Unexpectedly large energy gap in ZnO nanoparticles on a fused quartz support

Granulated zinc metal films on fused quartz and sapphire supports were obtained via pulsed laser deposition and then oxidized in air at different temperatures. We found that optical absorbance spectra of oxide films do not depend on the oxidation temperatures from 400 to 750 degrees C. The spectra are characterized by the fundamental absorption edge and the exciton maximum at 370 nm. At the oxidation temperature of 770 degrees C, the optical absorbance spectrum changes dramatically. The fundamental absorption edge was shown to shift to shorter wavelength, thereby heralding the growth of the band gap. To elucidate the origin of the observed phenomena, the films stoichiometry was studied via X-ray photoelectron spectroscopy (XPS). Band edge luminescence was observed independent of the oxidation temperature. On the other hand, visible luminescence at 500 nm occurs only at the oxidation temperatures higher than 600 degrees C and grows afterward. According to the results of XPS, the reason for the appearance of that luminescence peak is the appearance of oxygen vacancies at oxidation temperatures above 600 degrees C.

Automated summary

The optical absorbance spectra of oxide films do not change with different oxidation temperatures, except at 770°C. XPS analysis revealed the presence of oxygen vacancies at oxidation temperatures higher than 600°C, which are responsible for the visible luminescence at 500nm.