Size dependent optical properties in ZnO nanosheets

Photoluminescence (PL), scanning electronic microscopy (SEM) and Raman scattering have been studied in crystalline ZnO nanosheets with different sizes after the thermal annealing at 400 °C for 2 hours in ambient air. ZnO nanosheets were created by the electrochemical (anodization) method using the variation of the etching durations with obtained ZnO nanosheet sizes from the range 40-360 nm. Earlier it was shown using the X ray diffraction (XRD) method that thermal annealing performed the ZnO oxidation and crystallization with the creation of the wurtzite crystal lattice. Four PL bands are revealed in PL spectra with the PL peaks at 1.60, 2.08, 2.50 and 3.10 eV. Size decreasing of ZnO nanosheets stimulates tremendous changes of ZnO optical parameters. It is shown that decreasing the ZnO nanosheet sizes is accompanied by the intensity increase of a set of Raman peaks and the surface defect related PL bands. The reasons of emission transformation and the nature of optical transitions have been discussed as well.