Enhanced Photoelectric and Photothermal Responses on Silicon Platform by Plasmonic Absorber and Omni‐Schottky Junction

Recent progresses in plasmon‐induced hot electrons open up the possibility to achieve photon harvesting beyond the fundamental limit imposed by band‐to‐band transitions in semiconductors. To obtain high efficiency, both the optical absorption and electron emission/collection are crucial factors that need to be addressed in the design of hot electron devices. Here, we demonstrate a photoresponse as high as 3.3mA/W at 1500nm on a silicon platform by plasmonic absorber (PA) and omni‐Schottky junction integrated photodetector, reverse biased at 5V and illuminated with 10mW. The PA fabricated on silicon consists of a monolayer of random Au nanoparticles (NPs), a wide‐band gap semiconductor (TiO₂) and an optically thick Au electrode, resulting in broadband near‐infrared (NIR) absorption and efficient hot‐electron transfer via an all‐around Schottky emission path. Meanwhile, time and spectral‐resolved photoresponse measurements reveal that embedded NPs with superior absorption resembling plasmonic local heating sources can transfer their energy to electricity via the photothermal mechanism, which until now has not been adequately assessed or rigorously differentiated from the photoelectric process in plasmon‐mediated photon harvesting nano‐systems.     

Giant Rabi splitting in metal/semiconductor nanohybrids

We present strong coupling regime between localized plasmon in lithographed nanoparticles and excitons in an organic semiconductor. The lithographed nanoparticles allow a very good control of the particle size and environment, thereby avoiding a large inhomogeneous broadening of the plasmonic resonances which could partially mask the plasmon/exciton hybridization. The nanoparticles diameter ranges from 100 to 200 nm. A giant Rabi splitting energy of 450 meV is obtained, and typical behaviors of mixed states, i.e. anticrossing of their energies and crossing of their linewidths, are observed. Three-dimensional finite-difference time-domain simulations and coupled oscillator calculations are used to analyze and corroborate the experimental results.     

Dual plasmonic‐enhanced bulk‐heterojunction solar cell incorporating gold nanoparticles into solution‐processed anode buffer layer and active layer

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 solution‐processed polymer solar cells to achieve high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Stamping plasmonic nanoarrays on SERS‐supporting platforms

The dielectric property of a nanoparticle‐supporting film has recently garnered attention in the fabrication of plasmonic surfaces. A few studies have shown that the localized surface plasmon resonance (LSPR), and hence surface‐enhanced Raman scattering (SERS), strongly depends on the substrate refractive index. In order to create higher efficiency SERS‐active surfaces, it is therefore necessary to consider the substrate property along with nanoparticle morphology. However, due to certain limitations of conventional lithography, it is often not feasible to create well‐defined plasmonic nanoarrays on a substrate of interest. Here, an additive nanofabrication technique, i.e., nanotransfer printing (nTP), is implemented to integrate electron beam lithography (EBL) defined high‐aspect‐ratio nanofeatures on a variety of SERS‐supporting surfaces. With the aid of suitable surface chemistries, a wide range of plasmonic particles were successfully integrated on surfaces of three physically and chemically distinct dielectric materials, namely, polydimethyl siloxane (PDMS), SU‐8 photoresist, and glass surfaces, using silicon‐based relief pillars. These nTP‐created metal nanoparticles strongly amplify the Raman signal and complement the selection of suitable substrates for better SERS enhancement. Our experimental observations are also supported by theoretical calculations. The implementation of nTP to stamp out metal nanoparticles on a multitude conventional/unconventional substrates has novel applications in designing in‐built plasmonic microanalytical devices for SERS sensing and other related photonic studies. Copyright © 2011 John Wiley & Sons, Ltd.     

Review of Sn‐Doped Hematite Nanostructures for Photoelectrochemical Water Splitting

Hematite (α‐Fe₂O₃) nanostructures have been extensively studied as photoanodes for photoelectrochemical (PEC) water splitting. However, the photoactivity of pristine hematite nanostructures is limited by a number of factors, including poor electrical conductiviy and slow oxygen evolution reaction kinetics. Previous studies have shown that using tin (Sn) as an n‐type dopant can substantially enhance the photoactivity of hematite photoanodes by modifying their optical and electrical properties. Here, the recent accomplishments in using Sn‐doped hematite photoanodes for solar water splitting are highlighted.     

Switchable surface plasmon dichroic splitter modulated by optical polarization

For the miniaturization of optical devices, surface plasmon polaritons (SPPs) have been widely utilized due to their outstanding confinement and field‐enhancement characteristics. Analyzing a spectrum of optical signals and splitting certain regions of the spectrum range within a submicrometer‐scale structure are demanded for optical integrated systems. In this paper, a novel type of dichroic surface plasmon launcher that can switch the launching direction according to incident polarization states is demonstrated. Compared to the previously reported plasmonic dichroic splitters, the proposed schemes do not use any asymmetric geometry for directional launching. Hence, the direction of guided SPPs can be interchanged according to the polarization state. Such characteristics will be helpful to design switchable plasmonic devices that can be applied to active plasmonic integrated circuits.     

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron‐hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through interactions between the electron and hole with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron‐hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near‐term future for both fundamental research and the development of working devices which exploit MEG.     

Electron photoemission in plasmonic nanoparticle arrays: analysis of collective resonances and embedding effects

We theoretically study the characteristics of photoelectron emission in plasmonic nanoparticle arrays. Nanoparticles are partially embedded in a semiconductor, forming Schottky barriers at metal/semiconductor interfaces through which photoelectrons can tunnel from the nanoparticle into the semiconductor; photodetection in the infrared range, where photon energies are below the semiconductor band gap (insufficient for band-to-band absorption in semiconductor), is therefore possible. The nanoparticles are arranged in a sparse rectangular lattice so that the wavelength of the lattice-induced Rayleigh anomalies can overlap the wavelength of the localized surface plasmon resonance of the individual particles, bringing about collective effects from the nanoparticle array. Using full-wave numerical simulations, we analyze the effects of lattice constant, embedding depth, and refractive index step between the semiconductor layer and an adjacent transparent conductive oxide layer. We show that the presence of refractive index mismatch between media surrounding the nanoparticles disrupts the formation of a narrow absorption peak associated with the Rayleigh anomaly, so the role of collective lattice effects in the formation of plasmonic resonance is diminished. We also show that 5–20 times increase of photoemission can be achieved on embedding of nanoparticles without taking into account dynamics of ballistic electrons. The results obtained can be used to increase efficiency of plasmon-based photodetectors and photovoltaic devices. The results may provide clues to designing an experiment where the contributions of surface and volume photoelectric effects to the overall photocurrent would be defined.     

金属纳米颗粒对有机太阳能电池光吸收效率影响的研究

借助时域有限差分法,对几种常见金属纳米颗粒影响有机太阳能电池光吸收效率的因素及其内部物理机制进行了研究.首先对金属纳米颗粒激发局域表面等离子共振的场分布特点进行分析,对比其在电池不同功能层中对光吸收率的影响;其次基于米氏理论与电共振效应,得出金属纳米颗粒的结构参量对局域表面等离子共振位置及强度的影响规律,并以此进行优化设计.结果表明,具有高对称性形貌的金属纳米颗粒以小尺寸密堆积结构引入电池活性层,能够促进电池光吸收增强三倍以上.     

金属纳米颗粒对有机太阳能电池光吸收效率影响的研究

借助时域有限差分法,对几种常见金属纳米颗粒影响有机太阳能电池光吸收效率的因素及其内部物理机制进行了研究.首先对金属纳米颗粒激发局域表面等离子共振的场分布特点进行分析,对比其在电池不同功能层中对光吸收率的影响;其次基于米氏理论与电共振效应,得出金属纳米颗粒的结构参量对局域表面等离子共振位置及强度的影响规律,并以此进行优化设计.结果表明,具有高对称性形貌的金属纳米颗粒以小尺寸密堆积结构引入电池活性层,能够促进电池光吸收增强三倍以上.     

Plasmonic protection of the hot‐electron energy

It is shown, that hot electrons generated in a semiconductor can transfer their excess free energy into an embedded/adjacent plasmonic metallic structure (reservoir), before it is lost irreversibly to phonons in the semiconductor. Since the plasmon–phonon (and plasmon–photon) scattering in the metallic structure could be much slower than the electron–phonon scattering in the semiconductor, free energy of the hot electrons can be this way effectively protected from phonon emission for a significant amount of time. While the cubic point‐dipole crystal is proposed and studied here specifically as the plasmonic reservoir, other plasmonic structures including planar can be employed. It is also shown how the plasmon‐protected energy can by recycled in a novel, 3rd generation solar cell, be employing a planar plasmonic structure that is simultaneously also an electron collector of the cell. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Locally enhanced light–matter interaction of MoS2 monolayers at density-controllable nanogrooves of template-stripped Ag films

Transition metal dichalcogenide (TMD) monolayers, such as MoS_2, possess a direct optical bandgap are useful for emerging ultrathin optoelectronics in the visible light range, whereas their thin thickness limits light absorption and emission properties. To address this drawback, one promising approach is to hybridize plasmonic nanostructures with monolayer TMDs to utilize local field enhancement effects owing to localized surface plasmon resonance (LSPR). Herein, we propose a strong enhancement of the local light–matter interaction in MoS₂ monolayers on naturally generated nanoscale grooves. The nanogrooves are formed at grain boundaries (GBs) of template-stripped metal film substrates that are fabricated by mechanically stripping Ag films deposited on an ultra-flat Si substrate, wherein the nanogroove densities are systematically modulated by the Ag film thickness. We observe an effective photoluminescence enhancement factor of 758 and a Raman spectroscopy intensity enhancement of approximately 5 times in MoS₂ on the subwavelength-scale nanogrooves, compared with that on grain planes, which is attributed to a strong local field enhancement of the LSPR effect. Moreover, this plasmonic enhancement effect is elucidated by dark-field scattering spectroscopy and optical simulations. Our results can facilitate the utilization of density-controllable plasmonic nanogrooves synthesized without nanopatterning techniques for plasmonic hybrids on 2D semiconductors.     

Plasmonic nanoparticles and their analytical applications: A review

Plasmonic nanoparticles (NPs) have been reviewed herein for their fascinating optical properties in a wide spectral range and for their various applications. The surface plasmon resonance (SPR) bands of metal NPs can be tuned from visible to near infrared region by varying the shape of the metal NPs. As a result, the tuning of the SPR band over a spectral range is possible by making plasmonic NPs of different shapes. This review emphasizes fundamental studies of plasmonic NPs and nanocomposites with well-defined and controlled shapes that have several analytical applications such as molecular detection and determination in different fields. This review describes how oxidative etching and kinetic control can be utilized to manipulate the shape and optical properties of NPs. This review also describes the specific examples of the sensing applications of the localized surface plasmon resonance studies in which the researchers use both wavelength shift and surface-enhanced Raman scattering sensing to detect the molecules of chemical and biological relevance. The review ends with a perspective of the field, identifying the main challenges to be overcome and suggesting areas where the most promising developments are likely to happen in future.     

Surface exciton-plasmons and optical response of small-diameter carbon nanotubes

We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (≲1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption lineshape exhibits the line (Rabi) splitting ∼0.1–0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We expect this effect to open up paths to new tunable optoelectronic device applications of semiconducting carbon nanotubes.     

Towards Optimal Disorder in Gold Nanosponges for Long‐Lived Localized Plasmonic Modes

The potential of disorder to confine and enhance electromagnetic fields is well known and localized fields in turn can be used for non‐linear optical sensing and for studying quantum optics. Recently, nanoporous gold nanoparticles (nanosponges) were shown to support highly localized long‐lived plasmonic modes in the infrared spectral range. In this paper, we take first steps towards tailoring the disorder for optimal field localization and enhancement by calculating extinction and near‐field properties for different filling fractions and correlation lengths. We find that the filling fraction has not only a large effect on the fundamental dipolar surface‐plasmon resonance of the nanoparticle, but also on the frequency range in which localized modes of plasmonic nature occur. The influence of the correlation length is more subtle but is seen to influence the coupling between localized and far‐field modes as well. We briefly discuss first results on details of the localization process, which takes place on the same length scale as the typical structure size, so a simple cavity‐resonance picture cannot account for the relatively low frequency of the modes.     

Plasmonic fractals: ultrabroadband light trapping in thin film solar cells by a Sierpinski nanocarpet

Plasmonic Sierpinski nanocarpet as back structure for a thin film Si solar cell is investigated. We demonstrate that ultra-broadband light trapping can be obtained by placing square metallic nanoridges with Sierpinski pattern on the back contact of the thin film solar cell. The multiple-scale plasmonic fractal structure allows excitation of localized surface plasmons and surface plasmon polaritons in multiple wavelengths leading to obvious absorption enhancements in a wide frequency range. Full wave simulations show that 109 % increase of the short-circuit current density for a 200 nm thick solar cell, is achievable by the proposed fractal back structure. The amount of light absorbed in the active region of this cell is more than that of a flat cell with semiconductor thickness of 1,000 nm.     

Surface plasmon band tailoring of plasmonic nanostructure under the effect of water radiolysis by synchrotron radiation

Plasmonic metal nanostructures have a significant impact on a diverse domain of fields, including photocatalysis, antibacterial, drug vector, biosensors, photovoltaic cell, optical and electronic devices. Metal nanoparticles (MNps) are the simplest nanostructure promising ultrahigh stability, ease of manufacturing and tunable optical response. Silver nanoparticles (AgNp) dominate in the class of MNps because of their relatively high abundance, chemical activity and unique physical properties. Although MNps offer the desired physical properties, most of the synthesis and fabrication methods lag at the electronic grade due to an unbidden secondary product as a result of the direct chemical reduction process. In this paper, a facile protocol is presented for fabricating high‐yield in situ plasmonic AgNps under monochromatic X‐rays irradiation, without the use of any chemical reducing agent which prevents the formation of secondary products. The ascendancy of this protocol is to produce high quantitative yield with control over the reaction rate, particle size and localized surface plasmon resonance response, and also to provide the feasibility for in situ characterization. The role of X‐ray energy, beam flux and integrated dose towards the fabrication of plasmonic nanostructures has been studied. This experiment extends plasmonic research and provides avenues for upgrading production technologies of MNps.     

Colour tunability in a bimodal fluorescent hybrid nanostructure UCNPs@AuNPs@QDs

In the present work, lysine modified NaY_0.78Er_0.02Yb_0.2F₄ upconversion nanoparticles (UCNPs, positively charged) and lysine modified ZnSe:Mn²⁺ quantum dots (QDs, positively charged) are attached onto the surface of citrate reduced gold nanoparticles (AuNPs, negatively charged). The gold nanoparticles not only entangle the QDs and the UCNPs, through electrostatic interaction, but also tune the optical properties of UCNPs through the effect of surface plasmon resonance. The hybrid nanostructure gives green emission both through photoluminescence (under UV excitation) and through photon upconversion (under IR light excitation) process. The colour tuning is observed through variation in the size of QDs and through plasmonic effect of gold nanoparticles. In both the cases, the colour of emission gradually changes from green to red. The colour tunability and bi-modal photon conversion property of this material could be useful for its application in the field of bio-imaging and solar energy harvesting.     

Broadband and high‐efficiency conversion from guided waves to spoof surface plasmon polaritons

The conversion from spatial propagating waves to surface plasmon polaritons (SPPs) has been well studied, and shown to be very efficient by using gradient‐index metasurfaces. However, feeding energies into and extracting signals from functional plasmonic devices or circuits through transmission lines require the efficient conversion between SPPs and guided waves, which has not been reported, to the best of our knowledge. In this paper, a smooth bridge between the conventional coplanar waveguide (CPW) with 50 Ω impedance and plasmonic waveguide (e.g., an ultrathin corrugated metallic strip) has been proposed in the microwave frequency, which converts the guided waves to spoof SPPs with high efficiency in broadband. A matching transition has been proposed and designed, which is constructed by gradient corrugations and flaring ground, to match both the momentum and impedance of CPW and the plasmonic waveguide. Simulated and measured results on the transmission coefficients and near‐filed distributions show excellent transmission efficiency from CPW to a plasmonic waveguide to CPW in a wide frequency band. The high‐efficiency and broadband conversion between SPPs and guided waves opens up a new avenue for advanced conventional plasmonic integrated functional devices and circuits.     

Optical response of small-diameter semiconducting carbon nanotubes under exciton-surface-plasmon coupling

We analyze the optical response of small-diameter (?1 nm) semiconducting carbon nanotubes under the exciton-surface-plasmon coupling. Calculated optical absorption lineshapes exhibit the significant line (Rabi) splitting ∼0.1-0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We believe this effect may be used for the development of carbon nanotube based tunable optoelectronic device applications in areas such as nanophotonics and cavity quantum electrodynamics.