Chemical Interface Damping Depends on Electrons Reaching the Surface

Configuration-Modulated Hot Electron Dynamics of Gold Nanorod Assemblies

Comprehension and modulation of hot electron dynamics at an ultrafast time scale are crucial for exploring the hot electron-assisted energy transfer processes. Here, we report the hot electron dynamics of dispersed gold nanorods and their controlled assemblies measured by time-resolved pump-probe spectroscopy. Both assembly configurations are shown to accelerate the hot electron decay in comparison with dispersed nanorods. The hot electron dynamics exhibit different variations with aspect ratio in transverse and longitudinal polarizations. The hot electron lifetime and the spectral signature of the induced absorption modification are found to be highly sensitive to photon energy as well as assembly configuration and aspect ratios, showing different contributions of plasmon coupling and electron-surface scattering. This work not only improves the understanding of the underlying mechanisms of hot electron dynamics but also paves the way to optimize performance characteristics of hot carrier-assisted photocatalysis, photovoltaics, and all-optical high-rate photonic processing applications.

Nanoscale Surface Redox Chemistry Triggered by Plasmon-Generated Hot Carriers

Direct photoexcitation of charges at a plasmonic metal hotspot produces energetic carriers that are capable of performing photocatalysis in the visible spectrum. However, the mechanisms of generation and transport of hot carriers are still not fully understood and under intense investigation because of their potential technological importance. Here, spectroscopic evidence proves that the reduction of dye molecules tethered to a Au(111) surface can be triggered by plasmonic carriers via a tunneling mechanism, which results in anomalous Raman intensity fluctuations. Tip-enhanced Raman spectroscopy (TERS) helps to correlate Raman intensity fluctuations with temperature and with properties of the molecular spacer. In combination with electrochemical surface-enhanced Raman spectroscopy, TERS results show that plasmon-induced energetic carriers can directly tunnel to the dye through the spacer. This organic spacer chemically isolates the adsorbate from the metal but does not block photo-induced redox reactions, which offers new possibilities for optimizing plasmon-induced photocatalytic systems.

Kinetics and Mechanism of Metal Nanoparticle Growth via Optical Extinction Spectroscopy and Computational Modeling: The Curious Case of Colloidal Gold

An overarching computational framework unifying several optical theories to describe the temporal evolution of gold nanoparticles (GNPs) during a seeded growth process is presented. To achieve this, we used the inexpensive and widely available optical extinction spectroscopy, to obtain quantitative kinetic data. In situ spectra collected over a wide set of experimental conditions were regressed using the physical model, calculating light extinction by ensembles of GNPs during the growth process. This model provides temporal information on the size, shape, and concentration of the particles and any electromagnetic interactions between them. Consequently, we were able to describe the mechanism of GNP growth and divide the process into distinct genesis periods. We provide explanations for several longstanding mysteries, for example, the phenomena responsible for the purple-greyish hue during the early stages of GNP growth, the complex interactions between nucleation, growth, and aggregation events, and a clear distinction between agglomeration and electromagnetic interactions. The presented theoretical formalism has been developed in a generic fashion so that it can readily be adapted to other nanoparticulate formation scenarios such as the genesis of various metal nanoparticles.

Tunable gap plasmons in gold nanospheres adsorbed into a pH-responsive polymer film

HYPOTHESIS

Plasmon nanorulers are exquisitely sensitive distance sensors that are based on the electromagnetic interaction between metal nanoparticles and surfaces. We hypothesize that nanorulers can act as quantitative probes of processes such as particle aggregation and adsorption, and deploy them to investigate particle adsorption onto stimulus-responsive polymer films. While such systems have previously been qualitatively investigated with plasmon nanorulers, our quantitative analysis should provide deeper insights.

EXPERIMENT

Gold nanospheres are adsorbed from solution onto pH-responsive, amine-rich polyelectrolyte multilayer (PEM) films that are either directly deposited on a gold substrate or onto an intermediate self-assembled monolayer (SAM) of charged thiols. Fitting the optical scattering spectrum to a full-wave calculation, we quantify the sphere-substrate gap distance with good accuracy.

FINDINGS

We find that the gold spheres partially embed into the PEMs rather than ride on top of them, and that although the amount of actuation of the spheres afforded by tuning the pH is well controlled, it is significantly smaller than the corresponding thickness changes in unstrained films. Further, the presence of a SAM below the PEM increases the amount of polymer in the PEM, except for the thickest and most highly charged films, where the SAM instead appears to displace from the area below the nanospheres.

Plasmon-Driven Catalysis on Molecules and Nanomaterials

As a new class of photocatalysts, plasmonic noble metal nanoparticles with the unique ability to harvest solar energy across the entire visible spectrum and produce effective energy conversion have been explored as a promising pathway for the energy crisis. The resonant excitation of surface plasmon resonance allows the nanoparticles to collect the energy of photons to form a highly enhanced electromagnetic field, and the energy stored in the plasmonic field can induce hot carriers in the metal. The hot electron-hole pairs ultimately dissipate by coupling to phonon modes of the metal nanoparticles, resulting in a higher lattice temperature. The plasmonic electromagnetic field, hot electrons, and heat can catalyze chemical reactions of reactants near the surface of the plasmonic metal nanoparticles. This Account summarizes recent theoretical and experimental advances on the excitation mechanisms and energy transfer pathways in the plasmonic catalysis on molecules. Especially, current advances on plasmon-driven crystal growth and transformation of nanomaterials are introduced. The efficiency of the chemical reaction can be dramatically increased by the plasmonic electromagnetic field because of its higher density of photons. Similar to traditional photocatalysis, energy overlap between the plasmonic field and the HOMO-LUMO gap of the reactant is needed to realize resonant energy transfer. For hot-carrier-driven catalysis, hot electrons generated by plasmon decay can be transferred to the reactant through the indirect electron transfer or direct electron excitation process. For this mechanism, the energy of hot electrons needs to overlap with the unoccupied orbitals of the reactant, and the particular chemical channel can be selectively enhanced by controlling the energy distribution of hot electrons. In addition, the local thermal effect following plasmon decay offers an opportunity to facilitate chemical reactions at room temperature. Importantly, surface plasmons can not only catalyze chemical reactions of molecules but also induce crystal growth and transformation of nanomaterials. As a new development in plasmonic catalysis, plasmon-driven crystal transformation reveals a more powerful aspect of the catalysis effect, which opens the new field of plasmonic catalysis. We believe that this Account will promote clear understanding of plasmonic catalysis on both molecules and materials and contribute to the design of highly tunable catalytic systems to realize crystal transformations that are essential to achieve efficient solar-to-chemical energy conversion.

Plasmon-promoted electrocatalytic water splitting on metal-semiconductor nanocomposites: the interfacial charge transfer and the real catalytic sites

The interfacial charge transfer and real catalytic sites on metal–semiconductor hybrid catalysts for photoelectrocatalytic water splitting are studied.

Plasmonic metal nanoparticles (NPs) have emerged as promising visible light harvesters to facilitate solar-to-chemical energy conversion via the generation of hot electrons by non-radiative decay of plasmons. As one of the most promising renewable energy production methods for the future, electrocatalytic water splitting is an ideal chemical reaction in which plasmonic NPs can be utilized for direct solar-to-fuel conversion. Due to the rapid carrier recombination on plasmonic NPs, hybrid photocatalysts integrating metals and semiconductors are usually employed to separate the hot electrons and holes. However, an understanding of the catalytic mechanism, which is critical for rational design of plasmonic electrocatalysts, including the interfacial charge transfer pathway and real reactive sites, has been lacking. Herein, we report on the combination of plasmonic Au NPs and semiconductors (Ni and/or Co hydroxides) for plasmon-promoted electrocatalytic water splitting. By using surface-enhanced Raman spectroscopy (SERS), we find a strong spontaneous interfacial charge transfer between Au and NiCo layered double hydroxide (LDH), which facilitates both the oxygen and hydrogen evolution reactions. The real catalytic sites on the hybrid material are confirmed by selective blocking of the metal surface with a thiol molecular monolayer. It is found that the plasmon-promoted oxygen evolution occurs on the LDH semiconductor but surprisingly, the hydrogen evolution sites are mainly located on the Au NP surface. This work demonstrates the critical role of interfacial charge transfer in hot electron-driven water splitting and paves the way for rational design of high-performance plasmonic electrocatalysts.

Making waves: Radiation damping in metallic nanostructures

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Quantitative Measurement of the Optical Cross Sections of Single Nano-objects by Correlative Transmission and Scattering Microspectroscopy

The scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross sections of individual nano-objects. The method applies to microspectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects.

Role of Valence Band States and Plasmonic Enhancement in Electron-Transfer-Induced Transformation of Nitrothiophenol

Hot-electron-induced reactions are more and more recognized as a critical and ubiquitous reaction in heterogeneous catalysis. However, the kinetics of these reactions is still poorly understood, which is also due to the complexity of plasmonic nanostructures. We determined the reaction rates of the hot-electron-mediated reaction of 4-nitrothiophenol (NTP) on gold nanoparticles (AuNPs) using fractal kinetics as a function of the laser wavelength and compared them with the plasmonic enhancement of the system. The reaction rates can be only partially explained by the plasmonic response of the NPs. Hence, synchrotron X-ray photoelectron spectroscopy (XPS) measurements of isolated NTP-capped AuNP clusters have been performed for the first time. In this way, it was possible to determine the work function and the accessible valence band states of the NP systems. The results show that besides the plasmonic enhancement, the reaction rates are strongly influenced by the local density of the available electronic states of the system.

Site-selective CO disproportionation mediated by localized surface plasmon resonance excited by electron beam

Recent reports of hot-electron-induced dissociation of small molecules, such as hydrogen, demonstrate the potential application of plasmonic nanostructures for harvesting light to initiate catalytic reactions. Theories have assumed that plasmonic catalysis is mediated by the energy transfer from nanoparticles to adsorbed molecules during the dephasing of localized surface plasmon (LSP) modes optically excited on plasmonic nanoparticles. However, LSP-induced chemical processes have not been resolved at a sub-nanoparticle scale to identify the active sites responsible for the energy transfer. Here, we exploit the LSP resonance excited by electron beam on gold nanoparticles to drive CO disproportionation at room temperature in an environmental scanning transmission electron microscope. Using in situ electron energy-loss spectroscopy with a combination of density functional theory and electromagnetic boundary element method calculations, we show at the subparticle level that the active sites on gold nanoparticles are where preferred gas adsorption sites and the locations of maximum LSP electric field amplitude (resonance antinodes) superimpose. Our findings provide insight into plasmonic catalysis and will be valuable in designing plasmonic antennas for low-temperature catalytic processes.

Tuning Chemical Interface Damping: Interfacial Electronic Effects of Adsorbate Molecules and Sharp Tips of Single Gold Bipyramids

The optimization of the localized surface plasmon resonance (LSPR)-decaying channels of hot-electrons is essential for efficient optical and photochemical processes. Understanding and having the ability to control chemical interface damping (CID) channel contributions will bring about new possibilities for tuning the efficiency of plasmonic hot-electron energy transfer in artificial devices. In this scanning electron microscopy-correlated dark-field scattering study, the CID was controlled by focusing on the electronic nature of disubstituted benzene rings acting as adsorbates, as well as the effects of sharp tips on gold bipyramids (AuBPs) with similar aspect ratios to those of gold nanorods. The results showed that the sharp tips on single AuBPs, as well as the electronic effects of the adsorbate molecules, increase the interfacial contact between the nanoparticles and adsorbate molecules. Electron withdrawing groups (EWGs) on the adsorbates induce larger homogeneous LSPR line widths compared to those of electron donating groups (EDGs). Depending on the location (ortho, meta, and para) of the EDG, the effect of benzene rings with an EDG, which was considered to be induced by sulfur atoms bound to the nanoparticle surface, is weakened by the back transfer of electrons facilitated by the difference in the availability of the electrons of the EDG. Therefore, this study reports that the CID in the LSPR total decay channels can be tuned by controlling the electron withdrawing and electron donating features of adsorbate molecules with the surface topology of metal.

Plasmon-Induced Direct Hot-Carrier Transfer at Metal-Acceptor Interfaces

Plasmon-induced hot-carrier transfer from a metal nanostructure to an acceptor is known to occur via two key mechanisms: (i) indirect transfer, where the hot carriers are produced in the metal nanostructure and subsequently transferred to the acceptor, and (ii) direct transfer, where the plasmons decay by directly exciting carriers from the metal to the acceptor. Unfortunately, an atomic-level understanding of the direct-transfer process, especially with regard to its quantification, remains elusive even though it is estimated to be more efficient compared to the indirect-transfer process. This is due to experimental challenges in separating direct from indirect transfer as both processes occur simultaneously at femtosecond time scales. Here, we employ time-dependent density-functional theory simulations to isolate and study the direct-transfer process at a model metal-acceptor (Ag₁₄₇-Cd₃₃Se₃₃) interface. Our simulations show that, for a 10 fs Gaussian laser pulse tuned to the plasmon frequency, the plasmon formed in the Ag₁₄₇-Cd₃₃Se₃₃ system decays within 10 fs and induces the direct transfer with a probability of about 40%. We decompose the direct-transfer process further and demonstrate that the direct injection of both electrons and holes into the acceptor, termed direct hot-electron transfer (DHET) and direct hot-hole transfer (DHHT), takes place with similar probabilities of about 20% each. Finally, effective strategies to control and tune the probabilities of DHET and DHHT processes are proposed. We envision our work to provide guidelines toward the design of metal-acceptor interfaces that enable more efficient plasmonic hot-carrier devices.

Plasmon damping depends on the chemical nature of the nanoparticle interface

The chemical nature of surface adsorbates affects the localized surface plasmon resonance of metal nanoparticles. However, classical electromagnetic simulations are blind to this effect, whereas experiments are typically plagued by ensemble averaging that also includes size and shape variations. In this work, we are able to isolate the contribution of surface adsorbates to the plasmon resonance by carefully selecting adsorbate isomers, using single-particle spectroscopy to obtain homogeneous linewidths, and comparing experimental results to high-level quantum mechanical calculations based on embedded correlated wavefunction theory. Our approach allows us to indisputably show that nanoparticle plasmons are influenced by the chemical nature of the adsorbates 1,7-dicarbadodecaborane(12)-1-thiol (M1) and 1,7-dicarbadodecaborane(12)-9-thiol (M9). These surface adsorbates induce inside the metal electric dipoles that act as additional scattering centers for plasmon dephasing. In contrast, charge transfer from the plasmon to adsorbates-the most widely suggested mechanism to date-does not play a role here.

Direct hot-carrier transfer in plasmonic catalysis

An ab initio computational study of direct hot-carrier transfer at metal–molecule interfaces with relevance to plasmonic catalysis.

Influence of the capping material on pyridine-induced chemical interface damping in single gold nanorods

Chemical interface damping (CID) is one of the plasmon decay processes that occur in gold nanoparticles. With the aim of exploring new functional groups that can induce CID as an alternative to thiol groups, we performed dark-field (DF) scattering studies of gold nanorods (AuNRs) using pyridine as adsorbate. We found that the adsorption of pyridine molecules on single AuNRs though nitrogen-gold interactions leads to an increase of the localized surface plasmon resonance (LSPR) linewidth. However, pyridine molecules were not adsorbed effectively on AuNR surfaces having a capping reagent. This study allows us to gain insight into the effect and role of the capping reagent in pyridine-induced CID. Furthermore, pyridine was revealed to induce a strong CID through the interaction of the nitrogen atom with the Au surface, provided the capping material was previously removed from the AuNRs by oxygen plasma treatment. Finally, we demonstrated that CID could be used to sense pyridine and its derivatives in real-time.

Impact of chemical interface damping on surface plasmon dephasing

We characterized the change in photon absorption and scattering properties of plasmonic Au nanoparticles by chemical interface damping.

Plasmonic Nanosensors Reveal a Height Dependence of MinDE Protein Oscillations on Membrane Features

Single-particle plasmon spectroscopy has become a standard technique to detect and quantify the presence of unlabeled macromolecules. Here, we extend this method to determine their exact distance from the plasmon sensors with sub-nanometer resolution by systematically varying the sensing range into the surrounding by adjusting the size of the plasmonic nanoparticles. We improved current single-particle plasmon spectroscopy to record continuously for hours the scattering spectra of thousands of nanoparticles of different sizes simultaneously with 1.8 s time resolution. We apply this technique to study the interaction dynamics of bacterial Min proteins with supported lipid membranes of different composition. Our experiments reveal a surprisingly flexible operating mode of the Min proteins: In the presence of cardiolipin and membrane curvature induced by nanoparticles, the protein oscillation occurs on top of a stationary MinD patch. Our results reveal the need to consider membrane composition and local curvature as important parameters to quantitatively understand the Min protein system and could be extrapolated to other macromolecular systems. Our label-free method is generally easily implementable and well suited to measure distances of interacting biological macromolecules.

Quantum-Sized Metal Catalysts for Hot-Electron-Driven Chemical Transformation

Hot-electron-driven chemical transformation (HEDCT) represents an emerging research area in utilizing photoresponsive nanoparticles to enable efficient solar-to-chemical conversion. The unique properties of quantum-sized metal nanoparticles (QSMNPs) make them a class of photocatalysts that can generate hot electrons to drive surface chemical reactions with high quantum efficiency. Compared to the conventional thermal-driven chemical reactions, HEDCT offers the advantages of accelerating reaction rate, improving reaction selectivity, and possibly enabling the occurrence of thermodynamically endergonic reactions. Despite its embryonic stage of development, using QSMNPs for HEDCT shows great promise. Herein, a timely overview on the research progress is provided with a focus on the fundamental quantum processes involved in the photoexcitation of hot electrons and the following HEDCT on the surface of QSMNPs. The last section discusses the challenges, which also represent the opportunities for the materials research community, in designing robust QSMNP photocatalysts and understanding the fundamental quantum phenomena in HEDCT.

Single particle study: size and chemical effects on plasmon damping at the interface between adsorbate and anisotropic gold nanorods

Plasmon damping in gold nanorods (AuNRs) results in the broadening of the localized surface plasmon resonance (LSPR) linewidth. LSPR broadening of plasmonic nanoparticles is useful to maximize the fraction of light energy in light harvesting and energy conversion transferred to molecules attached on the surface. However, our understanding of plasmon decay channels in AuNRs is still limited, and chemical interface damping (CID) is the most poorly understood damping mechanism. Herein, to better understand plasmon damping including CID, we performed a single particle study of plasmonic anisotropic AuNRs using dark-field (DF) microscopy and spectroscopy. First, we examined the size-dependent broadening of the homogeneous LSPR linewidth of single AuNRs in water with three different aspect ratios (ARs) at a fixed diameter of 25 nm. The LSPR linewidth increased with a decrease in the AR of single AuNRs because of the reduced average distance of hot electrons to the surface. Second, we investigated the effect of refractive index variation of the surrounding medium on the LSPR linewidth in single AuNRs of three different sizes. The LSPR linewidth in single AuNRs remained almost constant regardless of their sizes while increasing the dielectric constant of the medium. Finally, we examined the effect of adsorbate thiol molecules on the homogeneous LSPR linewidth of single AuNRs in ethanol. The LSPR linewidth was broadened upon increasing the carbon chain length of 1-alkanethiol, and 4-nitrothiophenol with a strong electron withdrawing group induced a large broadening of the LSPR linewidth. Furthermore, single AuNRs with smaller ARs showed a larger broadening of the LSPR linewidth in the presence of adsorbate thiol molecules through CID. Therefore, this investigation provides a deeper insight into the size effect on plasmon damping including CID induced by the chemical interface effect in single AuNRs.

Homogeneous localized surface plasmon resonance inflection points for enhanced sensitivity and tracking plasmon damping in single gold bipyramids

The most polarizable localized surface plasmon resonance (LSPR) longitudinal mode of anisotropic metallic nanoparticles, such as gold bipyramids (AuBPs), is of high prominence. This optical response has tremendous applications from spectroscopy to photonics and energy devices to sensing. In conventional LSPR-based sensing, broadening and asymmetry in peaks due to chemical and instrument noise hinder obtaining a precise insight on shift positions, accordingly limiting the effectiveness and impact of LSPR sensors. Further, when investigating LSPR properties, utilizing more simplistic frequency dependent dielectric-type models can aberrantly impact the reliability of fundamental properties used for designing and fabricating efficient optical devices. For instance, more approximations can effectively limit screening intra-band and inter-band (IB) electronic transition contributions and other related optical properties. With an aim to find alternative methods to further improve their efficiency, as a first report, we devoted a particular focus on LSPR scattering inflection points (IFs) of single AuBPs. The findings reveal that tracking LSPR IFs exhibit high sensitivity over their counterpart LSPR peak shift locations. In addition, we newly detected IB transition contributions near the resonance energy in the range (1.50 eV-2.00 eV) dominated by intra-band transitions. A small increase in the local RI effectively enhances the LSPR quality factor due to IB transitions. Therefore, while neglecting IB transitions in the range below 2.4 eV can work for local air refractive index (RI), in high local RI media it can be aberrantly underestimated. Demonstrated by the use of the dielectric function based on Kramers-Kronig consistent Lorentz oscillators, our findings are in good agreement with the enhancing RI sensitivity effect. The results of this investigation support the idea that tracking curvature changes of an optical signal can be effectively used for LSPR longitudinal peak RI sensing as well as damping in the local RI environment of a single AuBP.

Surface-Enhanced Raman Scattering-Active Substrate Prepared with New Plasmon-Activated Water

Conventionally, reactions in aqueous solutions are prepared using deionized (DI) water, the properties of which are related to inert "bulk water" comprising a tetrahedral hydrogen-bonded network. In this work, we demonstrate the distinguished benefits of using in situ plasmon-activated water (PAW) with reduced hydrogen bonds instead of DI water in electrochemical reactions, which generally are governed by diffusion and kinetic controls. Compared with DI water-based systems, the diffusion coefficient and the electron-transfer rate constant of K₃Fe(CN)₆ in PAW in situ can be increased by ca. 35 and 15%, respectively. These advantages are responsible for the improved performance of surface-enhanced Raman scattering (SERS). On the basis of PAW in situ, the SERS enhancement of twofold higher intensity of rhodamine 6G and the corresponding low relative standard deviation of 5%, which is comparable to and even better than those based on complicated processes shown in the literature, are encouraging.

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.

Plasmonic metal-semiconductor photocatalysts and photoelectrochemical cells: a review

The incorporation of plasmonic metals into semiconductors is a promising route to improve the performance of photocatalysts and photoelectrochemical cells. This article summarizes the three major mechanisms of plasmonic energy transfer from a metal to a semiconductor, including light scattering/trapping, plasmon-induced resonance energy transfer (PIRET) and hot electron injection (also called direct electron transfer (DET)). It also discusses the rational design of plasmonic metal-semiconductor heterojunctions based on the underlying plasmonic energy transfer mechanisms. Moreover, this article highlights the applications of plasmonic photocatalysts and photoelectrochemical cells in solar water splitting, carbon dioxide reduction and environmental pollutant decomposition.

Transition-Metal Decorated Aluminum Nanocrystals

Recently, aluminum has been established as an earth-abundant alternative to gold and silver for plasmonic applications. Particularly, aluminum nanocrystals have shown to be promising plasmonic photocatalysts, especially when coupled with catalytic metals or oxides into "antenna-reactor" heterostructures. Here, a simple polyol synthesis is presented as a flexible route to produce aluminum nanocrystals decorated with eight varieties of size-tunable transition-metal nanoparticle islands, many of which have precedence as heterogeneous catalysts. High-resolution and three-dimensional structural analysis using scanning transmission electron microscopy and electron tomography shows that abundant nanoparticle island decoration in the catalytically relevant few-nanometer size range can be achieved, with many islands spaced closely to their neighbors. When coupled with the Al nanocrystal plasmonic antenna, these small decorating islands will experience increased light absorption and strong hot-spot generation. This combination makes transition-metal decorated aluminum nanocrystals a promising material platform to develop plasmonic photocatalysis, surface-enhanced spectroscopies, and quantum plasmonics.