Spectra of ZnO nanoparticles under low photon energy excitation

Time-integrated photoluminescence (PL) spectra between 1.2 and 2.25 eV of ZnO nanoparticles were observed at ambient temperatures when they were excited by a picosecond (ps) laser pulse at a low photon energy of 2.33 eV/532 nm, to show clear red shift when the excitation intensity increased. Gaussian analysis shows that the red shift is due to increase of the relative magnitudes of the Gaussian combination in the low energy region. Temporal evolution of the dominant emissions exhibited a similar double-exponential decay process, in which the respective two distinct decay durations of 189 ps at the corresponding amplitude of 82% and 2081 ps at 18% were identified. Speculation based on the surface-state emission due to the large surface-to-volume ratio of nanoscale materials is used to explain the phenomena.