Unveiling surface dynamics: in situ oxidation of defective WS2

Abstract

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.