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 ori