Photoluminescence of ZnO Nanopowders Doped with Ag Impurity

Abstract

The photoluminescence (PL) properties of Ag-doped ZnO nanopowders prepared by aqueous chemical deposition were investigated at room temperature. PL spectra were recorded over the 360–750 nm spectral range using excitation wavelengths from 250 to 750 nm. A pronounced ultraviolet emission centered at 380 nm is attributed to Ag_Zn acceptor-bound excitons. In addition to the near-band-edge emission, broad visible PL bands were detected, originating from radiative recombination through donor–acceptor pair transitions. The spectral distribution of the photoluminescence strongly depends on the excitation wavelength. As the excitation energy decreases, the intensity of the short-wavelength violet emission is suppressed, whereas the broad green/yellow/orange emission centered near 570 nm becomes increasingly dominant. This behavior is interpreted in terms of reabsorption of the higher-energy violet luminescence followed by energy transfer to defect-related recombination centers responsible for the broad visible PL band. A pronounced reduction in the overall photoluminescence intensity was observed for the sample containing the highest silver concentration. This reduction is attributed to a lower concentration of optically active Ag_Zn centers caused by the formation of a secondary silver oxide phase, which decreases the amount of substitutionally incorporated silver in the ZnO lattice. Under long-wavelength excitation, anti-Stokes luminescence is observed, which is attributed to sensitization of the corresponding luminescence centers.