Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver Citrate Solution by Monochromatic-Visible-Light Irradiation

Silver citrate solution was irradiated by monochromatic light from light-emitting diodes (LEDs). Under visible-light irradiation, a characteristic peak (Abs(hex)) was observed in the absorption spectrum above a certain value of fluence (Phi(thres)). The formation of hexagonal silver nanoprisms was confirmed by transmission electron microscopy observations; the size of these hexagons in the in-plane direction decreased with increasing E-LED (nominal peak energy). The formation process was investigated from the fluence evolution of Abs(hex), which was due to the localized surface plasmon resonance of the nanoprisms. With decreasing E-LED, Phi(thres) showed an exponential increase and the growth rate of Abs(hex) beyond Phi(thres) (Delta Abs(hex)/Delta Phi*,Phi*, = Phi - Phi(thres)) showed a linear decrease. The Delta Abs(hex)/Delta Phi* vs E-LED data indicated that Delta Abs(hex)/Delta Phi* became zero at E-LED = 1.9 eV. Generally, the peak energy of Abs(hex) expected at Phi* = 0 was close to E-LED. These results indicated that embryos were formed by visible-light irradiation below Phi(thres), and some of them were converted into hexagonal silver nanoprisms. The conversion of embryos into nanoprisms is governed by a resonant process induced by the incident light. Further, the formation of nanoprisms is induced by visible-light irradiation with photon energy greater than 1.9 eV.