Single‐Molecule Study of a Plasmon‐Induced Reaction for a Strongly Chemisorbed Molecule

H-Glass Supported Hybrid Gold Nano-Islands for Visible-Light-Driven Hydrogen Evolution

Flat panel reactors, coated with photocatalytic materials, offer a sustainable approach for the commercial production of hydrogen (H2) with zero carbon footprint. Despite this, achieving high solar-to-hydrogen (STH) conversion efficiency with these reactors is still a significant challenge due to the low utilization efficiency of solar light and rapid charge recombination. Herein, hybrid gold nano-islands (HGNIs) are developed on transparent glass support to improve the STH efficiency. Plasmonic HGNIs are grown on an in-house developed active glass sheet composed of sodium aluminum phosphosilicate oxide glass (H-glass) using the thermal dewetting method at 550 °C under an ambient atmosphere. HGNIs with various oxidation states (Au0, Au+, and Au-) and multiple interfaces are obtained due to the diffusion of the elements from the glass structure, which also facilitates the lifetime of the hot electron to be ≈2.94 ps. H-glass-supported HGNIs demonstrate significant STH conversion efficiency of 0.6%, without any sacrificial agents, via water dissociation. This study unveils the specific role of H-glass-supported HGNIs in facilitating light-driven chemical conversions, offering new avenues for the development of high-performance photocatalysts in various chemical conversion reactions for large-scale commercial applications.

Key Factors for Controlling Plasmon-Induced Chemical Reactions on Metal Surfaces

Plasmon-induced chemical reactions based on direct interactions between the plasmons of metal nanostructures and molecules have attracted increasing attention as a means of efficiently utilizing sunlight. In recent years, achievements in complex synthetic reactions as well as simple dissociation reactions of gaseous molecules using plasmons have been reported. However, recent research progress has revealed that multiple factors govern plasmon-induced chemical reactions. This perspective provides an overview of the key factors that influence plasmon-induced chemical reactions on metal surfaces and discusses the difficulty of controlling the reactions, which is caused by the entanglement of the key factors. A strategy for designing plasmonic metal catalysts to achieve the desired reactions is also discussed based on the current understanding, and directions for further research are provided.

Photocatalysis of Metallic Nanoparticles: Interband vs Intraband Induced Mechanisms

Photocatalysis induced by localized surface plasmon resonance of metallic nanoparticles has been studied for more than a decade, but photocatalysis originating from direct interband excitations is still under-explored. The spectral overlap and the coupling of these two optical regimes also complicate the determination of hot carriers' energy states and eventually hinder the accurate assignment of their catalytic role in studied reactions. In this Featured Article, after reviewing previous studies, we suggest classifying the photoexcitation via intra- and interband transitions where the physical states of hot carriers are well-defined. Intraband transitions are featured by creating hot electrons above the Fermi level and suitable for reductive catalytic pathways, whereas interband transitions are featured by generating hot d-band holes below the Fermi level and better for oxidative catalytic pathways. Since the contribution of intra- and interband transitions are different in the spectral regions of localized surface plasmon resonance and direct interband excitations, the wavelength dependence of the photocatalytic activities is very helpful in assigning which transitions and carriers contribute to the observed catalysis.

Recent Advances in Real-Time Time-Dependent Density Functional Theory Simulations of Plasmonic Nanostructures and Plasmonic Photocatalysis

Plasmonic catalysis provides a possible means for driving chemical reactions under relatively mild conditions. Rational design of these systems is impeded by the difficulty in understanding the electron dynamics and their interplay with reactions. Real-time, time-dependent density functional theory (RT-TDDFT) can provide dynamic information on excited states in plasmonic systems, including those relevant to plasmonic catalysis, at time scales and length scales that are otherwise out of reach of many experimental techniques. Here, we discuss previous RT-TDDFT studies of plasmonic systems, focusing on recent work that gains insight into plasmonic catalysis. These studies provide insight into plasmon dynamics, including size effects and the role of specific electronic states. Further, these studies provide significant insight into mechanisms underlying plasmonic catalysis, showing the importance of charge transfer between metal and adsorbate states, as well as local field enhancement, in different systems.

Intrinsic Optical Properties and Emerging Applications of Gold Nanostructures

The collective oscillation of free electrons at the nanoscale surface of gold nanostructures is closely modulated by tuning the size, shape/morphology, phase, composition, hybridization, assembly, and nanopatterning, along with the surroundings of the plasmonic surface located at a dielectric interface with air, liquid, and solid. This review first introduces the physical origin of the intrinsic optical properties of gold nanostructures and further summarizes stimuli-responsive changes in optical properties, metal-field-enhanced optical signals, luminescence spectral shaping, chiroptical response, and photogenerated hot carriers. The current success in the landscape of nanoscience and nanotechnology mainly originates from the abundant optical properties of gold nanostructures in the thermodynamically stable face-centered cubic (fcc) phase. It has been further extended by crystal phase engineering to prepare thermodynamically unfavorable phases (e.g., kinetically stable) and heterophases to modulate their intriguing phase-dependent optical properties. A broad range of promising applications, including but not limited to full-color displays, solar energy harvesting, photochemical reactions, optical sensing, and microscopic/biomedical imaging, have fostered parallel research on the multitude of physical effects occurring in gold nanostructures.

STM Visualization of N2 Dissociative Chemisorption on Ru(0001) at High Impinging Kinetic Energies

This paper examines the reactive surface dynamics of energy- and angle-selected N₂ dissociation on a clean Ru(0001) surface. Presented herein are the first STM images of highly energetic N₂ dissociation on terrace sites utilizing a novel UHV instrument that combines a supersonic molecular beam with an in situ STM that is in-line with the molecular beam. Atomically resolved visualization of individual N₂ dissociation events elucidates the fundamental reactive dynamics of the N₂/Ru(0001) system by providing a detailed understanding of the on-surface dissociation dynamics: the distance and angle between nitrogen atoms from the same dissociated N₂ molecule, site specificity and coordination of binding on terrace sites, and the local evolution of surrounding nanoscopic areas. These properties are precisely measured over a range of impinging N₂ kinetic energies and angles, revealing previously unattainable information about the energy dissipation channels that govern the reactivity of the system. The experimental results presented in this paper provide insight into the fundamental N₂ dissociation mechanism that, in conjunction with ongoing theoretical modeling, will help determine the role of dynamical processes such as energy transfer to surface phonons and nonadiabatic excitation of electron-hole pairs (ehps). These results will not only help uncover the underlying chemistry and physics that give rise to the unique behavior of this activated dissociative chemisorption system but also represent an exciting approach to studying reaction dynamics by pairing the angstrom-level spatiotemporal resolution of an in situ STM with nonequilibrium fluxes of reactive gases generated in a supersonic molecular beam to access highly activated chemical dynamics and observe the results of individual reaction events.

Plasmonic phenomena in molecular junctions: principles and applications

Molecular junctions are building blocks for constructing future nanoelectronic devices that enable the investigation of a broad range of electronic transport properties within nanoscale regions. Crossing both the nanoscopic and mesoscopic length scales, plasmonics lies at the intersection of the macroscopic photonics and nanoelectronics, owing to their capability of confining light to dimensions far below the diffraction limit. Research activities on plasmonic phenomena in molecular electronics started around 2010, and feedback between plasmons and molecular junctions has increased over the past years. These efforts can provide new insights into the near-field interaction and the corresponding tunability in properties, as well as resultant plasmon-based molecular devices. This Review presents the latest advancements of plasmonic resonances in molecular junctions and details the progress in plasmon excitation and plasmon coupling. We also highlight emerging experimental approaches to unravel the mechanisms behind the various types of light-matter interactions at molecular length scales, where quantum effects come into play. Finally, we discuss the potential of these plasmonic-electronic hybrid systems across various future applications, including sensing, photocatalysis, molecular trapping and active control of molecular switches.

Plasmonic Photocatalysis with Nonthermalized Hot Carriers

Hot carriers generated by plasmonic damping have been suggested to promote photocatalysis, yet it remains unclear how the nonthermalized hot carriers dynamically activate and promote the energy transfer processes. Here, we present an Anderson-Newns model to describe the vibrational excitation and bond dissociation induced by plasmonic hot carriers. The nonthermal distribution of the hot carriers generated by plasmon damping is accounted for on equal footing with thermal carriers at a given temperature in the electron-molecule scattering. We found that the nonthermal electrons in the high energy region can, albeit in much smaller populations, provide an efficient and dominant channel for photodissociation especially in the low-temperature and quantum plasmon regime. Our model captures the wavelength dependence and reproduces the enhancement factors observed by experiments for oxygen dissociation on silver nanoparticles. It also paves a way to harvesting nonthermal plasmonic energy for photocatalysis in the quantum regime.

Excited States of Metal-Adsorbed Dimethyl Disulfide: A TDDFT Study with Cluster Model

The optical near field refers to a localized light field near a surface that can induce photochemical phenomena such as dipole-forbidden transitions. Recently, the photodissociation of the S–S bond of dimethyl disulfide (DMDS) was investigated using a scanning tunneling microscope with far- and near-field light. This reaction is thought to be initiated by the lowest-energy highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition of the DMDS molecule under far-field light. In near-field light, photodissociation proceeds at lower photon energies than in far-field light. To gain insight into the underlying mechanism, we theoretically investigated the excited states of DMDS adsorbed on Cu and Ag surfaces modeled by a tetrahedral 20-atom cluster. The frontier orbitals of the molecule were delocalized by the interaction with the metal, resulting in narrowing of the HOMO–LUMO gap energy. The excited-state distribution was analyzed using the Mulliken population analysis, decomposing molecular orbitals into metal and DMDS fragments. The excited states of the intra-DMDS transitions were found over a wider energy range, but at low energies, their oscillator strengths were negligible, which is consistent with the experimental results. Sparse modeling analysis showed that typical electronic transitions differed between the higher and lower excited states. If these low-lying excited states are efficiently excited by near-field light with different selection rules, the S–S bond dissociation reaction can proceed.

Plasmonic reactivity of halogen thiophenols on gold nanoparticles studied by SERS and XPS

Localized surface plasmon resonances on noble metal nanoparticles (NPs) can efficiently drive reactions of adsorbed ligand molecules and provide versatile opportunities in chemical synthesis. The driving forces of these reactions are typically elevated temperatures, hot charge carriers or enhanced electric fields. In the present work the dehalogenation of halogenated thiophenols on the surface of AuNPs has been studied by surface enhanced Raman scattering (SERS) as a function of the photon energy to track the kinetics and identify reaction products. Reaction rates are found to be surprisingly similar for the different halothiophenols studied here, although the bond dissociation energies of the C-X bonds differ significantly. Complementary information about the electronic properties at the AuNP surface, namely work-function and valence band states, have been determined by X-ray photoelectron spectroscopy (XPS) of isolated AuNPs in the gas-phase. In this way, it is revealed how the electronic properties are altered by the adsorption of the ligand molecules, and we conclude that the reaction rates are mainly determined by the plasmonic properties of the AuNPs. SERS spectra reveal differences in the reaction product formation for the different halogen species and on this basis the possible reaction mechanisms are discussed to approach an understanding of opportunities and limitations in the design of catalytical systems with plasmonic NPs.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.

Dissociation of Single O2 Molecules on Ag(110) by Electrons, Holes, and Localized Surface Plasmons

A detailed understanding of the dissociation of O₂ molecules on metal surfaces induced by various excitation sources, electrons/holes, light, and localized surface plasmons, is crucial not only for controlling the reactivity of oxidation reactions but also for developing various oxidation catalysts. The necessity of mechanistic studies at the single-molecule level is increasingly important for understanding interfacial interactions between O₂ molecules and metal surfaces and to improve the reaction efficiency. We review single-molecule studies of O₂ dissociation on Ag(110) induced by various excitation sources using a scanning tunneling microscope (STM). The comprehensive studies based on the STM and density functional theory calculations provide fundamental insights into the excitation pathway for the dissociation reaction.

Recent Advances in Photochemical Reactions on Single-Molecule Electrical Platforms

The photochemical reaction is a very important type of chemical reactions. Visualizing and controlling photo-mediated reactions is a long-standing goal and challenge. In this regard, single-molecule electrical detection with label-free, real-time and in situ characteristics has unique advantages in monitoring the dynamic process of photoreactions at the single-molecule level. In this Review, we provide a valuable summary of the latest process of single-molecule photochemical reactions based on single-molecule electrical platforms. The single-molecule electrical detection platforms for monitoring photoreactions are displayed, including their fundamental principles, construction methods and practical applications. The single-molecule studies of two different types of light-mediated reactions are summarized as below: i) photo-induced reactions, including reversible cyclization, conformational isomerization and other photo-related reactions; ii) plasmon-mediated photoreactions, including reaction mechanisms and concrete examples, such as plasmon-induced photolysis of S-S/O-O bonds and tautomerization of porphycene. In addition, the prospects for future research directions and challenges in this field are also discussed. This article is protected by copyright. All rights reserved.

Controlling Localized Plasmons via an Atomistic Approach: Attainment of Site-Selective Activation inside a Single Molecule

Chemical reactions such as bond dissociation and formation assisted by localized surface plasmons (LSPs) of noble metal nanostructures hold promise in solar-to-chemical energy conversion. However, the precise control of localized plasmons to activate a specific moiety of a molecule, in the presence of multiple chemically equivalent parts within a single molecule, is scarce due to the relatively large lateral distribution of the plasmonic field. Herein, we report the plasmon-assisted dissociation of a specific molecular site (C-Si bond) within a polyfunctional molecule adsorbed on a Cu(100) surface in the scanning tunneling microscope (STM) junction. The molecular site to be activated can be selected by carefully positioning the tip and bringing the tip extremely close to the molecule (atomistic approach), thereby achieving plasmonic nanoconfinement at the tip apex. Furthermore, multiple reactive sites are activated in a sequential manner at the sub-molecular scale, and different sets of products are created and visualized by STM topography and density functional theory (DFT) modeling. The illustration of site-selective activation achieved by localized surface plasmons implies the realization of molecular-scale resolution for bond-selected plasmon-induced chemistry.

Atomic, Molecular and Hybrid Oxygen Structures on Silver

Interactions between oxygen and silver are important in many areas of science and technology, including materials science, medical, biomedical and environmental applications, spectroscopy, photonics, and physics. In the chemical industry, identification of oxygen structures on Ag catalysts is important in the development of environmentally friendly and sustainable technologies that utilize gas-phase oxygen as the oxidizing reagent without generating byproducts. Gas-phase oxygen adsorbs on Ag atomically by breaking the O-O bond and molecularly by preserving the O-O bond. Atomic O structures have Ag-O vibrations at 240-500 cm^-1. Molecular O₂ structures have O-O vibrations at significantly higher values of 870-1150 cm^-1. In this work, we identify hybrid atomic-molecular oxygen structures, which form when one adsorbed O atom reacts with one lattice O atom on the surface or in the subsurface of Ag. Thus, these hybrid structures require dissociation of adsorbed molecular oxygen into O atoms but still possess the O-O bond. The hybrid structures have O-O vibrations at 600-810 cm^-1, intermediate between the Ag-O vibrations of atomic oxygen and the O-O vibrations of molecular oxygen. The hybrid O-O structures do not form by a recombination of two adsorbed O atoms because one of the O atoms in the hybrid structure must be embedded into the Ag lattice. The hybrid oxygen structures are metastable and, therefore, serve as active species in selective oxidation reactions on Ag catalysts.

There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

Water Splitting Induced by Visible Light at a Copper-Based Single-Molecule Junction

Water splitting is an essential process for converting light energy into easily storable energy in the form of hydrogen. As environmentally preferable catalysts, Cu-based materials have attracted attention as water-splitting catalysts. To enhance the efficiency of water splitting, a reaction process should be developed. Single-molecule junctions (SMJs) are attractive structures for developing these reactions because the molecule electronic state is significantly modulated, and characteristic electromagnetic effects can be expected. Here, water splitting is induced at Cu-based SMJ and the produced hydrogen is characterized at a single-molecule scale by employing electron transport measurements. After visible light irradiation, the conductance states originate from Cu/hydrogen molecule/Cu junctions, while before irradiation, only Cu/water molecule/Cu junctions were observed. The vibration spectra obtained from inelastic electron tunneling spectroscopy combined with the first-principles calculations reveal that the water molecule trapped between the Cu electrodes is decomposed and that hydrogen is produced. Time-dependent and wavelength-dependent measurements show that localized-surface plasmon decomposes the water molecule in the vicinity of the junction. These findings indicate the potential ability of Cu-based materials for photocatalysis.

Electron-Catalyzed Dehydrogenation in a Single-Molecule Junction

Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.

State-of-the-art progress in tracking plasmon-mediated photoredox catalysis

Metal nanocrystals (NCs), particularly for plasmonic metal NCs with specific morphology and size, can strongly interact with ultraviolet-visible or even near-infrared photons to generate energetic charge carriers, localized heating, and electric field enhancement. These unique properties offer a promising opportunity for maneuvering solar-to-chemical energy conversion through different mechanisms. As distinct from previous works, in this review, recent advances of various characterization techniques in probing and monitoring the photophysical/photochemical processes, as well as the reaction mechanisms of plasmon-mediated photoredox catalysis are thoroughly summarized. Understanding how to distinguish and track these reaction mechanisms would furnish basic guidelines to design next-generation photocatalysts for plasmon-enhanced catalysis.

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.

Optical scanning tunneling microscopy based chemical imaging and spectroscopy

Through coupling optical processes with the scanning tunneling microscope (STM), single-molecule chemistry and physics have been investigated at the ultimate spatial and temporal limit. Electrons and photons can be used to drive interactions and reactions in chemical systems and simultaneously probe their characteristics and consequences. In this review we introduce and review methods to couple optical imaging and spectroscopy with scanning tunneling microscopy. The integration of the STM and optical spectroscopy provides new insights into individual molecular adsorbates, surface-supported molecular assemblies, and two-dimensional materials with subnanoscale resolution, enabling the fundamental study of chemistry at the spatial and temporal limit. The inelastic scattering of photons by molecules and materials, that results in unique and sensitive vibrational fingerprints, will be considered with tip-enhanced Raman spectroscopy. STM-induced luminescence examines the intrinsic luminescence of organic adsorbates and their energy transfer and charge transfer processes with their surroundings. We also provide a survey of recent efforts to probe the dynamics of optical excitation at the molecular level with scanning tunneling microscopy in the context of light-induced photophysical and photochemical transformations.