The PL spectra of the samples at various temperatures. (a) Photoluminescence spectra of the ZnO- NCs in the SiO2matrix at various RTP annealing temperatures. (b) The spectrum can be accounted for by two main contributions in the UV-blue and visible regions, respectively. (c) The evolution of various peaks as a function of annealing temperature is shown. For comparison, the volume evolution calculated from the NC size obtained from the TEM analysis is also shown. The decrease of the signal at high annealing temperature can be roughly accounted for by the decrease of the NC absorption cross section.