From ZnO Nanorods to Nanoplates:  Chemical Bath Deposition Growth and Surface-Related Emissions

In this paper, a low-temperature controllable chemical bath deposition method was demonstrated to prepare one-dimensional ZnO nanorods and two-dimensional nanoplates, and their surface-related emissions were studied by temperature-dependent cathodoluminescence spectra. By changing the precursor concentration, the ZnO morphology evolves from nanorods to nanoplates. ZnO nanorods grow fast along the c-axis direction due to the high surface energy of the polar (0001) plane when the concentration of OH- ions is low in the precursor solution. When the OH- concentration is increased, more OH- ions preferably adsorb on the (0001) plane of ZnO, and the growth of the ZnO nanocrystallite along the c axis is partially suppressed. However, they can still grow sideways along directions. Therefore, with the OH- concentration increased, the average aspect ratio (high/width) of ZnO nanorods is decreased. Finally, two-dimensional ZnO nanoplates are formed. Low-temperature cathodoluminescence spectra of such ZnO nanostructures exhibit donor-bound exciton emission and surface-state-related exciton emission caused by surface impurities. With increasing temperature, the bound exciton emission decreases gradually due to the ionization of donors and finally vanishes when the temperature is above 130 K. The near-band-gap ultraviolet emission at room temperature is dominated by surface-related exciton emission.