Plasmonics for improved photovoltaic devices

Two-dimensional ultrathin gold film composed of steadily linked dense nanoparticle with surface plasmon resonance

BACKGROUND

Noble metallic nanoparticles have prominent optical local-field enhancement and light trapping properties in the visible light region resulting from surface plasmon resonances.

RESULTS

We investigate the optical spectral properties and the surface-enhanced Raman spectroscopy of two-dimensional distinctive continuous ultrathin gold nanofilms. Experimental results show that the one- or two-layer nanofilm obviously increases absorbance in PEDOT:PSS and P3HT:PCBM layers and the gold nanofilm acquires high Raman-enhancing capability.

CONCLUSIONS

The fabricated novel structure of the continuous ultrathin gold nanofilms possesses high surface plasmon resonance properties and boasts a high surface-enhanced Raman scattering (SERS) enhancement factor, which can be a robust and cost-efficient SERS substrate. Interestingly, owing to the distinctive morphology and high light transmittance, the peculiar nanofilm can be used in multilayer photovoltaic devices to trap light without affecting the physical thickness of solar photovoltaic absorber layers and yielding new options for solar cell design.

Magnetic polarization in the optical absorption of metallic nanoparticles

We find remarkably strong absorption due to magnetic polarization in common colloidal and lithographic metallic nanoparticles. Our analysis is based upon a thorough examination of the dipolar electric and magnetic polarizabilities for representative combinations of nanoparticle composition, size, and morphology. We illustrate this concept by first discussing absorption in metallic spheres and then exploring ellipsoids, disks, and rings. Magnetic polarization reaches ~ 90% of the total absorption in 100 nm disks and rings for wavelengths above 1 μm under co-linear electric and magnetic irradiation. Our results demonstrate that the magnetic contribution to absorption cannot be naively overlooked, as it can largely exceed the contribution of electric polarization.

Oscillatory optical response of an amorphous two-dimensional array of gold nanoparticles

The optical response of metallic nanoparticle arrays is dominated by localized surface plasmon excitations and is the sum of individual particle contributions modified by interparticle coupling that depends on specific array geometry. We demonstrate a so far unexplored distinct oscillatory behavior of the plasmon peak position, full width at half maximum, and extinction efficiency in large area amorphous arrays of Au nanodisks, which depend on the minimum particle center-to-center distance in the array. Amorphous arrays exhibit short-range order and are completely random at long distances. In our theoretical analysis we introduce a film of dipoles approach, within the framework of the coupled dipole approximation, which describes the array as an average particle surrounded by a continuum of dipoles with surface densities determined by the pair correlation function of the array.

Altering the dewetting characteristics of ultrathin gold and silver films using a sacrificial antimony layer

Solid state dewetting of ultrathin films is the most straightforward means of fabricating substrate-supported noble metal nanostructures. This assembly process is, however, quite inflexible, yielding either densely packed smaller structures or widely spaced larger structures. Here, we demonstrate the utility of introducing a sacrificial antimony layer between the substrate and noble metal overlayer. We observe an agglomeration process which is radically altered by the concurrent sublimation of antimony. In stark contrast with conventional dewetting, where the thickness of the deposited metal film determines the characteristic length scales of the assembly process, it is the thickness of the sacrificial antimony layer which dictates both the nanoparticle size and interparticle spacing. The result is a far more flexible self-assembly process where the nanoparticle size and areal density can be varied widely. Demonstrations show nanoparticle areal densities which are varied over four orders of magnitude assembled from the identical gold layer thickness, where the accompanying changes to nanostructure size see a systematic shift in the wavelength of the localized surface plasmon resonance. As a pliable self-assembly process, it offers the opportunity to tailor the properties of an ensemble of nanostructures to meet the needs of specific applications.

Contrast between surface plasmon polariton-mediated extraordinary optical transmission behavior in epitaxial and polycrystalline Ag films in the mid- and far-infrared regimes

In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

Plasmon resonance tuning in metallic nanocavities

Nanocavities fabricated in a metallic surface have important and technologically useful properties of complete light absorption and strong field enhancement. Here, we demonstrate how a nanometerthick alumina deposition inside such a cavity can be used to gain an exquisite control over the resonance wavelength. This process allows achieving a precise control over the spectral response and is completely reversible allowing many tuning attempts to be made on a single structure until the optimum performance is achieved.

Parallel assembly of particles and wires on substrates by dictating instability evolution in liquid metal films

Liquid metal wires supported on substrates destabilize into droplets. The destabilization exhibits many characteristics of the Rayleigh-Plateau model of fluid jet breakup in vacuum. In either case, breakup is driven by unstable, varicose surface oscillations with wavelengths greater than the critical one (λ(c)). Here, by controlling the nanosecond liquid lifetime as well as stability of a rivulet as a function of its length by lithography, we demonstrate the ability to dictate the parallel assembly of wires and particles with precise placement.

2D quasiperiodic plasmonic crystals

Nanophotonic structures with irregular symmetry, such as quasiperiodic plasmonic crystals, have gained an increasing amount of attention, in particular as potential candidates to enhance the absorption of solar cells in an angular insensitive fashion. To examine the photonic bandstructure of such systems that determines their optical properties, it is necessary to measure and model normal and oblique light interaction with plasmonic crystals. We determine the different propagation vectors and consider the interaction of all possible waveguide modes and particle plasmons in a 2D metallic photonic quasicrystal, in conjunction with the dispersion relations of a slab waveguide. Using a Fano model, we calculate the optical properties for normal and inclined light incidence. Comparing measurements of a quasiperiodic lattice to the modelled spectra for angle of incidence variation in both azimuthal and polar direction of the sample gives excellent agreement and confirms the predictive power of our model.

Pitch-dependent resonances and near-field coupling in infrared nanoantenna arrays

We investigate coupling in arrays of nanoparticles resonating as half-wave antennas on both silicon and sapphire, and find a universal behavior when scaled by antenna length and substrate index. Three distinct coupling regimes are identified and characterized by rigorous finite-difference time domain simulations. As interparticle pitch is reduced below the oft-described radiative to evanescent transition, resonances blue shift and narrow and exhibit an asymmetric band consistent with a Fano lineshape. Upon further pitch reduction, a transition to a third regime, termed here as near-field coupling, is observed in which the resonance shifts red, becomes more symmetric, and broadens dramatically. This latter regime occurs when the extension of the resonant mode beyond the physical antenna end overlaps that of its neighbor. Simulations identify a clear rearrangement of field intensity accompanying this regime, illustrating that longitudinal modal fields localize in the air gap rather than in the higher index substrate at a pitch consistent with the experimentally observed transition.

Plasmonic nano-antenna a-Si:H solar cell

In this work the effects of plasmonics, nano-focusing, and orthogonalization of carrier and photon pathways are simultaneously explored by measuring the photocurrents in an elongated nano-scale solar cell with a silver nanoneedle inside. The silver nanoneedles formed the support of a conformally grown hydrogenated amorphous silicon (a-Si:H) n-i-p junction around it. A spherical morphology of the solar cell functions as a nano-lens, focusing incoming light directly on the silver nanoneedle. We found that plasmonics, geometric optics, and Fresnel reflections affect the nanostructured solar cell performance, depending strongly on light incidence angle and polarization. This provides valuable insight in solar cell processes in which novel concepts such as plasmonics, elongated nanostructures, and nano-lenses are used.

Enhanced electrical conductivity of silver nanoparticles for high frequency electronic applications

An enhancement in the electrical performance of low temperature screen-printed silver nanoparticles (nAg) has been measured at frequencies up to 220 GHz. We show that for frequencies above 80 GHz the electrical losses in coplanar waveguide structures fabricated using nAg at 350 °C are lower than those found in conventional thick film Ag conductors consisting of micrometer-sized grains and fabricated at 850 °C. The improved electrical performance is attributed to the better packing of the silver nanoparticles resulting in lower surface roughness by a factor of 3. We discuss how the use of silver nanoparticles offers new routes to high frequency applications on temperature sensitive conformal substrates and in sub-THz metamaterials.

Photon management in two-dimensional disordered media

Elaborating reliable and versatile strategies for efficient light coupling between free space and thin films is of crucial importance for new technologies in energy efficiency. Nanostructured materials have opened unprecedented opportunities for light management, notably in thin-film solar cells. Efficient coherent light trapping has been accomplished through the careful design of plasmonic nanoparticles and gratings, resonant dielectric particles and photonic crystals. Alternative approaches have used randomly textured surfaces as strong light diffusers to benefit from their broadband and wide-angle properties. Here, we propose a new strategy for photon management in thin films that combines both advantages of an efficient trapping due to coherent optical effects and broadband/wide-angle properties due to disorder. Our approach consists of the excitation of electromagnetic modes formed by multiple light scattering and wave interference in two-dimensional random media. We show, by numerical calculations, that the spectral and angular responses of thin films containing disordered photonic patterns are intimately related to the in-plane light transport process and can be tuned through structural correlations. Our findings, which are applicable to all waves, are particularly suited for improving the absorption efficiency of thin-film solar cells and can provide a new approach for high-extraction-efficiency light-emitting diodes.

Electron accumulation on metal nanoparticles in plasmon-enhanced organic solar cells

Plasmonic metal nanoparticles have been used to enhance the performance of thin-film devices such as organic photovoltaics based on polymer/fullerene blends. We show that silver nanoprisms accumulate long-lived negative charges when they are in contact with a photoexcited bulk heterojunction blend composed of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM). We report both the charge modulation and electroabsorption spectra of silver nanoprisms in solid-state devices and compare these spectra with the photoinduced absorption spectra of P3HT/PCBM blends containing silver nanoprisms. We assign a previously unidentified peak in the photoinduced absorption spectra to the presence of photoinduced electrons on the silver nanoprisms. We show that coating the nanoprisms with a 2.5 nm thick insulating layer can completely inhibit this charging. These results may inform methods for limiting metal-mediated losses in plasmonic solar cells.

Fano resonant ring/disk plasmonic nanocavities on conducting substrates for advanced biosensing

By introducing a conducting metal layer underneath a Fano resonant asymmetric ring/disk plasmonic nanocavity system, we demonstrate that electromagnetic fields can be strongly enhanced. These large electromagnetic fields extending deep into the medium are highly accessible and increase the interaction volume of analytes and optical fields. As a result, we demonstrate high refractive index sensitivities as large as 648 nm/RIU. By exciting Fano resonances with much sharper spectral features, as narrow as 9 nm, we experimentally show high figure of merits as large as 72 and reliable detection of protein mono- and bilayers. Furthermore, the conducting substrate enables strong interaction between fundamental and higher order modes of the system by minor structural asymmetries. This is very advantageous for experimental realization of systems supporting resonances with well-defined Fano-like line shape without requiring challenging fabrication resolution. Exploiting conducting metallic substrates and the associated propagating surface plasmons at their interface could be extended to other Fano resonant cavity geometries for improved biosensing performance.

Broadband and broadangle SPP antennas based on plasmonic crystals with linear chirp

Plasmonic technology relies on the coupling of light to surface electromagnetic modes on smooth or structured metal surfaces. While some applications utilise the resonant nature of surface polaritons, others require broadband characteristics. We demonstrate unidirectional and broadband plasmonic antennas with large acceptance angles based on chirped plasmonic gratings. Near-field optical measurements have been used to visualise the excitation of surface plasmon polaritons by such aperiodic structures. These weakly aperiodic plasmonic crystals allow the formation of a trapped rainbow-type effect in a two-dimensional geometry as surface polaritons of different frequencies are coherently excited in different locations over the plasmonic structure. Both the crystal's finite size and the finite lifetime of plasmonic states are crucial for the generation of broadband surface plasmon polaritons. This approach presents new opportunities for building unidirectional, broadband and broad-angle plasmonic couplers for sensing purposes, information processing, photovoltaic applications and shaping and manipulating ultrashort optical pulses.

Controlling surface plasmon interference in branched silver nanowire structures

Using quantum dot fluorescence imaging, we investigated the interference of surface plasmon beams in branched silver nanowire structures. Depending on the phases and polarizations of the incident light, interferences of plasmon beams modulate the plasmon propagation in the branched structures and the output light intensity in the distal ends. The interference visibility is strongly dependent on the incident polarization at the main wire terminal, and the mechanism is revealed by quantum dot fluorescence imaging of the near field distribution of propagating plasmons. The near field distribution pattern resulting from the beating of different plasmon modes plays a critical role in the plasmon interference. The overlap of the antinode in the near field pattern with the connection junction in the nanowire structure is required for a large interference visibility, since the overlap makes the electric field intensity difference of the two plasmon beams smaller. It is found that the plasmon interference is strongly dependent on the polarization of the excitation light at the main wire terminal, but weakly dependent on the polarization at the branch wire terminal.

Crystalline structure-dependent growth of bimetallic nanostructures

Morphological control of multimetallic nanostructures is crucial for obtaining shape-dependent physical and chemical properties. Up to date, control of the shapes of multimetallic nanostructures has remained largely empirical. Multimetallic nanostructures have been produced mostly through seed-mediated growth. Understanding the role played by starting nanocrystal seeds can help in controlling the shape and in turn the plasmonic and catalytic properties of multimetallic nanostructures. In this work, we have studied the effect of the crystalline structure and shape of Au nanocrystal seeds on the morphology of the resultant bimetallic nanostructures. Single-crystalline Au nanorods, multiply twinned Au nanorods, and multiply twinned Au nanobipyramids were employed as the starting seeds. Both silver and palladium exhibit highly preferential growth on the side surfaces of the single-crystalline Au nanorods, giving rise to bimetallic cuboids, whereas they prefer to grow at the ends of the multiply twinned Au nanorods and nanobipyramids, giving rise to bimetallic nanorods. These results indicate that the morphology of the bimetallic nanostructures is highly dependent on the crystalline structure of the Au nanocrystal seeds. Our results will be useful for guiding the preparation of multimetallic nanostructures with desired shapes and therefore plasmonic properties for various plasmon-based applications.

Shape transformation and visible region plasmonic modulation of silver nanoplates by graphene oxide

The plasmon band of Ag nanoplates can be continuously tuned over the visible region by simply stirring with graphene oxide (GO) nanosheets, accompanied with the shape transformation from triangle to round. The ability of GO to competitively adsorb ligands (from Ag nanoplates) has been considered to response for this interesting change. Through the protection of sufficient citrate ions, the resultant Ag nanoplates exhibit the excellent stability over a long time without noticeable change of their optical properties.

Fano-like resonances arising from long-lived molecule-plasmon interactions in colloidal nanoantennas

We examine ultrafast dynamics in a coupled molecule-plasmon system. Using a new ultrafast Raman technique called surface enhanced-femtosecond stimulated Raman spectroscopy (SE-FSRS), we prove that plasmonic nanoparticles and adsorbed molecules are coupled by the appearance of Fano-like lineshapes, which arise from the interaction of narrowband vibrational coherences and the broadband plasmon resonance. We probe the effect of plasmon energy on the vibrational lineshapes and observe changes in the phase of the line shape dispersion. Finally, we examine the effect of plasmon-molecule coupling on the molecular vibrational coherence lifetime. Surprisingly, coupling of the molecular vibration to the plasmon does not significantly shorten the vibrational coherence dephasing time. Better understanding of the ultrafast dynamics of excited vibrational states and vibrational coherences in coupled molecular-plasmonic systems should assist in developing a mechanism-based view of plasmonically enhanced photovoltaic and photocatalytic systems.

Aluminum plasmonic nanoantennas

The use of aluminum for plasmonic nanostructures opens up new possibilities, such as access to short-wavelength regions of the spectrum, complementary metal-oxide-semiconductor (CMOS) compatibility, and the possibility of low-cost, sustainable, mass-producible plasmonic materials. Here we examine the properties of individual Al nanorod antennas with cathodoluminescence (CL). This approach allows us to image the local density of optical states (LDOS) of Al nanorod antennas with a spatial resolution less than 20 nm and to identify the radiative modes of these nanostructures across the visible and into the UV spectral range. The results, which agree well with finite difference time domain (FDTD) simulations, lay the groundwork for precise Al plasmonic nanostructure design for a variety of applications.

Design principles for photovoltaic devices based on Si nanowires with axial or radial p-n junctions

Semiconductor nanowires (NWs) are a developing platform for electronic and photonic technologies, and many demonstrated devices utilize a p-type/n-type (p-n) junction encoded along either the axial or radial directions of the wires. These miniaturized junctions enable a diverse range of functions, from sensors to solar cells, yet the physics of the devices has not been thoroughly evaluated. Here, we present finite-element modeling of axial and radial Si NW p-n junctions with total diameters of ~240 nm and donor/acceptor doping levels ranging from 10(16) to 10(20) cm(-3). We evaluate the photovoltaic performance of horizontally oriented NWs under 1 sun illumination and compare simulated current-voltage data to experimental measurements, permitting detailed analysis of NW performance, limitations, and prospect as a technology for solar energy conversion. Although high surface-to-volume ratios are cited as detrimental to NW performance, radial p-n junctions are surprisingly insensitive to surface recombination, with devices supporting open-circuit voltages (V(OC)) of ~0.54 V and internal quantum efficiencies of 95% even with high surface recombination velocities (SRVs) of 10(5) cm/s. Axial devices, in which the depletion region is exposed to the surface, are far more sensitive to SRV, requiring substantially lower values of 10(3)-10(4) cm/s to produce the same level of performance. For low values of the SRV (<100 cm/s), both axial and radial NWs can support V(OC) values of >0.70 V if the bulk minority carrier lifetime is 1 μs or greater. Experimental measurements on NWs grown by a vapor-liquid-solid mechanism yield V(OC) of 0.23 and 0.44 V for axial and radial NWs, respectively, and show that axial devices are limited by a SRV of ~7 × 10(3) cm/s while radial devices are limited by a bulk lifetime of ~3 ns. The simulations show that with further development the electrical characteristics of 200-300 nm Si NWs are sufficient to support power-conversion efficiencies of 15-25%. The analysis presented here can be generalized to other semiconductor homo- and heterojunctions, and we expect that insights from finite element modeling will serve as a powerful method to guide the design of advanced nanoscale structures.

Dark plasmonic breathing modes in silver nanodisks

We map the complete plasmonic spectrum of silver nanodisks by electron energy loss spectroscopy and show that the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Therefore, this mode does not couple to light and has escaped from observation in optical experiments. This radial breathing mode has the character of an extended two-dimensional surface plasmon with a wavenumber determined by the circular disk confinement. Its strong near fields can impact the hybridization in coupled plasmonic nanoparticles as well as couplings with nearby quantum emitters.

Theory of three-dimensional nanocrescent light harvesters

The optical properties of three-dimensional crescent-shaped gold nanoparticles are studied using a transformation optics methodology. The polarization insensitive, highly efficient, and tunable light harvesting ability of singular nanocrescents is demonstrated. We extend our approach to more realistic blunt nanostructures, showing the robustness of their plasmonic performance against geometric imperfections. Finally, we provide analytical and numerical insights into the sensitivity of the device to radiative losses and nonlocal effects. Our theoretical findings reveal an underlying relation between structural bluntness and spatial dispersion in this particular nanoparticle configuration.

Generalized Routes to Mesostructured Silicates with High Metal Content

Nanostructured materials with high metal content are interesting for a number of applications, including catalysis as well as energy conversion and storage. Here we elaborate an approach that combines the advantages of simple silica sol-gel chemistry with the ability to tailor metal composition and structure by introducing a ligand that connects a silane with an amino acid or hydroxy acid. Reacting this ligand with a metal acetate generates a precursor for a range of metal-silica nanocomposites. Comparing this chemistry with conventional organic ligand-metal complexes that can be physically mixed into sol-gel derived silicates elucidates advantages, e.g. of going to high metal loadings. Resulting nanomaterials are characterized by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (NMR) to reveal structural characteristics on multiple lengths scales, i.e. from the microscopic (molecular) level (NMR) all the way to the mesoscale (SAXS) and macroscale (TEM).

LSPR enhanced MSM UV photodetectors

We fabricated localized surface plasmon resonance enhanced UV photodetectors on MOCVD grown semi-insulating GaN. Plasmonic resonance in the UV region was attained using 36 nm diameter Al nanoparticles. Extinction spectra of the nanoparticles were measured through spectral transmission measurements. A resonant extinction peak around 300 nm was obtained with Al nanoparticles. These particles gave rise to enhanced absorption in GaN at 340 nm. Spectral responsivity measurements revealed an enhancement factor of 1.5. These results provided experimental verification for obtaining field enhancement by using Al nanoparticles on GaN.

Fabrication of plasmonic devices using femtosecond laser-induced forward transfer technique

Using femtosecond laser-induced forward transfer techniques we have fabricated gold dots and nanoparticles on glass substrates, as well as nanobumps on gold thin film. The surface morphologies of these structures with different laser fluences and film thicknesses are investigated. We also study the focusing and defocusing properties of the nanofence-an arranged nanobump pattern-by the total-internal reflection microscope. Observations reveal that surface plasmon waves can be highly directed and focused via this nanofence pattern. Results are in good agreement with the simulation results using the finite-element method and demonstrate the potential applications of these nanophotonic devices. Furthermore, we utilize high laser energy to fabricate plasmonic waveguides, and also succeed in transferring the waveguides to another substrate. The attenuation rates of the light propagating in the waveguides are observed to achieve 0.31 dB μm(-1) and 0.48 dB μm(-1) on the target and receiver sides, respectively.

Plasmon-plasmon interaction: controlling light at nanoscale

This paper investigates the effect of the locally induced bulk plasmon resonance on light guiding of a plasma-like medium. It is demonstrated that, by inducing or suppressing the bulk plasmon resonance, one can manipulate the light propagation. This concept is then employed for a hybrid dielectric-loaded plasmonic waveguide with the aim to efficiently control the surface plasmon polaritons with a highly doped silicon as an active material. The proposed approach is shown to have a high potential for nanoscale light manipulation and development of a fully CMOS-compatible electro-optical plasmonic modulator.

Synergistic effect of surface plasmon resonance and constructed hierarchical TiO2 spheres for dye-sensitized solar cells

We demonstrate a strategy for incorporating plasmon resonant metallic nanoparticles in the construction of hierarchical TiO(2) spheres. Localized electric fields can be produced by the addition of Au nanoparticles, which can excite dye molecules more effectively than incident far-field light. The synergistic effect of surface plasmon resonance with constructed TiO(2) nanostructures has been investigated, and was confirmed by optical spectroscopy, J-V characteristics, EIS analysis and OCVD measurements. When Au nanoparticles are incorporated into the constructed TiO(2) spheres, the device achieves a power conversion efficiency of 6.62%, a 4.6% increase compared to the device based on constructed TiO(2) spheres without plasmon resonant Au nanoparticles, and a 17.4% increase compared to that without any treatment.

A combination of concave/convex surfaces for field-enhancement optimization: the indented nanocone

We introduce a design strategy to maximize the Near Field (NF) enhancement near plasmonic antennas. We start by identifying and studying the basic electromagnetic effects that contribute to the electric near field enhancement. Next, we show how the concatenation of a convex and a concave surface allows merging all the effects on a single, continuous nanoantenna. As an example of this NF maximization strategy, we engineer a nanostructure, the indented nanocone. This structure, combines all the studied NF maximization effects with a synergistic boost provided by a Fano-like interference effect activated by the presence of the concave surface. As a result, the antenna exhibits a NF amplitude enhancement of ~ 800, which transforms into ~1600 when coupled to a perfect metallic surface. This strong enhancement makes the proposed structure a robust candidate to be used in field enhancement based technologies. Further elaborations of the concept may produce even larger and more effective enhancements.

Nanotechnology-enabled energy harvesting for self-powered micro-/nanosystems

Health, infrastructure, and environmental monitoring as well as networking and defense technologies are only some of the potential areas of application of micro-/nanosystems (MNSs). It is highly desirable that these MNSs operate without an external electricity source and instead draw the energy they require from the environment in which they are used. This Review covers various approaches for energy harvesting to meet the future demand for self-powered MNSs.

Ultrasmooth metallic films with buried nanostructures for backside reflection-mode plasmonic biosensing

We present a new plasmonic device architecture based on ultrasmooth metallic surfaces with buried plasmonic nanostructures. Using template-stripping techniques, ultrathin gold films with less than 5 Å surface roughness are optically coupled to an arbitrary arrangement of buried metallic gratings, rings, and nanodots. As a prototypical example, we present linear plasmonic gratings buried under an ultrasmooth 20 nm thick gold surface for biosensing. The optical illumination and collection are completely decoupled from the microfluidic delivery of liquid samples due to the backside, reflection-mode geometry. This allows for sensing with opaque or highly scattering liquids. With the buried nanostructure design, we maintain high sensitivity and decoupled backside (reflective) optical access as with traditional prism-based surface plasmon resonance (SPR) sensors. In addition, we also gain the benefits offered by nanoplasmonic sensors such as spectral tunability and high-resolution, wide-field SPR imaging with normal-incidence epi-illumination that is simple to construct and align. Beyond sensing, our buried plasmonic nanostructures with ultrasmooth metallic surfaces can benefit nanophotonic waveguides, surface-enhanced spectroscopy, nanolithography, and optical trapping.

Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks

Wide wavelength ranges of light localization and scattering characteristics can be attributed to shape-dependent longitude surface plasmon resonance in complicated nanostructures. We have studied this phenomenon by spectroscopic measurement and a three-dimensional numerical simulation, for the first time, on the high-density branched silver nanowires and nanomeshworks at room temperature. These nanostructures were fabricated with simple light-induced colloidal method. In the range from the visible to the near-infrared wavelengths, light has been found effectively trapped in those trapping sites which were randomly distributed at the corners, the branches, and the junctions of the nanostructures in those nanostructures in three dimensions. The broadened bandwidth electromagnetic field enhancement property makes these branched nanostructures useful in optical processing and photovoltaic applications.

Thermodynamic upper bound on broadband light coupling with photonic structures

The coupling between free space radiation and optical media critically influences the performance of optical devices. We show that, for any given photonic structure, the sum of the external coupling rates for all its optical modes are subject to an upper bound dictated by the second law of thermodynamics. Such bound limits how efficient light can be coupled to any photonic structure. As one example of application, we use this upper bound to derive the limit of light absorption in broadband solar absorbers.

Large-scale fabrication of a continuous gold network for use as a transparent conductive electrode in photo-electronic devices

Large-scale periodic gold network electrodes were fabricated using the developed and versatile nanosphere lithography technique. The fabrication processes, structural characterizations and network formation mechanism were described in detail. An enhanced optical transmission peak was observed from the transmission spectrum, which could be assigned to the extraordinary transmission mediated commonly by (a) localized surface plasmon resonance (LSPR) and (b) surface plasmon polaritons. The effects of film thickness, sphere diameter (periodicity) and reactive ion etching time on their optical and electrical properties were also investigated. By controlling these three independent variables, we could tune the SPR peak position and their light transmission distributions flexibly. Our large-scale continuous gold network can serve as a transparent conductive electrode, while possessing the role of a surface plasmonic resonance component can make it very attractive for potential photo-electric device applications in a range from plasmon-enhanced broadband photovoltaics to SPR-based chemo- and biosensors.

Salt-mediated kinetics of the self-assembly of gold nanorods end-tethered with polymer ligands

Studies on the self-assembly of metal nanoparticles (NPs) in the presence of ions are motivated by the biosensing applications of NP clusters and the capability to control the morphology of clusters of oppositely charged NPs. The effect of ions has been explored for the self-assembly of metal NPs capped solely with ionic ligands, whereas, in general, the surface of NPs can be coated with a mixture of ligands interacting with each other by non-electrostatic forces. In the present work, we examined the kinetics of self-assembly of gold nanorods capped with a mixture of low-molecular weight ionic molecules and nonpolar polymer ligands. We show that in contrast with earlier reports on the effect of electrolytes on NP self-assembly, the driving force for the accelerated self-assembly of nanorods is the reduction in polymer solubility in the presence of ions, rather than the screening of the electric double layer of the charged ligands. The reported results are important for NP self-assembly occurring in mixed solvents, in which attraction forces between nonpolar ligands are governed by the balance between solvent-solvent and solvent-salt interactions. Furthermore, the addition of salts can be used to increase the rate of nanorod self-assembly, which, otherwise, is an intrinsically slow process.

Design of a plasmonic back reflector for silicon nanowire decorated solar cells

This Letter presents a crystalline silicon thin film solar cell model with Si nanowire arrays surface decoration and metallic nanostructure patterns on the back reflector. The nanostructured Ag back reflector can significantly enhance the absorption in the near-infrared spectrum. Furthermore, by inserting a ZnO:Al layer between the silicon substrate and nanostructured Ag back reflector, the absorption loss in the Ag back reflector can be clearly depressed, contributing to a maximum J(sc) of 28.4 mA/cm(2). A photocurrent enhancement of 22% is achieved compared with a SiNW solar cell with a planar Ag back reflector.

High efficiency thin upgraded metallurgical-grade silicon solar cells on flexible substrates

We present a thin film (<20 μm) solar cell based on upgraded metallurgical-grade polycrystalline Si that utilizes silver nanoparticles atop silicon nanopillars created by block copolymer nanolithography to enhance light absorption and increase cell efficiency η > 8%. In addition, the solar cells are flexible and semitransparent so as to reduce balance of systems costs and open new applications for conformable solar cell arrays on a variety of surfaces. Detailed studies on the optical and electrical properties of the resulting solar cells suggest that both antireflective and light-trapping mechanisms are key to the enhanced efficiency.