Ubiquitous shallow trap states and lattice hydrogenation of ZnO particles

ZnO nanoparticle powders are an important and common starting material for functional devices that require the highest purity for electronic and optical applications. In this combined experimental and ab initio study, we use electron paramagnetic resonance (EPR) spectroscopy to detect shallow donor states in the ZnO lattice via their characteristic paramagnetic resonance at g = 1.96. Using DFT calculations that account for the chemical environment and the high temperatures during the gas-phase synthesis in conjunction with control experiments with atomic hydrogen, we demonstrate that these paramagnetic defects correspond to neutral hydrogenated oxygen sites (OH⁰ _O). As a key implication of these findings, ZnO particle powders, although produced under the pure conditions of gas-phase synthesis and subsequently annealed in a vacuum to 873 K, were found to be doped with hydrogen at a base concentration of c > 10^-5 at%. This, in turn, highlights the overlooked role of unwanted hydrogen in nanoscale ZnO as components for applications in optics, electronics, and sensing.