Unveiling surface dynamics: in situ oxidation of defective WS₂

Applications of transition-metal dichalcogenides (TMDs) are affected by defects and oxidation in air. In this work, we clarify the relationship between oxidation dynamics and O₂ availability for highly defective (and therefore reactive) surfaces of WS₂ crystals. Grazing incidence Ar⁺ sputtering was used to induce a significant concentration of S vacancies in the sample, rendering it highly susceptible to oxidative degradation. In this paper we observe that oxidation occurs slowly under low O₂ pressures (<10^-4 mbar) due to reduced O₂-vacancy interactions. At higher O₂ pressures, the reaction progresses rapidly, as tracked by changes in the oxidation state of W using XPS. The density functional theory calculations support the experimentally observed changes in the oxidation state of W after sputtering and oxidation. They provide the mechanisms of O₂ dissociation on S vacancy clusters, demonstrating that the reaction barrier depends on the coordination of surface W atoms. These results can be useful for protecting samples from degradation in device applications.