Localized surface plasmons and hot electrons

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Fundamentals and applications of photo-thermal catalysis

Photo-thermal catalysis has recently emerged as an alternative route to drive chemical reactions using light as an energy source.

Recent Advances in Plasmon-Promoted Organic Transformations Using Silver-Based Catalysts

Plasmonics has emerged as a promising methodology to promote chemical reactions and has become a field of intense research effort. Ag nanoparticles (NPs) as plasmonic catalysts have been extensively studied because of their remarkable optical properties. This review analyzes the emergence and development of localized surface plasmon resonance (LSPR) in organic chemistry, mainly focusing on the discovery of novel reactions with new mechanisms on Ag NPs. Initially, the basics of LSPR and LSPR-promoted photocatalytic mechanisms are illustrated. Then, the recent advances in plasmonic nanosilver-mediated photocatalysis in organic transformations are highlighted with an emphasis on the related reaction mechanisms. Finally, a proper perspective on the remaining challenges and future directions in the field of LSPR-promoted organic transformations is proposed.

Fast Photoelectric Conversion in the Near-Infrared Enabled by Plasmon-Induced Hot-Electron Transfer

Interfacial charge transfer is a fundamental and crucial process in photoelectric conversion. If charge transfer is not fast enough, carrier harvesting can compromise with competitive relaxation pathways, e.g., cooling, trapping, and recombination. Some of these processes can strongly affect the speed and efficiency of photoelectric conversion. In this work, it is elaborated that plasmon-induced hot-electron transfer (HET) from tungsten suboxide to graphene is a sufficiently fast process to prevent carrier cooling and trapping processes. A fast near-infrared detector empowered by HET is demonstrated, and the response time is three orders of magnitude faster than that based on common band-edge electron transfer. Moreover, HET can overcome the spectral limit of the bandgap of tungsten suboxide (≈2.8 eV) to extent the photoresponse to the communication band of 1550 nm (≈0.8 eV). These results indicate that plasmon-induced HET is a new strategy for implementation of efficient and high-speed photoelectric devices.

Time-Resolved Study of Site-Specific Corrosion in a Single Crystalline Silver Nanoparticle

We followed over 24 h a corrosion process in monocrystalline triangular-shaped nanoparticles at a single-particle level by atomic force microscopy and optical spectroscopy techniques under ambient laboratory conditions. The triangular-shaped form of the particles was selected, because the crystallographic orientation of the particles is well defined upon their deposition on a substrate. We observed that the particles already start to alter within this time frame. Surprisingly, the corrosion starts predominantly from the tips of the particles and it creates within few hours large protrusions, which strongly suppress the plasmon character of the particles. These observations support the crystallographic model of these particles consisting of a high-defect hexagonal closed packed layer, and they could help material scientists to design more stable silver nanoparticles. Moreover, this described technique can be used to reveal kinetics of the corrosion in the nanoscale of other materials.

Harnessing Plasmon-Induced Hot Carriers at the Interfaces With Ferroelectrics

This article reviews the scientific understanding and progress of interfacing plasmonic particles with ferroelectrics in order to facilitate the absorption of low-energy photons and their conversion to chemical fuels. The fundamental principles of hot carrier generation and charge injection are described for semiconductors interfaced with metallic nanoparticles and immersed in aqueous solutions, forming a synergistic juncture between the growing fields of plasmonically-driven photochemistry and semiconductor photocatalysis. The underlying mechanistic advantages of a metal-ferroelectric vs. metal-nonferroelectric interface are presented with respect to achieving a more optimal and efficient control over the Schottky barrier height and charge separation. Notable recent examples of using ferroelectric-interfaced plasmonic particles have demonstrated their roles in yielding significantly enhanced photocurrents as well as in the photon-driven production of molecular hydrogen. Notably, plasmonically-driven photocatalysis has been shown to occur for photon wavelengths in the infrared range, which is at lower energies than typically possible for conventional semiconductor photocatalysts. Recent results thus demonstrate that integrated ferroelectric-plasmonic systems represent a potentially transformative concept for use in the field of solar energy conversion.

Quantifying Wavelength-Dependent Plasmonic Hot Carrier Energy Distributions at Metal/Semiconductor Interfaces

Hot carriers generated from the nonradiative decay of localized surface plasmons are capable of driving charge-transfer reactions at the surfaces of metal nanostructures. Photocatalytic devices utilizing plasmonic hot carriers are often based on metal nanoparticle/semiconductor heterostructures owing to their efficient electron-hole separation ability. The rapid thermalization of hot carriers generated at the metal nanoparticles yields a distribution of carrier energies that determines the capability of the photocatalytic device to drive redox reactions. Here, we quantify the thermalized hot carrier energy distribution generated at Au/TiO₂ nanostructures using wavelength-dependent scanning electrochemical microscopy and a series of molecular probes with different redox potentials. We determine the quantum efficiencies and oxidizing power of the hot carriers from wavelength-dependent reaction rates and photocurrent across the metal/semiconductor interface. The wavelength-dependent reaction efficiency tracks the surface plasmon resonance spectrum of the Au nanoparticles, showing that the reaction is facilitated by plasmon excitation, while the responses from molecules with different redox potentials shed light on the energy distribution of the hot holes generated at metal nanoparticle/semiconductor heterostructures. The results provide important insight into the energies of the plasmon-generated hot carriers and quantum efficiencies of plasmonic photocatalytic devices.

Simple Unbiased Hot-Electron Polarization-Sensitive Near-Infrared Photodetector

Plasmonic nanostructures can generate energetic "hot" electrons from light in a broad band fashion depending on their shape, size, and arrangement. Such structures have a promising use in photodetectors, allowing high speed, broad band, and multicolor photodetection. Because they function without a band gap absorption, photon detection at any energy would be possible through engineering of the plasmonic nanostructure. Herein, a compact hot-electron-based photodetector that combines polarization sensitivity and circularly polarized light detection in the near-infrared region was fabricated using an indium tin oxide (ITO)-Au hybrid layer. Furthermore, the sensitivity of the device was significantly increased by adding a poled Azo molecular glass film in a capacitor configuration. The resulting device is capable of detecting light below the ITO band gap at ambient temperature without any bias voltage. This photodetector, which is amenable to large-area fabrication, can be integrated with other nanophotonic and nanoplasmonic structures for operation at telecom wavelengths.

Correlated Absorption and Scattering Spectroscopy of Individual Platinum-Decorated Gold Nanorods Reveals Strong Excitation Enhancement in the Nonplasmonic Metal

Bimetallic nanocatalysts have the potential to surmount current limitations in industrial catalysis if their electronic and optical properties can be effectively controlled. However, improving the performance of bimetallic photocatalysts requires a functional understanding of how the intricacies of their morphology and composition dictate every element of their optical response. In this work, we examine Au and Pt-decorated Au nanorods on a single-particle level to ascertain how Pt influences the plasmon resonance of the bimetallic nanostructure. We correlated scattering, photoluminescence, and pure absorption of individual nanostructures separately to expose the impact of Pt on each component. We found that the scattering and absorption spectra of uncoated Au nanorods followed expected trends in peak intensity and shape and were accurately reproduced by finite difference time domain simulations. In contrast, the scattering and absorption spectra of single Pt-decorated Au nanorods exhibited red-shifted, broad features and large deviations in line shape from particle to particle. Simulations using an idealized geometry confirmed that Pt damps the plasmon resonance of individual Au nanorods and that spectral changes after Pt deposition were a consequence of coupling between Au and Pt in the hybrid nanostructure. Simulations also revealed that the Au nanorod acts as an antenna and enhances absorption in the Pt islands. Furthermore, comparing photoluminescence spectra from Au and Pt-decorated Au nanorods illustrated that emission was significantly reduced in the presence of Pt. The reduction in photoluminescence intensity indicates that Pt lowers the number of hot carriers in the Au nanorod available for radiative recombination through either direct production of hot carriers in Pt following enhanced absorption or charge transfer from Au to Pt. Overall, these results confirm that the Pt island morphology and distribution on the nanorod surface contribute to the optical response of individual hybrid nanostructures and that the damping observed in ensemble measurements originates not only from structural heterogeneity but also because of significant damping in single nanostructures.

Plasmon-Induced Conductance Switching of an Electroactive Conjugated Polymer Nanojunction

A plasmonic molecular electronic device, consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires bridging an ultramicroelectrode and an indium tin oxide (ITO) substrate covered by gold nanoparticles (Au NPs), has been developed. Light irradiation of this device has a dramatic impact on its conductance. Polymer strands, maintained electrochemically in their oxidized, conducting state, reversibly switch to their insulating state upon irradiation by visible-wavelength light, resulting in a sharp decrease in the conductance. The high-conductance state is restored when the light is turned off. Switching depends on the wavelength and the intensity of the incident light. It is due to reversible reduction of the nanosized region of PEDOT nanowires in contact with a gold NP and is attributed to plasmon-induced hot-electron injection into the PEDOT. The high/low conductance ratio can be as great as 1000, and switching requires low light intensity (220 W/m²). These results could open the way to the design of a new family of optoelectronic switches.

Harnessing Hot Electrons from Near IR Light for Hydrogen Production Using Pt-End-Capped-AuNRs

Gold nanorods show great potential in harvesting natural sunlight and generating hot charge carriers that can be employed to produce electrical or chemical energies. We show that photochemical reduction of Pt(IV) to Pt metal mainly takes place at the ends of gold nanorods (AuNRs), suggesting photon-induced hot electrons are localized in a time-averaged manner at AuNR ends. To use these hot electrons efficiently, a novel synthetic method to selectively overgrow Pt at the ends of AuNRs has been developed. These Pt-end-capped AuNRs show relatively high activity for the production of hydrogen gas using artificial white light, natural sunlight, and more importantly, near IR light at 976 nm. Tuning of the surface plasmon resonance (SPR) wavelength of AuNRs changes the hydrogen gas production rate, indicating that SPR is involved in hot electron generation and photoreduction of hydrogen ions. This study shows that gold nanorods are excellent for converting low-energy photons into high-energy hot electrons, which can be used to drive chemical reactions at their surfaces.

Reversible light-dependent molecular switches on Ag/AgCl nanostructures

Nanostructured Ag/AgCl substrates were used to generate reversible and highly efficient light-dependent chemical switches based on adsorbed 4,4'-dimercaptoazobenzene (DMAB). DMAB was formed in situ via laser-induced dimerization either from 4-nitrothiophenol (4-NTP) or 4-aminothiophenol (4-ATP). The subsequent reaction pathways of DMAB, however, were quite different as monitored by surface enhanced Raman spectroscopy. In the 4-NTP/DMAB system, AgCl catalyses the reversal of the dimerization. Conversely, irradiation of adsorbed 4-ATP first generated cis-DMAB attached to the surface via two Ag-S bonds, followed by AgCl-catalysed cleavage of one Ag-S bond and cis → trans photoisomerisation of DMAB. In the dark, the trans-isomer thermally reverts to cis-DMAB. The here presented light-dark chemical switches, which work without changing other parameters (e.g., pH, anaerobic vs. aerobic), are based on the (photo)catalytic properties of the Ag/AgCl substrate and do not function on pure metal surfaces.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

The effect of hot electrons and surface plasmons on heterogeneous catalysis

Hot electrons and surface-plasmon-driven chemistry are amongst the most actively studied research subjects because they are deeply associated with energy dissipation and the conversion processes at the surface and interfaces, which are still open questions and key issues in the surface science community. In this topical review, we give an overview of the concept of hot electrons or surface-plasmon-mediated hot electrons generated under various structural schemes (i.e. metals, metal-semiconductor, and metal-insulator-metal) and their role affecting catalytic activity in chemical reactions. We highlight recent studies on the relation between hot electrons and catalytic activity on metallic surfaces. We discuss possible mechanisms for how hot electrons participate in chemical reactions. We also introduce controlled chemistry to describe specific pathways for selectivity control in catalysis on metal nanoparticles.

Plasmonically Enhanced Photocatalytic Hydrogen Production from Water: The Critical Role of Tunable Surface Plasmon Resonance from Gold-Silver Nanoshells

Gold-silver nanoshells (GS-NSs) having a tunable surface plasmon resonance (SPR) were employed to facilitate charge separation of photoexcited carriers in the photocalytic production of hydrogen from water. Zinc indium sulfide (ZnIn2S4; ZIS), a visible-light-active photocatalyst, where the band gap varies with the [Zn]/[In] ratio, was used as a model ZIS system (E(g) = 2.25 eV) to investigate the mechanisms of plasmonic enhancement associated with the nanoshells. Three types of GS-NS cores with intense absorptions centered roughly at 500, 700, and 900 nm were used as seeds for preparing GS-NS@ZIS core-shell structures via a microwave-assisted hydrothermal reaction, yielding core-shell particles with composite diameters of ∼200 nm. Notably, an interlayer of dielectric silica (SiO2) between the GS-NSs and the ZIS photocatalyst provided another parameter to enhance the production of hydrogen and to distinguish the charge-transfer mechanisms. In particular, the direct transfer of hot electrons from the GS-NSs to the ZIS photocatalyst was blocked by this layer. Of the 10 particle samples examined in this study, the greatest hydrogen gas evolution rate was observed for GS-NSs having a SiO2 interlayer thickness of ∼17 nm and an SPR absorption centered at ∼700 nm, yielding a rate 2.6 times higher than that of the ZIS without GS-NSs. The apparent quantum efficiencies for these core-shell particles were recorded and compared to the absorption spectra. Analyses of the charge-transfer mechanisms were evaluated and are discussed based on the experimental findings.

Reversible coupling of 4-nitroaniline molecules to 4-aminothiophenol functionalized on Ag nanoparticle/graphene oxide nanocomposites through the plasmon assisted chemical reaction

In this study, we demonstrate the formation of azo compounds on a composite of silver nanoparticles and graphene oxides (Ag@G) through a plasmon-assisted coupling reaction between different amine compounds.

Infrared hot-carrier photodetection based on planar perfect absorber

Hot-carrier based photodetectors are independent on the semiconductor bandgap, thus paving a new paradigm of photovoltaic conversion. Herein, we propose a non-nanostructured and multilayered metal/insulator/transparent conductive oxide/silica/reflector system, and explore in detail the optical response and the electrical transport in the device via the finite-element electromagnetic simulation and the probability-based analytical carrier-transport calculation. Results show that the planar system can function as a planar perfect absorber at the targeted wavelength under the inbuilt cavity resonance with a very high tunability by tailoring the cavity length and the metal thickness. Moreover, a strong asymmetrical absorption is formed in the two electrode layers, yielding strong unidirectional photocurrents and output power densities. This Letter suggests a more simple and feasible way to realize hot-carrier infrared photodetectors.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Controllable Tuning Plasmonic Coupling with Nanoscale Oxidation

The nanoparticle on mirror (NPoM) construct is ideal for the strong coupling of localized plasmons because of its simple fabrication and the nanometer-scale gaps it offers. Both of these are much harder to control in nanoparticle dimers. Even so, realizing controllable gap sizes in a NPoM remains difficult and continuous tunability is limited. Here, we use reactive metals as the mirror so that the spacing layer of resulting metal oxide can be easily and controllably created with specific thicknesses resulting in continuous tuning of the plasmonic coupling. Using Al as a case study, we contrast different approaches for oxidation including electrochemical oxidation, thermal annealing, oxygen plasma treatments, and photo-oxidation by laser irradiation. The thickness of the oxidation layer is calibrated with depth-mode X-ray photoemission spectroscopy (XPS). These all consistently show that increasing the thickness of the oxidation layer blue-shifts the plasmonic resonance peak while the transverse mode remains constant, which is well matched by simulations. Our approach provides a facile and reproducible method for scalable, local and controllable fabrication of NPoMs with tailored plasmonic coupling, suited for many applications of sensing, photochemistry, photoemission, and photovoltaics.