Surface Plasmons In Metallic Nanostructures Research Articles

Plasmoelectronic properties of self-assembled gold nanoparticles: impedance spectroscopy experiments combined with numerical simulations

Plasmoelectronics is a fast developing field driven by the combination of charge transport and surface plasmons in metal nanostructures. In this paper, we report on theoretical and experimental investigations of the plasmoelectronic properties of self-assembled monolayers of colloidal gold nanoparticles (NPs). A local plasmonic-electronic nanojunction is introduced, as the fundamental building block of a mesoscale electrical network, and serves as the beginning point for the development of a multiscale modeling strategy. The interparticle charge tunneling and accumulation account for the dynamical charge transport; the plasmoelectronic properties are implemented at the nanoscale using electrodynamic calculations based on the discrete dipole approximation (DDA) method. The electric characteristics of the macroscopic NP network are calculated using a numerical resolution of the current-bias equations based on Kirchhoff's laws. Disorder effects due to size and position fluctuations of the NPs within the network as well as dislocations and point defects in their spatial arrangement are taken into account. The effects of light excitation intensity and wavelength on the nanojunction photoconductance and on the macroscopic photoresistance of the NP network are addressed. In particular, plasmoelectronic conduction paths are extracted from the coupled DDA-Kirchhoff numerical simulations, and their statistical distribution is investigated as well as their dependence on path length. The theoretical results are compared with impedance spectroscopy measurements performed under optical excitation. We found a good agreement between the predicted impedance Nyquist plots and resonance properties, and the measurements. In particular, we were able to estimate the surface plasmon-assisted photoelectric conversion efficiency involving first neighbor NPs. We found that the photoconductance of a single nanojunction formed by a dimer of nearly touching NPs is around 18 nS/W/cm2 under resonant plasmonic excitation. The presented work provides insight into the photoinduced charge transport in self-assembled NP networks and opens the route to novel applications in the field of plasmoelectronics.

Plasmon-Induced Hot Carriers from Interband and Intraband Transitions in Large Noble Metal Nanoparticles

Hot electrons generated from the decay of localized surface plasmons in metallic nanostructures have the potential to transform photocatalysis, photodetection and other optoelectronic applications. However, the understanding of hot-carrier generation in realistic nanostructures, in particular the relative importance of interband and intraband transitions, remains incomplete. Here we report theoretical predictions of hot-carrier generation rates in spherical nanoparticles of the noble metals silver, gold and copper with diameters up to 30 nanometers obtained from a novel atomistic linear-scaling approach. As the nanoparticle size increases the relative importance of interband transitions from d-bands to sp-bands relative to surface-enabled sp-band to sp-band transitions increases. We find that the hot-hole generation rate is characterized by a peak at the onset of the d-bands, while the position of the corresponding peak in the hot-electron distribution can be controlled through the illumination frequency. In contrast, intraband transitions give rise to hot electrons, but relatively cold holes. Importantly, increasing the dielectric constant of the environment removes hot carriers generated from interband transitions, while increasing the number of hot carriers from intraband transitions. The insights resulting from our work enable the design of nanoparticles for specific hot-carrier applications through their material composition, size and dielectric environment.

Recent Progress and Future Opportunities for Hot Carrier Photodetectors: From Ultraviolet to Infrared Bands

AbstractThe hot carriers generated from the nonradiative decay of surface plasmons in metallic nanostructures can inject into the conduction band of a semiconductor, allowing for the sub‐bandgap photodetection under room temperature. By the controllable interfacial barrier height between the plasmonic and semiconductor/insulator materials, the hot carrier photodetectors working from ultraviolet to infrared bands are extensively demonstrated with significant progress. In this review, hot carrier dynamics are briefly discussed from generation, transport, and emission perspectives. The state‐of‐the‐art progress of hot carrier photodetectors with various configurations, material constitutions, and plasmonic nanostructures are surveyed. To further promote hot carrier extraction efficiency toward the practical applications, the thermodynamic loss analysis, and the potential strategies from the optical, electrical, and material perspectives are addressed. The performances of the developed hot carrier photodetectors are also summarized, particularly addressing the novel functionalities, challenges, and future opportunities.

Plasmon–Exciton Interactions: Spontaneous Emission and Strong Coupling

AbstractThe extraordinary optical properties of surface plasmons in metal nanostructures provide the possibilities to enhance and accelerate the spontaneous emission, and manipulate the decay and emission processes of quantum emitters. The extremely small mode volume of plasmonic nanocavities also benefits the realization of plasmon–exciton strong coupling. Here, the progress on the study of plasmon modified spontaneous emission and plasmon–exciton strong coupling are reviewed. The fundamentals of surface plasmons and quantum emitters, and the methods for assembling coupling systems of plasmonic nanostructures and quantum emitters are first introduced. Then the major aspects of plasmon modified spontaneous emission, including emission intensity, lifetime, spectral profile, direction, polarization, and energy transfer are reviewed. The coupling of quantum emitters and plasmonic waveguides is then discussed. Next, the developments of strong coupling between plasmonic structures and various quantum emitters are reviewed. Finally a few applications are highlighted followed by conclusions and outlook.

Strong Plasmon-Exciton Coupling in Ag Nanoparticle-Conjugated Polymer Core-Shell Hybrid Nanostructures.

Strong plasmon–exciton coupling between tightly-bound excitons in organic molecular semiconductors and surface plasmons in metal nanostructures has been studied extensively for a number of technical applications, including low-threshold lasing and room-temperature Bose-Einstein condensates. Typically, excitons with narrow resonances, such as J-aggregates, are employed to achieve strong plasmon–exciton coupling. However, J-aggregates have limited applications for optoelectronic devices compared with organic conjugated polymers. Here, using numerical and analytical calculations, we demonstrate that strong plasmon–exciton coupling can be achieved for Ag-conjugated polymer core-shell nanostructures, despite the broad spectral linewidth of conjugated polymers. We show that strong plasmon–exciton coupling can be achieved through the use of thick shells, large oscillator strengths, and multiple vibronic resonances characteristic of typical conjugated polymers, and that Rabi splitting energies of over 1000 meV can be obtained using realistic material dispersive relative permittivity parameters. The results presented herein give insight into the mechanisms of plasmon–exciton coupling when broadband excitonic materials featuring strong vibrational–electronic coupling are employed and are relevant to organic optoelectronic devices and hybrid metal–organic photonic nanostructures.

Orbital Angular Momentum and Energy Loss Characterization of Plasmonic Excitations in Metallic Nanostructures in TEM

Recently, a new device to measure the orbital angular momentum (OAM) electronic spectrum after elastic/inelastic scattering in a transmission electron microscope has been introduced. We modified the theoretical framework needed to describe conventional low-loss electron energy loss spectroscopy (EELS) experiments in transmission electron microscopes (TEM) to study surface plasmons in metallic nanostructures, to allow for an OAM post selection and devise new experiments for the analysis of these excitations in nanostructures. We found that unprecedented information on the symmetries and on the chirality of the plasmonic modes can be retrieved even with limited OAM and energy resolutions.

Quasinormal mode analysis of extremely localized optical field in body-of-revolution plasmonic structures

Surface plasmons in metallic nanostructures can confine the optical field within the region of subwavelength, even nanometer scale, and thus enhance the light-matter interaction and other physical processes, which will lead the plasmon optics to possess attractive applications in many areas. However, the " mode volume” often used to characterize field confinement in plasmonic structures is only defined phe-nomenologically and suffers ambiguity when applied to complex structures. In this work, we develop a theoretical method to characterize the field confinement based on quasi-normal mode analysis. We recognize the fact that a plasmonic resonance may result from many eigen-modes, which together contribute to the observed field confinement. An effective mode volume is introduced for quasi-normal modes and used to characterize the field confinement when the plasmonic resonance is dominated by a single quasi-normal mode. Two typical kinds of plasmonic structures are systematically examined, and the field confinement on the order of 10 nm<sup>3</sup>–100 nm<sup>3</sup> is confirmed. In pursuit of the ultimate field confinement, we revisit the so-called " pico-cavity” formed by an atomistic protrusion in the nano gap of the particle-on-mirror configuration. The apparent hot spot is shown to have contributions from several quasi-normal modes. The dominant one exhibits a further squeezed mode volume compared with the scenario without the protrusion, but is still well above 10 nm<sup>3</sup>.

Advances in surface-enhanced optical forces and optical manipulations

The localized surface plasmons in metal nanostructures under optical excitation will lead to near-field localization and enhancement, which have shown important applications in surface enhancement spectroscopy, ultra-sensitive sensing, microfluidic chip, enhanced optical force, etc. The plasmon resonance and the resulting electric field enhancement strongly depend on wavelength and structure geometry. As a result, the optical force will be closely related to the field distribution, that is, the optical force can be used to manipulate and sort plasmonic metal structures. The large near-field enhancement and gradient of metal nanoparticle aggregates can also be used as a " plasmonic tweezer” to manipulate other particles. Furthermore, in the case of changing the incident polarization and even for a new type of structured laser beam, the optical manipulation has a higher degree of freedom. In this review, having briefly introduced the plasmon-enhanced optical force, we focus on the recent advances in the following three aspects: 1) the manipulation of plasmonic nanoparticles by optical tweezer, 2) the manipulation of other particles by plasmonic tweezer, and 3) dependence of plasmonic optical force on the polarization, optical angular momentum, structured light and the structured chirality. Comparing with other topics of plasmon- enhanced light-interactions, there is plenty of room for further developing the plasmon-enhanced optical force and optical manipulation. Several research trends can be foreseen. 1) More precise optical manipulating and sorting of nanoparticles (even sub-nanometer). For example, more sensitive special resonant modes (e.g. Fano resonance) of plasmonic nanostructure can be utilized. For some nanostructures with small feature sizes, especially when the gap size is close to 1 nm, the non-local effect has a certain effect on the plasmon resonance. Therefore, when calculating the optical force in this case, non-local effects and possibly other quantum effects should be considered. 2) Richer laser fields, that is, using various new structured fields and chiral structures provides a higher degree of freedom for the optical forces and optical manipulation. Also, the localized surface plasmons can be combined with propagating surface plasmons. 3) Wider applications of plasmonic optical forces, especially in combination with other effects and even interdiscipline, e.g. enhanced spectroscopy, enhanced single particle chemical reactions, nonlinear optical effects, and photothermal manipulations.

Microsphere Assisted Super-resolution Optical Imaging of Plasmonic Interaction between Gold Nanoparticles

Conventional far-field microscopy cannot directly resolve the sub-diffraction spatial distribution of localized surface plasmons in metal nanostructures. Using BaTiO3 microspheres as far-field superlenses by collecting the near-field signal, we can map the origin of enhanced two-photon photoluminescence signal from the gap region of gold nanosphere dimers and gold nanorod dimers beyond the diffraction limit, on a conventional far-field microscope. As the angle θ between dimer’s structural axis and laser polarisation changes, photoluminescence intensity varies with a cos4θ function, which agrees quantitatively with numerical simulations. An optical resolution of about λ/7 (λ: two-photon luminescence central wavelength) is demonstrated at dimer’s gap region.

On quantum approach to modeling of plasmon photovoltaic effect

Surface plasmons in metallic nanostructures including metallically nanomodified solar cells are conventionally studied and modeled by application of the Mie approach to plasmons or by the finite element solution of differential Maxwell equations with imposed boundary and material constraints (e.g., upon commercial COMSOL software system). Both approaches are essentially classical ones and neglect quantum particularities related to plasmon excitations in metallic components. We demonstrate that these quantum plasmon effects are of crucial importance especially in theoretical simulations of plasmon-aided photovoltaic phenomena. Quantum corrections considerably improve both the Mie and COMSOL approaches in this case. We present the semiclassical random phase approximation description of plasmons in metallic nanoparticles and apply the quantum Fermi golden rule scheme to assess the sunlight energy transfer to the semiconductor solar cell mediated by surface plasmons in metallic nanoparticles deposited on the top of the battery. In addition, short-ranged electron–electron interaction in metals is discussed in the framework of the semiclassical hydrodynamic model. The significance of the related quantum corrections is illustrated by the quantumly improved COMSOL simulations.

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle-TiO2 Films.

Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (<1%), and the physics underlying this low yield remains unclear. Here, we use transient absorption spectroscopy to quantify the efficiency of the initial electron transfer in model systems composed of gold nanoparticles (NPs) fully embedded in TiO2 or Al2O3 films. In independent experiments, we measure free carrier absorption and electron-phonon decay in the model systems and determine that the electron-injection efficiency from the Au NPs to the TiO2 ranges from about 25% to 45%. While much higher than some previous estimates, the measured injection efficiency is within an upper-bound estimate based on a simple approximation for the Au hot-electron energy distribution. These results have important implications for understanding the achievable injection efficiencies of hot-electron plasmonic devices and show that the injection efficiency can be high for Au NPs fully embedded within a semiconductor with dimensions less than the Au electron mean free path.

Coupling Solar Energy into Reactions: Materials Design for Surface Plasmon-Mediated Catalysis.

Enabled by surface plasmons, noble metal nanostructures can interact with and harvest incident light. As such, they may serve as unique media to generate heat, supply energetic electrons, and provide strong local electromagnetic fields for chemical reactions through different mechanisms. This solar-to-chemical pathway provides a new approach to solar energy utilization, alternative to conventional semiconductor-based photocatalysis. To provide readers with a clear picture of this newly recognized process, this review presents coupling solar energy into chemical reactions through plasmonic nanostructures. It starts with a brief introduction of surface plasmons in metallic nanostructures, followed by a demonstration of tuning plasmonic features by tailoring their physical parameters. Owing to their tunable plasmonic properties, metallic materials offer a platform to trigger and drive chemical reactions at the nanoscale, as systematically overviewed in this article. The design rules for plasmonic materials for catalytic applications are further outlined based on existing examples. At the end of this article, the challenges and opportunities for further development of plasmonic-mediated catalysis toward energy and environmental applications are discussed.

Plasmonic effects of au/ag bimetallic multispiked nanoparticles for photovoltaic applications.

In recent years, there has been considerable interest in the use of plasmons, that is, free electron oscillations in conductors, to boost the performance of both organic and inorganic thin film solar cells. This has been driven by the possibility of employing thin active layers in solar cells in order to reduce materials costs, and is enabled by significant advances in fabrication technology. The ability of surface plasmons in metallic nanostructures to guide and confine light in the nanometer scale has opened up new design possibilities for solar cell devices. Here, we report the synthesis and characterization of highly monodisperse, reasonably stable, multipode Au/Ag bimetallic nanostructures using an inorganic additive as a ligand for photovoltaic applications. A promising surface enhanced Raman scattering (SERS) effect has been observed for the synthesized bimetallic Au/Ag multispiked nanoparticles, which compare favorably well with their Au and Ag spherical nanoparticle counterparts. The synthesized plasmonic nanostructures were incorporated on the rear surface of an ultrathin planar c-silicon/organic polymer hybrid solar cell, and the overall effect on photovoltaic performance was investigated. A promising enhancement in solar cell performance parameters, including both the open circuit voltage (VOC) and short circuit current density (JSC), has been observed by employing the aforementioned bimetallic multispiked nanoparticles on the rear surface of solar cell devices. A power conversion efficiency (PCE) value as high as 7.70% has been measured in a hybrid device with Au/Ag multispiked nanoparticles on the rear surface of an ultrathin, crystalline silicon (c-Si) membrane (∼ 12 μm). This value compares well to the measured PCE value of 6.72% for a similar device without nanoparticles. The experimental observations support the hope for a sizable PCE increase, due to plasmon effects, in thin-film, c-Si solar cells in the near future.

Single-Molecule Surface-Enhanced Raman Scattering Sensitivity of Ag-Core Au-Shell Nanoparticles: Revealed by Bi-Analyte Method

Single-molecule surface-enhanced Raman scattering (SM-SERS) is an important application of localized surface plasmons in metallic nanostructures. Conventionally, Ag nanoparticles are used in solution-based SM-SERS experiments, but their usage is limited due to toxicity and oxidation issues. Au nanoparticle solutions are relatively biocompatible and SERS-active, but they do not facilitate large-scale SERS enhancement factors, which is an important prerequisite for SM-SERS. Under such constraints, silver-core gold-shell nanoparticles can be an excellent alternative for SM-SERS. Motivated by this, herein we report on the experimental evidence of SM-SERS sensitivity of Ag-core Au-shell nanoparticles by employing bianalyte method. Additionally, by detecting resonant molecules at femtomolar concentrations, we show that Ag-core Au-shell nanoparticle can be harnessed for ultrasensitive detection of molecules. The provided evidence will further motivate usage of such gold-shell-based bimetallic nanostructures for SM-SERS in biological environments.

DNA‐Directed Self‐Assembly of Core‐Satellite Plasmonic Nanostructures: A Highly Sensitive and Reproducible Near‐IR SERS Sensor

AbstractThe excitation of surface plasmons in metallic nanostructures provides an opportunity to localize light at the nanoscale, well below the scale of the wavelength of the light. The high local electromagnetic field intensities generated in the vicinity of the nanostructures through this nanofocusing effect are exploited in surface enhanced Raman spectroscopy (SERS). At narrow interparticle gaps, so‐called hot‐spots, the nanofocusing effect is particularly pronounced. Hence, the engineering of substrates with a consistently high density of hot‐spots is a major challenge in the field of SERS. Here, a simple bottom‐up approach is described for the fabrication of highly SERS‐active gold core‐satellite nanostructures, using electrostatic and DNA‐directed self‐assembly. It is demonstrated that well‐defined core‐satellite gold nanostructures can be fabricated without the need for expensive direct‐write nanolithography tools such as electron‐beam lithography (EBL). Self‐assembly also provides excellent control over particle distances on the nanoscale. The as‐fabricated core‐satellite nanostructures exhibit SERS activities that are superior to commercial SERS substrates in signal intensity and reproducibility. This also highlights the potential of bottom‐up self‐assembly strategies for the fabrication of complex, well‐defined functional nanostructures with future applications well beyond the field of sensing.

EELS and EFTEM of Surface Plasmons in Metallic Nanostructures

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Controlling Light Localization and Light–Matter Interactions with Nanoplasmonics

Nanoplasmonics is the emerging research field that studies light-matter interactions mediated by resonant excitations of surface plasmons in metallic nanostructures. It allows the manipulation of the flow of light and its interaction with matter at the nanoscale (10(-9) m). One of the most promising characteristics of plasmonic resonances is that they occur at frequencies corresponding to typical electronic excitations in matter. This leads to the appearance of strong interactions between localized surface plasmons and light emitters (such as molecules, dyes, or quantum dots) placed in the vicinity of metals. Recent advances in nanofabrication and the development of novel concepts in theoretical nanophotonics have opened the way to the design of structures aimed to reduce the lifetime and enhance the decay rate and quantum efficiency of available emitters. In this article, some of the most relevant experimental and theoretical achievements accomplished over the last several years are presented and analyzed.

Metal-enhanced fluorescence of colloidal nanocrystals with nanoscale control

Engineering the spectral properties of fluorophores, such as the enhancement of luminescence intensity, can be achieved through coupling with surface plasmons in metallic nanostructures. This process, referred to as metal-enhanced fluorescence, offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies. It becomes even more appealing when applied to colloidal semiconductor nanocrystals, which exhibit size-dependent optical properties, have high photochemical stability, and are characterized by broad excitation spectra and narrow emission bands. Other approaches have relied upon the coupling of fluorophores (typically organic dyes) to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. Here, we develop a new strategy based on the highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals dispersed in a polymer blend. We demonstrate the possibility of obtaining precise control and a high spatial selectivity of the fluorescence enhancement process.

Multiple multipole method with automatic multipole setting applied to the simulation of surface plasmons in metallic nanostructures.

Highly accurate computations of surface plasmons in metallic nanostructures with various geometries are presented. Calculations for cylinders with irregular cross section, coupled structures, and periodic gratings are shown. These systems exhibit a resonant behavior with complex field distribution and strong field enhancement, and therefore their computation requires a very accurate numerical method. It is shown that the multiple multipole (MMP) method, together with an automatic multipole setting (AMS) procedure, is well suited for these computations. An AMS technique for the two-dimensional MMP method is presented. It relies on the global topology of each domain boundary to generate a distribution of numerically independent multipole expansions. This technique greatly facilitates the MMP modeling.