Optical absorption properties and control method of nanostructures

Abstract

Due to the localized surface plasmon resonance (LSPR) effect excited at the nanostructure surface, plasmonic nanostructures have strong scattering and absorption properties, which have important applications in photothermal conversion, photochemical conversion, photoelectric conversion, and so on. Compared to single-material nanoparticles, core/shell plasmonic composite nanoparticles have unique optical properties. The plasmon resonance features are strongly affected by nanoparticle size, shape, layer thickness, and the surrounding medium. In this study, Ag-core/SiO2-shell and SiO2-core/Ag-shell nanostructures were examined, with water as the surrounding medium. The effects of structure size and layer thickness on optical absorption properties were investigated. Moreover, the differences in the optical properties between the metal/dielectric and dielectric/metal nanostructures, and the main causes of the differences, will be discussed in this paper. The optical absorption properties of core/shell plasmonic nanostructures were simulated by the finite-difference time-domain (FDTD) method. The inner and outer radii of the core/shell nanostructures were R 1 and R 2, respectively. For the Ag/SiO2 core/shell nanostructure, as outer radius R 2 increased, the absorption intensity gradually decreased, while the plasmon resonance frequency remained approximately constant. A slight change in SiO2 shell thickness could significantly affect resonance intensity, but had little effect on resonance frequency. As the LSPR effect was excited at the Ag core surface, the electric-field was strongly enhanced. The strong near-field effect enhanced light absorption and scattering. By altering the core and shell materials, forming metal-coated dielectric composite nanostructures, the optical absorption properties can be changed significantly. The SiO2-core and Ag-shell composite nanostructure was studied to investigate the effect of nanostructure parameters on optical absorption properties. The metal shell thickness has a great influence on the resonance features. A little change in the Ag shell thickness can lead to a great shift of the plasmon resonance peak. As the metal shell thickness increased, the resonance frequency gradually shifted to shorter wavelengths, and the resonance intensity weakened. This indicates that the metal shell thickness plays an important role in tuning the plasmon resonance features. The electric-field and energy density are much stronger in the dielectric-core/metal-shell composite nanostructure than in the metal-core/dielectric-shell composite nanostructure. By controlling the relative thicknesses of the core and shell layers, the absorption peak can be tuned to desired wave bands. Thus, the absorption band will be broadened by combining nanostructures of different sizes.