Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 14162−14167
Electrochemical Fine Tuning of the Plasmonic Properties of Au Lattice Structures Hiro Minamimoto, Shunpei Oikawa, Takahiro Hayashi, Alice Shibazaki, Xiaowei Li, and Kei Murakoshi* Department of Chemistry, Faculty of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
Downloaded via KAOHSIUNG MEDICAL UNIV on July 5, 2018 at 10:15:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: 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).
■
INTRODUCTION Localized surface plasmon resonance (LSPR), which is the collective oscillation of free electrons in metal nanostructures induced by light illumination onto the metal nanostructure, can realize nanoscale confinement of light energy.1−4 This confined light leads to the formation of a strong optical field, and especially for the dimer case, it can produce intensity magnification of up to ∼105.5,6 The excited plasmonic optical properties of the metal nanostructure are sensitive to the choice of the metal, shape, and size.1,5,7−9 Owing to the improvement in various nanoscale fabrication techniques, such as electron beam lithography,10−12 the plasmonic properties can be precisely controlled for various applications, such as biochemical sensors and surface-enhanced Raman scattering measurements.13−15 To achieve efficient use of the nanoscale confined light, further suppression of light scattering and a welltuned energy with extended lifetime of the excited states are required to make use of the highly confined light energy. Regarding the plasmonic properties, dephasing of the coherent oscillation is induced by electronic/vibrational excitation, radiation damping (energy loss by scattering), and damping owing to surface collisions.16−19 For nanostructures with diameters larger than 20 nm, radiation damping is predominant.16 To discuss the lifetime of the plasmonic excited state, evaluation of the line width, which is defined as the full width at half-maximum (fwhm), is effective because the line width depends on nonradiative and radiative damping of the plasmon modes.20 Many measurements using different sized and/or shaped structures have revealed that typical plasmon lifetimes are 2−10 fs.19,21 One approach to obtain a longer © 2018 American Chemical Society
lifetime of the excited state is to use the Fano resonance, which results from interaction of the narrow dark modes with the broad bright modes.22−24 For excitation of the Fano resonance, overlap of the broad dipolar mode with the narrow dark mode is required. Because of this coupling, the Fano resonance exhibits a distinct asymmetric shape, leading to apparent simultaneous excitation of both the bright and dark modes. The nonradiative dark mode can be used as a propagating mode in the waveguide to confine the light energy in an ultrasmall space beyond the diffraction limit. Recently, two-dimensionally arranged metal nanoparticles have been received much attention as another approach to control plasmon lifetimes.25−29 In such a two-dimensional (2D) square grating of metal nanoparticles (lattice structure), the scattered light from one particle is absorbed by neighboring nanoparticles to excite the plasmon, resulting in a transformation from radiative to evanescent. By this suppression of radiative loss, strongly coupled nanoparticles in the lattice structures show the surface lattice resonance (SLR) mode with an extremely narrow line width (
