Size-Dependent Optical Properties of Colloidal ZnO Nanoparticles Charged by Photoexcitation

The above-bandgap illumination of colloidal ZnO nanoparticles (NPs) in ethanol solutions is found to lead to reversible shifts of the absorption and photoluminescence (PL) excitation spectra, indicating charging of the nanoparticles with electrons. A rapid drop of deep-level PL intensity at the early stage of illumination is observed simultaneously with the splitting-off and growth of a new red-shifted near-band-edge PL band. Such a splitting of the near-bandgap PL band under illumination is observed for the first time and corroborates with the previous assumptions about the behavior of the NP ensemble emission upon gradual NPs’ charging with electrons. The possible relation between the new PL band and photoinduced charging of NPs with excess electrons is discussed on the basis of the dependence of the PL spectrum evolution and absorption band shift relaxation on the NP size and controllable access of oxygen during illumination.