Fabrication and visible emissions of ZnO nanocrystal doped transparent zinc silicate glass-ceramics

ZnO nanocrystals doped transparent glass and glass-ceramics with visible emissions were successfully synthesized through melt-quenching method. Structures and defect-related photoluminescence of quenched glasses and glass-ceramics were significantly dependent on ZnO content in precursor and post heat treatment time. Nano-sized ZnO crystals could generate in quenched glass before heat treatment with a particular precursor composition (44SiO₂-11Al₂O₃-40ZnO-10K₂O, mol%), and this quenched glass emitted strong green emission. For glass-ceramics, short time (5 min) heat treatment was conducive to intensive green or yellow emission depending on the excitation energy. Further extending of heat treatment resulted in yellow emission as the dominant one, together with a decline of intensity due to reduced defects. Moreover, the emission peaks shifted towards longer wavelength, from violet, blue, to green and yellow, with an increase of heat treatment time, which can be ascribed to different defect states along with crystal growth during heat treatment. Green light corresponded to oxygen vacancies (V_o), the main defect of ZnO before heat treatment, whereas yellow light was associated with the transition from zinc interstitials (Zn_i) to oxygen interstitials (O_i) produced with the structure transformation during heat treatment. Yellow emission can emit with both above- and below-bandgap excitation, and preferentially excited by energy (395 nm, ∼3.14 eV) below the bandgap, near Zn_i level. Green emission was only efficient under the excitation larger than bandgap. Defect-based mechanisms of visible emissions with different initial states were proposed.