Unique Electronic Excitations at Highly Localized Plasmonic Field.

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.

Inherent Promotion of Ionic Conductivity via Collective Vibrational Strong Coupling of Water with the Vacuum Electromagnetic Field.

Hydrogen bonding interactions among water molecules play a critical role in chemical reactivity, dynamic proton mobility, static dielectric behavior, and the thermodynamic properties of water. In this study, we demonstrate the modification of ionic conductivity of water through hybridization with a vacuum electromagnetic field by strongly coupling the O─H stretching mode of H2O to a Fabry-Perot cavity mode. The hybridization generates collective vibro-polaritonic states, thereby enhancing the proton conductivity by an order of magnitude at resonance. In addition, the dielectric constants increase at resonance in the coupled state. The findings presented herein reveal how a vacuum electromagnetic environment can be engineered to control the ground-state properties of water.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces.

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Potential energy shift of the Fermi level at plasmonic structures for light-energy conversion determined by graphene-based Raman measurements.

Single layer graphene was used to determine the electrochemical potential of plasmonic nano-structures for photoelectrochemical energy conversions. From electrochemical Raman measurements of the graphene layer under near-infrared light, illumination has revealed the relationship between the photoenergy conversion ability and the Fermi level of the plasmonic structure. The determination is based on in situ monitoring of G and 2D Raman bands of the graphene layer on plasmonic structures. The correlation plots of G and 2D bands show the dependence on the photoconversion ability. The present electrochemical Raman measurements provide detailed understanding of the plasmon-induced charge transfer process for further developments on the ability.

Nonzero Wavevector Excitation of Graphene by Localized Surface Plasmons.

Electrochemical surface-enhanced Raman scattering measurements of single layer graphene provide unique information on resonant excitation induced by localized surface plasmons under controlled electron or hole doping. The highly confined electromagnetic field from the LSPs of the Au nanodimer structures prepared on defect-free graphene can generate holes and electrons of the electrochemical potentials beyond the limit of far-field light illumination. The electrochemical in situ SERS spectra prove nonzero wavevector excitation through the observation of normally forbidden Raman bands in graphene. The present findings point to a novel approach to breaking the limit of optoelectronic interactions and photochemical reactions of graphene and other semiconductors.

Nanoscale control of plasmon-active metal nanodimer structures via electrochemical metal dissolution reaction.

Herein, we report the control of the optical properties of metal nanodimer structures using electrochemical metal dissolution reactions. The reaction rate could be precisely tuned by changing the electrochemical potential and, as a consequence, fine tuning of the size and gap distance of metal nanodimers was achieved as the functions of applied potential and polarization time. The observed linear correlation between the scattering intensity and charge resulting from nanostructure dissolutions suggested that the surface dissolution rate was 0.30 nm min-1, corresponding to the surface dissolution of a single atomic layer per min. The present method can control the change in the volume of the structures, leading to the change in the gap distance of nanodimers at an atomic-scale level.

Plasmonically enhanced electromotive force of narrow bandgap PbS QD-based photovoltaics.

Electromotive force of photovoltaics is a key to define the output power density of photovoltaics. Multiple exciton generation (MEG) exhibited by semiconductor quantum dots (QDs) has great potential to enhance photovoltaic performance owing to the ability to generate more than one electron-hole pairs when absorbing a single photon. However, even in MEG-based photovoltaics, limitation of modifying the electromotive force exists due to the intrinsic electrochemical potential of the conduction band-edges of QDs. Here we report a pronouncedly improved photovoltaic performance by constructing a PbS QD-sensitized electrode that comprises plasmon-active Au nanoparticles embedded in a titanium dioxide thin film. Significant enhancement on electromotive force is characterized by the onset potential of photocurrent generation using MEG-effective PbS QDs with a narrow bandgap energy (Eg = 0.9 eV). By coupling with localized surface plasmon resonance (LSPR), such QDs exhibit improved photoresponses and the highest output power density over the other QDs with larger bandgap energies (Eg = 1.1 and 1.7 eV) under visible light irradiation. The wavelength-dependent onset potential and the output power density suggest effective electron injection owing to the enhanced density of electrons excited by energy overlapping between MEG and LSPR.

Highly Sensitive Detection of Organic Molecules on the Basis of a Poly(N-isopropylacrylamide) Microassembly Formed by Plasmonic Optical Trapping.

We demonstrate that a poly(N-isopropylacrylamide) (PNIPAM) microassembly, formed by plasmonic optical trapping, can provide the platform for a highly sensitive detection technique for fluorescent and nonfluorescent organic molecules dissolved in aqueous solution. PNIPAM microassemblies can be easily formed by a combination with a photothermal effect and an enhanced optical force. These physical phenomena were obtained through resonant excitation of localized surface plasmon (LSP). Sparsely distributed fluorescent or nonfluorescent molecules dissolved in solution can be extracted into the PNIPAM assembly, resulting in an increase in fluorescence or Raman signals. In particular, we successfully detected quite small amounts of analytes (rhodamine B) at the 10-9 mol/L level. Using LSP is an alternative approach in analytical chemistry and can be used in addition to surface enhanced Raman scattering and surface enhanced fluorescence.

Out-of-Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111).

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS2 and MoSe2 monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out-of-plane strain in both MoS2 and MoSe2 layers, whereas no strain generation is observed on polycrystalline Au or SiO2 /Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the SMoS (SeMoSe) structure and Au atoms via unique moiré superstructure formation for MoS2 and MoSe2 layers on Au(111). This observation indicates that the specific adsorption sites give rise to out-of-plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS2 or MoSe2 layer.

Plasmonic optical trapping of nanometer-sized J- /H- dye aggregates as explored by fluorescence microspectroscopy.

In the present study, we explored plasmonic optical trapping (POT) of nanometer-sized organic crystals, carbocyanine dye aggregates (JC-1). JC-1 dye forms both J- and H- aggregates in aqueous solution. POT behavior was analyzed using fluorescence microspectroscopy. POT of JC-1 aggregates was realized in an increase in their fluorescence intensity from the focus area upon plasmon excitation. Repeating on-and-off plasmonic excitation resulted in POT of JC-1 aggregates in a trap-and-release mode. Such POT of nanometer-sized dye aggregates lying in a Rayleigh scattering regime (< 100 nm) is important toward molecular manipulation. Furthermore, interestingly, we found that the J-aggregates were preferentially trapped than H-aggregates. It possibly indicates semi-selective optical trapping of nanoparticles on the basis of molecular alignments.

Electrochemical control of strong coupling states between localized surface plasmons and molecule excitons for Raman enhancement.

The intensity of Raman scattering from dye molecules strongly coupled with localized surface plasmons of metal nanostructures was controlled by the electrochemical potential. Through in situ electrochemical extinction and surface-enhanced Raman scattering measurements, it is found that the redox state of the molecules affects the coupling strength, leading to the change in the intensity of the Raman scattering. Analysis of the Raman spectrum provides information on the molecules in strong coupling states showing effective enhancement of Raman scattering.

Effective Brownian ratchet separation by a combination of molecular filtering and a self-spreading lipid bilayer system.

A new molecular manipulation method in the self-spreading lipid bilayer membrane by combining Brownian ratchet and molecular filtering effects is reported. The newly designed ratchet obstacle was developed to effectively separate dye-lipid molecules. The self-spreading lipid bilayer acted as both a molecular transport system and a manipulation medium. By controlling the size and shape of ratchet obstacles, we achieved a significant increase in the separation angle for dye-lipid molecules compared to that with the previous ratchet obstacle. A clear difference was observed between the experimental results and the simple random walk simulation that takes into consideration only the geometrical effect of the ratchet obstacles. This difference was explained by considering an obstacle-dependent local decrease in molecular diffusivity near the obstacles, known as the molecular filtering effect at nanospace. Our experimental findings open up a novel controlling factor in the Brownian ratchet manipulation that allow the efficient separation of molecules in the lipid bilayer based on the combination of Brownian ratchet and molecular filtering effects.

Plasmon-assisted water splitting using two sides of the same SrTiO3 single-crystal substrate: conversion of visible light to chemical energy.

A plasmon-induced water splitting system that operates under irradiation by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two solution chambers to separate hydrogen (H2) and oxygen (O2). To promote water splitting, a chemical bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a reduction co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge separation at the Au/SrTiO3 interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO3 substrate. The chemical bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.

Plasmonically nanoconfined light probing invisible phonon modes in defect-free graphene.

We present a simple plasmonic method that enables tuning of accessibility to the dipole-forbidden transition states of matter. This technique is realized by well-controlled plasmonic dimers, which can confine optical fields on the order of molecular dimensions. As an example, the approach is applied to activate invisible noncenter phonon modes of defect-free graphene in resonance Raman spectra. The relative intensity of the normally forbidden modes with respect to the dipole allowed modes progressively increases as the degree of field confinement increases. This opens up a novel avenue for both photochemical excitation of molecular systems and nanoscale characterization of materials.

Single molecule dynamics at a mechanically controllable break junction in solution at room temperature.

The in situ observation of geometrical and electronic structural dynamics of a single molecule junction is critically important in order to further progress in molecular electronics. Observations of single molecular junctions are difficult, however, because of sensitivity limits. Here, we report surface-enhanced Raman scattering (SERS) of a single 4,4'-bipyridine molecule under conditions of in situ current flow in a nanogap, by using nano-fabricated, mechanically controllable break junction (MCBJ) electrodes. When adsorbed at room temperature on metal nanoelectrodes in solution to form a single molecule junction, statistical analysis showed that nontotally symmetric b(1) and b(2) modes of 4,4'-bipyridine were strongly enhanced relative to observations of the same modes in solid or aqueous solutions. Significant changes in SERS intensity, energy (wavenumber), and selectivity of Raman vibrational bands that are coincident with current fluctuations provide information on distinct states of electronic and geometrical structure of the single molecule junction, even under large thermal fluctuations occurring at room temperature. We observed the dynamics of 4,4'-bipyridine motion between vertical and tilting configurations in the Au nanogap via b(1) and b(2) mode switching. A slight increase in the tilting angle of the molecule was also observed by noting the increase in the energies of Raman modes and the decrease in conductance of the molecular junction.

Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light.

The use of localized surface plasmons (LSPs) for highly sensitive biosensors has already been investigated, and they are currently being applied for the optical manipulation of small nanoparticles. The objective of this work was the optical trapping of λ-DNA on a metallic nanostructure with femtosecond-pulsed (fs) laser irradiation. Continuous-wave laser irradiation, which is generally used for plasmon excitation, not only increased the electromagnetic field intensity but also generated heat around the nanostructure, causing the DNA to become permanently fixed on the plasmonic substrate. Using fs laser irradiation, on the other hand, the reversible trapping and release of the DNA was achieved by switching the fs laser irradiation on and off. This trap-and-release behavior was clearly observed using a fluorescence microscope. This technique can also be used to manipulate other biomolecules such as nucleic acids, proteins, and polysaccharides and will prove to be a useful tool in the fabrication of biosensors.

Single-molecule observations for determining the orientation and diffusivity of dye molecules in lipid bilayers.

The molecular orientation and diffusion of dye molecules in artificial lipid bilayers were observed using total internal reflection fluorescence microscopy. An artificial lipid bilayer composed of a ternary lipid mixture of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), and cholesterol was used. The molecular orientation, which was obtained through defocused imaging, clarified the microscopic features, including cholesterol-induced changes in the local packing structure. Diffusion analysis gave insights into the macroscopic aspects of phase distribution in the heterogeneous bilayer system. Combining these two independent investigations, we revealed the effect of cholesterol addition on microscopic local packing and macroscopic phase structures. Our observations showed a transition from a DLPC-network-like structure to a DPPC-network-like structure upon the addition of cholesterol, which was not evident from previous domain shape observations. The present single-molecule observations yielded the actual phase structure that controls the motion of molecules in the membrane. The results imply that the orientation and diffusivity of molecules offer useful information regarding the phase distribution, which may be hindered by the apparent phase structure in a heterogeneous lipid bilayer that contains cholesterol.

Local thermal elevation probing of metal nanostructures during laser illumination utilizing surface-enhanced Raman scattering from a single-walled carbon nanotube.

Localized laser-induced heating of an individual Au nano-dimer was quantitatively evaluated by measuring the surface-enhanced Raman scattering (SERS) from an isolated single-walled carbon nanotube (SWNT) supported above the nano-gap between the two metal centres of the dimer. The SERS measurement showed an apparent wavenumber shift in the G-band of the Raman spectra with an increase in the power of the illuminated laser light, indicating the laser-induced local thermal elevation of the Au nano-dimer. In addition, it was found that the effect of the laser illumination on the thermal elevation in air was larger than that in aqueous solution, indicating that the localized laser-heating effect is strongly influenced by the surrounding environment. The present technique provides a measure of the highly localized heating effect of plasmonic metal nanostructures under photo-illumination.

Unveiling the Hidden Energy Profiles of the Oxygen Evolution Reaction via Machine Learning Analyses.

The oxygen evolution reaction (OER) is a crucial electrochemical process for hydrogen production in water electrolysis. However, due to the involvement of multiple proton-coupled electron transfer steps, it is challenging to identify the specific elementary reaction that limits the rate of the OER. Here we employed a machine-learning-based approach to extract the reaction pathway exhaustively from experimental data. Genetic algorithms were applied to search for thermodynamic and kinetic parameters using the current-electrochemical potential relationship of the OER. Interestingly, analysis of the datasets revealed the energy state distributions of reaction intermediates, which likely originated in the interactions among intermediates or the distribution of multiple sites. Through our exhaustive analyses, we successfully uncovered the hidden energy profiles of the OER. This approach can reveal the reaction pathway to activate for efficient hydrogen production, which facilitates the design of catalysts.

Understanding Spatial Distributions of Dye Molecules Coupled to the Surface Lattice Resonance Mode through Electrochemical Reaction Control.

The strong coupling, which is the light-matter interaction, leads to changes in the energy landscape of the chemical dynamics, resulting in the modulation of the reaction pathways. In this study, we achieved strong coupling between dye molecules dispersed in the polymer films and the surface lattice resonance mode, which is excited on plasmonic lattice arrays. In addition, we successfully tuned the coupling strength by introducing the electrochemical potential control method. Reversible decreases and increases in the coupling strength were observed as a result of the reversible electrochemical redox reactions of dye molecules. It is important that the spatial distribution of the molecules coupled to the lattice resonance mode was clarified by using various polymer film thicknesses. Our present electrochemical method for controlling strong coupling states represents a promising method for tuning the light-absorption properties of systems.