Fig. 1. (Colour on-line) PL spectra of the ZnO bulk crystal at 10 K.(a) Full-range spectrum under the excitation of a 325 nm He-Cd laser. The inset reveals the detailed components of the sharp UV peak, including the principal lines of free exciton (FX), donor-bound exciton (DX) and two-electron satellites (TES), as well as their longitudinal optical (LO) phonon sidebands. (b) Defects induced visible emissions of the sample for the excitation wavelengths of 325 nm, 385 nm, and 450 nm.

Fig. 2. (a, b) (Colour on-line) Decaying traces of the visible luminescence of the sample at different temperatures after ceasing the 385 nm excitation. (c, d) show the recorded time-resolved phosphorescence images at 50 and 150 K, respectively.

Fig. 3. (Colour on-line) Variable-temperature PL spectra of the sample under the continuous excitation of 450 nm laser.