Nonmonotonic excitation power dependence of the UV photoluminescence rate from large ZnO nanoparticle assemblies

Abstract

The photoluminescence from solid-state ZnO nanoparticles (NPs) assemblies was studied as a function of the number on NPs in the assembly. With increasing number of NPs, the UV emission band broadened and redshifted, while the UV/visible emission rate ratio decreased. Furthermore, while samples with few NPs displayed a typical linear dependence for the UV emission rate as a function of the excitation power, samples with large number of deposited NPs showed unusual non-linear, nonmonotonic dependences with maxima at intermediate excitation powers. Also, for samples with large number of NPs, the UV band became broader as the excitation power was increased. These unusual results can be understood by considering the interplay between photon self-absorption effects, interface trap states between NPs and illumination power dependent potential barriers at NP/NP interfaces. The exposure of samples with large number of NPs to ethanol vapor during the photoluminescence measurements resulted in partial removal of the non-monotonic behavior of the UV emission rate dependence on excitation power.