Abstract
Zinc oxide is a versatile semiconductor with an expansive range of applications including lighting, sensing and solar energy conversion. Two central phenomena coupled to its performance that remain heavily investigated are the origin of its sub-band-gap green emission and the nature of its conductivity. We report photoluminescence and dark conductivity measurements of zinc oxide nanoparticle films under various atmospheric conditions that demonstrate the vital role of adsorbates. We show that the UV emission and conductivity can be tuned reversibly by facilitating the adsorption of species that either donate or extract electrons from the conduction band. When the conductivity data are compared with photoluminescence spectra taken under the same ambient conditions, the green emission can be directly linked to surface superoxide formation, rather than surface hydroxylation or native defects such as oxygen vacancies. This demonstrates how and explains why the green emission can be controlled by surface reactivity and chemical environment.
Highlights
Zinc oxide is a versatile semiconductor with an expansive range of applications including lighting, sensing and solar energy conversion
Since a significant decrease in the conductivity under these same oxygen-rich conditions is observed, and since the green emission is caused by trap-assisted recombination, we propose that the origin of green emission in Zinc oxide (ZnO) is due to adsorbed OÀ2, which, to the authors knowledge has never been considered
The relative intensities of green (~2.3 eV) and UV emission (~3.3 eV) have been used to characterize emissive defects in ZnO19,31. This has typically been done to compare samples annealed at different temperatures or conditions, e.g., in zinc- or oxygen-rich environments, to manipulate the concentrations of specific native defects[22,23,24]
Summary
This led to a rapid decrease in the green emission intensity. This change led to an initial rapid increase in the green emission due to the sudden injection of oxygen into the system. This led to a quick drop, and an eventual quench, of the green emission and an abrupt increase in the UV emission.
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