Photoluminescence of ZnO quantum dots produced by a sol–gel process

We report synthesis and spectroscopic characterization of sol–gel derived zinc oxide (ZnO) quantum dots (Qdots) of 3–8 nm size. The Qdots exhibited a broad and strong visible emission peak centered near 520 nm with a quantum efficiency of approximately 5%. A weak ultraviolet (UV) emission peak was also observed near the band gap at 370 nm. The integrated area under the curve for the visible emission band was 35 times greater than that of the band-edge UV emission band of ZnO Qdots at room temperature. The photophysical characteristics (luminescence wavelength and intensity) of ethanol-dispersed Qdots were greatly affected over time (aging), suggesting the continuation of particle growth. In contrast, no such spectroscopic changes were noticed for air-dried ZnO Qdots. Time-resolved emission spectra of Qdots recorded at 520 nm emission were fitted with two sequential exponential decay curves with lifetimes of 14 ns and 77 ns. Visible photoluminescence of ZnO at low temperature (80 K) exhibited 150% higher intensity than that at room temperature. This was discussed based on the standard model of competitive radiative and non-radiative relaxation processes at various temperatures.