Influence of morphology and surface conditions on defect properties of nanocrystalline zinc oxide

Abstract

Performance of nanoscale ZnO-based systems depends on the nanomorphology and surface conditions, in particular surface defect states. We investigated the impact of the surface/interface phenomena on the defect-related properties for ZnO-containing nanocrystalline systems. To probe these surface-related effects we employed photoluminescence (PL) spectroscopy. Among others, we studied ZnO-containing nanocrystalline specimens grown by wet precipitation with size and morphology controlled by the synthesis parameters. We observed a strong correlation between defect-related luminescence and morphological sample variations. For example, there was a consistent relationship between the surface/volume ratio and the relative intensity of the PL defect emission, indicating strong influence of the optically-active surface states. Commercially available ZnO nanopowders from several vendors were investigated by PL.^Observation of the size effects was somewhat overshadowed by the sample-to-sample differences in quality, and thus defects' content and abundance. Temperature-dependent PL measurements in the bound-exciton (BEx) range were performed to elucidate surface-related corrections to the excitons thermodynamics. Specially, calculations for the excitonic activation energies indicated strong dependences of the nanocrystal size on the predominant BEx dissociation channels. Also, we observed nonlinear dependences of BEx peak broadening on temperature suggesting surface phonon contributions. We used remote plasma treatments to tailor surface defect properties of ZnO nanopowders. We report on the plasma-driven modifications of defect optical signatures such as BEx and visible luminescence. Besides, plasma treatments revealed size-dependent effects in the studied specimens. PL was used to study ZnO nanoparticles embedded into a polymethyl methacrylate (PMMA) matrix by bulk polymerization.^We found that the polymer encapsulation enhances room-temperature excitonic luminescence by several orders of magnitude, similar to the effects of low temperatures on the as-received nanoparticles. At the same time, relative intensities of the visible defect luminescence did not change noticeably after the PMMA embedding. Intensity-dependent PL experiments showed no indication of a random lasing threshold, thus we attributed the observed phenomena to the influence of the PMMA/ZnO interfaces.