au coated glass slide with 5nm Ti as a counter electrode with 10 mm gap between the two electrodes. Phosphate-buffered saline (PBS) and deionized (DI) water were used as solvents where nanorods were well dispersed with 5 mins of sonication. For the colloidal stability, zeta potential was maintained to be ~3 mV. By measuring the suspension absorbance, nanorods show two absorption peaks, longitudinal at 513 nm and transverse at 700nm wavelength, which match well with the LC circuit prediction. Under 10V applied electric field, positively charged gold nanorods at the concentration of 6.52E+13 (#/mL) deposit MSPhC surface with the density of 230 #/µm², which was reasonably uniform and sparse. Preliminary tests show a reflectance dip near 700nm. Photocurrent measurement is under way to demonstrate the enhanced hot electron transfer over full visible light and near-infrared solar spectrum.
References
[1] Chou, J. B., et al. " Enabling Ideal Selective Solar Absorption with 2D Metallic Dielectric Photonic Crystals " Advanced Materials, V. 26, Issue 47, p.7922, 2014 (Inside front cover article)
[2] Chou, J.B., et al., “Broadband Photoelectric Hot Carrier Collection with Wafer-Scale Metallic-Semiconductor Photonic Crystals”, 42th IEEE Photovoltaic Specialist Conference, New Orleans, 2015

Hybrid nanostructures consisting of metal nanoparticles (MNPs) and quantum dots (QDs), have been found to exhibit unique, new optical features as a result of the interaction between the MNPs and QDs. When QDs are in close proximity to a MNP, the enhanced electric field generated due to the excitation of localized surface plasmons of the MNP can vary the QD exciton cycles, affecting QD emission. The focus of this work is to understand how the exciton-plasmon interaction in these hybrid nanostructures is dependent on the excitation wavelength. The hybrid nanostructures consist of Au NPs coated with a SiO₂ shell of a controlled thickness and CdSe/ZnS QDs adsorbed onto the SiO₂ shells. To study the exciton-plasmon interactions, QD fluorescence lifetimes are monitored under various excitation wavelengths utilizing time-correlated single photon counting. We find the emission lifetimes of the hybrid nanostructures are highly dependent on the excitation wavelength relative to the localized surface plasmon resonance of the Au NPs. Our work illustrates that by tuning the excitation wavelength, there is either an enhancement or weakening in the exciton-plasmon coupling between Au NPs and QDs.

Plasmonic nanoparticles can enhance visible light absorption and reduce electron-hole recombination in semiconductors, but an overly large amount of plasmonic nanoparticles may lead to defects and therefore charge recombination. Plasmonic nanoparticles may also act as electron traps along electron transfer paths due to the Schottky barrier at the metal/semiconductor interface. We report the fabrication of a photoanode based on multi-segment CdS-Au nanorod array (NRA), in which the position and the number of Schottky barriers, as well as arrangement of energy level can be programmed. The charge transportation path and surface plasmon resonance (SPR) effect can be rationally designed and optimized by controlling the size and sequence of the different components along the nanorod length. Electrons are injected from the CdS segment to the adjacent Au segment to maintain Fermi level equilibrium as the CdS Fermi level is higher than that of Au. Upon irradiation, the increase in the carrier density in CdS realigns the relative energy levels between the CdS and Au, leading to a reduced Schottky barrier height. The photo-generated electrons can #64258;ow easily across the series of height-reduced Schottky barriers when positive bias is applied, and photocatalytic activity can be linearly increased by increasing the number of segments in the NRA. The photocurrent of a nine-segmented CdS-Au NRA was nearly 10.5 mA/cm² at 0 V (vs. Ag/AgCl) in H⁺ reduction, rendering them promising photoanodes for hydrogen evolution in PEC cells. The photoanode was also highly stable; retaining 96.3% of its initial performance after 40 on-off chopped lighting cycles.

Engineering interfacial photo-induced charge transfer for highly synergistic photocatalysis is successfully realized based on nanobamboo array architecture. Programmable assemblies of various components and heterogeneous interfaces, and, in turn, engineering of the energy band structure along the charge transport pathways, play a critical role in generating excellent synergistic effects of multiple components for promoting photocatalytic efficiency.

The decay of surface plasmon resonances is usually considered to be a detriment in the field of plasmonics, but the capture of excited carriers whose energy is normally lost to heat offers a new opportunity in optoelectronic and energy conversion devices. In particular, the large extinction cross-section at a surface plasmon resonance enables nanostructures to absorb a significant fraction of the solar spectrum in very thin metallic films and generate energetic carriers that can be collected by hot carrier injection across a metal-semiconductor interface, or can be exploited to drive photochemical processes. Despite the broad interest in solar-driven reactions, the majority of fundamental studies on plasmonic hot-carriers has so far focused on Si-based devices with plasmonic nanostructures resonant at near infrared wavelengths, whereas fewer works have explored analogous phenomena operating in the UV-visible range.
Recently we analyzed the quantum mechanical decay processes for surface plasmon polaritons, with and without phonons, and found that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. In particular, the onset of interband transitions, occurring in the VIS regime (around 2 eV) for most common plasmonic metals (Au, Cu, Ag), is expected to significantly modify the hot-carriers distribution. Based on our theoretical work, we have designed an experimental system using GaN/noble-metal nanoscale heterostructures to investigate the photocurrent generated upon plasmon decay across the extended UV-visible regime. In particular, use of GaN as a semiconductor electrode enables hot carrier collection without interband absorption in GaN for carrier energies >1 eV. By using photolithography methods we fabricated planar Schottky diodes where the Schottky contact was patterned to concurrently serve as plasmonic resonator. Using either a Hg-Xe lamp or a supercontinuum laser source to excite plasmonic/GaN structures in the energy range from 3.3 eV to 0.8 eV, we will report photocurrent measurements as a function of photon energy for metal/semiconductor combinations with similar Schottky-barrier heights and with similar spectral position of the plasmonic absorption resonances. By direct comparison with ab-initio calculations, we can assess the contribution of direct and indirect interband transitions in the metal to hot carrier generation. By employing metals with distinctly different band structures, such as Au and Al, we identify guidelines for engineering the generation of plasmonic hot-carriers in the UV-VIS regime.

A dual plasmonic resonance effect on the performance of poly(3-hexylthiophene) (P3HT):phenyl C61-butyricacid methyl ester (PC61BM) based polymer solar cells (PSCs) has been demonstrated by selectively incorporating 25 nm colloidal gold nanoparticles (Au NPs) in a solution-processed molybdenum oxide (MoO3) anode buffer layer and 5 nm colloidal Au NPs in the active P3HT :PCBM layer. The devices exhibit up to ~20% improvement in power conversion efficiency which is attributed to the dual effect of localized surface plasmon resonance (LSPR) of Au NPs with enhanced light absorption and exciton generation. Our report shows a guideline on the usage of dual LSPR effect for the solutionprocessed polymer solar cells to achieve high efficiencies.

Hybrid organic-inorganic composites combining the advantage of organic and inorganic have emerged as promising candidates for cost-effective photovoltaics (PVs). Silicon-based hybrid solar cells with the power conversion efficiency exceeding 17% play a key role in this field. Si nanostructures (NSs) such as nanowire array can provide a low reflectivity over a broad range of solar spectrum, which promise them as efficient light harvesting substrates. However, the light-trapping nanostructures significantly enlarge the surface area of Si substrates and result in additional surface trap states, leading to a high surface recombination velocity. Alternatively, antireflection layers such as SiN_x are typically deposited by plasma enhanced chemical vapor deposition method, which can suppress light reflection. Unfortunately, this technique is incompatible with the organic layer on Si. Therefore, absorption enhancement is one of the primary challenges to achieve high efficient organic/Si solar cells. Most recently, the metallic NSs with novel plasmonic properties in terms of optical response have been widely employed in the PV field. Firstly, we proposed poly(3,4 ethylenedioxythiophene) /poly(styrenesulfonate) (PEDOT:PSS)/Au nanoparticles (NPs) composite films to effectively enhance the light harvesting properties for organic/Si PVs. Au NPs were prepared by simple and cost-effective solution-processed techniques. With tuning the size of the particle, the scatting and excitation intensity dramatically affect the light coupling in the conducting polymer layer. The finite different time domain (FDTD) simulation as also used for exploring the surface plsmonic effect theoretically.Secondly, the composite of Au NPs and grapheme oxide was utilized into the hybrid solar cell. The addition of grapheme oxide greatly improves the concentration of Au NPs without aggregation. Finally, the effect of gold nanorods (Au NRs) induced surface plasmons on the performance of devices was investigated by introducing hot carrier generation. When the hot electron with sufficient momentum was generated in the Au NRs indirect contact with silicon where an interface potential is formed, the excited electron in the nanostructures might have sufficient energy to pass the barrier, which will induce photocurrent of the device.

Shell-isolated nanoparticles (SHINs) were firstly reported by Li et al[1] for enhanced Raman signal, termed shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). The key factor was the coating the Au nanoparticles with an ultrathin silica shell preventing the direct contact with the probed surface. By increasing the nanoparticle-target molecule distance with thicker silica shells one can observe a continuous transition from fluorescence quenching to fluorescence enhancement[2]. Herein, we report controlled experiments using Au-SHINs for shell-isolated nanoparticle enhanced fluorescence (SHINEF)[3,4]. It is shown that Au-SHINs of about 100 nm in diameter produce higher enhancement factor than smaller Au-SHINs in solution and on a Langmuir-Blodgett (LB) monolayer. The enhancement factor is further increased by aggregation of the large Au-SHINs in solution, and the findings are supported by numerical calculations .Therefore, two important factors: nanoparticle size and aggregation, can be used to further increase the observed enhancement factor in plasmon enhanced fluorescence.
[1] J. F. Li, Y. F. Huang, Y. Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y. Zhou, D. Y. Wu, B. Ren, Z. L. Wang, Z. Q. Tian, Nature 2010, 464, 392.
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[4] I. Osorio-Román, A.R. Guerrero, P. Albella, R. F. Aroca. Analytical chemistry 2014, 86, 10246-51

Nano-photonics provide powerful platforms for strong light trapping, extreme subwavelength confinement and extraordinary optical responses for various applications such as sensors [1-2], photodetectors [3-4] and light harvesting devices [5-6]. Especially for organic solar cells (OSCs) which use optically thin active layers (< 300 nm) - restricted from the very short carrier diffusion lengths (10~100nm), low carrier mobility and strong charge recombinations -, diverse strategies have been exploited to increase the optical absorption of OSCs. Various approaches including nano-plasmonic structures, surface texturing, resonance cavity and ray-optical light trapping systems have been studied so far, to achieve a more efficient photon collection leading toward the best achievable final optical-electrical performance of the OSCs [5-6].
Nevertheless, most of past studies on nano-photonics based OSCs have been carried out by employing relatively thick active layers (> 100 nm), from the difficulty in photonic mode confinement in extreme regime; thus failing to provide the full optical and electrical performance gains for the device. For the OSCs of ultrathin active layers (le; 70 nm), which could offer efficient collection of photo-generated charge carriers without strong recombination loss, the optical design achieving broadband and strong absorption in extreme confinement remains still as a challenge.
In this work, we demonstrate a broadband, strong optical absorption and efficient collection of photo-generated charge carrier, with the use of broadband and highly confined plasmonic mode capsulated in the ultra-thin (=70 nm) active layer cavity of ITO-free PCDTBT:PC₇₀BM OSCs. Enhancement in optical absorption of ~ 48% (AM1.5G weighted) and improvement in power-conversion efficiency (PCE) of ~ 42% are achieved, when compared to a control reference structure. These improvements are explained in terms of the absorption spectrum broadening with the use of frustrated Fabry-Perot structure, broadband phase control, and short traveling distance of charge carriers in the active layer. The application of suggested structure, to ultra-thin (<100 nm) film solar cells based on other material systems will be discussed as well.
[1] Liu, N., et al. Nano Lett., 2010, 10(7), 2342-2348.
[2] Jung, K., et al. Adv. Mater., 2014, 26(34), 5924-5929.
[3] Chalabi, H., et al. Nano Lett., 2014, 14(3), 1374-1380.
[4] Yi, J. M., et al. ACS photon., 2014, 1(4), 365-370.
[5] Lee, S., et al. Opt. express, 2014, 22(104), A1145-A1152.
[6] In, S., et al. ACS Photon., 2014, 2(1), 78-85.

Metamaterials are artificially engineered structures designed to offer extraordinary electromagnetic properties that is not easily obtained with naturally occurring materials [1-2]. Capable of accessing to uncommon values of permittivity and permeability, metamaterials thus also provide a route toward designer artificial media, for the ideal control of reflection, absorption and transmission properties of electromagnetic waves. Various forms of wave control including perfect absorption [3], extraordinary transmission [4] and sub-diffractional light focusing [5] have been demonstrated so far.
In this study, we focus on the design of ultra-thin metamaterials achieving wideband and perfect reflection, especially for the application-rich visible regime of solar energy harvesting. Targeting a perfect back-reflector for very-thin active layer (50nm) organic solar cells, we demonstrate a ultrathin (~30nm) metamaterial film; also functioning as a hole transport layer (HTL) on top of the metallic anode substrate, which together derives a non-resonant broadband (400 nm < lambda; < 800 nm) reflection of 93.3% in average.
As a result of the photon re-use from wideband perfect reflection, very efficient optical absorption of ~66.0% (AM1.5G solar spectrum weighted power) is achieved, corresponding to ~39% enhancement when compared to a reference OSC (PTB7:PC₇₀BM) without the reflector. Providing optical absorption values comparable to PTB7:PC₇₀BM based organic solar cells of relatively thick active layers (100~300nm) [6-7], and also offering the chances of mitigating the recombination loss of organic materials [7] in combination with the use of very-thin active layer (50nm), our proposal will stimulate the application of perfect reflecting metamaterials toward highly efficient and ultra-thin devices for energy-harvesting, photo detecting, sensing and filtering systems.
[1] Sihvola, A. Metamaterials, 2007, 1(1), 2-11.
[2] Poddubny, A., et al. Nature Photon., 2013, 7(12), 948-957.
[3] Landy, N. I., et al. PRL, 2008, 100(20), 207402.
[4] Gao, W., et al. Nano lett., 2014, 14(3), 1242-1248.
[5] Asatsuma, T., Opt. express, 2008, 16(12), 8711-8719.
[6] Zhou, L., et al. Sci. Rep., 2014, 4.
[7] Stolterfoht, M., et al. Sci. Rep., 2015 5, 9949.

TiO₂ is a well-known photocatalytic material. Its combination with plasmonic nanoparticles (NPs) has been demonstrated in the literature in various ways. A vast majority of the publications have focused on reactions involving photocatalytic decomposition of organic compounds and water splitting. However, there is little evidence on the efficiency of such composite materials in the intermediate photo-oxidation step required to achieve the removal of heavy metals during water purification (e.g. As, Mn). In the present study we investigate the enhanced photocatalytic activity of TiO₂ (E_g = 3.2 eV) and TiO_xN_y (N content set to provide samples with E_g either 2.8 eV or 2.4 eV) with optically active Ag and Au NPs towards oxidation of As and Mn aqueous species (10 mg/L). The photocatalytic templates were immersed on aqueous solutions of As(III) and Mn(II) oxy-anions. For the illumination of the samples we used a white LED lamp. XPS has been used to identify the retention of the metal ions and their oxidation state during photocatalysis. We demonstrate that the performance of the photocatalysts is a strong function of the Eg of the semiconductor and the properties of the NPs. This can be explained by the specific optical activity of the Ag or Au NPs on top of the different photoactive materials. We show that by tailoring the Eg and the size of the NPs it is possible to maximize the photochemical activity of a semiconductor and create more efficient devices for heavy metal purification of water.

The approach using localized surface plasmon resonance (LSPR) from metallic nanoparticles is attractive as one of the promising method to enhance the internal quantum efficiency of organic light emitting diodes (OLEDs). LSPR, derived from the collective oscillation of free electrons, results in a strong light-scattering and amplified local electromagnetic field, hereby increases electromagnetic density of states which contribute to more efficient light emission of OLEDs. We have employed monodispersed silver nanoparticles prepared by polyol process, where nucleation and growth of the nanocrystals could be simply manipulated by the change of salts and reaction temperatures. While those randomly dispersed nanoparticles were simply spin-coated onto indium-tin oxide (ITO) electrode, more ordered metal arrays were formed by the solvent annealing-induced phase separation of block copolymer self-assembly process. Silver nanoparticles layers were given a particular morphology, which is driven by self-assembly of polystyrene-block-poly(2-vinyl pyridine) copolymer (S2VP) thin film. Controlling the annealing time and solvent type of the block copolymer self-assembly results in either dot array or line array with controllable dot/line contents. For example, using tetrahydrofuran during the solvent annealing, ordered dot array can be achieved, while line array can be formed by solvent annealing using chloroform. Nanoparticle layers with various array pattern were prepared by the substitution of pyridine part of S2VP thin film. Each of nanoparticle layers has different silver contents. We have investigated LSPR-induced enhancement of light emission for white OLED composed of blue-orange tandem phosphorescent devices. Structure of white OLEDs is Glass/ITO/coupling agent for adhesion/metal plasnonic layer/PEDOT:PSS(60nm)/DNTPD(50nm) /TAPC(20nm)/mCP:Firpic-10wt%(30nm)/Bphen(30nm)/LiF(1nm)/Ag(1nm)/HATCN(50nm)/ DNTPD(50nm) /TAPC(20nm)/CBP:Ir(piq)₂acac(30nm)/Bphen(30nm) /LiF(1nm)/Al(100nm) [abbreviations for OLED materials were not fully described]. Time-resolved photoluminescence and electroluminescence studies were performed to prove the enhancement of light by LSPR. Litht emission efficiency shows notable improvement with the contents of plasmonic nanoparticles, about 35 % in terms of maximum current efficiency for dot/line mixed patterns of silver nanoparticles. We believe that LSPR-aided efficiency enhancement of white OLED light can be useful to take a further step to achieve higher efficiency of full color/large area active matrix OLEDs using white OLED/color filter or white OLED/quantum dot color converting materials.

We report a general protocol for synthesizing Ag based hybrid metal chalcogenide nanorods in organic solution by using Ag nanoparticles as seeds. From the experimental observations, the precursor molar ratio of sulfur to transition metal plays an important role in controlling the growth of metal sulfide and the sulfurization of the Ag seeds. Transmission electron microscopy and control experiments have revealed that in the growth process, metal sulfide firstly grows on one side of the Ag seeds. The other side of Ag seed is sulfurized and Ag₂S shell is formed when there are excess sulfur precursors. The length of metal sulfide can be controlled by adjusting the stoichiometric ratio between the precursors and Ag seeds. This novel technique exhibits wide applicability for metal chalcogenide nanorods synthesis in organic phase, including CdS, ZnS, MnS, and CdSe. Moreover, these hybrid metal-semiconductor nanorods exhibit significant improvement in the photocatalytic activity.

This work represents a study of galvanic replacement reaction between Au-Cu bimetallic nanorods and HAucl₄ in organic media. In this approach, Au-Cu rods act as sacrificial templates to form hollow structures, followed by dumbbell shaped particles, and finally they break into spheres. The concentration of HAucl₄ is found to play a major role in formation of different structures. The intermediates obtained are further investigated for their unique composition and shape dependent plasmonic and catalytic properties. Moreover, the intermediates naturally possess many “hot spots” making them promising substrates for surface enhanced Raman scattering.

Bulk heterojunction organic photovoltaics (OPVs) have emerged as a promising solar energy harvesting technology due to efficiencies so far measured up to 11.5%. P3HT:PCBM is thus far the most widely studied and commonly used donor/acceptor compound. Despite the promise of bulk heterojunctions in providing facilitating efficient charge separation and transport to the respective electrodes, thin film geometries are required for continuous interfaces between the donor/acceptor materials. This thin film requirement limits the achievable optical density of the active layer ultimately compromising the performance of the devices due to poor absorption of solar light. In this work we show that light harvesting in P3HT:PCBM OPVs can be significantly enhanced by incorporating Au/Ag core/shell bimetallic plasmonic nanostructures into the PEDOT:PSS buffer layer of the solar cell. This enables coupling of the forward scattered light from the plasmonic nanostructures with the active layer effectively increasing the optical path length in the solar cell. We investigate the behavior of the plasmon enhanced OPVs with optical and electrical characterizations, as well as time resolved photoluminescence to demonstrate the impact of metal nanostructures in the charge transport properties. We performed finite difference time domain calculations to understand their fundamental optical properties. Our experiments and simulations suggest the enhanced near-field, far-field, and multipolar resonances of bimetallic nanostructures facilitating broadband absorption of solar radiation collectively give rise to their superior performance in OPVs.

Plasmonic nanoparticles have become an increasingly common research area as well as becoming a key component in many important applications, such as solar energy harvesting, chemical sensing via surface enhanced Raman scattering, cancer treatment and optical encoding of information to name but a few. The main reason behind their adaptability to these and other prominent applications is their unique optical properties that allow for the manipulation of light below the diffraction limit. This effect, known as Local Surface Plasmon Resonance (LSPR), a resonance phenomenon occurring when the frequency of the incident photons match the frequency of the surface electrons oscillating against the restoring force of the positive nuclei. The optical identity of the LSPR is extremely sensitive to the particle&’s size, shape, distribution and the dielectric functions of the metal and surrounding medium. The ability of this plasmon resonance to scatter light (Mie scattering), finds great utility in optical and imaging fields. The scattering process can be orientated towards specific angles (primarily specular) but depending on the nanostructuring can be diffuse as well. The separate identification and quantification of specular and diffuse scattering is of great importance in order to design nanoparticle configurations that favour a specific enhanced scattering regime. A prime example of this is to consider the variation to the performance of a photovoltaic device by considering the incident angle onto the device, akin to the angular variation of the sun striking a solar cell throughout a day. To date, there is very limited literature relating to this full optical characterisation and the angular properties of nanoparticle arrays. Here we present a novel investigation into the diffuse and specular reflection of light from laser fabricated plasmonic nanoparticles on various substrates, including flexible and transparent polymers. These nanoparticles presented a single LSPR at normal incidence but also a strong variation to colouration over a wide range of angles when viewed with bare eyes. For the full characterisation of these samples we custom built a goniometric system capable of illuminating the samples at various angles and identifying both the specular and diffuse reflection separately. A range of specular angles, in addition to a series of diffuse angles from each specular position were investigated for all the above samples.

Harvesting energy from sun light has always been a splendid goal for human beings. To convert solar energy into chemical energy, semiconductor with suitable electronic band structure and band gap engineering play an indispensable role in water splitting and photocatalysis. Many transition metal oxides combined with noble metal have been reported as excellent candidates for water splitting and photocatalysis in the past decade. In this report, we present a novel approach, epitaxial thin film process, to understand the mechanism of water splitting and photocatalysis using multiferroics, BiFeO₃ and noble metal, Au, heterostructures. Through advanced thin film process, we have successfully controlled the crystalline orientations of BiFeO₃ thin film and Au nano particle. Under this circumstances, the efficiency in three different orientations: (100), (110), and (111) of BiFeO₃ can be studied independently and exclude lots of factors which can not be controlled in other processes, such as, grain size, crystalline size, and orientation. Moreover, localized surface plasmon resonance induced by gold nanoparticles creates the additional absorption band which located at 600 nm. This additional absorption can be tuned by the polarization direction of BiFeO₃ and effectively reduces the band gap of Au/BeFiO₃ heterostructure to around 2 eV, which should harness the most solar spectrum. The present fabrication process will provide copious information and profound understanding of photocatalysis and water splitting based on crystalline facets and surface plasmon effect. On a broader perspective, our work emphasizes the effects of hybrid noble metal and oxide systems and paves the way toward an environmentally friendly technology.

Nanoframes are novel and interesting class of hollow nanostructures both for fundamental studies and applications. Due to the strong plasmonic fields resulting from the coupling between two surface plasmon modes with the external and internal fields, the overall electromagnetic fields in hollow nanostructures were found to be higher with more hot spots than in solid ones with same dimensions.
In the present article, triangular gold nanoframes (TGNFs) are synthesized with a facile high-yield synthesis method and the plasmon field of these TGNFs with different size and ridge thickness have been determined by theoretical computation using the Finite-Difference Time-Domain method. To better understand how these nanoframes can be incorporated for different applications, the detailed relationship between the optical response and structural properties of single TGNF is studied with electron energy-loss spectroscopy (EELS) and correlated to the theoretical computation. Gold nanoframes are much stable than silver due to chemical inertness of gold, therefore, TGNFs are more sensitive both for localized surface plasmon resonance sensing and Surface-Enhanced Raman Scattering (SERS) applications. The results from TGNFs presented here could be used to engineer and design other type of nanoframe-based structures for specific plasmonic applications.

Surface enhanced Raman scattering (SERS) is emerging as a powerful analytical and trace detection technique for chemical and biological sensing. SERS activity of anisotropic and coupled metal nanostructures is highly dependent on the relative orientation of the nanostructures with respect to the polarization of excitation source. In this work, we systematically investigate the polarization-dependent SERS activity of two different anisotropic plasmonic nanostructures, namely, Au@Ag nanocuboids and cuboid Au nanorattles. Au@Ag nanocuboids are comprised of Au nanorod (AuNR) core and a thin Ag shell. Cuboid Au nanorattles are comprised of AuNR core and porous and hollow cuboid Au shell. Although perceivably different, both nanostructures exhibited strong polarization-dependent SERS activity. The SERS activity of Au@Ag nanocuboids was found to be dominated by the cuboidal shape of the shell, exhibiting maximum SERS intensity when the source polarization is along the sharp corners. On the other hand, in the case of cuboid Au nanorattles, the internal electromagnetic hotspot formed between AuNR (core) and porous Au shell resulted in the maximum SERS activity when the polarization of the source is parallel to the longitudinal axis of the nanostructures. Finite-difference time-domain electromagnetic simulations were employed to underpin the experimental findings. Comprehensive understanding of the SERS activity of anisotropic nanostructures with built-in electromagnetic hotspots will enable the rational design of nanostructures for ultrasensitive chemical and biological sensors.

Demands for quick diagnosis for point of care is on increase. Colorimetric sensor provides a way to sense and analyze various chemicals and biomaterial targets based on local refractive index changes. However, the conventional diagnosis systems which requires additional signal analysis instruments are not suitable to the source-limited countries. Therefore, the appropriate and reasonable point-of-care system should have include the signal read-out with bare eyes. For colorimetric sensors, metal nanoparticles were used, which reacted with glucose oxidase to exhibit change of colors by localized surface plasmon. However, both size and shape of enzymatically synthesized nanoparticles varies in the nucleation and growth process of nanoparticles, leading to lower the efficiency of detection and transform indistinctive colors.
Here we designed a structure of Si/Au with hole patterns for effective color transition with finite elemental method of optical simulation. When the normally incident light reaches the bottom Si layer, the reflected light forms interference with incoming light. If the constructively interfered light is located at top gold layer, the light is effectively absorbed due to localized surface plasmon effect of Au holes. Thus the color depends on the height of the hole patterned Si depth which determines the interference length. Also as the top Au layer get thicker, the effective color transition is observed. The periodic nano pattern is easily demonstrated by laser interference lithography (LIL) and deep reactive-ion etching (DRIE) processes. On Si substrate, HMDS and nLOF photoresist are sequentially spin-coated. 2^nd exposure of 325nm laser on 90#730; rotated si substrate after 1^st exposure makes it able to develop hole patterns with period of 250nm to 1um depending on the incident angle of the laser. The holes on photoresist is transferred to Si later by Bosh process, the commercially known DRIE method. Top Au layer is deposited by electron-beam. Additional Au on the structure causes color transition from the original color which provides a way to sense the signal produced by incoming antigen. Finally, the color transition is demonstrated by using addition of reduced Au from the reduction of hydrogen peroxide produced by glucose-glucose oxidase enzymatic reaction.

Over the past few years, we have demonstrated plasmon resonant enhancement of several photocatalytic processes (e.g., water splitting¹, methyl orange decomposition², CO oxidation³, and chemical vapor deposition⁴) by integrating plasmon resonant nanoparticles with strongly photocatalytic semiconductors. When the incident photon energy matches the plasmon resonance of the Au nanoparticles, we observe a significant enhancement in the photocatalytic activity due to the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles. Here, the Au nanoparticles improve the photocatalytic performance by coupling light very effectively from the far field to the near field, right at the surface of the photocatalyst. We model the plasmon excitation at the Au nanoparticle-photocatalytic semiconductor interface using finite difference time domain (FDTD) simulations, which provide a rigorous analysis of the electric fields and charge at the interface. We have also demonstrated plasmonic enhancement of dye sensitized solar cells, which also benefit from the focusing of light to the near field regime at a very thin layer of adsorbed dye molecule.⁵ While most of our earlier work utilized TiO₂ as the photocatalytic semiconductor, we have recently demonstrated plasmon-enhanced water splitting on TiO₂-passivated GaP photocatalysts, which makes more efficient use of the solar spectrum than bare TiO₂.⁶ Furthermore, the TiO₂ passivation layer makes the actively absorbing GaP photochemically stable and robust to corrosion.
1. Liu, Hou, Pavaskar, Aykol and Cronin, Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination. Nano Lett., 11, 1111 (2011).
2. Hou, Liu, Pavaskar, Hung and Cronin, Plasmonic Enhancement of Photocatalytic Decomposition of Methyl Orange under Visible Light. J. Catal., 277, 149 (2011).
3. Hung, Aykol, Valley, Hou and Cronin, Plasmon Resonant Enhancement of Carbon Monoxide Catalysis. Nano Letters, 10, 1314 (2010).
4. Hung, Hsu, Bushmaker, Kumar, Theiss and Cronin, Laser Directed Growth of Carbon-Based Nanostructures by Plasmon Resonant Chemical Vapor Deposition. Nano Letters, 8, 3278 (2008).
5. Hou, Pavaskar, Liu, Theiss, Aykol and Cronin, Plasmon resonant enhancement of dye sensitized solar cells. Energy & Environmental Science, 4, 4650 (2011).
6. Qiu, Zeng, Pavaskar, Li and Cronin, Plasmon-Enhanced Water Splitting on TiO₂-Passivated GaP Photocatalysts. Phys. Chem. Chem. Phys., 16, 3115 (2014).

Next generation mesoporous solar cells (MSCs) including dye-sensitized solar cells (DSSCs) and perovskite-sensitized solar cells (PSSCs) have rapidly emerged with efficiencies reported up to 13% for DSSCs and 19% for PSSCs. Current efforts to improve the performance of MSCs are focused on altering the photoanode morphology, and manipulating the sensitizer composition. In contrast to these approaches that often lead to incremental improvement tailored to each MSC system, plasmonic enhancement provides a universal route applicable to the whole family of MSCs that can significantly boost the optical absorption and carrier generation. Through small additions of metal nanoparticles to MSCs (< 2%), the amount of sensitizer required to achieve high efficiency can be drastically reduced enabling thin film architectures. Here we demonstrate enhanced light harvesting in both DSSCs and PSSCs by embedding silica-coated gold/silver bimetallic nanostructures (Au@Ag@SiO₂-NSs) in the photoanodes. In DSSCs Au@Ag@SiO₂-NSs led to 7.8% efficiency relative to 5.8% for reference (no nanostructures) devices, resulting in 34% improvement in performance; while in MAPbBr3 PSSCs Au@Ag@SiO₂-NSs resulted in 5.6 % efficiency relative to 3.5 % for reference devices giving rise to 60% enhancement. Photocurrent and IPCE spectra revealed that device performance in MSCs is controlled by particle density of Au@Ag@SiO₂-NSs and monotonically decreases at high concentrations. Transient absorption pump-probe spectroscopy provides a mechanistic understanding of how the presence of nanostructures contributes to the superior performance of MSCs by examining charge transport in the solar cells. This work shows that bimetallic plasmonic nanostructures can be employed as a universal platform for enhanced light-trapping in a range of solar devices.

Light interactions with material is a very interesting topic and its application in engineering is active. With light interaction of material at some noble metals invokes collective oscillation of electrons widely recognized as surface plasmons. Especially gold nanoparticles have been attracting much attention because their tunable resonance frequency, extended absorption spectra to visible light and excellent stability in various environments. With semiconductor, these nanoparticles could inhere exceptional properties such as plasmon band driven hot-electron injection (or direct electron transfer, DET), local electromagnetic field enhancement (LEMF) and plasmon induced resonant electron transfer (PIRET) like fo#776;rster resonance energy transfer.
Recently, solar water splitting being utilized by photoelectrochemical cells (PECs) has greatly recognized its importance because of the effective production of hydrogen for renewable energy. Through the past years, the technology has been amazingly meliorated, and theoretical basis have been enormously proceeded. Nevertheless, the theoretical performance has not been achieved. Gold nanoparticles are recognized as one of the keys to unknot this theoretical gap due to its exceptional properties that previously stated. For instance, Zewei Liu et al. reported 66-fold increase of Au-TiO₂ film photocurrent with respect to TiO₂ film at 633nm in Nano Letters (2011, 11, 1111-1116). Notwithstanding some major progress of understanding plasmonic solar water splitting, the interpretation of enhancement mechanism is still under ambiguity.
We provide coverage control of randomly-shaped and octahedron gold nanoparticles and that method can be expanded to various morphologically tailored nanoparticle shapes. Also, we found that octahedral gold nanoparticle attached TiO₂ thin film showed excellent current enhancement with respect of TiO₂ thin film not only in visible (VIS) wavelength, but also in ultra-violet (UV) wavelength region. At the moment of our understanding of the photocurrent enhancement mechanism is hot-electron injection mechanism works well in the visible wavelength region and LEMF mechanism works well with entire UV-VIS wavelength region. Due to LEMF mechanism, gold nanoparticle can act as a plasmonic oxygen catalyst. This photocurrent enhancement induced by LEMF mechanism is applicable to various research fields such as PECs, photovoltaics and photocatalysts.

Band alignments in a conventional device geometry (electrolyte/metal/TiO₂/electrolyte) are an important element to determine charge transfer, initially from the metal to the oxide, then across the oxide/electrolyte interface where they can participate in the OER. More specifically, the efficiency of a multilayer device strongly depends on photon capture efficiency and the Internal Quantum Efficiency (IQE, number of electrons injected into the semiconductor conduction band per absorbed photon). IQE is deeply influenced by the electrostatic barriers formed at the metal-semiconductor interface (M/SC), and that controls electron and hole transfer, i.e. by the Schottky barrier heights (SBH).
Using DFT we develop a detailed atomic understanding of well-known TiO₂ surfaces with different orientations and stoichiometries, and their corresponding interfaces formed with several noble metals commonly used in optical devices, and that show generation of hot carriers by plasmon decay. We calculate band alignments, Schottky barrier heights, and electronic structure properties of these ideal interfaces. With this information in hand,estimate photo-generated charge carrier energy density distributions by plasmon decay, and compare them among the different heterostructures. We discuss the properties of the estimated generated carriers in terms of the stoichiometry of the interface, the nature of its energy distribution (production of high energy electron vs high energy hole), and its origin within band structure features. We provide guidelines for choosing combinations of metal/oxide/stoichiometry to optimize the energy density distribution of photo-generated charge carriers for catalytic processes.

We present an integrated solar module that can improve the absorption of above-bandgap longer wavelength photons for ultrathin silicon solar cells by exploiting plasmonically enhanced spectral upconversion. Ultrathin (~8 um) nanostructured silicon solar cells are embedded in a thin polymeric waveguide medium containing NaYF₄:Yb³⁺,Er³⁺ nanocrystals as upconverion luminophores, coated on a nanostructured plasmonic substrate implemented with cylindrical nanoholes and truncated-cone-shaped nanoposts. Both excitation and emission processes of upconversion luminophores are effectively enhanced under a standard one-sun illumination by combined effects of surface plasmon resonance to amplify the light intensity at the excitation wavelength as well as to facilitate the far-field out-coupling at the emission wavelengths, respectively. The performance of the integrated upconversion solar module improved by ~13 % compared to devices on a nanostructured plasmonic substrate without upconversion luminophores due to the collective contributions from plasmonically enhanced spectral upconversion, as well as additional photon flux from wave-guiding and fluorescence of upconversion luminophores.

Enabled by surface plasmon, noble metal nanostructures can interact with and harvest incident light. As such, they may serve as unique media to generate heat, supply energetic electrons and provide strong local electromagnetic field for chemical reactions through different mechanisms. In this talk, I will demonstrate two different approaches to couple solar energy into chemical reactions. In the first approach, we have designed a series of metal nanostructures that can directly harvest broad-spectrum solar light and convert it into heat for organic reactions. The role of plasmonic hot electrons in the reactions has been also assessed. The findings provide insights into controlling the nanostructures for efficient direct solar-to-chemical energy conversion via surface design. In the second system, we have integrated the plasmonic nanostructures with semiconductor photocatalysts, which extends the working spectrum for photocatalytic reactions. Well designing the interfaces between metal and semiconductor nanostructures, we have been able to synergize the metal-semiconductor Schottky junction with the plasmonic effect (including hot carrier injection and electromagnetic field enhancement), leading to enhanced photocatalytic performance in full solar spectrum. It is anticipated that this series of works open a new window to rationally designing plasmonic nanostructures for catalytic applications.
References:
1. S. Bai, X. Li, Q. Kong, R. Long, C. Wang, J. Jiang, Y. Xiong, Adv. Mater.2015, 27, 3444.
2. R. Long, Z. Rao, K. Mao, Y. Li, C. Zhang, Q. Liu, C. Wang, Z. Y. Li, X. Wu, Y. Xiong, Angew. Chem. Int. Ed.2015, 54, 2425.
3. S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Chem. Soc. Rev. 2015, 44, 2893.

TiO₂, a photoactive semiconductor (E_g= 3.2 eV), cannot generate enough current to sustain power in a photovoltaic device due to its limited spectral activity, which is restricted to the UV. Typically it is used in conjunction with light absorbing materials that extend the spectral response of the photovoltaic device. Recently, it was shown that plasmonic nanoparticles can inject hot electrons into TiO₂ and extend the photovoltaic device spectral response. However, in order to obtain these nanoparticles in a homogeneous and highly organized way, their formation requires either patterning or special preparation techniques. Furthermore, to complete the solar cell structure usually a hole transport material is used.
Using a combinatorial investigation we show that hot electron generation can occur in Ag nanostructures formed on TiO₂ using sputtering. We prepared a solid state plasmonic solar cell, based on TiO₂ and Ag, without complicated nanoparticle deposition techniques and with no hole transport material. To obtain these solar cells, silver was deposited with different thicknesses by sputtering as a back contact for the solar cells. The silver nanostructures are formed by the rough nature of the TiO_2, which was prepared by spray pyrolysis. The Ag has a dual role both as the back contact current conductor and as the metal forming a Schottky barrier with the TiO₂. The Ag generates hot electrons upon illumination that are injected into the TiO₂, enhancing the photovoltaic activity of the solar cells into the visible region of the solar spectrum.
We provide evidence for the plasmonic photovoltaic activity by carrying out IPCE measurements that show a current onset at around 700 nm, while XPS results reveal a wide spread plasmonic peak for the Ag. I-V measurements show photovoltaic activity dependent on the Ag back contact thickness. The best cell performances give short circuit currents up to 1.18 mA cm^-2 and open circuit voltages up to 400 mV. To our knowledge the obtained currents are much higher than any previous report for TiO₂/Ag solar cells.

Use of nanoparticles to convert photons into thermal energy necessitates the heating of the suspending fluid embedding the particles. High temperature media can influence the energy and bandwidth of the nanoparticle plasmon resonance thus altering the absorption capabilities of the nanoparticles as they absorb more photons. Additionally, heated nanoparticles can reshape due to unstable surface energies or they can fall out of suspension due to surfactant boiling or instability at high temperature. Here, we directly measured the optical properties of Au nanorods and indium tin oxide (ITO) nanocrystals at a range of temperatures up to 300 degrees Celsius in the heat transfer fluids Duratherm S and Dynalene MT. Measurements were done using a combination of UV-Vis and Fourier transform infrared spectroscopy (FTIR) to obtain optical absorbance from 300 to 4000 nm. Changes to optical properties are reported from ambient to heated conditions. Effects of thermal cycling were also analysed by taking nanoparticles to 300 degrees Celsius and back to ambient multiple times and comparing the optical properties for lifetime testing. Applications for direct heating of nanoparticles include solar conversion, catalysis, steam generation, photothermal storage, and biological systems including photothermal imaging and therapy.

Organic solar cell devices (OSCs) are a viable next-generation energy source due to their appealing properties, such as their low cost, light weight, and good mechanical flexibility. However, OSC devices have a very thin active layer because the most polymeric materials have low carrier mobility and a short exciton diffusion length. This thin active layer can lead to poor absorption of solar light, resulting in low power conversion efficiency (PCE). It is thus necessary to find a way to improve the PCE of OSCs without increasing the thickness of the active layer. Currently, surface plasmon resonance (SPR) has received considerable attention as an alternative method that offers interesting optical properties for the thin active layer of OSCs and OLEDs [1-3]. The SPR-induced localized enhanced electromagnetic field increases the optical absorption in the active layer without increasing its thickness.
In this work, we propose two simple methods to improve the efficiency of OSCs by applying SPR. The first method involves the application of Au nano rods (NRs) to OSCs to increase the fill factor (FF) and the PCE of OSCs using the near-infrared (NIR) wavelength, which is typically wasted in current OSC devices. In order to fabricate a plasmonic OSC device, we blended Au NRs into the anodic buffer layer. The SPR effect of metal nanoparticles has shape- and size-dependent properties, and cylindrical particles are more apt to have higher amounts of forward scattered light as opposed to spherical particles. Thus, Au NRs have advantages over the differently shaped nanoparticles. The value of V_oc for the plasmonic OSC device was nearly unchanged, but the FF value was increased by inserting the Au nano rods into the anodic buffer layer. The coupling between photo-induced excitons and the plasmons of the Au NRs increased the number of hot electrons for efficient exciton dissociation, causing the FF of the plasmonic OSC device to increase.
We also suggested another method to improve the efficiency of OSC devices using the SPR effect by creating a nanobump structured metal electrode without an additional complex fabrication process or an expensive fabrication process. In order to fabricate the nanobump structured metal electrode, we inserted silica nanoparticles into an OSC device. The nanobump structure of the metal electrode acts as subwavelength antenna and generates SPR. Hence, a SPR-induced strongly localized electromagnetic field enhancement occurs in close proximity to the nanobump structure, causing the optical absorption in the active layer to increase and the efficiency of the OSC device to improve.
Reference:
[1] H.A. Atwater and A. Polman, Nat. Mater. 9 (2010) 205-213
[2] C.S. Choi, D.Y. Kim, S.M. Lee, M.S. Lim, K.C. Choi, H. CHo, T.W. Koh and S. Yoo, Adv. Opt. Mater. 1 (2013) 687-691
[3] S.M. Lee, Y. Cho, D.Y. Kim, J.S. Chae and K.C. Choi, Adv. Opt. Mater. DOI: 10.1002/adom.201500103

Photoelectrochemical (PEC) water splitting is one of the most promising way to convert solar energy directly into fuel. PEC water splitting is achieved by light absorbing semiconductors driving the oxygen evolution (OER) and hydrogen evolution (HER) reactions. Metal oxide semiconductors such as TiO₂ (Eg:3.2 eV), Fe₂O₃ (Eg:1.9 eV) , WO₃ (Eg:2.8 eV), BiVO₄ (Eg:2.4 eV) are considered as prominent semiconductor photoanodes for PEC water splitting due to their low cost and high stability in aqueous environments. BiVO₄ with monoclinic scheellite structure appears to be a prominent metal oxide material showing strong potential on water splitting and organic pollutants decomposition under visible light irradiation. Nevertheless, most of metal oxide semiconductors show limited overall reaction efficiency due to their narrow light absorption spectrum and inefficient catalytic performance.
An effective method to improve this limited efficiency is functionalizing these semiconductors with plasmonic metal nanoparticles. Metal nanostructures have attracted increasing interest, since they exhibit localized surface plasmon resonance (LSPR) that can dramatically increase the light absorption and the near-field intensity. Recently, employment of plasmonic Au and Ag nanoparticles of different shapes and dimensions, such as nanoshells of a dielectric core and a metallic shell (SiO₂@Au, SiO₂@Ag), appears to be a prominent way of utilizing LSPR. The plasmonic Au or Ag nanoshell particles allows for matching the wavelength of LSPR with the light absorption spectrum of the metal oxide.
In this study, PECF devices have been fabricated by functionalizing metal oxide semiconductors with plasmonic metal nanoparticles and a correlation between LSPR of the nanoshell and photocurrent of PEC devices is explored. The microstructure and crystal structure of plasmonic particle-decorated metal oxide semiconductor (BiVO₄) film were investigated. The effect of the surface plasmons on light absorption and scattering were investigated. In addition, photogenerated current of the plasmonic PEC devices was systematically characterized. Plasmonic SiO₂@Ag core@shell and SiO₂@Ag@SiO₂ core@shell@shell nanoparticles that were successfully deposited on metal oxide semiconductors thin films, improved optical absorption and increased photocurrent density. Also, a negative shift of onset potential for photoelectrochemical water oxidation was observed upon modification of metal oxide semiconductor electrode surface with plasmonic nanoparticles. We will also present the results of the theoretical simulation supporting our experimental observations.

In this presentation, we will discuss our recent progresses in the development of novel plasmonic nanocomposite materials for energy conversion. Our focus will be firstly on the synthesis of highly active and chemically stable plasmonic nanostructures by using a number of methods, including for example protecting reactive metals such as silver with gold, and forming alloys. Then we will discuss how to incorporate the plasmonic nanostructures into semiconducting materials such as titania for energy conversion. In particular, we will discuss a sandwich-structured photocatalyst that shows an excellent performance in degradation reactions of a number of organic compounds under UV, visible light, and direct sunlight. We will also show the photocatalytic reaction can be used to initiate direct growth of plasmonic nanostructures on titania nanostructures and produce nanocomposites that can find use as electron transport layers in the fabrication of organic solar cells. Due to the improved charge separation and transfer by the presence of plasmonic nanoparticles, we have observed notable enhancement in power conversion efficiency and external quantum efficiency.

The absorption and conversion of photons below the bandgap of a semiconductor can lead to significant performance improvements in photoelectrochemistry (PEC), solar driven water-splitting, and thermal energy conversion. However no material exists today that is both stable in electrolyte solutions and absorbing over the entire visible and adjacent (near-UV and near-infrared) ranges of solar spectrum.
Photogenerated hot carriers can solve this issue by absorbing low energy photons in the thin metal and by injecting those hot electrons over the Schottky barrier at metal/semiconductor interfaces. Here we present our finite-difference time-domain (FDTD) simulation result of a metallic-semiconductor photonic crystal (MSPhC) which shows broad absorption range up to ~1.5 mu;m, covering a large portion of the solar spectrum.[1] The absorption of different materials in the composite structure can be decomposed by calculating divergence of the Poynting vectors, which refers to the spatial power absorption. The result show Au as the dominant absorbing material in MSPhC with at least three orders of magnitude higher absorption compared to other MSPhC materials. Au absorption under different thicknesses from 0 - 50 nm are studied and compared with experiments. The absorption dramatically decreases with increasing Au thickness with a peak value at the thickness ~5nm.[1] This is an interesting finding since Au is considered as the absorbing layer. Thin Au layer allows for light to penetrate into the MSPhC structure and couple into waveguide, cavity, and surface plasmon polariton (SPP) modes. Thin metal layers will also help to minimize the electron-electron scattering before hot electrons inject into the catalyst layer. Simulations shows that the waveguide mode inside the walls of the MSPhC cavity structure is the dominant mode, while the SPP modes which propagates along the air/Au/TiO₂ interface has a larger full-width at half-max (FWHM) value indicating a higher loss mode compared to the waveguide mode.[2] The high loss is due to the peak E-field of SPP modes located in the metal.
Reference
[1] Wang, Y, “Study on the Photoelectric Hot Electrons Generation and Transport with Metallic Semiconductor Photonic Crystals”, 2015 (Master Thesis)
[2] Chou, J.B., et al., “Broadband Photoelectric Hot Carrier Collection with Wafer-Scale Metallic-Semiconductor Photonic Crystals”, 42th IEEE Photovoltaic Specialist Conference, New Orleans, 2015

Proton-coupled electron transfer (PCET) reactions involve the transfer of a proton and an electron and play an important role in a number of chemical and biological processes [1]. Here, we describe a novel phenomenon, plasmon-enhanced PCET, which is manifested using SiO₂-coated Ag nanoparticles [2] functionalized with gallic acid (GA) [3], a natural antioxidant molecule that can perform PCET. These GA-functionalized nanoparticles show enhanced plasmonic response at near-IR wavelengths, due to particle coupling [4] through agglomeration caused by the GA molecules. Near-IR 785 nm laser irradiation induces strong local hot-spots on the SiO₂-coated Ag nanoparticles and plasmon energy transfer occurs to the grafted GA molecules, as evidenced by surface enhanced Raman scattering (SERS). This energy transfer lowers the GA-OH bond dissociation enthalpy by at least 2 kcal/mole. The occurrence of nanoparticle-driven plasmon-enhancement of PCET brings together the -so far unrelated- research domains of nanoplasmonics, electron/proton translocation with significant impact on a variety of applications including interfacial electron/proton transfer.
References
[1] J. M. Mayer. Annu. Rev. Phys. Chem.55, 363-390 (2004).
[2] G. A. Sotiriou, T. Sannomiya, A. Teleki, F. Krumeich, J. Vörös & S. E. Pratsinis. Adv. Funct. Mater.20, 4250-4257 (2010).
[3] Y. Deligiannakis, G. A. Sotiriou & S. E. Pratsinis. ACS Appl. Mater. Interfaces4, 6609-6617 (2012).
[4] G. A. Sotiriou, F. Starsich, A. Dasargyri, M. C. Wurnig, F. Krumeich, A. Boss, J.-C. Leroux & S. E. Pratsinis. Adv. Funct. Mater.24, 2818-2827 (2014).

Photocatalysts of metal nanoparticles on catalytically inert support has attracted great interest in recent years although it has been known for long time that metal nanoparticles exhibit intensive light absorption. Nanoparticles s of plasmonic metals (gold, silver and copper) can intensely absorb visible light due to the localized surface plasmon resonance (LSPR) effect yield “hot electrons”. Besides, the electrons in nanoparticles of all metals can absorb a photon of wide wavelength range become “hot electron” since metals have continuous electronic energy levels. The hot electrons can induce chemical reactions of reactions of the molecules on the particles under moderate conditions. Thus, with these catalysts we can drive the syntheses of fine chemicals with focused sunlight at ambient temperature. Since the absorption due to LSPR is very intensive, a smart approach was also developed: incorporated palladium, the metal widely used for various organic reactions under heating, with gold nanoparticle base. Photocatalysis of metal nanoparticle has been successfully applied to over 30 chemical transformations driven by visible light, including reductive coupling of nitro aromatic compound to corresponding azo compounds, selective reduction nitro aromatic compounds to functionalized anilines, seven cross coupling reactions, selective oxidation of aliphatic alcohols, oxidative addition of benzylamine, dehydrogenation of aromatic alcohols to corresponding aldehydes and ketones etc. These photocatalysis processes achieved high reaction rate and product selectivity. If the photocatalysis process is successfully applied to even a moderate fraction (e.g. 10%) of the fine chemical production processes, it will bring profound change in chemical engineering science.
References
Xi Chen, Huaiyong Zhu, Jin-Cai Zhao, Zhan-Feng Zheng, Xue-Ping Gao, Angew. Chem. Int. Ed. 47, 5353, 2008.
Huaiyong Zhu, Xuebin Ke, Xuzhuang Yang, Sarina Sarina, Hongwei Liu, Angew. Chem. Int. Ed. 49, 9657, 2010.
Sarina, Sarina; Zhu, Huaiyong; Jaatinen, Esa; Xiao, Qi; Liu, Hongwei; Jia, Jianfeng; Chen, Chao; Zhao, Jian, J. Am. Chem. Soc.135, 5793, 2013.
Sarina Sarina, Huai-Yong Zhu, Qi Xiao, Esa Jaatinen, Jianfeng Jia, Yiming Huang, Zhanfeng Zheng, Haishun Wu, Angew. Chem. Int. Ed. 53, 2979, 2014.
Xiao, Qi; Liu, Zhe; Bo, Arixin; Zavahir, Sifani; Sarina, Sarina; Bottle, Steven; Riches, James; Zhu, Huaiyong, J. Am. Chem. Soc., 137, 1956, 2015.

This work focuses on the plasmonic effect on the exciton/multiexciton emission properties of colloidal quantum dots (QDs). When CdSe/CdS core/shell QDs were placed near Au@SiO₂ core@shell nanoparticles, their exciton/multiexciton emission lifetimes and quantum yields were modified due to the exciton-plasmon interaction. At the single QD level, we found that the ratio between the biexciton and exciton quantum yields of the QDs near Au nanoparticles was increased, leading to a small photon antibunching “dip”. Electrodynamics modeling revealed that biexciton quantum yield of single QDs was increased while the exciton quantum yield decreased due to the plasmonic structures. Moreover, we found the change in the exciton/multiexciton emission lifetimes of the QD/Au nanoparticle system depended on the excitation wavelength. The studies suggest that plasmonic structures can be used to modify the exciton/multiexciton emission efficiency of QDs.

Dye-sensitized solar cells (DSSCs) present a promising alternative to traditional silicon photovoltaics (SiPVs) due to their low-cost and straightforward manufacturing processes and the use of less toxic and inexpensive constituent materials. However the comparatively low power conversion efficiencies (PCEs) achieved so far, ~12% for DSSCs vs. ~25% for SiPVs, have prevented them from becoming commercially competitive. Recent efforts to improve the PCE of DSSCs have largely focused on strategies such as altering the TiO₂ architecture, incorporating new cobalt-based or solid-state electrolytes, and synthesizing new panchromatic sensitizers. In contrast to these approaches that often lead to incremental improvement tailored to each DSSC system, plasmonic enhancement provides a universal route applicable to the whole family of DSSC materials that can significantly boost the optical absorption and carrier generation. In this work, we demonstrate wet-chemically synthesized gold nanostructures can be straightforwardly incorporated into the mesoporous TiO₂ layer without altering the material or electronic properties of the sensitizers. Gold nanocubes of ~45 nm edge length were coated with a thin (~5 nm), uniform layer of silica (Au@SiO₂ NCs) and were embedded homogeneously throughout the photoanode of a DSSC over a range of nanoparticle loadings. We have shown a maximum PCE of 7.8% achieved for plasmon-enhanced devices relative to 5.8% of TiO₂-only reference devices, resulting in 34% improvement in performance. We observed a systematic dependence of device performance on the particle density of Au@SiO₂ NCs, where efficiencies decreased at very high concentrations. Finite difference time domain calculations indicate that nanocubes primarily absorb incident light and generate intense electromagnetic near fields at the edges and corners due to the quasi-static lightning-rod effect. These intense near fields couple with nearby sensitizer molecules resulting in increased light harvesting and carrier generation and improved device performance.

Plasmonic nanocomposites consisting of metal nanoparticles embedded in a dielectric organic or inorganic matrix have a host of applications ranging from simple color filters to complex metamaterials. The present talk is concerned with vapor phase deposition of nanocomposites and the resulting plasmonic properties. Deposition techniques include evaporation and magnetron co-sputtering of the matrix and metallic components and the combination of a high-rate gas phase aggregation cluster source with plasma polymerization or magnetron sputtering. In contrast to the production of plasmonic nanostructures by e-beam lithography, the nanocomposites form by self-organization which allows large-scale deposition, although the application of the high-rate cluster source for upscaling remains challenging. It will be shown how the plasmonic properties can be tailored via the incorporation of metallic clusters with well-defined composition, filling factor, and filling factor profile into various matrices. Examples involve transparent conducting metal coatings, perfect plasmonic absorbers and photoswitchable nanocomposites.
Acknowledgements - Financial support by DFG within the Collaborative Research Centers TR24 and SFB 677 is acknowledged.

Flexible devices are in much greater demand than ever before. A driving force for this increased interest is the desirability for wearable devices and especially for those that can provide rapid and accurate information on demand. A key milestone to achieving this is the ability to fabricate the desired components on a flexible substrate. Here we present a platform for the fabrication of plasmonic nanoparticles (NP) on a polymer substrates (PET, polycarbonate) via laser annealing, in an ultra-fast approach that is compatible with the temperature sensitive substrate. Although the simplest methodology for the probing the self-assembly of nanoparticles from thin metal films is thermal annealing, the high temperatures necessary makes this process non-transferable to a polymer substrates. Due to the low mass and associated low thermal characteristics (capacity and conductance) of the polymer substrate, the produced nanoparticles have a much diminished LSPR response compared to those produced on a solid substrate such as a silicon wafer. In order to compensate for this, we tested various oxide barrier interlayers (Y₂O₃, TiO₂, CeO₂, SiO₂, Ta₂O₅, ITO, AZO and ZnO), grown by sputtering or electron-beam evaporation, between the polymer substrate and the metal film (Ag, Au). For a given application, this barrier layer need not only act as a buffer, it could be used as a functional thin film as well, consider the example of a layer of ITO or AZO in a flexible photovoltaic device; such flexible plasmonic devices offer great potential for various applications such as flexible photovoltaics with enhanced efficiency, biomedical sensors via Surface Enhanced Raman Scattering (SERS), and coloured ophthalmic lenses. The different oxide interlayers are exhibiting various thermal conductivities, surface morphology and chemistry resulting in various contact angles of the melted metals upon them. Consequently, we investigate thoroughly the interplay between these intrinsic characteristics of the oxide surfaces with the laser annealing process parameters (such as laser wavelength, fluence, ambient and number of pulses) that dictates the final morphology and the plasmonic performance of the produced nanoparticles. The far-field and near-field optical performance of the produced nanoparticles on various oxide layers are studied by optical reflectance spectroscopy at normal incidence and by SERS experiments, respectively; in particular, the SERS tests were performed using solutions of R6G molecules in the 1nM - 1mM concentration range.

Degenerately doped metal oxide semiconductors, like ITO, exhibit plasmonic resonance at near and mid-infrared wavelengths tunable by varying their composition. Nanocrystals of many such materials have now been synthesized and applications are emerging that leverage the responsiveness of their localized surface plasmon resonance (LSPR) to electronic charging and discharging. I will describe how these phenomena are enabling the development of a new class of electrochromic glass that can dynamically control heat loads and daylighting in buildings to save energy and enhance comfort of building occupants. Further applications of these novel plasmonic nanocrystals will hinge, in part, on their ability to concentrate infrared light into nanoscale volumes and to enhance electronic and vibrational state transitions via associated field enhancement effects. Through simulations using the discrete dipole approximation, we have predicted high enhancement factors exceeding 300x for faceted nanocrystals. The potential for field enhancement is greatest for low loss materials with long plasmon dephasing times. Since the homogeneous linewidth of the LSPR is inversely related to the dephasing time, we have sought to distinguish this intrinsic linewidth from the heterogeneous broadening that is always present for colloidal nanoparticles. Measuring LSPR spectra of individual nanocrystals by tip-enhanced synchrotron FTIR spectroscopy we find single nanocrystals can have linewidths less than half of the corresponding ensembles. Thus, the dephasing times are long and plasmonic oxide nanocrystals have great potential for diverse applications in energy.

A crucial target for the development of key enabling technologies to be implemented in the next generation of building envelopes is identified in the diffusion of intelligent multifunctional windows triggering efficient energy management and cost savings without unreasonable effort and complexity. Photovoltachromic devices integrate photovoltaic and electrochromic functionalities allowing both their separate or conjugate use. [1] The recent development in the field of doped oxide nanocrystals [2-3] may allow to develop a new class of transparent devices able to shield the infrared heat load carried by sunlight. Their optical features may be finely tuned by changing the nature of the nanomaterials engaged in the dynamic shift of the plasmonic scattering with specific parts of the solar spectrum. We report the fabrication of high-quality mesoporous electrodes based on surfactant-capped Indium Tin Oxide (ITO) colloidal nanocrystals and their implementation in self-powered “plasmochromic” devices which are simultaneously capable of generating electric energy as a photovoltaic system as well as of perceiving small variations in external radiation and controlling the energy fluxes by means of a smart variation of their optical transmittance. The effect of electrochemical charging on the optical features of the ITO-NCs mesoporous films has been systematically investigated by making them to work as anodes in a set of lab-scale EC cells filled with three different batches of electrolytes, respectively based on lithium perchlorate, lithium iodide and 2-dimethyl-3-propylimidazolium iodide in propylencarbonate. Several batches of dye-sensitized photovoltachromic cells have been implemented, featuring a broad spectral coverage thank to four different sensitizing dyes having almost complementary absorption spectra have been explored along with a couple of specifically designed device architectures. A reversible modulation of the solar transmittance (in the range 700-2500nm) higher than 30% (Tvis/Tsol from 78/65 to 70/33, coloration efficiency > 350 cm²/C in the NIR) has been demonstrated as well as an independent control of the electrochromic behavior over the visible and near-infrared regions.

Recent studies on plasmonic semiconductor nanocrystals opened a new possibility of near infrared (NIR)-selective electrochromism. The light extinction by localized surface plasmon resonance (LSPR) can be modulated via electrochemical charging and discharging^[1], and the target LSPR wavelength range can be fine-tuned by selecting the nanocrystal matrix and dopants^[2]. Among a variety of doped metal oxide nanocrystals that we have synthesized, vacancy-doped tungsten oxide (WO_3-x) shows an intense LSPR absorption optimized to match the solar NIR radiation (700~1300 nm). We present that a mesoporous film of WO_3-x nanocrystals exhibits fully reversible switching of LSPR upon charging and discharging of Li⁺ ions. This allows a dynamic control of solar NIR transmittance (i.e. heat gain) through windows, which can significantly reduce the immense energy demand for air-conditioning. Furthermore, we fabricated a composite film consisting of WO_3-x nanocrystals and a conventional visible (VIS)-electrochromic material, niobia glass (NbO_x), to realize NIR-VIS dual-band modulation. Deliberately designed nanocomposite architecture with optimized charge transport pathways provides a capability to selectively and rapidly switch each component. In result, unprecedented dual-band modulation performance was achieved covering the entire solar spectrum. The composite device exhibits 36%T_NIR : 73%T_Vis at the ‘Cool mode&’ and 7%T_NIR : 22%T_Vis at the neutral color ‘Dark mode&’^[3].
[1] Garcia, G., Buonsanti, R., Runnerstrom, E. L., Mendelsberg, R. J., Llordes, A., Anders, A., Richardson, T. J., Milliron, D. J., Nano Letters, 11, 4415-4420
[2] Lounis, S. D., Runnerstrom, E. L., Bergerud, A., Nordlund, D. & Milliron, D. J., J. Am. Chem. Soc.136, 7110-7116
[3] Kim, J., Ong, G. K., Wang, Y., LeBlanc, G., Williams, T. E., Mattox. T. M., Helms, B. A., Milliron, D. J., Submitted

Surface plasmon polaritons are collective charge oscillations coupled to photons at the interface between a metal and a dielectric. When structured appropriately, thin metal films exhibit surface plasmon resonances that drastically alter their optical properties. Such resonances can be exploited to develop more efficient photovoltaic devices, for example by replacing the conducting oxide layer used in traditional solar cells by a metallic contact that exhibits enhanced optical transmission.
Here, we present a systematic exploration of the design space of thin metal films perforated with arrays of sub-wavelength holes. Such arrays are known to exhibit transmission resonances above the cutoff wavelength of an individual hole of the same dimensions. Using electron-beam lithography, we fabricate sub-wavelength hole arrays in metal films made of gold and silver, which are the most often utilized plasmonic metals, but also less explored materials such as copper and aluminum. We then characterize the optical properties of our fabricated structures using a high-brightness Fourier-transform spectrometer. Experimental results are compared to finite-difference time-domain simulations in order to understand the various surface plasmon modes supported by these structures.
We demonstrate that, on resonance, the transmission normalized to the open area of the holes can exceed unity, meaning that each hole transmits more light than is impinging on it directly. Increasing the pitch of the array red-shifts the resonances while increasing the hole radius results in higher overall transmission and broader peaks. As the thickness of the film increases, the total transmissivity decreases, while in thinner films a broadening of the resonance due to the coupling between the surface plasmons on the two interfaces is observed. We also compare arrays of sub-wavelength holes and their inverted structures, i.e. arrays of metal nano-islands, and observe complementary behavior.
For photovoltaic applications, sub-wavelength hole arrays have the dual benefit of providing an electrically conducting anode while simultaneously aiding in photon management. By appropriately tuning the design parameters, we are able to maximize the absorption of solar radiation in structures that are also suitable for efficient charge separation. We therefore explore the use of these transparent contacts in unconventional solar cell architectures, such as plasmonic photovoltaic devices where the carriers are generated by plasmonic absorption. We have implemented this approach in a metal-insulator-metal stack comprised of a perforated gold film, a thin zinc oxide spacer and an indium-tin-oxide rear contact. The significant advantage of this design is that such devices are considerably more economical to fabricate than traditional solar cells that require high-purity semiconductors.

Plasmons - as collective photon-electron oscillations - are naturally positioned to provide an ideal intermediate step in the conversion of photon energy into electrical current [1]. Surface plasmons can be excited by thermal fluctuations at high temperatures, while optically-excited plasmons eventually dissipate their energy as heat. As such, surface plasmons also provide a bridge for efficient energy transfer between phonons and photons [1]. We will report on our recent work on engineering efficient energy transfer and conversion between phonons, photons and electrical charge carriers with the aid of plasmonic thin films.
In particular, we will discuss our recent results on tailoring radiative heat transfer across narrow vacuum gaps between thin plasmonic films [2]. We predict that plasmon-mediated heat flux can exceed the blackbody radiation limit by many orders of magnitude, and the benchmark established by the polar dielectric materials by an order of magnitude. We attribute this enhancement to the significant spectral broadening of radiative heat transfer due to coupling between surface plasmon polaritons on both sides of thin films. We compare the performance of thin films of conventional plasmonic materials such as noble metals and news ones such as metal oxides, and identify best material and geometrical properties to maximize the radiative heat flux. We also predict that the use of phase change materials allows for dynamic switching of the heat flux spectrum [2,3].
We will also discuss the use of plasmonic materials in harvesting solar energy by collection of photo-generated hot electrons via the processes of internal photoemission as an alternative approach to traditional photovoltaics. We will compare the limits to the maximum achievable energy conversion efficiency with various metals and semimetals, and will outline the ways to increase the efficiency limits by using quantum confinement effects in thin films as well as by splitting the solar spectrum [4].
This work has been supported by the US Department of Energy, Office of Science, and Office of Basic Energy via ‘Solid State Solar-Thermal Energy Conversion Center (S3TEC)&’, Award No. DE-SC0001299/DE-FG02-09ER46577 (for solar thermal applications) and Grant no. DE-FG02-02ER45977 (for near-field transport and photon DOS tailoring via confinement effects).
1. S. V. Boriskina, H. Ghasemi & G. Chen, 'Plasmonic materials for energy: from physics to applications,' Mater. Today 16, 375, 2013.
2. S.V. Boriskina, J. Tong, Y. Huang, J. Zhou, V. Chiloyan & G. Chen, 'Enhancement and tunability of near-field radiative heat transfer mediated by surface plasmon polaritons in thin plasmonic films,' Photonics 2, 659, 2015.
3. Y. Huang, S.V. Boriskina & G. Chen, 'Electrically tunable near-field radiative heat transfer via ferroelectric materials,' Appl. Phys. Lett. 105, 244102, 2014.
4. S.V. Boriskina, et al., 'Roadmap on Optical Energy Conversion,' to appear in J. Opt. 2015.

While noble metals have been the primary focus of plasmonics for more than a decade, for applications that may involve large-area light harvesting, aluminum is emerging as a highly sustainable alternative plasmonic material of great promise. Aluminum plasmonic hot-electron-based photodetection has been demonstrated in a prototype device, to investigate carrier transfer in hot carrier-based metal-semiconductor photodetectors, where the distinction between plasmonically generated carriers and those generated by optical excitation of interband transitions in the metal can be observed. Another important energy-relevant application is plasmonic photocatalysis. We have shown that Aluminum nanocrystals offer photocatalytic activity similar to that observed on noble metal nanoparticles for simple diatomic molecule dissociation reactions at room temperature. We observe that the electronic structure of Aluminum modifies its plasmonic response relative to noble metals in both these applications.

Surface plasmon amplification by stimulated emission of radiation (spaser) is an emerging field of photonics which may lead to the development of nanoscale sources of coherent light. Infrared (IR) emitting quantum dots (QDs) based on IV-VI and III-V semiconductors with bandgaps in the 0.5 - 1.5 eV range represent a promising type of gain media for plasmon amplification. In addition, IR QDs hold great promise for applications in light emitting devices, IR sensing, and photon-management in photovoltaics. While synthetic methods of IR QDs have greatly advanced recently, there is still a limited knowledge of the emitting states, intrinsic radiative lifetimes, and exact factors that control emission efficiencies in these materials. A particular challenge in the investigation of the emissive properties of IR QDs is the existence of multiple channels for nonradiative recombination competing with radiative decay. Current methods commonly employ time-resolved measurements of photoluminescence (PL) via time-correlated photon counting. Temporal resolution of these experiments is typically limited by ~100 ps, which is often insufficient to detect short-lived transients associated with surface trapping or other fast nonradiative processes such as Auger recombination in charged QDs. Furthermore, since these measurements usually utilize Si avalanche photodiodes they can only be applied to materials with emission wavelengths shorter than 1 mu;m.
In this work, we employ ultrafast gated PL frequency-upconversion for the characterization of IR-emitting QDs. This technique provides sub-picosecond temporal resolution and allows us to access emission wavelengths up to ~3 mu;m. Using this method we assess the intrinsic exciton recombination rates in PbSe, PbS, InAs, and InSb QDs. While the Fermi&’s golden rule predicts the linear increase of the intrinsic radiative rates in QDs with increasing their bandgap, we observe the opposite trend which is especially pronounced for bandgaps >1 eV. Moreover, for the III-V QDs (InAs and InSb based) the measured radiative lifetimes are at least an order of magnitude longer compared to calculations based on the bulk transition dipole moment. These resutls point towards the existence of a complex structure of band-edge exciton states. This presentation will introduce the model which provides a plausible explanation of experimental observations taking into account the confinement-enhanced electron-hole exchange interaction which leads to large, tens of meV splitting between “dark” and “bright” exciton states.

Metallic nanostructures have been widely implemented to increase the photocurrent of photovoltaic (PV) devices, including Ag and, most recently, Al. Despite the remarkable progress in the field, there are no measurements demonstrating how the plasmonic nanostructures can boost the open-circuit voltage (Voc) of PV devices. Here, we experimentally show how the presence of Ag and Al locally affect the Voc of ultra-thin GaAs solar cells. For that, we implement illuminated Kelvin probe force microscopy [1]. When subtracting the contact potential difference signal of illuminated- from dark-measurements we quantify the splitting of the quasi-Fermi level of the solar cell under operation conditions. Further, our scanning probe microscopy approach allows us to spatially resolve how the Voc of the device is influenced by the presence of the plasmonic nanostructures. We also quantify and spectrally resolve the local photocurrent enhancement due to the nanoparticles by locally mapping the current generated by the device using near-field scanning optical microscopy [2].
[1] E. M. Tennyson et al., in review.
[2] M. S. Leite et al., ACS Nano11, 11883 (2014).

Localized surface plasmon resonances (LSPRs), the collective oscillations of conduction electrons in metallic nanoparticles, can produce intense near-fields at the resonance wavelengths. Plasmonic nanoparticles have been incorporated in the design of photovoltaic (PV) and photocatalytic devices, where they have been shown to enhance solar energy harvesting efficiency. Research has shown that the addition of plasmonic nanoparticles improves the efficiency of solar light harvesting via one or more of the following mechanisms¹: (1) LSPR excitation leads to an increase in path length for incoming light via scattering, thereby increasing light absorption by the semiconductors; (2) energy transfer from the decay of an LSPR directly creates an electron-hole pair in the semiconductor, a process known as plasmon-induced resonant energy transfer (PIRET). Its efficiency relies on the overlap between the LSPR emission and the band gap absorption of the semiconductor²; (3) direct electron transfer (DET) from the nanoparticle to a semiconductor, in which an LSPR decays, through Landau damping, into a “hot” electron that may then scatter into the semiconductor if it has sufficient energy to overcome the Schottky barrier formed at the interface³. Mechanism (1) is only effective for photon energies above the band gap, while mechanism (2) and (3) involve photons with energies below or above the band gap, therefore, are of particular interest and importance. However, despite its importance, little is known about how PIRET and DET operate at the nanoscale, particularly at the level of a single nanoparticle.
In this paper, we present a nanoscale EELS study of PIRET and DET on several Ag nanocube@substrate systems, where the cubes and substrates serve as plasmonic energy donors and acceptors, respectively. The substrates are carefully chosen to turn off or isolate the PIRET and DET energy transfer channels. To simplify the discussion, we focus on only the cube@silicon dioxide (SiO₂)/boron phosphide (BP)/amorphous silicon (a-Si) results. By monitoring the change in the substrate-localized dipole (D) mode of the Ag nanocube on different substrates, we find that SiO2 does not affect the D mode while both BP and a-Si significantly damp the D mode at the cube-substrate interface, indicating the presence of energy transfer. This is because SiO₂ is a transparent large band-gap (9 eV) insulator which is closed to both PIRET and DET; BP has the PIRET off (optically transparent) but the DET on (small band gap); a-Si has both PIRET and DET on.
We introduce a novel method to spatially map energy transfer in plasmon-assisted solar energy harvesting systems, suggesting new materials and device geometries.

There have been some remarkable progress over the past few years with regard to silver nanocrystals, including the demonstration of new methods for the synthesis of silver nanocubes with edge length down to tens of nanometers, silver nanowires with diameters as thin as tens of nanometers, silver nanorods with no absorption in the entire visible region from 400-800 nm. It has also become feasible to achieve shape-controlled synthesis of silver nanocrystals such as nanocubes using an aqueous system. It is really an exciting time to explore the utilization of silver nanocrystals in various applications, including energy harvesting, plamonics, display, and flexible electroncis. In this talk, I will highlight some of these new developments, with a focus on mechanistic understanding and scalable production.

Cu_2-xE, a p-type of semiconductor, show stoichiometry-dependent optical and electronic properties. The LSPR of Cu_2-xE NCs originates from the collective oscillation of positive free carriers, copper vacancies. The density of copper vacancies not only directly influences the frequency and intensity of LSPR, but also determines its charge transport properties. For Cu_2-xSe NCs, the tuning of copper vacancy can be realized by simply adjusting the Cu/Se ratio, i.e., x in the composition. Generally, the larger the x is, the higher the copper vacancy density will be.
We present here a simple approach to prepare monodispersed Cu_2-xSe NCs with tunable copper vacancy density. By changing the molar ratio of OA/OAm in copper precursor solution, Cu_2-xSe NCs with controlled stoichiometry but the same crystal structure were obtained. The LSPR of Cu_2-xSe NCs can be synthetically tuned from 980 nm to roughly 1400 nm. The control of stoichiometry is based on the different coordinating ability of OA and OAm toward cuprous ions. As far as we know, this is the first report to directly tune the copper vacancy density of Cu_2-xSe NCs through synthesis. Furthermore, we developed additional post treatment methods to engineer the LSPR of Cu_2-xSe NCs. When treated with OAm, the Cu⁺ could be extracted out from Cu_2-xSe NCs with the format of copper-amine complex, which increased copper vacancy density and resulted in a blue-shift of LSPR band. CS₂ could lead to a similar response in LSPR, the insertion of S^2- into Cu_2-xSe lattice towards a Cu_2-xSe_yS_1-y ternary alloy, which provides more cationic vacancies, may be responsible for the observed LSPR. And for four-element copper chalcogenides, copper zinc tin sulfide (CZTS) and copper indium tin sulfide (CITS) nanoparticles, the LSPR red-shifted with decreased Cu/(In+Sn).

Metallic nanostructures, used in an ever-increasing number of applications from medicine to sensing, typically exhibit a narrow-bandwidth response to optical excitations due to their small sizes. We computationally design and experimentally synthesize aspect-ratio tailored distributions of silver nanodisks to extinguish light over three broad and tunable frequency windows in the visible range (400nm-600nm, 600nm-800nm, and 400nm-800nm). We demonstrate that metallic nanodisks are up to 20x more efficient in absorbing and scattering visible light than more common structures, with increasing per-volume efficiency at longer wavelengths (further away from the plasma frequency of the underlying metal). For our synthesized particles, broadband extinction per volume closely approaches theoretical predictions, confirming the high efficiency of nanodisks and demonstrating the collective power of computational design and experimental precision for developing new photonics technologies.

Nanosynthesis is an emerging field studying the creation of nanostructures. Despite tremendous progress, the synthetic capabilities are still largely limited to simple component and symmetrical nanocrystals. Going beyond individual shapes, additional growth of a shaped domain onto an existing structure would enable arbitrary structural manipulation towards tailored nano-hybrids. In this talk, I will discuss a method to maintain a ligand-deficient “fresh” surface to confine the site of growth in a colloidal system. With the symmetry broken, Ag nanowire and triangular prism are sequentially grown from a colloidal Au seed with well-defined shapes. Our ability in the dynamic control of “fresh” and “old” surfaces allows selective growth at a single site/direction, thus opening doors to sophisticated synthetic designs and broadening the horizon of our search for synergistic effects and functional architectures.

Recently Ag-Ag₂X(X= S, Se) hybrid nanostructures have attracted a great deal of attention due to their enhanced chemical and thermal stability and also their morphology and composition dependent tunable local surface plasmon resonance. It has been shown that these Ag-Ag₂X nanostructures can be synthesized via the sulfidation of pre-grown silver anisotropic nanoparticles. Yet, this process is poorly understood, often leading to anomalous syntheses and unaccountable physical properties. In this work we use theory and experiment to investigate the structural and plasmonic evolution of Ag-Ag₂X nanoprisms during the sulfidation of Ag precursors. The previously observed red-shifted extinction of the Ag-Ag₂S hybrid nanoprism as sulfidation occurs is in contradiction to theoretical predictions, indicating that reaction doesn&’t just involve the prism tips as previously speculated.
By elucidating the correlation between the final structure and morphology of the synthesized Ag-Ag₂X nanoprisms, we find that depending on the reaction conditions sulfidation occurs on both the prism tips and the (111) surfaces, leading to a core(Ag)-anisotropic shell(Ag₂X) prism nanostructure. Additionally, we demonstrate that, depending on the relative amounts of Ag₂X at the prism tips and Ag₂X shell thickness around the prism, the dipole plasmon can shift either way.

Nanostructures presenting optical resonances present a strong potential for energy applications. This potential has been first developed with noble metal nanostructures. At their plasmonic resonances, they can be used as scatterers for improved light trapping into photovoltaic photonic structures or as near-field enhancers boosting photocarrier excitation in photovoltaic media.¹ Very recently, plasmoelectric potentials have been measured in resonant noble metal nanostructures, thus allowing a novel opto-electrical conversion scheme.²
Optical resonances can be excited in nanostructures beyond noble metals. Indeed, most of the metals of the periodic table can support plasmonic resonances.³ Moreover, non-Drude plasmonic-like resonances can also be achieved: for instance the so-called interband polaritonic resonances in nanostructures presenting sharp interband transitions, such as bismuth nanostructures.^4,5 Based on such resonances, the potential of bismuth nanostructures for photocatalysis has been demonstrated.^6,7 In the reported works, photocatalysis was achieved using bismuth nanospheres. At present, the underlying mechanism has to be discussed together with the photocatalytic potential of bismuth nanostructures in a broad range of sizes and shapes.
In this presentation, we provide a detailed description of the optical response of bismuth nanostructures as a function of their size and shape, with dimensions ranging from 50 nm to 500 nm. We demonstrate a strong dependence of the absorption, scattering and extinction cross-sections, near-field, surface charges and currents that will impact the efficiency of photocatalytic solutions based on bismuth nanostructures.
1 Polman, A. et al.; Photonic design principles for ultrahigh efficiency photovoltaics, Nature Materials 2012, 11, 174
2 Sheldon, M.T. et al.; Plasmoelectric potentials in metal nanostructures, Science 2014, 346, 828
3 Naik, G. et al.; Alternative plasmonic materials: Beyond gold and silver, Advanced Materials 2013, 25, 3264
4 Toudert, J. et al.; Exploring the optical potential of nano-bismuth: tunable surface plasmon resonances in the near ultraviolet-to-near infrared range, Journal of Physical Chemistry C 2012, 116, 20530
4 Toudert, J. et al.; Spectroscopic ellipsometry for active nano- and meta- materials, Nanotechnology Reviews 2014, 3, 223
6 Wang, Z. et al.; Investigation of the optical and photocatalytic properties of bismuth nanospheres prepared by a facile thermolysis method, Journal of Physical Chemistry C 2014, 118, 1155
7 Dong, F.; A semimetal bismuth element as a direct plasmonic photocatalyst, Chemical Communications 2014, 50, 10386

There is a great deal of interest in using anisotropic nanomaterials for optoelectronic energy conversion because of their tunable optical properties. In particular, plasmonic nanorods have been introduced into semiconductor-based energy harvesting and conversion devices using a host of both top-down (such as nanoimprint lithography, anodization, and ion-etching) and bottom-up (such as solution-based processing, electrochemical deposition, and self-assembly of molecules) fabrication. Bottom-up synthesized plasmonic nanorods have the potential to be easily integrated directly into the active layer of cheaper, lower-process-energy, solution-processable thin-film energy devices. Having nanostructures that can be introduced directly within semiconductor thin films during solution-processing, rather than dispersed on top of the semiconducting layer, as is common for conventional semi-crystalline inorganic devices, is advantageous because local electric fields associated with the nanoparticles can directly overlap with the semiconducting material. For plasmonic nanorods, gold tends to be the material most commonly employed due to facile bottom-up synthesis, good stability, and reproducibility. However silver nanorods, while not as commonly utilized due to the lack of well-developed or reproducible synthesis methods, have more advantageous optical properties, such as lower parasitic absorption losses and surface plasmon resonances that can be tuned to span the entire visible range, thus providing better spectral overlap with the semiconductor&’s absorption and emission bands and increased efficiency.
The goal of this research project is to synthesize silver nanorods that can be directly integrated within solution-processable conjugated polymer semiconductors for low-process-energy, energy-efficient light-emitting devices. There are many methods for synthesis of nanorods, however the polyol method was chosen in keeping with sustainability efforts. It is considered a green synthesis since it employs polyethylene glycol as both the solvent and reducing agent, thereby eliminating the use of harsh reducing agents such as sodium borohydride. This method was also chosen for its claimed ease of reproducibility and controllability of shape, size, and aspect ratios of nanoparticles. Many aspects of the synthesis are investigated to determine their effect on the nanoscale morphology of the nanorods, including: varying the viscosity of the solvent, heating conditions, and time which the synthesis proceeds. Throughout the synthesis of nanorods the reaction is monitored qualitatively and quantitatively, by visual color changes, and UV/Visible extinction spectroscopy. The nanorods will be integrated into two main nanorod/semiconducting polymer composite designs using solution-based processing: thin film heterostructures and core-shell nanoparticles, which are of interest for large-area and sub-wavelength light sources, respectively.

The constant strive towards procuring environment friendly energy sources leads many green technologies to evolve. Currently, solar energy assisted water splitting for the production of hydrogen fuel has been regarded as one of the most sustainable platform in terms of cost-effectiveness and abundance. Titanium oxide (TiO₂) loaded with noble metals as Gold, is an economic water splitting catalyst that has been widely investigated for water splitting process nowadays [1-3]. The metal photonic crystal (MPhC) nanostructures used in our work has TiO₂ as catalysis with Al₂O₃ as the supporting figures in the form of cylindrical cup on the base of SiO₂/Si substrate. The shape and size of the nanostructures seem to have advantages in the absorption of photons and advertently increase the efficiency of water splitting. The other parameter, which can significantly play a key role in the process, is the degree of crystallization of the catalyst, the supporting structures and the interfaces among the materials. A rapid high-energy electron beam induced crystallization of the supporting figures has been observed in-situ inside High-Resolution Transmission Electron Microscope (HRTEM). The live crystallization of the nanomaterial occurring under electron beam heating (300KV) affects the interface of the material and could change the optical properties of the material and hence enhancing the water splitting capabilities of MPhC. In this communication, we are reporting our in-details study of the phenomenon of rapid crystallization of the supporting figures of water splitting catalyst and its subsequent effect in the overall performance of water splitting and the production of hydrogen fuel.

A number of energy-relevant processes, ranging from battery charging to hydrogen storage to memory switching, rely on nanomaterial phase transitions induced by solute intercalation. However, many of these phase transitions are poorly understood, since observing them in nanomaterials - and in particular in individual nanoparticles - can be extremely challenging. This presentation will describe a novel technique to investigate intercalation-driven phase transitions in individual nanoparticles, based on in-situ environmental transmission electron microscopy (TEM) and plasmon electron energy loss spectroscopy (EELS). By combining the high spatial resolution of TEM with environmental plasmonic spectroscopy, we can correlate a material&’s atomic structure with its local phase with nanometer-scale resolution.
As a model system, we focus on the hydrogenation of palladium nanoparticles. To unravel the thermodynamics of phase transitions in individual Pd nanocrystals, we use the plasmon-EEL signal at varying hydrogen pressures as a proxy for hydrogen concentration in the particle. First, we investigate the hydriding properties of single-crystalline particles, free from defects and grain boundaries, and free from elastic interactions with the substrate. We obtain single particle loading and unloading isotherms for particles ranging from approximately 10 nm to 100 nm, allowing us to address outstanding questions about the nature of phase transitions and surface energy effects in zero-dimensional nanomaterials. We find that hydrogen loading and unloading isotherms of single crystals are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that thermodynamic phases do not coexist in single-crystalline nanoparticles, in striking contrast with ensemble measurements of Pd nanoparticles. Then, we extend our single-particle techniques to explore the hydriding properties of polycrystalline and multiply-twinned nanoparticles, including Pd nanorods and icosahedra. In contrast to single crystalline nanoparticles, these particles exhibit sloped isotherms and narrowed hysteretic gaps. Based on these results, we develop a model to deconvolve the effects of disorder and strain on the phase transitions in nanoscale systems. Lastly, we describe techniques to generate high-resolution plasmon-EELS (and hence phase) maps of nanoparticles. These mapping studies promise unprecedented insight into the internal phase of nanomaterials, and can be complemented with diffraction and dark-field imaging studies. We will discuss how these results could be used to interpret the thermodynamics of Li-ion insertion in battery electrodes, hydrogen absorption in state-of the-art metal hydride catalysts, or ion exchange reactions in quantum dot syntheses.

Coherent control over the propagation of light well beyond the diffraction limit has been so far realized using metallic nanostructures which sustain surface plasmons. Besides the ability to control light propagation at nanoscales, localization of light in small volumes has found a large variety of applications in harmonic generation and optical circuitry. A critical point of field localization at nanoscales is the coupling efficiency of the optical far-field radiation into the volume fraction of interest. It has been proposed that tapered metallic waveguides can offer a highly efficient control over the delivery of energy at their apex through an adiabatic reduction of the plasmon velocity, which asymptotically leads to field localization and stopping.
As a model system we use single-crystalline gold tapers, whose particularly smooth surfaces eliminate surface plasmon localization and scattering losses along the taper shaft. We experimentally investigate the energy bands of these tapers as plasmonic light trappers by electron energy-loss spectroscopy (EELS) and energy-filtering transmission electron microscopy (EFTEM). The experiments were conducted at the Zeiss SESAM microscope operated at an accelerating voltage of 200 kV. The microscope is equipped with an electron monochromator and the MANDOLINE energy filter.
The striking evidence is that plasmons are excited both at the apex and along the taper shaft. By moving the electron beam along the taper shaft, we probe the evolution of the optical modes versus the distance from the apex. At the very apex, we resolve an extremely broadband spectral feature covering the whole energy range from 0.5 eV to 2.0 eV. By moving the electron beam away from the apex, further resonances are excited by the electron beam. Using the finite-difference time-domain technique with an embedded electron probe [1], the experimental EELS patterns are perfectly reproduced in our simulations, including both the broadband spectrum at the apex and the occurrence of higher-order resonances.
In summary, our results show higher-order angular momentum modes with unexpected resonance dispersions. These findings suggest that the three dimensional gold tapers can be used for an efficient coupling of the far-field to the near-field which makes them particularly interesting for future quantum-optical applications [2].
References
[1] N. Talebi et al., New J. Phys.15 (2013) 053013
[2] N. Talebi et al., submitted (2015)
Acknowledgements
NT gratefully acknowledges Alexander von Humboldt Foundation for the research scholarship. Financial support by the European Union Seventh Framework Program [FP/2007/2013] under grant agreement no 312483 (ESTEEM2), the European Union project CRONOS (Grant number 280879-2), the Deutsche Forschungsgemeinschaft (SPP1391, DFGLi580/8-1, INST184/107-1) and the Korea Foundation for International Cooperation of Science and Technology (Global Research Laboratory project, K20815000003) is gratefully acknowledged.

Spatially resolved electron energy loss spectroscopy (EELS) with a scanning transmission electron microscope (STEM) enables nanometer scale mapping of surface and volume plasmon modes across nanostructures (NSs). Volume plasmons (VPs) are high energy longitudinal oscillations of the free-electron gas that propagate through the bulk of a material and can be excited by fast electrons. They have energies in the vacuum UV so may be useful for nano-optics in this energy range. These excitations also have sub-femtosecond lifetimes and propagation distances below 50 nm, necessitating high-resolution nanofabrication techniques to control and confine them. Controlling surface plasmon (SP) resonances is also of interest; high energy SP materials have applications in UV nano-optics and UV surface enhanced spectroscopy. It has been previously demonstrated that plasmon energies in chemically synthesized NSs undergo increasing blue shifts with decreasing NS diameter.¹ However, VP energy shifts in Al NSs have not been previously reported.² Al is of interest not only because is it earth-abundant and low-cost, but also because its negative ε₁ and low ε₂ in the UV make it an excellent material for UV plasmonics. Its high plasma frequency of 15 eV allows for SPs in the UV range, and its sharp VP resonance peak indicates a long VP lifetime. The ability to control plasmon resonances in Al NSs can lead to development of UV nano-optical components, but without a strategy for SP and VP mapping on the nanoscale, it is not possible to correlate the spatial distribution of plasmon modes to NS shape and size.
EELS has been used to map SP modes in lithographic Au and Ag NSs, yet this technique has not been used to map VPs, nor has it been demonstrated in lithographic Al.^3,4 Lithography is not as limiting in shape or resolution as chemical synthesis; without lithographic control, investigation of plasmon confinement in complex NSs with high plasmon energies would not be possible.
Our work provides a top down method for controlling and mapping the VP and SP energy in Al NSs of 6-120 nm in diameter. We used EELS to map the SP and VP resonant energies and amplitudes across Al nanodisks. EELS spectra were acquired with an aberration-corrected Hitachi HD2700C STEM with angstrom-scale spatial resolution and 0.35 eV resolution in energy. We observed a VP mode and dipolar, quadrupolar, and hexapolar SPs and SP polaritons. Plasmon peaks were isolated by background subtraction and fit to a Gaussian to find the intensity and energy of the plasmon modes across the disks. Numerical simulations and analytical methods verified these plasmon modes. The boundary effect, in which the VP mode is modified at an interface due to interaction with the SP, is clearly demonstrated here.
1 M. Li et al. Solid State Commun. 152 (16) 1564-1566 (2012)
2 P. Batson. Solid State Commun. 34 477-480 (1980)
3 F.P. Schmidt et al. Nano Lett. 14 (8) 4810-4815 (2014)
4 A.L. Koh et al. Nano Lett. 11 (3) 1323-1330 (2011)

Noble metal nanoparticles have been widely investigated for enhancement of optical processes, such as Raman scattering, frequency conversion, luminescence, and absorption. Local enhancement of these processes occurs through the highly concentrated optical fields that accompany excitation of plasmon resonances in the metal nanoparticles. Significant enhancement can be obtained globally, and not just locally, for processes such as Raman scattering and harmonic generation that depend nonlinearly on local intensities. The situation is less clear for processes that depend linearly on intensity, such as absorption - the basis for plasmon-enhanced solar cells - because the metal nanoparticles simply redistribute optical energy from one spatial location to another. Further complicating matters is the fact that enhanced recombination will necessarily compete with enhanced absorption.
In order to understand these tradeoffs, we performed calculations for a simplified model of a dye-sensitized solar cell. A sensitizer, such as a molecule or quantum dot, is coupled to a plasmonic metal nanostructure through its near field, and electrons in the excited state of the sensitizer can transfer into the states of a semiconductor electrode. Using a model-Hamiltonian approach, we found that there is an optimal coupling between the sensitizer and the plasmons that maximizes the electron-transfer efficiency. This optimum occurs because the coupling modifies not only the amount of incident light absorbed by the sensitizer, but also the dynamics of electron transfer from the sensitizer to the semiconductor.
If the coupling between a molecule or quantum dot and a plasmonic metal nanostructure is strong enough, it will lead not only to enhanced absorption and emission, but also to strongly modified spectral and dynamical properties. For example, realistic classical calculations show that a single colloidal quantum dot has the potential to induce nearly complete transparency in the much stronger absorption and scattering spectrum of a metal nanostructure. This can be understood as the result of Fano-like interference between the dipoles of the quantum-dot exciton and the plasmon. Quantum-mechanical calculations show that excitation with short laser pulses con remove or even reverse this induced-transparency dip by changing the interference from destructive to constructive. This illustrates the possibility of dynamically controlling the spatiotemporal flow of optical energy on the nanoscale, suggesting that larger systems of coupled metal nanoparticles and quantum dots could be designed for optimal harvesting of solar energy.

Plasmonics has long suffered a reputation as the province of lossy structures for nanophotonic applications --to be reluctantly tolerated and carefully managed. However plasmon decay processes harbor interesting opportunities for new photonic energy conversion structures and mechanisms. First principles calculations of decay rate for surface plasmon polaritons via and phonon-mediated processes indicate that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. In particular, the onset of interband transitions, occurring in the visible regime for the noble plasmonic metals (Au, Cu, Ag), and is expected to significantly modify the hot-carriers distribution. Results of experiments that test these calculations, using GaN/noble-metal nanoscale heterostructures, will be presented. Resonant excitation of periodic antenna arrays coupled to waveguides in a guided mode resonance configuration give rise to narrowband absorption with resonant absorption wavelength defined by the array geometrical parameters. When these antennas are defined as thermoelectric junctions on a low thermal conductivity membrane substrate, the temperature rise associated with resonant optical absorption induces a measurable thermoelectric potential. These resonant thermoelectric antennas have potential to serve as component elements of hyperspectral detectors with sensitivity over a very broad spectral range. Near-resonant excitation of metallic antennas induces a plasmoelectric potential, distinct from the thermoelectric potential, due to changes in carrier concentration in response to excitation. We describe several experiments to measure plasmoelectric potentials in metal nanostructures.

The engineering of localized surface plasmon resonance (LSPR) near-field interactions is complicated by the need to position neighboring resonators with nanometer-scale precision. For isotropic dielectric environments, inter-particle separation distances far smaller than nanoparticle size are required to maximize local field intensity. Here, we show that the anisotropic dielectric environment inherent to 1-D nanowires can enhance near-field coupling by more than 5x. We demonstrate this effect by measuring the far-field spectral response of Si nanowires containing two phosphorus-doped segments with in situ infrared spectroscopy. Analysis of LSPR peak position as a function of resonator separation (between 0 and 130 nm) according to the “plasmon ruler” equation enables determination of a near-field decay length scaling parameter. While conventional noble metal resonators in isotropic environments commonly exhibit decay parameters near 0.2-0.3, we find that phosphorus-doped segments in Si nanowires yield a decay parameter exceeding 1.4. Discrete dipole approximation (DDA) calculations, analyzed in the same manner, show that the near-field interactions of doped-Si resonators are strongly dependent on the resonator aspect ratio, carrier density, and nanowire material (e.g., Si, Ge, GaAs, etc.). Our findings show that strong near-field interactions, and thus extreme light confinement, is possible with relaxed lithographic precision, thus simplifying the use of LSPR coupling interactions in next generation chemical sensors, photodetectors, and optical interconnects.

Doped colloidal metal oxide nanocrystals (MONCs) enjoy scientific attention for their ability to support tunable localized surface plasmon resonance (LSPR). This phenomenon can potentially be harnessed for applications including surface enhanced Raman spectroscopy (SERS), chemical sensing, and dynamic plasmonic electrochromism in the near- and mid-infrared spectrum. However, if doped MONCs are ever to be successfully used for applications like SERS, we must be able to carefully tune LSPR properties such as peak energy, line width, and line shape. Control over these properties requires additional and thorough understanding of how the interplay between synthetic control, nanocrystal structure, carrier dynamics and scattering dictates LSPR properties.¹ We have already examined and manipulated these relationships in tin-doped indium oxide (ITO) nanocrystals,² where we found that synthetic conditions strongly influence ionized impurity scattering and thus the LSPR line shape for nanocrystals of otherwise similar size, shape, and composition.
This discovery inspired us to develop strategies to intentionally minimize ionized impurity scattering in doped MONCs through rational dopant selection and defect engineering. Recent literature shows that doping metal oxides with cations that are well matched in radius to the host cation (e.g. Dy-doped CdO),³ minimizes concentration of oxygen vacancies and local strain fields, resulting in low ionized impurity scattering and high mobility. In this light, we synthesized and characterized a novel doped metal oxide nanocrystal, Ce-doped indium oxide (CIO), where Ce⁴⁺ is a good match in ionic radius to In³⁺. In nanocrystal form, CIO does indeed display mid-IR LSPR absorption with exceptionally narrow line widths and high quality factors, indicating low ionized impurity scattering and high electron mobility. Furthermore, the LSPR absorption energy is easily tuned with Ce concentration, and multiple nanocrystal sizes and shapes can be accessed synthetically. The quality factors achieved here are much greater than the other mid-IR plasmonic MONCs such as aluminum-doped zinc oxide and ITO, which points to their potential as candidates for mid-IR devices like SERS substrates, dynamic notch filters, and heat scavengers.
¹ S. D. Lounis, E. L. Runnerstrom, A. Llordés, and D. J. Milliron, J. Phys. Chem. Lett., 2014, 5, 1564-1574.
² S. D. Lounis, E. L. Runnerstrom, A. Bergerud, D. Nordlund, and D. J. Milliron, J. Am. Chem. Soc., 2014, 136, 7110-7116.
³ E. Sachet, C. T. Shelton, J. S. Harris, B. E. Gaddy, D. L. Irving, S. Curtarolo, B. F. Donovan, P. E. Hopkins, P. A. Sharma, A. L. Sharma, J. Ihlefeld, S. Franzen, and J.-P. Maria, Nature Mater., 2015, 14, 414-420.

We report an investigation of surface plasmon resonances (SPRs) in ZnO/MgO core/shell nanowires decorated with Ag nanoparticles, combining the techniques of cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) within a scanning transmission electron microscope (STEM). The combination of CL and EELS within a STEM represents a powerful characterization technique, due to the fact that CL is a measure of decay and EELS is a measure of excitation allowing us to use both to track optical excitations throughout their whole lifetime. Within the STEM, nanometer scale mapping of SPRs using both spectroscopies is possible, and the resulting comparison of the two-dimensional plasmon maps allows us to thoroughly analyze plasmonic behavior within the system in all three spatial dimensions from purely experimental observations. Our analysis is then confirmed by measuring the approximate geometry of the nanoparticle in three dimensions and performing finite-difference time-domain simulations. The rich complementarity of STEM-CL and STEM-EELS provides an analysis of phenomena that are unattainable from each technique individually, and one that can be applied to complex functional optical structures with nanometer precision.
This work was funded by NSF-EPS-1004083, NSF-TN-SCORE, DE-FG02-09ER46554, DE-FG02-01ER45916 and the DOE Office of Science BES Materials Science and Engineering Division.

We investigate the effects of generation of hot plasmonic carriers and heat in metal nanostructures. The problem of generation of energetic plasmonic electrons is treated using the quantum mechanical approaches based on the kinetic DFT and the equation of motion of the density matrix [1-4], whereas heat release from optically-excited plasmonic nanocrystals is calculated classically [5,6]. The energy distribution of optically-excited plasmonic carriers is very different in metal nanocrystals with large and small sizes. In large nanocrystals, most excited carriers have very small excitation energies and the electron distribution resembles the case of the plasmon wave in bulk. For metal nanocrystal with smaller sizes (less than 10nm) or in nanostructures with hot spots [3,7], the energy distribution of hot carriers becomes flat and has a large number of carriers with high energy [1-4,7]. Therefore, smaller nanocrystals and nanostructures with plasmonic hot spots are preferable for injection of plasmonic carriers into semiconductors or into molecules on a surface. The physical reasons for the above properties are the non-conservation of linear momentum in a nanocrystal and the plasmonic field-enhancement effect. The geometry, type of metal, and orientation of the external electric field are important to obtain high quantum efficiencies of generation and injection of plasmonic electrons [1-4]. Other important properties and limitations are the following: (1) Intra-band transitions are preferable for generation of energetic electrons and dominate the absorption for relatively long wavelengths (approximately >600 nm) [1], (2) inter-band transitions efficiently generate energetic holes in the d-band of gold [1,8] and (3) the carrier-generation and absorption spectra can be significantly different [1-3]. The d-band hole generation can be used for efficient plasmonic photochemistry [8]. Nanostructures with strong plasmonic hot spots generate unusually large numbers of energetic electrons, which can be observed using the ultra-fast transient spectroscopy [7]. The results obtained in this study can be used to design a variety of plasmonic nanodevices based on hot electron injection and heat generation for photo-catalysis, photo-detectors and solar-energy applications.
[1] A.O. Govorov, H. Zhang, V. Demi, and Y. K. Gun&’ko, NanoToday, 9, 85 (2014).
[2] A. O. Govorov, H. Zhang and Y. K. Gun'ko, J. Phys. Chem. C 117, 16616 (2013).
[3] H. Zhang and A. O. Govorov, J. Phys. Chem. C118, 7606 (2014).
[4] A.O. Govorov and H. Zhang, J. Phys. Chem. C, 119, 6181 (2015).
[5] A. Govorov and H.H. Richardson, NanoToday 2, 20 (2007).
[6] S. Baral, A. Green, M. Livshits, A. Govorov and H.H. Richardson, ACS Nano 8, 1439-1448 (2014).
[7] H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, Lucas V. Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology, accepted.
[8] L. Weng, H. Zhang, A. O. Govorov, and M. Ouyang, Nature Communications 5, 4792 (2014).

Plasmonic sensitivity of nanomaterials towards surrounding dielectric change is an important criterion to assess its capabilities for the biological and chemical molecule detection. It has been seen that plasmonic sensitivity can be altered by changing the nanoparticle (NP) geometry from simple (spherical) to more complicated ones (triangle, cube or star-like etc.). There are various plasmonic modes excited in these rather complicated structures at different energies which are suspected to show distinct behavior towards dielectric change of the surrounding. Therefore, it seems critical to study the dielectric sensitivity of individual plamonic mode to improve the molecule detection limits.
In the present study, we chose electron energy-loss spectroscopy (EELS) technique in a monochromator equipped scanning transmission electron microscope (STEM) as a tool to excite various plasmonic modes in silver (Ag) nanotriangles and study the dielectric sensitivity of each mode. To perform these measurements, Ag nanotriangles were synthesized and then coated with 20 nm thin films of HfO₂ (ε = 25) and TiO₂ (ε = 86). The experiments were benefited by the in-built monochromator in the STEM which improved the energy resolution (0.15 eV) drastically for quantitative study. This is found that three distinct modes at (i) corners, (ii) edge center and (iii) entire edge are excited in the Ag nanotriangles which show exclusive dependency on triangle size. The dielectric sensitivity of each plasmon mode is also quite distinct where in-plane dipole mode show higher sensitivity compare to out-of-plane dipole mode as well as quadripolar modes. This study practically puts STEM/EELS technique to do sensitivity measurements which is yet an untouched territory for electron microscopy community, and shows that such studies can be done with high spatial and energy resolution.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Metallic nanoantennas have been used as nanophotonic detectors of infrared and visible radiation [1] [2]. The extension of these elements to the ultraviolet (UV) range has not been satisfactory due to the poor optical absorption showed by metals at these frequencies. The low values of electrical conductivity of metals at UV frequencies compromise the generation of currents along the resonant geometries. In change, several non-conventional materials show a larger value of electrical conductivity, increasing notably the absorption of nanoantennas in the UV [3]. This electrical conductivity increment is more noticeable in liquid semimetals as Bismuth or Gallium. Another way to improve optical absorption of these resonant elements is to arrange them with a high spatial density of semimetal nanoantennas.
In this contribution we evaluate numerically, using multiphysics simulation, the light to heat conversion performance of a vertical nanoantenna arrangement embedded in a dielectric matrix. The so-obtained UV metamaterial enabling strong electromagnetic plasmonic absorption and heating effects. The use of nanoantennas allows a polarization and frequency selectivity that can be adequate to generate ultraviolet sensors. These selectivities are strongly related with the shape of the resonant elements. Furthermore, since the vertical antennas are embedded within a robust dielectric matrix, this arrangement allows to change from solid to liquid phase maintaining the nanoantenna geometry. This is possible for materials as Bismuth or Gallium which show a lower melting temperature than dielectric substrates [4]. This phase transition makes the metamaterial active upon control of temperature.
[1] L. Novotny and N. van Hulst, “Antennas for light,” Nat. Photon. 5 (2), 83-90, (2011).
[2] A. Cuadrado, E. Briones, F.J. González, J. Alda, “Polarimetric Pixel using Seebeck Nanoantenna”. Opt Exp, 22, No 11, 13835-13845, (2014).
[3] J.Toudert, R.Serna, and M. Jiménez de Castro, “Exploring the optical potencial of nano-Bismuth: Tunable surface Plasmon resonances in the near Ultraviolet-to-Near Infrared range”, J. Phys. Chem. C, 116 (38), pp 20530-20539, (2012).
[4] M. Jiménez de Castro, F. Cabello, J. Toudert, R. Serna and E.Haro-Poniatowski, “Potential of bismuth nanoparticles embedded in a glass matrix for spectral-selective thermo-optical devices” Appl. Phys. Lett. 105, 113102 (2014).

Hot electrons and holes can be generated when high-energy photons are absorbed in a metal. These carriers have excess kinetic energy, which is dissipated through sample heating. Recovery of hot carriers is important for detectors, sensors, and power convertors; however, the design and implementation of these devices is difficult due to strict requirements on the device geometry, angle of illumination, and incident photon wavelength. Here, we present a simple, angle-independent device based on transparent conducting electrodes that allows for the generation and collection of hot carriers [1]. We show experimental photocurrent generation from both monochromatic and broadband light sources, show uniform absorption for incident illumination at up to 60° from the surface normal, and find an expected open-circuit voltage in the range 1.5-3.0 V. Under solar illumination, the device is 1 order of magnitude more efficient than previous metal-insulator-metal designs, and power conversion efficiencies >10% are predicted with optimized structures. I will also discuss our recent work in IR detection based on sub-bandgap photon absorption. This approach opens the door to new hot carrier collection devices and detectors based on transparent conducting electrodes.
[1] Tao Gong and Jeremy N. Munday, Nano Lett., 15, 147-152 (2015)

A chiral object, i.e., one that is distinguishable from its mirror image, interacts differently with photons of left-hand (+) and right-hand (-) circular polarizations. The difference in the dissipation part is known as circular dichroism (CD), which is usually observed for organic molecules with one or more chiral centers and is widely used to probe their molecular stereometry. On the contrary, CD is generally not associated with surface plasmons as conventionally synthesized noble metal nanoparticles usually have reflection symmetry. This talk will cover an intriguing case in which non-chiral plasmonic nanoparticles are surrounded by a CD medium that give surface plasmon chirality. It will be demonstrated that the subtle difference in the chiral medium&’s response to photons of (+) and (-) polarizations could lead to significant CD at the nanoparticles&’ plasmon resonance band that is far apart from the medium&’s native CD band. Experimentally, the chiral medium is provided by decorating the plasmonic nanoparticles with a monolayer of DNA molecules that have chirality arising from its individual nucleobases and helix structure. It is found that the surface plasmon-induced CD is very sensitive to the nonchiral plasmonic nanoparticles&’ shape and chemical composition. We identify gold/silver core/shell nanocubes as most effective for providing the largest CD enhancement in the near-visible region, while DNA&’s native CD signal appears at much higher energy. On one hand, the discovered phenomenon opens novel opportunities in ultrasensitive probing of chiral molecules and for novel optical nanomaterials based on the chiral elements. On the other hand, it presents a new question on how to quantitatively understand the plasmon-induced CD from a theoretical perspective. In this talk we will present a full, Mie-like, solution to the case of spherical particles and discuss the possible extension to nanoparticles of more complicated shapes.

There has recently been much interest in the extraction of hot electrons generated from non-radiative surface plasmon decay as this process can provide additional bandwidth for both photodetectors and photovoltaics. The hot electrons are typically injected into semiconductors over a Schottky barrier, enabling the generation of photocurrent with below bandgap photon illumination. MoS₂, a two-dimensional semiconductor, is an attractive hot electron acceptor due to its ability to provide internal photoamplification. Here, we investigate the injection of hot electron into bilayer MoS₂ that has been integrated with plasmonic resonators. In addition to demonstrating photoresponsivity reaching 5.2 A/W with below bandgap photon excitation, we provide experimental evidence showing that the source of the photocurrent is hot electron injection opposed to photovoltaic or photothermoelectrical effects. We demonstrate a photogain reaching 10⁴ and an injection efficiency that is comparable with Si-based hot electron injection photodetectors. With proper engineering of the trap states in the system this technique can be further implemented in applications including photodetection and memristor devices.

Due to thermalization of charge carriers created by high-energy photons, silicon-based solar cells show a poor yield in the UV-blue range of the solar spectrum [1]. The deposition of a thin layer of a quantum-cutter (QC) material on top of silicon-based solar cells seems to be a promising solution to reduce these losses. In this case each incident UV-blue photon is converted into two near infra-red (NIR) photons both able to generate one electron-hole pair.
The fluoride materials appropriately doped by rare earth (RE) ions (Pr³⁺, Ce³⁺, Nd³⁺) ions are promising for QC materials especially due to their wide-band gap and low phonon energy compared to oxides [2].
One of the way how to enhance the absorption in UV and luminescence properties of RE ion is using plasmonic behavior of metallic nanoparticles (NPs) Most of the plasmonic research has so far focused on “classical” materials Ag and Au. Beyond this noble metals however, other, less used metals exist, whose specific characteristic could significantly bring new functionalities. For example Al NPs yield an localised surface plasmon (LSP) resonance within the deep ultraviolet optical range [3] and by tuning the size of the NPs the resonance frequency could be shifted form UV up visible spectral range. Those optical properties could perfectly suite QC demands. Despite potential and low cost of Al, the exploitation of its plasmonics is very recent and still facing both scientific and technical challenges. One of the crucial problems is the degradation of plasmonic properties induced by aluminum rapid oxidation. This problem could be solved by using fluorides matrices, which will embedded the Al NPs
In our work we demonstrated successfully fabrication of Al NPs embedded in Pr³⁺:CaF₂ fluoride matrices by means of combination of two PVD deposition techniques: Pulsed Laser Deposition (PLD) and Electron Beam Evaporation (EBE). This technique is very flexible and allows nanometrtic size control of NPs distribution of different metal as well as doping of the films by RE ions. The fabricated layers were analyzed by spectral ellipsometers (M2000 and VUV-Vase) in the broad spectral range from 146 nm to 1000 nm wavelength. The analysis of the measured date revealed an absorption band corresponds to LSP resonance from 220 nm to 380 nm wavelength depends on the NPs size. The results were compared and discussed with the results of analysis structural properties performed by methods (AFM, SEM, HRTEM and XRD). The enhancement of luminescence and down conversion properties of Pr³⁺:CaF₂ by means of LPS resonance will be presented.
[1] B. S. Richards, Sol. Energy Mater. Sol. Cells 90, 1189 (2006).
[2] A. Guille, A. Pereira, A. Bensalah-Ledoux, B. Moine, M. Novotny#769;, J. Buli#769;#345;, P. Fitl, J. Lancok, J Appl Phys, 114 (2013) 203509
[3] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander N.J. Halas, ACS NANO 8 (2014) 834

Nanostructures composed of noble metals (i.e. Au, Ag and Cu) are known to absorb visible light leading to the generation of localized surface plasmons (LSPs). This collective oscillation of free electrons can eventually decay into what are known as hot electrons. These energetic electrons have been shown to participate in chemical reactions when irradiated with a flux of low intensity visible photons. One such application is the dissociation of hydrogen which has been shown to occur via the transfer of hot electrons from Au to physisorbed H₂ under visible illumination creating a transient negative H₂^- ion and subsequent dissociation. We have leveraged hot electron induced dissociation to develop fast optical H₂ sensors based on changes in optical transmission and a spectral shift in the localized surface plasmon resonance associated with thin films of nanoparticles. The mechanism of sensing, however, is still not fully understood.
One hypothesis is that the formation of a metastable gold hydride layer follows the dissociation event, which leads to a change in the dielectric constant of the Au nanostructures, thereby causing the optical transmission to change. The results of Density Functional Theory and in situ spectroscopic ellipsometry provide support for the proposed mechanism and shed light on the effects of the interaction of Au with atomic hydrogen. These results open up new avenues towards the design of efficient plasmonic catalysts based on surfactant free nanostructures, for example nanoparticle alloys (Ag-Au) and the activation of other chemical species e.g., CH₄.
Reference
(1) Seeing Is Believing: Hot Electron Based Gold Nanoplasmonic Optical Hydrogen Sensor, Sil, D.; Gilroy, K. D.; Niaux, A.; Boulesbaa, A.; Neretina, S.; Borguet, E. ACS Nano 2014, 8, 7755.
(2) How does Atomic Hydrogen Interact with Au under Optical Excitation? D. Sil.; Gilroy, K. D.; Glor, E.; Fakhraai, Z.; Lane, Christopher.; Barbiellini, B.; Bansil, A.; Neretina, S. and Borguet, E. (In Preparation 2015)
(3) Optical Nanopalsmonic Detection of Methane on Surfactant -Free Au-Ag Nanostructures, D. Sil.; Sylla, S.; Gilroy, K. D.; Neretina, S. and Borguet, E.(In Preparation 2015)

Well monodispersed metal nanoparticles are widely used to study the organization into ordered or monodispersed assemblies, in order to enhance the resulting optical properties. The gold bipyramids is an example of a highly monodispersed non-spherical nanoparticle which is acquiring more significance as a practical seed for creating different and still highly monodispersed particles. The optical study of dimers of bipyramids shows the large redshifts expected when two longitudinal plasmons interact in line with one another. The organization of rod-like nanoparticles in the slightly corrugated surface of diblock copolymers leads to very high alignment due to strong capillary forces. The structures are then used for enhancing the optical fluorescence or the nonlinear optical response.

As a fundamental electrostatic limit, the space charge limit (SCL) for photocurrent is a universal feature and of paramount importance in organic semiconductors with unbalanced electron/hole mobility and high exciton generation. Here, we propose a new concept of plasmonic-electrical effect to manipulate the electrical properties (photocarrier generation, recombination, transport, and collection) of semiconductor devices with the help of plasmonically induced light redistribution. As a proof-of-concept, organic solar cells (OSCs) incorporating metallic planar and grating anodes are systematically investigated for normal and inverted device structures. Interestingly, although strong plasmonic resonances induce abnormally dense photocarriers around a grating anode, the grating-inverted OSC is exempt from space charge accumulation (limit) and degradation of electrical properties in contrast to the planar-inverted and planar-normal ones. It is because abnormally redistributed holes by the plasmonic-electrical effect, despite of the typically low mobility of holes, shorten hopping path of low mobility holes to reach the grating anode. Consequently, the work contributes to the evolution of device architecture to break the SCL with detailed multiphysics explanations. Moreover, the proposed plasmonic-electrical concept will open up a novel way to manipulate both optical and electrical properties of organic semiconductor devices for optoelectronic applications.

Plasmonic engineering of metallic nanoparticles (MNPs) have attracted considerable interest in optoelectronics due to their unique optical and electronic properties [1-3]. Especially, localized surface plasmon resonance (LSPR) effect induced by MNPs has been widely used to improve the electroluminescence (EL) efficiency of organic light-emitting diodes (OLEDs) through interaction with excited species in the emitting layer [1]. However, to date, there have been few reports on the enhancement of the overall EL characteristics including the carrier injection efficiency, luminance, current efficiency, and power efficiency of devices when adopting the LSPR effect of MNPs in OLEDs.
Here, we show the overall EL efficiency-enhanced OLEDs by introducing silver nanoparticles (Ag NPs) in the hole injection layer. The device structure is composed of indium tin oxide (ITO)-coated glass/PEDOT:PSS with and without Ag NPs/Alq₃ (100 nm)/LiF (1 nm)/Al (100 nm). First, we did steady-state photoluminescence (PL) and transient PL decay profile measurements of control samples without Ag NPs and plasmonic samples with Ag NPs to compare the optical characteristics in the ITO-coated glass/PEDOT:PSS with and without the Ag NPs/Alq₃ (100 nm) sample structure. In this case, the PL intensity and exciton lifetime of the plasmonic sample increased ~30% and decreased 2.15 ns, respectively, compared to the control sample. This reduced exciton lifetime can be explained by the spontaneous emission rate enhancement of the exciton according to the Purcell effect, resulting in the enhanced PL intensity of the plasmonic samples. In the case of the device, the operating voltage of the plasmonic device was reduced by 0.28 V compared to that of the control device because of an inhomogeneous local electric field caused by the Ag NPs, resulting from the increased carrier injection. Moreover, the luminance, current efficiency, and power efficiency of the plasmonic device at 25 mA/cm² was 15%, 15%, and 19% higher than those of the control device, respectively, without any angular dependency due to the optical microcavity effect induced by the introduced Ag NPs.
In conclusion, we have shown that the resonant coupling between LSP by Ag NPs and excited species in the emitting layer can increase the spontaneous emission rate of the exciton, simultaneously improving the PL and EL efficiency. We believe that this plasmonic effect can have a great impact on the field for highly efficient organic optoelectronic devices.
Acknowledgments
This work was supported by a grant from the National Research Foundation of South Korea (NRF) funded by the South Korean government (MSIP) (CAFDC/Kyung Cheol Choi/No. 2007-0056090).
References
[1] K.Y. Yang, K.C. Choi, C.W. Ahn, Applied Physics Letters. 94 (2009) 173301.
[2] S.M. Lee, D. Kim, D.Y. Jeon, K.C. Choi, Small. 8 (2012) 1350-1354.
[3] J.B. Shin, S.M. Lee, M. Kim, D. Kim, D.Y. Jeon, K.C. Choi, IEEE Photonics Technology Letters. 25 (2013) 2342-2345.

Hybrid nanoparticles combine disparate materials onto a single nanosystem thus providing a powerful approach for bottom-up design of novel architectures. Beyond the fundamental development in synthesis, the interest in HNPs arises from their combined and often synergetic properties exceeding the functionality of the individual components. These ideas are well demonstrated in hybrid semiconductor-metal nanoparticles, which are the focus of this talk. The synergistic optical and chemical properties of hybrid nanoparticles resulting in light-induced charge separation and charge transfer, allow photocatalytic activity which can promote surface chemistry redox reactions, and open a pathway for converting solar energy to chemical energy stored in a fuel.
We will report on the effects of the surface coating and the co-catalyst metal size on the photocatalytic function of metal tipped semiconductor nanorods as a model hybrid nanoparticle system. Both tested parameters were found to influence the photocatalytic efficiency and charge transfer dynamics. Different types of surface coatings including various types of thiolated alkyl ligands and different polymer coatings were explored. A significant increase in the apparent quantum yield was detected for polyethylenimine (PEI) coated NPs compared to other surface coatings, such as L-Glutathione and mercaptoundecanoic acid. This pronounced increase in the photocatalytic efficiency is attributed to the improved surface passivation by the PEI polymer minimizing the surface trapping of charge carriers, compared to alternative surface coatings. Furthermore, investigation of different sizes of Au metal component reveal an optimum behavior in which an intermediate size of metal domain provides the optimal hydrogen evolution rate for the photocatalytic water reduction. Ultrafast transient absorbance measurements along with steady-state emission and time-resolved spectral measurements support these observations. A model was devised to capture the essential effects of the size of the metal tip on the photoctalytic efficiency.
The understanding of the effects of the hybrid nanosystems properties on the photocatalytic processes contribute to the rational design of hybrid nanostructures in photocatalytic applications.

Understanding charge separation and recombination processes and developing materials that allow efficient/effective manipulation of charge carriers with nanoscale precision are of fundamental importance in advancing next-generation electronics, optoelectronics and energy technologies. As semiconductor heterostructures have enabled today&’s optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities in colloidal quantum dots for high-performance, solution-processable devices and systems. With anisotropic shapes that can be exploited for assembly, charge carrier manipulation and optical anisotropy, incorporating heterojunctions in colloidal semiconductor nanorods presents a promising direction. However, there are challenges in utilizing some of the unique features of semiconductor heterojunction nanorods. For example, staggered band offset in type-II heterojunction nanorods can provide fast and directional photoinduced charge separation but the small absorption cross-section of charge separated state is less than ideal for photovoltaic applications. Incorporating plasmonic materials into devices that utilize heterojunction nanorods may be a potential solution to overcome some of these challenges. Our recent efforts to integrate semiconductor heterojunction nanorods with plamonic nanoparticles within photovoltaic and optoelectronic device settings will be presented.

Plasmonic nanocavity arrays with strong coupling between nanoparticles can both trap light in the plane of the array and localize optical fields around each nanoparticle. The optical confinement in a nanoscale volume is useful for plasmon-based lasing and sensing applications. Plasmon lasers can support ultrasmall mode confinement and ultrafast dynamics with device feature sizes below the diffraction limit. However, most plasmon-based lasers rely on solid gain materials, e.g., inorganic semiconducting nanowire or organic dye in a solid matrix, that preclude the possibility of dynamic tuning. In addition, most plasmon-based lasers show bi-directional lasing emission or emission with limited far-field directionality. Here we show that arrays of gold nanoparticles surrounded by liquid dye molecules exhibit lasing emission that can be modulated by the dielectric environment. By integrating gold nanoparticle arrays within microfluidic channels and flowing in liquid gain materials with different refractive indices, we achieve dynamic tuning of the plasmon lasing wavelength. We further demonstrate that two-dimensionalplasmonic crystals (gold or silver) incorporated with dye molecules exhibit lasing in a single emission direction. Beyond tunable lasing, we explore the structure engineering of a subwavelength unit and show how arrays ofplasmonic hetero-oligomers consisting of strong plasmonic materials (gold) and reactant-specific elements (palladium) reveal new architectures for enhanced hydrogen gas sensing.

Here we demonstrate the first integration of plasmonic nanoantennas in a polymer light-emitting field-effect transistor, enabling direct control, via the applied voltage, over both the spatial positon of the electron-hole recombination zone as well as the number of recombination events. By simply changing the bias we can switch repeatedly between no coupling and efficient coupling with the nanoantennas and thus enhanced emission in a single device.
The charge transporting and emitting semiconducting polymer DPPT-BT has a small HOMO-LUMO gap of 1.2 eV, which enables balanced electron and hole injection with carrier mobilities of up to 0.3 cm²V^-1s^-1. Its photoluminescence yield in the near infrared (emission wavelength 1000-1300 nm) is only 0.05 % and thus efficient enhancement based on the Purcell effect is both wanted and possible. Colloidal gold nanorods were synthesized to match the emission wavelength of the polymer and acted as plasmonic nanoantennas. They were directly integrated into the desired parts of the transistor channel without affecting the charge transport. For a relatively low density of gold nanorod antennas an average electroluminescence enhancement of three was found, which was consistent with the observed photoluminescence enhancement and FDTD simulations. The achieved voltage control over the coupling of emission to the gold nanorods demonstrates the potential of this type of planar electroluminescent device for plasmonic waveguide integration and optoelectronic circuits.

In recent years, plasmonic nanomaterials have shown their promising applications in photovoltaic devices. Light trapping for enhancement of absorption and device performance can be achieved by integration of plasmonic nanomaterials into solar cells. Perovskite absorbers have attracted substantial interest as they have many advantages; one of these advantages is the effective light absorption in well fabricated solar cells. However, the absorption near the band gap can be further improved by nanooptical concepts, which also allow thinning the absorbers. We previously developed a versatile low temperature spray chemical vapor deposition (spray-CVD) process to deposit size- and density-controlled Ag nanoparticles (Ag NPs) on different kinds of substrates as well as combined with extremely thin absorbers. In this work, modelling and experimental integration of Ag NPs into thin-film perovskite solar cells and comprehensive characterization will be presented. Using a 3D finite element method, the Ag nanoparticles&’ size, density and position within the solar cell are varied to study the influence of the nanoparticles on the absorption of the perovskite layer. Absorption enhancement was achieved with Ag NPs of 70 nm diameter (with contact angle of 44 °) and 100 nm diameter (with contact angle of 70 °) at the interface between perovskite and hole conducting layers. Assuming full conversion of the absorbed photons into photocurrent under standard AM 1.5 illumination condition, the corresponding increase of short circuit current density J_sc can reach 0.6 mA/cm² for 70 nm Ag NPs and 1.3 mA/cm² for 100 nm Ag NPs. For experiment, corresponding sizes of Ag NPs grown by spray-CVD were integrated into the solar cell structure to study the absorption enhancement of the perovskite absorber. Optical properties of the absorption enhancement of Ag NPs were systematically studied by UV-Vis spectroscopy. The morphology (SEM), crystal structure (XRD), composition (EDX) and electronic properties (surface photovoltage spectroscopy) of the layers were investigated as well.

In this presentation, we report a first hand study of plasmon enhanced photocurrent observed in hybrid nanostructures based heterojunction solar cell. The heterojunction solar cell was fabricated, using chemically synthesized narrow gap, IV-VI group semiconductor nanoparticles (PbS and PbSe) of 3~6nm diameter, wide gap semiconductor ZnO nanowires of ~1 mu;m length and ~50nm diameter, and gold nanoparticles (~5nm to 100nm), by spin-coating (~20cycles) onto FTO glasses, in ambient conditions (25C, 1atm). The synthesized nanostructures were characterized by XRD, UV-VIS absorption, SEM, AFM, TEM, solar simulator, etc. Nanostructures of variant sizes were integrated in to the heterojunction devices to study the effects on photocurrent and solar cell performance. The sizes, lengths, thickness of nanostructures were optimized to have best solar cell devices. The effects of fabrication conditions (such as growth temperature, growth time, anneal temperature, ligand treatments, in air or in N₂, etc.) on device performance were also studied. The architecture of film stack, i.e., the positions of Au nanoparticles and PbS, PbSe nanoparticles were also studied. It is confirmed that introducing Au nanopartiles with proper size will lead to increase of photocurrent as well as solar cell devices. The key challenges are to minimize the trap states and optimize the interface of nanostructures.