Interaction of ultra-violet radiation with surface energy levels of the ZnO-Li 2 O-O 2 system as revealed by electron spin resonance spectroscopy

E.s.r. examination of the influence of u.v.-irradiation on surface processes at Li-doped ZnO shows that several paramagnetic centres are involved in the interaction with the gas phase. Annealing in oxygen at 773 K results in formation of zinc vacancies, with signal at g= 2.0130. On oxygen adsorption at 293 K, zinc vacancies are formed at the surface with g= 2.0145. Both signals are visible only in Li-doped samples of low Fermi-level, their intensity depending on irradiation. The radical oxygen signals at g1= 2.054, g2= 2.0077 and g3= 2.0033 accompany the formation of vacancies and increase on irradiation due to photo-adsorption. Signals at g= 1.96 and 1.975 are also present. On grinding, F-centres are formed with g= 1.997. They reversibly adsorb oxygen and interact with radiation. X-ray photoelectron spectra reveal two kinds of oxygen, the lattice oxygen 1 s level depending on defect structure.