Plasmon-induced hot carriers in metallic nanoparticles

Theory of "Hot" Photoluminescence from Drude Metals

We provide a complete quantitative theory for light emission from Drude metals under continuous wave illumination, based on our recently derived steady-state nonequilibrium electron distribution. We show that the electronic contribution to the emission exhibits a dependence on the emission frequency which is very similar to the energy dependence of the nonequilibrium distribution, and characterize different scenarios determining the measurable emission line shape. This enables the identification of experimentally relevant situations, where the emission lineshapes deviate significantly from predictions based on the standard theory (namely, on the photonic density of states), and enables the differentiation between cases where the emission scales with the metal object surface or with its volume. We also provide an analytic description (which is absent from the literature) of the (polynomial) dependence of the metal emission on the electric field, its dependence on the pump laser frequency, and its nontrivial exponential dependence on the electron temperature, both for the Stokes and anti-Stokes regimes. Our results imply that the emission does not originate from either Fermion statistics (due to e-e interactions), and even though one could have expected the emission to follow boson statistics due to involvement of photons (as in Planck's Black Body emission), it turns out that it deviates from that form as well. Finally, we resolve the arguments associated with the effects of electron and lattice temperatures on the emission, and which of them can be extracted from the anti-Stokes emission.

Hot Electrons in TiO2-Noble Metal Nano-Heterojunctions: Fundamental Science and Applications in Photocatalysis

Plasmonic photocatalysis enables innovation by harnessing photonic energy across a broad swathe of the solar spectrum to drive chemical reactions. This review provides a comprehensive summary of the latest developments and issues for advanced research in plasmonic hot electron driven photocatalytic technologies focusing on TiO2-noble metal nanoparticle heterojunctions. In-depth discussions on fundamental hot electron phenomena in plasmonic photocatalysis is the focal point of this review. We summarize hot electron dynamics, elaborate on techniques to probe and measure said phenomena, and provide perspective on potential applications-photocatalytic degradation of organic pollutants, CO2 photoreduction, and photoelectrochemical water splitting-that benefit from this technology. A contentious and hitherto unexplained phenomenon is the wavelength dependence of plasmonic photocatalysis. Many published reports on noble metal-metal oxide nanostructures show action spectra where quantum yields closely follow the absorption corresponding to higher energy interband transitions, while an equal number also show quantum efficiencies that follow the optical response corresponding to the localized surface plasmon resonance (LSPR). We have provided a working hypothesis for the first time to reconcile these contradictory results and explain why photocatalytic action in certain plasmonic systems is mediated by interband transitions and in others by hot electrons produced by the decay of particle plasmons.

Surface plasmon mediates the visible light-responsive lithium-oxygen battery with Au nanoparticles on defective carbon nitride

Aprotic lithium-oxygen (Li-O₂) batteries have gained extensive interest in the past decade, but are plagued by slow reaction kinetics and induced large-voltage hysteresis. Herein, we use a plasmonic heterojunction of Au nanoparticle (NP)-decorated C₃N₄ with nitrogen vacancies (Au/N_V-C₃N₄) as a bifunctional catalyst to promote oxygen cathode reactions of the visible light-responsive Li-O₂ battery. The nitrogen vacancies on N_V-C₃N₄ can adsorb and activate O₂ molecules, which are subsequently converted to Li₂O₂ as the discharge product by photogenerated hot electrons from plasmonic Au NPs. While charging, the holes on Au NPs drive the reverse decomposition of Li₂O₂ with a reduced applied voltage. The discharge voltage of the Li-O₂ battery with Au/N_V-C₃N₄ is significantly raised to 3.16 V under illumination, exceeding its equilibrium voltage, and the decreased charge voltage of 3.26 V has good rate capability and cycle stability. This is ascribed to the plasmonic hot electrons on Au NPs pumped from the conduction bands of N_V-C₃N₄ and the prolonged carrier life span of Au/N_V-C₃N₄ This work highlights the vital role of plasmonic enhancement and sheds light on the design of semiconductors for visible light-mediated Li-O₂ batteries and beyond.

Control of Chemical Reaction Pathways by Light-Matter Coupling

Because plasmonic metal nanostructures combine strong light absorption with catalytically active surfaces, they have become platforms for the light-assisted catalysis of chemical reactions. The enhancement of reaction rates by plasmonic excitation has been extensively discussed. This review focuses on a less discussed aspect: the induction of new reaction pathways by light excitation. Through commentary on seminal reports, we describe the principles behind the optical modulation of chemical reactivity and selectivity on plasmonic metal nanostructures. Central to these phenomena are excited charge carriers generated by plasmonic excitation, which modify the energy landscape available to surface reactive species and unlock pathways not conventionally available in thermal catalysis. Photogenerated carriers can trigger bond dissociation or desorption in an adsorbate-selective manner, drive charge transfer and multielectron redox reactions, and generate radical intermediates. Through one or more of these mechanisms, a specific pathway becomes favored under light. By improved control over these mechanisms, light-assisted catalysis can be transformational for chemical synthesis and energy conversion.

First-Principles Insights into Plasmon-Induced Catalysis

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

Plasmonic Au Nanoparticle@Ti3C2Tx Heterostructures for Improved Oxygen Evolution Performance

As we know, in plasmonic-enhanced heterogeneous catalysis, the reaction rates could be remarkably accelerated by generating hot carriers in the constituent nanostructured metals. To further improve the reaction rate, well-defined heterostructures based on plasmonic gold nanoparticles on MXene Ti₃C₂T_x nanosheets (Au NPs@Ti₃C₂T_x) were rationally designed and systematically investigated to improve the performance of the oxygen evolution reaction (OER). The results demonstrated that the catalysis performance of the Au NPs@Ti₃C₂T_x system could be easily tuned by simply varying the concentration and size of Au NPs, and Au NPs@Ti₃C₂T_x with an average Au NP diameter (∼10 nm) exhibited a 2.5-fold increase in the oxidation or reduction current compared with pure Ti₃C₂T_x. The enhanced OER performance can be attributed to the synergistic effect of the plasmonic hot hole injection and Schottky junction carrier trapping. Owing to easy fabrication of Au NPs@Ti₃C₂T_x, the tunable size and concentration of Au NPs loaded on MXene nanosheets, and the significantly enhanced OER, it is expected that this work can lay the foundation to the design of multidimensional MXene-based heterostructures for highly efficient OER performance.

Plasmon-assisted click chemistry at low temperature: an inverse temperature effect on the reaction rate

Plasmon assistance promotes a range of chemical transformations by decreasing their activation energies. In a common case, thermal and plasmon assistance work synergistically: higher temperature results in higher plasmon-enhanced catalysis efficiency. Herein, we report an unexpected tenfold increase in the reaction efficiency of surface plasmon-assisted Huisgen dipolar azide-alkyne cycloaddition (AAC) when the reaction mixture is cooled from room temperature to -35 °C. We attribute the observed increase in the reaction efficiency to complete plasmon-induced annihilation of the reaction barrier, prolongation of plasmon lifetime, and decreased relaxation of plasmon-excited-states under cooling. Furthermore, control quenching experiments supported by theoretical calculations indicate that plasmon-mediated substrate excitation to an electronic triplet state may play the key role in plasmon-assisted chemical transformation. Last but not least, we demonstrated the possible applicability of plasmon assistance to biological systems by AAC coupling of biotin to gold nanoparticles performed at -35 °C.

Plasmonic photothermal catalysis for solar-to-fuel conversion: current status and prospects

Solar-to-fuel conversion through photocatalytic processes is regarded as promising technology with the potential to reduce reliance on dwindling reserves of fossil fuels and to support the sustainable development of our society. However, conventional semiconductor-based photocatalytic systems suffer from unsatisfactory reaction efficiencies due to limited light harvesting abilities. Recent pioneering work from several groups, including ours, has demonstrated that visible and infrared light can be utilized by plasmonic catalysts not only to induce local heating but also to generate energetic hot carriers for initiating surface catalytic reactions and/or modulating the reaction pathways, resulting in synergistically promoted solar-to-fuel conversion efficiencies. In this perspective, we focus primarily on plasmon-mediated catalysis for thermodynamically uphill reactions converting CO2 and/or H2O into value-added products. We first introduce two types of mechanism and their applications by which reactions on plasmonic nanostructures can be initiated: either by photo-induced hot carriers (plasmonic photocatalysis) or by light-excited phonons (photothermal catalysis). Then, we emphasize examples where the hot carriers and phonon modes act in concert to contribute to the reaction (plasmonic photothermal catalysis), with special attention given to the design concepts and reaction mechanisms of the catalysts. We discuss challenges and future opportunities relating to plasmonic photothermal processes, aiming to promote an understanding of underlying mechanisms and provide guidelines for the rational design and construction of plasmonic catalysts for highly efficient solar-to-fuel conversion.

Ultrafast Carrier and Lattice Dynamics in Plasmonic Nanocrystalline Copper Sulfide Films

Excited carrier dynamics in plasmonic nanostructures determines many important optical properties such as nonlinear optical response and photocatalytic activity. Here it is shown that mesoscopic plasmonic covellite nanocrystals with low free-carrier concentration exhibit a much faster carrier relaxation than in traditional plasmonic materials. A nonequilibrium hot-carrier population thermalizes within first 20 fs after photoexcitation. A decreased thermalization time in nanocrystals compared to a bulk covellite is consistent with the reduced Coulomb screening in ultrathin films. The subsequent relaxation of thermalized, equilibrium electron gas is faster than in traditional plasmonic metals due to the lower carrier concentration and agrees well with that in a bulk covellite showing no evidence of quantum confinement or hot-hole trapping at the surface states. The excitation of coherent optical phonon modes in a covellite is also demonstrated, revealing coherent lattice dynamics in plasmonic materials, which until now was mainly limited to dielectrics, semiconductors, and semimetals. These findings show advantages of this new mesoscopic plasmonic material for active control of optical processes.

Direct and Broadband Plasmonic Charge Transfer to Enhance Water Oxidation on a Gold Electrode

Plasmonic photocatalysis via hot charge carriers suffers from their short lifetime compared with the sluggish kinetics of most reactions. To increase lifetime, adsorbates on the surface of a plasmonic metal may create preferential states for electrons to be excited from. We demonstrate this effect with O adsorbates on a nanoporous gold electrode. Nanoporous gold is used to obtain a broadband optical response, to increase the obtained photocurrent, and to provide a SERS-active substrate. Only with adsorbates present, we observe significant photocurrents. Illumination also increases the adsorbate coverage above its dark potential-dependent equilibrium, as derived from a two-laser in situ SERS approach. Density functional theory calculations confirm the appearance of excitable states below the Fermi level. The photocurrent enhancement and broadband characteristics reveal the potential of the plasmonic approach to improve the efficiency of photoelectrochemical water splitting.

Plasmonic nano-antimicrobials: properties, mechanisms and applications in microbe inactivation and sensing

Bacterial, viral and fungal infections pose serious threats to human health and well-being. The continuous emergence of acute infectious diseases caused by pathogenic microbes and the rapid development of resistances against conventional antimicrobial drugs necessitates the development of new and effective strategies for the safe elimination of microbes in water, food or on surfaces, as well as for the inactivation of pathogenic microbes in human hosts. The need for new antimicrobials has triggered the development of plasmonic nano-antimicrobials that facilitate both light-dependent and -independent microbe inactivation mechanisms. This review introduces the relevant photophysical mechanisms underlying these plasmonic nano-antimicrobials, and provides an overview of how the photoresponses and materials properties of plasmonic nanostructures can be applied in microbial pathogen inactivation and sensing applications. Through a systematic analysis of the inactivation efficacies of different plasmonic nanostructures, this review outlines the current state-of-the-art in plasmonic nano-antimicrobials and defines the application space for different microbial inactivation strategies. The advantageous optical properties of plasmonic nano-antimicrobials also enhance microbial detection and sensing modalities and thus help to avoid exposure to microbial pathogens. Sensitive and fast plasmonic microbial sensing modalities and their theranostic and targeted therapeutic applications are discussed.

All-Optically Reconfigurable Plasmonic Metagrating for Ultrafast Diffraction Management

Hot-electron dynamics taking place in nanostructured materials upon irradiation with fs-laser pulses has been the subject of intensive research, leading to the emerging field of ultrafast nanophotonics. However, the most common description of nonlinear interaction with ultrashort laser pulses assumes a homogeneous spatial distribution for the photogenerated carriers. Here we theoretically show that the inhomogeneous evolution of the hot carriers at the nanoscale can disclose unprecedented opportunities for ultrafast diffraction management. In particular, we design a highly symmetric plasmonic metagrating capable of a transient symmetry breaking driven by hot electrons. The subsequent power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm²) laser fluence. Our theoretical investigation also indicates that the recovery time of the symmetric configuration can be controlled by tuning the geometry of the metaatom, and can be as fast as 2 ps for electrically connected configurations.

Visible-Light Induced Sustainable Water Treatment Using Plasmo-Semiconductor Nanogap Bridge Array, PNA

The development of sustainable methods for energy-intensive water treatment processes continues to be a challenging issue. Plasmonic-semiconductor nanoparticles, which absorb large amounts of sunlight in the visible range for conversion into chemical energy efficiently, can form the basis of a sustainable water treatment method. However, the potential uses of plasmonic semiconductor particles for water treatment have not been fully explored yet because of the limitations associated with the imbalance between light capture, charge transfer, and the required recycling steps for the particles themselves. Herein, a significantly improved visible-light-induced water treatment method that uses a plasmo-semiconductor nanogap bridge array (PNA) is reported. As an arrangement of antenna-reactors, the PNA enables the balancing of the largely enhanced electromagnetic field in the plasmonic nanogap coupling region and optimal separation of charge carriers in the semiconductor. The simultaneous effects of visible-light absorption and charge transfer lead to the generation of a highly enhanced visible-light-induced OH radical (•OH). Consequently, visible-light-induced 5-log N/N₀ water disinfection and 100% chemical decomposition for sustainable water treatment were demonstrated. Owing to the large light absorption, charge carrier utilization, and array-oriented scalability, the PNA will be valuable in various sustainable energy and environmental applications.

Confined Hot Electron Relaxation at the Molecular Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster and Layered C60

The plasmonic response of metallic nanostructures plays a key role in amplifying photocatalytic and photoelectric conversion. Since the plasmonic behavior of noble metal nanoparticles is known to generate energetic charge carriers such as hot electrons, it is expected that the hot electrons can enhance conversion efficiency if they are transferred into a neighboring molecule or semiconductor. However, the method of transferring the energized charge carriers from the plasmonically generated hot electrons to the neighboring species remains controversial. Herein, we fabricated a molecularly well-defined heterointerface between the size-selected plasmonic noble-metal nanoclusters (NCs) of Ag_n (n = 3-55)/Au_n (n = 21) and the organic C₆₀ film to investigate hot electron generation and relaxation dynamics using time-resolved two-photon photoemission (2PPE) spectroscopy. By tuning the NC size and the polarization of the femtosecond excitation photons, the plasmonic behavior is characterized by 2PPE intensity enhancement by 10-100 times magnitude, which emerge at n ≥ 9 for Ag_n NCs. The 2PPE spectra exhibit contributions from low-energy electrons forming coherent plasmonic currents and hot electrons with an excitation energy up to photon energy owing to two-photon excitation of an occupied state of the Ag_n NC below the Fermi level. The time-resolved pump-probe measurements demonstrate that plasmon dephasing generates hot electrons which undergo electron-electron scattering. However, no photoemission occurs via the charge transfer state forming Ag_n⁺C₆₀^- located in the vicinity of the Fermi level. Thus, this study reveals the mechanism of ultrafast confined hot electron relaxation within plasmonic Ag_n NCs at the molecular heterointerface.

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.

Challenges in Plasmonic Catalysis

The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light-matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years.

Plasmon-Mediated Intramolecular Methyl Migration with Nanoscale Spatial Control

Plasmonic materials interact strongly with light to focus and enhance electromagnetic radiation down to nanoscale volumes. Due to this localized confinement, materials that support localized surface plasmon resonances are capable of driving energetically unfavorable chemical reactions. In certain cases, the plasmonic nanostructures are able to preferentially catalyze the formation of specific photoproducts, which offers an opportunity for the development of solar-driven chemical synthesis. Here, using plasmonic environments, we report inducing an intramolecular methyl migration reaction, forming 4-methylpyridine from N-methylpyridinium. Using both experimental and computational methods, we were able to confirm the identity of the N-methylpyridinium by making spectral comparisons against possible photoproducts. This reaction involves breaking a C-N bond and forming a new C-C bond, highlighting the ability of plasmonic materials to drive complex and selective reactions. Additionally, we observe that the product yield depends strongly on optical illumination conditions. This is likely due to steric hindrance in specific regions on the nanostructured plasmonic substrate, providing an optical handle for driving plasmonic catalysis with spatial specificity. This work adds yet another class of reactions accessible by surface plasmon excitation to the ever-growing library of plasmon-mediated chemical reactions.

Machine-Free Polymerase Chain Reaction with Triangular Gold and Silver Nanoparticles

Photothermal effects of metal nanoparticles (NPs) are used for various biotechnological applications. Although NPs have been used in a polymerase chain reaction (PCR), the effects of shape on the photothermal properties and its efficiency on PCR are less explored. The present study reports the synthesis of triangular gold and silver NPs, which can attain temperatures up to ∼90 °C upon irradiation with 808 nm laser. This photothermal property of synthesized nanoparticles was evaluated using various concentrations, irradiation time, and power to create a temperature profile required for variable-temperature PCR. This study reports a cost-effective, machine-free PCR using both gold and silver triangular NPs, with efficiency similar to that of a commercial PCR machine. Interestingly, addition of triangular NPs increases PCR efficiency in commercial PCR reactions. The higher PCR efficiencies are due to the direct binding and unfolding of double-stranded DNA as suggested by circular dichroism and UV spectroscopy. These findings suggest that triangular NPs can be used to develop cost-effective, robust machine-free PCR modules and can be used in various other photothermal applications.

Hybrid theoretical models for molecular nanoplasmonics

The multidisciplinary nature of the research in molecular nanoplasmonics, i.e., the use of plasmonic nanostructures to enhance, control, or suppress properties of molecules interacting with light, led to contributions from different theory communities over the years, with the aim of understanding, interpreting, and predicting the physical and chemical phenomena occurring at molecular- and nano-scale in the presence of light. Multiscale hybrid techniques, using a different level of description for the molecule and the plasmonic nanosystems, permit a reliable representation of the atomistic details and of collective features, such as plasmons, in such complex systems. Here, we focus on a selected set of topics of current interest in molecular plasmonics (control of electronic excitations in light-harvesting systems, polaritonic chemistry, hot-carrier generation, and plasmon-enhanced catalysis). We discuss how their description may benefit from a hybrid modeling approach and what are the main challenges for the application of such models. In doing so, we also provide an introduction to such models and to the selected topics, as well as general discussions on their theoretical descriptions.

Increased Intraband Transitions in Smaller Gold Nanorods Enhance Light Emission

Photoinduced light emission from plasmonic nanoparticles has attracted considerable interest within the scientific community because of its potential applications in sensing, imaging, and nanothermometry. One of the suggested mechanisms for the light emission from plasmonic nanoparticles is the plasmon-enhanced radiative recombination of hot carriers through inter- and intraband transitions. Here, we investigate the nanoparticle size dependence on the photoluminescence through a systematic analysis of gold nanorods with similar aspect ratios. Using single-particle emission and scattering spectroscopy along with correlated scanning electron microscopy and electromagnetic simulations, we calculate the emission quantum yields and Purcell enhancement factors for individual gold nanorods. Our results show strong size-dependent quantum yields in gold nanorods, with higher quantum yields for smaller gold nanorods. Furthermore, by determining the relative contributions to the photoluminescence from inter- and intraband transitions, we deduce that the observed size dependence predominantly originates from the size dependence of intraband transitions. Specifically, within the framework of Fermi's golden rule for radiative recombination of excited charge carriers, we demonstrate that the Purcell factor enhancement alone cannot explain the emission size dependence and that changes in the transition matrix elements must also occur. Those changes are due to electric field confinement enhancing intraband transitions. These results provide vital insight into the intraband relaxation in metallic nanoconfined systems and therefore are of direct importance to the rapidly developing field of plasmonic photocatalysis.

In Situ Visualization of Localized Surface Plasmon Resonance-Driven Hot Hole Flux

Nonradiative surface plasmon decay produces highly energetic electron-hole pairs with desirable characteristics, but the measurement and harvesting of nonequilibrium hot holes remain challenging due to ultrashort lifetime and diffusion length. Here, the direct observation of LSPR-driven hot holes created in a Au nanoprism/p-GaN platform using photoconductive atomic force microscopy (pc-AFM) is demonstrated. Significant enhancement of photocurrent in the plasmonic platforms under light irradiation is revealed, providing direct evidence of plasmonic hot hole generation. Experimental and numerical analysis verify that a confined |E|-field surrounding a single Au nanoprism spurs resonant coupling between localized surface plasmon resonance (LSPR) and surface charges, thus boosting hot hole generation. Furthermore, geometrical and size dependence on the extraction of LSPR-driven hot holes suggests an optimized pathway for their efficient utilization. The direct visualization of hot hole flow at the nanoscale provides significant opportunities for harnessing the underlying nature and potential of plasmonic hot holes.

Plasmonic Hot Electron-Mediated Hydrodehalogenation Kinetics on Nanostructured Ag Electrodes

An attractive field of plasmon-mediated chemical reactions (PMCRs) is developing rapidly, but there is still incomplete understanding of how to control the kinetics of such a reaction related to hot carriers. Here, we chose 8-bromoadenine (8BrAd) as a probe molecule of hot electrons to investigate the influence of the electrode potential, laser wavelength, and power on the PMCR kinetics on silver nanoparticle-modified silver electrodes. Plasmonic hot electron-mediated cleavage of the C-Br bond in 8BrAd has been investigated by combining in situ electrochemical surface-enhanced Raman spectroscopy and density functional theory calculations. The experimental and theoretical results reveal that the energy position of plasmon relaxation-generated hot electrons can be modulated conveniently by applied potentials and laser light. This allows the proposal of a mechanism of modulating the matching energy of the hot electron of plasmon relaxation to promote the efficiency of PMCRs in electrochemical interfaces. Our work will be helpful to design surface plasmon resonance photoelectrochemical reactions on metal electrode surfaces of nanostructures with higher efficiency.

Photomodulated Spatially Confined Chemical Reactivity in a Single Silver Nanoprism

Single-atom and single-particle catalysis is an area of considerable topical interest due to their potential in explaining important fundamental processes and applications across several areas. An interesting avenue in single-particle catalysis is spatial control of chemical reactivity within the particle by employing light as an external stimulus. To demonstrate this concept, we report galvanic replacement reactions (GRRs) as a spatial marker of subparticle chemical reactivity of a silver nanoprism with AuCl₄^- ions under optical excitation. The location of a GRR within a single Ag nanoprism can be spatially controlled depending on the plasmon mode excited. This leads to chemomorphological transformation of Ag nanoprisms into interesting Ag-Au structures. This spatial biasing effect is attributed to localized hot electron injection from the tips and edges of the silver nanoprisms to the adjacent reactants that correlate with excitation of different surface plasmon modes. The study also employs low-energy-loss EELS mapping to additionally probe the spatially confined redox reaction within a silver nanoprism. The findings presented here allow the visualization of a plasmon-driven subparticle chemical transformation with high resolution. The selective optical excitation of surface plasmon eigenmodes of anisotropic nanoparticles offers opportunities to spatially modulate chemical transformations mediated by hot electron transfer.

Shining Light on Aluminum Nanoparticle Synthesis

ConspectusAluminum in its nanostructured form is generating increasing interest because of its light-harvesting properties, achieved by excitation of its localized surface plasmon resonance. Compared to traditional plasmonic materials, the coinage metals Au and Ag, Al is far more earth-abundant and, therefore, more suitable for large-area applications or where cost may be an important factor. Its optical properties are far more flexible than either Au or Ag, supporting plasmon resonances that range from UV wavelengths, through the visible regime, and into the infrared region of the spectrum. However, the chemical synthesis of Al nanocrystals (NCs) of controlled size and shape has historically lagged far behind that of Au and Ag. This is partially due to the high reactivity of Al precursors, which react readily with O₂, H₂O, and many reagents used in traditional NC syntheses. The first chemical synthesis of Al NCs was demonstrated by Haber and Buhro in 1998, decomposing AlH₃ using titanium isopropoxide (TIP), with a number of subsequent reports refining this protocol. The role of a catalyst in Al NC synthesis is, we believe, unique to this synthetic approach. In 2015, the first synthesis of size controlled Al NCs was published by our group. Since then, we have significantly advanced Al NC synthesis, postsynthetic modifications, and applications of Al nanoparticles (NPs)-NCs with additional surface modifications-in chemical sensing and photocatalysis. Colloidal Al synthesis has its unique challenges, differing markedly from the far more familiar Au and Ag syntheses, which currently appears to present a de facto barrier to broader research activity in this field.The goal of this Account is to highlight developments in controlled synthesis of Al NCs and applications of Al NPs over the last five years. We outline techniques for successful Al NC synthesis and address some of the problems that may be encountered in this synthesis. A mechanistic understanding of AlH₃ decomposition using TIP has been developed, while new directions have been discovered for synthetic control. Facet-binding ligands, alternate Al precursors, new titanium-based reduction catalysts, even solvent composition have all been shown to control reaction products while also opening doors to future developments. A variety of postsynthetic modifications to the Al NC native oxide surface, including polymer, MOF, and transition metal island coatings have been demonstrated for applications in molecular sensing and photocatalysis. In this Account, we hope to convey that Al synthesis is more accessible than generally perceived and to encourage new synthetic development based on underlying mechanisms controlling size and shape. High selectivity in particle faceting and twinning, implementation of seeded growth principles for monodisperse samples, and the demonstration of new, practical applications of Al nanoparticles remain primary challenges in the field. As Al nanoparticle synthesis is refined and new applications emerge, colloidal Al will become an accessible and low-cost plasmonic nanomaterial complementary to Au and Ag.

Hot-Carrier Generation in Plasmonic Nanoparticles: The Importance of Atomic Structure

Metal nanoparticles are attractive for plasmon-enhanced generation of hot carriers, which may be harnessed in photochemical reactions. In this work, we analyze the coherent femtosecond dynamics of photon absorption, plasmon formation, and subsequent hot-carrier generation through plasmon dephasing using first-principles simulations. We predict the energetic and spatial hot-carrier distributions in small metal nanoparticles and show that the distribution of hot electrons is very sensitive to the local structure. Our results show that surface sites exhibit enhanced hot-electron generation in comparison to the bulk of the nanoparticle. Although the details of the distribution depend on particle size and shape, as a general trend, lower-coordinated surface sites such as corners, edges, and {100} facets exhibit a higher proportion of hot electrons than higher-coordinated surface sites such as {111} facets or the core sites. The present results thereby demonstrate how hot carriers could be tailored by careful design of atomic-scale structures in nanoscale systems.

Long-Lived Hot Electron in a Metallic Particle for Plasmonics and Catalysis: Ab Initio Nonadiabatic Molecular Dynamics with Machine Learning

Multiple experiments provide evidence for photovoltaic, catalytic, optoelectronic, and plasmonic processes involving hot, i.e., high energy, electrons in nanoscale materials. However, the mechanisms of such processes remain elusive, because electrons rapidly lose energy by relaxation through dense manifolds of states. We demonstrate a long-lived hot electron state in a Pt nanocluster adsorbed on the MoS₂ substrate. For this purpose, we develop a simulation technique, combining classical molecular dynamics based on machine learning potentials with ab initio nonadiabatic molecular dynamics and real-time time-dependent density functional theory. Choosing Pt₂₀/MoS₂ as a prototypical system, we find frequent shifting of a top atom in the Pt particle occurring on a 50 ps time scale. The distortion breaks particle symmetry and creates unsaturated chemical bonds. The lifetime of the localized state associated with the broken bonds is enhanced by a factor of 3. Hot electrons aggregate near the shifted atom and form a catalytic reaction center. Our findings prove that distortion of even a single atom can have important implications for nanoscale catalysis and plasmonics and provide insights for utilizing machine learning potentials to accelerate ab initio investigations of excited state dynamics in condensed matter systems.

Using Hot Electrons and Hot Holes for Simultaneous Cocatalyst Deposition on Plasmonic Nanostructures

Hot electrons generated in metal nanoparticles can drive chemical reactions and selectively deposit cocatalyst materials on the plasmonic hotspots, the areas where the decay of plasmons takes place and the hot electrons are created. While hot electrons have been extensively used for nanomaterial formation, the utilization of hot holes for simultaneous cocatalyst deposition has not yet been explored. Herein, we demonstrate that hot holes can drive an oxidation reaction for the deposition of the manganese oxide (MnO_x) cocatalyst on different plasmonic gold (Au) nanostructures on a thin titanium dioxide (TiO₂) layer, excited at their surface plasmon resonance. An 80% correlation between the hot-hole deposition sites and the simulated plasmonic hotspot location is showed when considering the typical hot-hole diffusion length. Simultaneous deposition of more than one cocatalyst is also achieved on one of the investigated plasmonic systems (Au plasmonic nanoislands) through the hot-hole oxidation of a manganese salt and the hot-electron reduction of a platinum precursor in the same solution. These results add more flexibility to the use of hot carriers and open up the way for the design of complex photocatalytic nanostructures.

Plasmon-Enhanced Resonant Photoemission Using Atomically Thick Dielectric Coatings

By proposing an atomically thick dielectric coating on a metal nanoemitter, we theoretically show that the optical field tunneling of ultrafast-laser-induced photoemission can occur at an ultralow incident field strength of 0.03 V/nm. This coating strongly confines plasmonic fields and provides secondary field enhancement beyond the geometrical plasmon field enhancement effect, which can substantially reduce the barrier and enable more efficient photoemission. We numerically demonstrate that a 1 nm thick layer of SiO₂ around a Au-nanopyramid will enhance the resonant photoemission current density by 2 orders of magnitude, where the transition from multiphoton absorption to optical field tunneling is accessed at an incident laser intensity at least 10 times lower than that of the bare nanoemitter. The effects of the coating properties such as refractive index, thickness, and geometrical settings are studied, and tunable photoemission is numerically demonstrated by using different ultrafast lasers. Our approach can also directly be extended to nonmetal emitters, to-for example-2D material coatings, and to plasmon-induced hot carrier generation.

Nanoscale Photocatalytic Activity of Gold and Gold-Palladium Nanostructures Revealed by Tip-Enhanced Raman Spectroscopy

The new paradigm of solid-state catalysis is that coupling of plasmonic and catalytic metals can be used to achieve much higher catalytic efficiency relative to their counterparts. Chemical reactions on such bimetallic nanostructures are light-driven, which essentially enables "green catalysis" in organic synthesis. The catalytic efficiency of bimetallic platforms directly depends on their nanoscale structures, which remain poorly understood. We used tip-enhanced Raman spectroscopy (TERS) to investigate nanoscale plasmonic and photocatalytic properties of novel gold-palladium microplates (Au@PdMPs), along with their monometallic counterparts. We found that 4-nitrobenzenethiol (4-NBT) can be catalyzed to p,p'-dimercaptoazobisbenzene (DMAB) and 4-aminothiophenol (4-ATP) on Au@PdMPs, whereas monometallic AuMPs produce exclusively DMAB as a result of such photocatalytic reduction of 4-NBT. Using 4-NBT as a molecular reporter, we found that the efficiency of these catalytic reactions has strong correlation with the nanoscale structure of microplates. Coupling TERS to GC-MS, we also found that Au@PdMPs were capable of catalyzing the Suzuki-Miyaura coupling reaction.

Au-Nanoplasmonics-Mediated Surface Plasmon-Enhanced GaN Nanostructured UV Photodetectors

The nanoplasmonic impact of chemically synthesized Au nanoparticles (Au NPs) on the performance of GaN nanostructure-based ultraviolet (UV) photodetectors is analyzed. The devices with uniformly distributed Au NPs on GaN nanostructures (nanoislands and nanoflowers) prominently respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoflower-based device yields the highest photoresponsivity of ∼ 380 mA/W, detectivity of ∼ 10¹⁰ jones, reduced noise equivalent power of ∼ 5.5 × 10^-13 W Hz^-1/2, quantum efficiency of ∼ 145%, and fast response/recovery time of ∼40 ms. The report illustrates the mechanism where light interacts with the chemically synthesized nanoparticles guided by the surface plasmon to effectively enhance the device performance. It is observed that the Au NP-stimulated local surface plasmon resonance effect and reduced channel resistance contribute to the augmented performance of the devices. Further, the decoration of low-dimensional Au NPs on GaN nanostructures acts as a detection enhancer with a fast recovery time and paves the way toward the realization of energy-efficient optoelectronic device applications.

Plasmonic hot electrons for sensing, photodetection, and solar energy applications: A perspective

In plasmonic metals, surface plasmon resonance decays and generates hot electrons and hot holes through non-radiative Landau damping. These hot carriers are highly energetic, which can be modulated by the plasmonic material, size, shape, and surrounding dielectric medium. A plasmonic metal nanostructure, which can absorb incident light in an extended spectral range and transfer the absorbed light energy to adjacent molecules or semiconductors, functions as a "plasmonic photosensitizer." This article deals with the generation, emission, transfer, and energetics of plasmonic hot carriers. It also describes the mechanisms of hot electron transfer from the plasmonic metal to the surface adsorbates or to the adjacent semiconductors. In addition, this article highlights the applications of plasmonic hot electrons in photodetectors, photocatalysts, photoelectrochemical cells, photovoltaics, biosensors, and chemical sensors. It discusses the applications and the design principles of plasmonic materials and devices.

Plasmonic-enhanced photocatalysis reactions using gold nanostructured films

This work shows the enhancement of the visible photocatalytic activity of TiO₂ NPs film using the localized surface plasmonic resonance of Au nanostructures. We adopted a simple yet effective surface treatment to tune the size distribution, and plasmonic resonance spectrum of Au nanostructured films on glass substrates, by hot plate annealing in air at low temperatures. A hybrid photocatalytic film of TiO₂:Au is utilized to catalyse a selective photodegradation reaction of Methylene Blue in solution. Irradiation at the plasmonic resonance wavelength of the Au nanostructures provides more effective photodegradation compared to broadband artificial sunlight of significantly higher intensity. This improvement is attributed to the active contribution of the plasmonic hot electrons injected into the TiO₂. The broadband source initiates competing photoreactions in the photocatalyst, so that carrier transfer from the catalyst surface to the solution is less efficient. The proposed hybrid photocatalyst can be integrated with a variety of device architectures and designs, which makes it highly attractive for low-cost photocatalysis applications.

This work shows the enhancement of the visible photocatalytic activity of TiO₂ NPs film using the localized surface plasmonic resonance of Au nanostructures.

Efficient Hot Electron Transfer from Small Au Nanoparticles

Many important chemical transformations enabled by plasmonic hot carrier photocatalysis have been reported, although their efficiencies are often too low for practical applications. We examine how the efficiency of plasmon-induced hot electron transfer depends on the Au particle size in Au-tipped CdS nanorods. We show that with decreasing Au size, the plasmon width increases due to enhanced surface damping contributions. The excitation of Au nanoparticles leads to an instrument response time-limited ultrafast hot electron transfer process to CdS (≪140 fs). The quantum efficiency of this process increases from ∼1% to ∼18% as the particle size decreases from 5.5 ± 1.1 to 1.6 ± 0.5 nm due to both enhanced hot electron generation and transfer efficiencies in small Au particles. Our finding suggests that decreasing plasmonic particle size is an effective approach for improving plasmon-induced hot carrier transfer efficiency and provides important insight for the rational improvement of plasmonic hot carrier-based devices.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

Photoactivation of catalytic materials through plasmon-coupled energy transfer has created new possibilities for expanding the scope of light-driven heterogeneous catalysis. Here we present a nanoengineered plasmonic photocatalyst consisting of catalytic Pd islands preferentially grown on vertices of Al nanocubes. The regioselective Pd deposition on Al nanocubes does not rely on complex surface ligands, in contrast to site-specific transition-metal deposition on gold nanoparticles. We show that the strong local field enhancement on the sharp nanocube vertices provides a mechanism for efficient coupling of the plasmonic Al antenna to adjacent Pd nanoparticles. A substantial increase in photocatalytic H₂ dissociation on Pd-bound Al nanocubes relative to pristine Al nanocubes can be observed, incentivizing further engineering of heterometallic antenna-reactor photocatalysts. Controlled growth of catalytic materials on plasmonic hot spots can result in more efficient use of the localized surface plasmon energy for photocatalysis, while minimizing the amount and cost of precious transition-metal catalysts.

Infrared driven hot electron generation and transfer from non-noble metal plasmonic nanocrystals

Non-noble metal plasmonic materials, e.g. doped semiconductor nanocrystals, compared to their noble metal counterparts, have shown unique advantages, including broadly tunable plasmon frequency (from visible to infrared) and rich surface chemistry. However, the fate and harvesting of hot electrons from these non-noble metal plasmons have been much less explored. Here we report plasmon driven hot electron generation and transfer from plasmonic metal oxide nanocrystals to surface adsorbed molecules by ultrafast transient absorption spectroscopy. We show unambiguously that under infrared light excitation, hot electron transfers in ultrafast timescale (<50 fs) with an efficiency of 1.4%. The excitation wavelength and fluence dependent study indicates that hot electron transfers right after Landau damping before electron thermalization. We revealed the efficiency-limiting factors and provided improvement strategies. This study paves the way for designing efficient infrared light absorption and photochemical conversion applications based on non-noble metal plasmonic materials.

Harvesting of hot electrons in non-noble metal plasmonic materials is still little explored. Here the authors investigate plasmon-driven hot electron generation in doped metal oxide nanocrystals and the mechanism of transfer to surface adsorbed molecules by ultrafast transient absorption spectroscopy.

Determining plasmonic hot-carrier energy distributions via single-molecule transport measurements

Hot carriers in plasmonic nanostructures, generated via plasmon decay, play key roles in applications such as photocatalysis and in photodetectors that circumvent bandgap limitations. However, direct experimental quantification of steady-state energy distributions of hot carriers in nanostructures has so far been lacking. We present transport measurements from single-molecule junctions, created by trapping suitably chosen single molecules between an ultrathin gold film supporting surface plasmon polaritons and a scanning probe tip, that can provide quantification of plasmonic hot-carrier distributions. Our results show that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The technique developed in this work will enable quantification of plasmonic hot-carrier distributions in nanophotonic and plasmonic devices.

Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

Devices driven by above-equilibrium "hot" electrons are appealing for photocatalytic technologies, such as in situ H₂O₂ synthesis, but currently suffer from low (<1%) overall quantum efficiencies. Gold nanostructures excited by visible light generate hot electrons that can inject into a neighboring semiconductor to drive electrochemical reactions. Here, we designed and studied a metal-insulator-metal (MIM) structure of Au nanoparticles on a ZnO/TiO₂/Al film stack, deposited through room-temperature, lithography-free methods. Light absorption, electron injection efficiency, and photocatalytic yield in this device are superior in comparison to the same stack without Al. Our device absorbs >60% of light at the Au localized surface plasmon resonance (LSPR) peak near 530 nm-a 5-fold enhancement in Au absorption due to critical coupling to an Al film. Furthermore, we show through ultrafast pump-probe spectroscopy that the Al-coupled samples exhibit a nearly 5-fold improvement in hot-electron injection efficiency as compared to a non-Al device, with the hot-electron lifetimes extending to >2 ps in devices photoexcited with fluence of 0.1 mJ cm^-2. The use of an Al film also enhances the photocatalytic yield of H₂O₂ more than 3-fold in a visible-light-driven reactor. Altogether, we show that the critical coupling of Al films to Au nanoparticles is a low-cost, lithography-free method for improving visible-light capture, extending hot-carrier lifetimes, and ultimately increasing the rate of in situ H₂O₂ generation.

Sensing Biomarkers with Plasmonics

The detection of biomarkers is critical for enabling early disease diagnosis, monitoring the progression, and tracking the effectiveness of therapeutic intervention. Plasmonic sensors exhibit a broad range of analytical capabilities, from the rapid generation of colorimetric readouts to single-molecule sensitivity in ultralow sample volumes, which have led to their increased exploration in bioanalysis and point-of-care applications. This perspective presents selected accounts of recent developments on the different types of plasmonic sensing platforms, the pervasive challenges, and outlook on the pathway to translation. We highlight the sensing of upcoming biomarkers, including microRNA, circulating tumor cells, exosomes, and cell-free DNA, and discuss the opportunity of utilizing plasmonic nanomaterials and tools for biomarker detection beyond biofluids, such as in tissues, organs, and disease sites. The integration of plasmonic biosensors with established and upcoming technologies of instrumentation, sample pretreatment, and data analysis will help realize their translation to clinical settings for improving healthcare and enhancing the quality of life.

Ultrafast relaxation dynamics in bimetallic plasmonic catalysts

Combining a plasmonic metal, such as gold, with other popular catalysts, such as Ni or Pt, can extend its benefits to many energy-extensive reactions catalyzed by those metals. The efficiency of a plasmon-enhanced catalytic reaction is mainly determined by the light absorption cross section and the photoexcited charge carrier relaxation dynamics of the nanoparticles. We have investigated the charge carrier relaxation dynamics of gold/nickel (Au/Ni) and gold/platinum (Au/Pt) bimetallic nanoparticles. We found that the addition of Ni or Pt to gold can reduce light absorption in gold nanoparticles. However, electron-phonon coupling rates of Au/Ni and Au/Pt nanoparticles are significantly faster than that of pure Au nanoparticles. This is due to the fact that both Ni and Pt possess significantly larger electron-phonon coupling constants and higher densities of states near the Fermi level in comparison with Au. Additionally, the phonon-phonon coupling rate of bimetallic Au/Pt and Au/Ni nanoparticles was significantly different from that of pure gold nanoparticles, due to the acoustic impedance mismatch at the nanoparticle/substrate interface. Our findings provide important insights into the rational design of bimetallic plasmonic catalysts.

Hot electron and thermal effects in plasmonic catalysis of nanocrystal transformation

Plasmonic metal nanoparticles have the ability to harvest visible light and cause effective energy conversion, and they are considered as promising catalysts to drive chemical reactions. Although plasmonic catalysis has been widely used to mediate the reaction of organic molecules, the mechanism of contribution of thermal and hot carriers remains unclear. The catalysis of hot carriers is normally proposed as the dominant role of plasmonic catalysis, while the contribution of plasmonic thermal effects is often ignored, since the molecules on the metal surface are unstable at high temperatures. Here, plasmon catalytic nanocrystal transformation including oxidation reaction and optimization of the crystal structure is employed to investigate the plasmonic contributions of hot electron and thermal effects in plasmonic catalysis. It is found that the transformation rate and the corresponding product are very different with and without the assistance of hot electron catalysis. The thermal effect plays a dominant role in plasmon-catalyzed material transformation, and hot electrons can promote the oxidation reaction by facilitating the generation of active oxygen. The investigation provides insight into the specific role of hot electron and thermal effects in plasmonic catalysis, which is critically important for exploiting the highly localized fast plasmonic thermal effect and for designing energy-efficient plasmonic catalysts.

Mapping Hot Electron Response of Individual Gold Nanocrystals on a TiO2 Photoanode

Incorporating metal nanocrystals with semiconductor photoanodes significantly enhances the efficiency of the energy conversion in the visible range during water splitting due to the excitation of hot electrons. While extensively studied on ensemble samples, hot electron response of metal nanocrystals in a photoelectrochemical cell remains unexploited at the single-particle level. Herein, we systematically investigate hot electron response of individual single-crystalline gold nanocrystals (AuNCs) on a TiO₂ photoanode during water splitting. We directly correlate the morphology of the AuNC and its plasmonic property to the efficiencies involving hot electrons with the help of single-particle dark-field microscopy and photocurrent mapping. Our results show that the efficiencies of individual AuNCs are dependent on a variety of factors including interface condition, applied bias, excitation power, incident angle, and AuNC size. Our research may shed light on optimizing the light-harvesting capability of metal/semiconductor photoanodes by providing insights into the photocatalytic processes.