Optical properties of ZnO nanocrystals under photo-induced electron doping

The absorption and photoluminescence (PL) spectra of negatively photodoped ZnO nanocrystals dispersed in ethanol are measured under a stepwise photodoping process at room temperature. The spectra are measured almost simultaneously at each photodoping level and compared in detail to discuss the origin of the spectral change. The redshift of the PL spectrum and the increase of its intensity are clearly observed in the early stage of photodoping and are attributed to the trion luminescence. The PL spectrum is found to split into two peaks for highly photodoped nanocrystals. The cause of the splitting is discussed in terms of its dependence on the nanocrystal size.