Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.