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.

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.

Steering the Reaction Pathways of Terminal Alkynes by Introducing Oxygen Species: From C-C Coupling to C-H Activation.

Selective regulation of chemical reactions is crucial in chemistry. Oxygen, as a key reagent in ubiquitous oxidative chemistry, exhibits great potential in regulating molecular assemblies, and more importantly, chemical reactions in molecular systems supported by metal surfaces. However, the unique catalytic performance and reaction mechanisms of oxygen species remain elusive, which are essential for understanding reaction selection and regulation. In this study, by a combination of scanning tunneling microscopy (STM) imaging/manipulations and density functional theory (DFT) calculations, we showed that the on-surface reaction pathways of terminal alkynes could be steered from C-C coupling to C-H activation with high selectivity by introducing O2 into the molecular system. The catalytic performance and reaction mechanisms of oxygen species were explored in the C-H activation processes, and both molecular O2 and atomic O could efficiently steer the reaction pathways. These results would provide a fundamental understanding of interfacial catalytic reaction processes.

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.

Dissociation Mechanism of a Single O2 Molecule Chemisorbed on Ag(110).

The dissociation of O2 molecules chemisorbed on silver surfaces is an essential reaction in industry, and the dissociation mechanism has long attracted attention. The detailed dissociation mechanism was studied at the single-molecule level on Ag(110) by using a scanning tunneling microscope (STM). The dissociation reaction was found to be predominantly triggered by inelastically tunneled holes from the STM tip due to the significantly distributed density of states below the Fermi level of the substrate. A combination of action spectroscopy with the STM and density functional theory calculations revealed that the O2 dissociation reaction is caused by direct ladder-climbing excitation of the high-order overtones of the O-O stretching mode arising from anharmonicity enhanced by molecule-surface interactions.

Localized Graphitization on Diamond Surface as a Manifestation of Dopants.

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes. Graphitized domains of a few nanometers are shown to manifest the effects of boron dopants on the BDD surface. Confirmed by first-principles calculations, local density of states measurements reveal that the electronic structure of these features is characterized by in-gap states induced by boron-related lattice deformation. The dopant-related graphitization is uniquely observed in BDD (111), which explains its electrochemical superiority over the (100) facet. These experimental observations provide atomic-scale information about the role of dopants in modulating the conductivity of diamond, as well as, possibly, other functional doped materials.

Chemical Identification and Bond Control of π-Skeletons in a Coupling Reaction.

Highly unsaturated π-rich carbon skeletons afford versatile tuning of structural and optoelectronic properties of low-dimensional carbon nanostructures. However, methods allowing more precise chemical identification and controllable integration of target sp-/sp2-carbon skeletons during synthesis are required. Here, using the coupling of terminal alkynes as a model system, we demonstrate a methodology to visualize and identify the generated π-skeletons at the single-chemical-bond level on the surface, thus enabling further precise bond control. The characteristic electronic features together with localized vibrational modes of the carbon skeletons are resolved in real space by a combination of scanning tunneling microscopy/spectroscopy (STM/STS) and tip-enhanced Raman spectroscopy (TERS). Our approach allows single-chemical-bond understanding of unsaturated carbon skeletons, which is crucial for generating low-dimensional carbon nanostructures and nanomaterials with atomic precision.

Single-Molecule Study of a Plasmon-Induced Reaction for a Strongly Chemisorbed Molecule.

Chemical reactions induced by plasmons achieve effective solar-to-chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism. To obtain mechanistic knowledge, we investigated the plasmon-induced dissociation of a single-molecule strongly chemisorbed on a metal surface, two O2 species chemisorbed on Ag(110) with different orientations and electronic structures, using a scanning tunneling microscope (STM) combined with light irradiation at 5 K. A combination of quantitative analysis by the STM and density functional theory calculations revealed that the hot carriers are transferred to the antibonding (π*) orbitals of O2 strongly hybridized with the metal states and that the dominant pathway and reaction yield are determined by the electronic structures formed by the molecule-metal chemical interaction.

Single-molecule resonance Raman effect in a plasmonic nanocavity.

Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.

Atomic-Scale Visualization of the Stepwise Metal-Mediated Dehalogenative Cycloaddition Reaction Pathways: Competition between Radicals and Organometallic Intermediates.

Dehalogenative cycloaddition reaction is a powerful strategy to generate new ring scaffolds with π-conjugated features on a surface, and thus holds great promise toward atomically precise electronic devices or nanomaterials. The ortho-dihalo substitution provides a good strategy to realize cycloaddition. However, the limited understanding of intermediate states involved hinders mechanistic exploration for further precise design and optimization of reaction products. Now, the evolutions of competing surface-stabilized radicals and organometallic intermediates in real space were visualized toward the formation of dominant conjugated four-membered ring connections. From the interplay of scanning tunneling microscopy and density functional theory calculations, the stepwise metal-mediated dehalogenative cycloaddition pathway is elucidated both experimentally and theoretically. The results provide fundamental insights into the intermediate states involved in on-surface synthesis.

Scanning probe microscopy for real-space observations of local chemical reactions induced by a localized surface plasmon.

Localised surface plasmon (LSP) resonance has attracted considerable attention in recent years as an efficient driving force for chemical reactions. The chemical reactions induced by LSP are classified into two types, namely, redox reactions based on plasmon-induced charge separation (PICS) and chemical reactions induced by the direct interaction between LSP and molecules (plasmon-induced chemical reactions). Although both types of reactions have been extensively studied, the mechanisms of PICS and plasmon-induced chemical reactions remain unexplained and controversial because conventional macroscopic methods can hardly grasp the local chemical reactions induced by LSP. In order to obtain mechanistic insights, nanoscale observations and investigations are necessary. Scanning probe microscopy (SPM) is a powerful experimental tool to investigate not only the surface morphology but also the physical and chemical properties of samples at a high spatial resolution. In this perspective review, we first explain SPM combined with optical excitation, and then, review the recent studies using SPM techniques for real-space observations of the chemical reactions induced by LSP.

Mechanistic Studies of Plasmon Chemistry on Metal Catalysts.

Chemical reactions induced by the localized surface plasmon (LSP) of metal nanostructures could be important for a sustainable society to achieve highly efficient conversion from solar energy to chemical energy. However, the reaction mechanism of plasmon chemistry in metal catalysis is still controversial. Mechanistic studies of plasmon chemistry involving direct interactions between the LSP and molecules are reviewed and discussed in terms of the excitation mechanisms of the molecules. We focus on the studies performed using plasmonic metal nanoparticles and highlight the recent progress in plasmon chemistry investigated using scanning probe microscopy with high spatial resolution to obtain mechanistic insights that cannot be obtained by macroscopic analytical methods. This Minireview delivers an overview of the mechanistic understanding of plasmon chemistry in metal catalysis at the current stage, and provides guidance for future studies with respect to clarifying reaction mechanisms.

Real-space and real-time observation of a plasmon-induced chemical reaction of a single molecule.

Plasmon-induced chemical reactions of molecules adsorbed on metal nanostructures are attracting increased attention for photocatalytic reactions. However, the mechanism remains controversial because of the difficulty of direct observation of the chemical reactions in the plasmonic field, which is strongly localized near the metal surface. We used a scanning tunneling microscope (STM) to achieve real-space and real-time observation of a plasmon-induced chemical reaction at the single-molecule level. A single dimethyl disulfide molecule on silver and copper surfaces was dissociated by the optically excited plasmon at the STM junction. The STM study combined with theoretical calculations shows that this plasmon-induced chemical reaction occurred by a direct intramolecular excitation mechanism.

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light.

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (nS)) to the LUMO-derived MO (the antibonding orbital localized on the S-S bond (σ*SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule-metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes.

Direct output of electrical signals from LSPR sensors on the basis of plasmon-induced charge separation.

Potentiometric and conductometric sensors based on localized surface plasmon resonance were developed. The sensors can be applied to coloured and turbid samples because light need not pass through the sample solution.

Elucidation of Isomerization Pathways of a Single Azobenzene Derivative Using an STM.

The predominant pathway for the isomerization between cis- and trans-azobenzenes-either (i) inversion by the bending of an NNC bond or (ii) rotation by the torsion of two phenyl rings-continues to be a controversial topic. To elucidate each isomerization pathway, a strategically designed and synthesized azobenzene derivative was investigated on a Ag(111) surface. This was achieved by exciting the molecule with tunneling electrons from the tip of a scanning tunneling microscope (STM). Structural analyses of the molecularly resolved STM images reveal that both inversion and rotation pathways are available for isomerization on a metal surface and strongly depend on the initial adsorption structures of the molecule. On the basis of the potential energy diagrams for the isomerization, it is concluded that isomerization pathways on a metal surface are not simply related to the excited states.

Photoelectrochemical synthesis, optical properties and plasmon-induced charge separation behaviour of gold nanodumbbells on TiO2.

Chemically synthesized, commercially available Au nanorods were adsorbed on a TiO2 thin film, and photoelectrochemically transformed to Au nanodumbbells by photoelectrochemical deposition of Au at both ends of the NRs under UV irradiation. The nanodumbbells show about fourfold greater light absorption than the nanorods based on localized surface plasmon resonance (LSPR) in the visible to near infrared region. The absorption intensities and wavelengths of the Au nanodumbbells depend on the size of their spheroidal caps, which can be controlled by UV exposure time. The nanodumbbells can be applied to LSPR sensors, as their absorption peak redshifts with increasing local refractive index near the metal surface. The Au nanodumbbells on TiO2 are also suitable for photofunctional materials and devices based on plasmon-induced charge separation (PICS) at the Au-TiO2 interface, because of their higher photoabsorption intensity, better wavelength tunability and greater PICS efficiency than nanorods.

Localized surface plasmon resonance sensors based on wavelength-tunable spectral dips.

Localized surface plasmon resonance (LSPR) sensors serve as sensitive analytical tools based on refractive index changes, which can be applied to affinity-based chemical sensing and biosensing. However, to select the monitoring wavelength, monodisperse Au or Ag nanoparticles must be synthesized. Here we developed LSPR sensors that operate at arbitrary wavelengths after preirradiation at the corresponding wavelength. Polydisperse plasmonic Ag nanospheroids or nanorods are photocatalytically deposited on TiO2. The nanoparticle ensemble shows a broad absorption band over the visible and near infrared regions, and absorption dips can be formed at desired wavelengths simply by photoexciting the ensemble at the wavelengths, on the basis of plasmon-induced charge separation. The dips redshift linearly in response to a positive change of refractive index, and the refractive index sensitivity linearly increases with increasing dip wavelength (e.g., 356 nm RIU(-1) at 1832 nm). The dip-based sensor is applied to monitoring of selective binding between biotin and streptavidin. The present system would allow development of miniaturized and cost-effective sensors that operate at the optimum wavelength at which the sensitivity is highest within the optical window of the sample.

Photoinduced reversible changes in morphology of plasmonic Ag nanorods on TiO2 and application to versatile photochromism.

We achieved reversible changes in length and spectrum of Ag nanorods based on plasmon-induced photoelectrochemical reactions. The changes are applied to multi-wavelength and dual-polarization photochromism in visible-infrared regions. It allows display of invisible images viewable only by infrared cameras. Also possible is display of superimposed visible and invisible images.