Improvements in light energy confinement and the improvement of the plasmon excitation lifetime are effective ways to achieve efficient light energy usage on plasmonic nanostructures. Herein, we propose a facile method to improve the quality (Q) factor of an Au nanostructure array using a relatively low annealing temperature method. Annealing at a temperature below 200 degrees C drastically changed the extinction spectra of Au nanoprisms so that they exhibited a narrow full width at half-maximum while maintaining structural anisotropy. X-ray diffraction and electrochemical measurements proved the improvement of the crystallinity of the Au nanoprisms as the origin of relatively high Q factor. After annealing, sharp peaks were observed owing to a decrease in the electron scattering induced by the decrease in the grain boundaries and the surface roughness. Moreover, low-temperature annealing was applied to the well-defined Au lattice structure prepared using electron-beam lithography, which resulted in a 5-fold improvement of Q factors in the surface lattice resonance mode. The present method was demonstrated to be important for obtaining high-quality plasmonic structures.

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Highly reproducible control of metal plasmonic nanostructures has been achieved via precise tuning of the electrochemical Au dissolution reaction that occurs at the surfaces of well-defined bridged nanodisk dimer structures on an atomic scale. It was found that the scattering intensity is strongly suppressed during the transition from the conductive mode to the gap mode of the localized surface plasmon resonance during the period when the gap is formed and increased between Au nanodisks. The characteristic shift of the plasmon mode during this suppression of the scattering intensity verifies the excitation of the bonding quadrupolar mode, which appears only at sub-nanometer gap distances (d < 1 nm). Electrochemical potential control demonstrates that the scattering suppression states with estimated gap distances of less than 1 nm can be maintained for more than 100 s under ambient conditions. The method and phenomena presented here will be useful in the preparation of plasmonic structures for ultimate light confinement applications.

Ultra-small nanogap of plasmonic metal dimer can confine light into molecular size. In this study, we have tried to establish the method for the control of the gap distance of Au nanodimer structures using electrochemical method. Au nanodimer structures were fabricated on a conductive glass substrate by the electron beam lithography method. Electrochemical dissolution of bridged Au nanodisk dimer was conducted under in-situ dark-field microscopy observation. The electrode potential was kept at the potential for Au dissolution. SEM images proved the formation of the gap between Au nanoparticles. From the time series scattering spectra of Au nanodimer, the drastic optical property changes has been observed at the gap formation. This optical property change can be assigned to the plamon mode change from the charge transfer mode to bonding dipolar mode due to the formation of the sub-nm gap at Au nanodimer. The present method is a useful tool for controlling the shape and the gap distance between Au nanoparticles in a single nanometer scale.

The optical property of plasmon-active metal nano dimer structure strongly depends on its shape and gap distance. Thus, the precise control of metal nano structure has been receiving much attention in various field. In the present study, we have tried to control the plasmonic property by combining electrochemical method with in-situ dark-field microscopy. Controlled metal dissolution in the size range below a few nm leads to the successful switching from the charge transfer plasmon (CTP) to the bonding dipolar plasmon (BDP) mode. The highly localized plasmonic field generated during the switching could be applied for various applications including molecular optical trapping in solution at room temperature.

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.

We tuned the plasmonic properties of the Au lattice structure by electrochemical potential control. Au lattice structures with different values of the spacing, diameter, and height show characteristic optical properties determined by the surface lattice resonance of the localized surface plasmon mode. Electrochemical potential control can change the metal structures through metal dissolution, as well as the energy of the electrons in metals. In situ real time observation of the optical properties of Au lattice structures by electrochemical dark-field scattering microscopy shows the fine-tuning of the plasmonic properties with characteristic resonance energy and controlled spectral width. By controlling surface dissolution of the Au lattice structure at a rate of a few nanometers per minute, we tuned the plasmonic properties and achieved a spectral width of 0.145 eV at a maximum resonance of 1.74 eV (714 nm).

The effect of plasmon excitation on metal nanostructures was investigated under electrochemical potential control with intense pulsed laser illumination. The well-defined Au nano-rod structures showed a distinct thermal effect in the form of shape transformation, in the absence of the plasmon-induced electron transfer reaction. It was found that the shape transformation only occurred under the resonance condition of the localized surface plasmon mode excitation.

Efficient use of light energy is regarded as a key factor in solving energy challenges to create a sustainable society. The highly concentrated photon energy generated by localized surface plasmon resonance excitation in the vicinity of metal nanostructures can enhance light-matter interaction. Optimization of the interactions between plasmons and electrons in materials can lead to novel light energy applications. To overcome the current limitations for these interactions, the plasmon field must be focused to an extremely small size close to the molecular scale. Formation of the plasmonic field at the quantum limit may cause interesting phenomena with unique photoresponses. Recently, following the development of nanofabrication techniques, detailed investigations have been undertaken to understand these processes. In this focus review, we describe recent advances in the strong interactions between highly localized photons and electrons in nanomaterials, including molecules, nanocarbons, and quantized nanoparticles. First, we outline the plasmonic properties that depend on the metal nanostructures. In addition, we describe surface-enhanced Raman scattering (SERS), which is used to detect interactions between plasmons and materials. The importance of the resonant electronic excitation process, which is a chemical effect based on the charge transfer contribution, is discussed while considering the unique molecular selectivity in SERS. We then highlight the unique photoresponse properties that are used for ultra-sensitive detection of single molecules by the localized plasmon field. These properties are major advantages of the plasmon field. Next, we introduce strong coupling between plasmons and excitons. This coupling state is promising because of its ability to modify the intrinsic optical properties of materials via creation of a novel absorption wavelength region to accumulate the light energy. Finally, we discuss the use of plasmon excitation for effective chemical reactions accompanied by electron transfer. We conclude that reduction of light to the molecular scale would open novel routes for energy manipulation required by the next generation.

Here we report the optical property control of Au nano-bridged structures by the single-atom metal layer deposition onto the surface. The deposition and dissolution of the Cu atom layer were performed by the electrochemical under potential deposition method. Through the control of the reversible electrochemical deposition and dissolution process, the reproducible photoswitching behavior was observed.