Atomic-Scale Oxygen-Mediated Etching of 2D MoS₂ and MoTe₂

Oxidation is the main cause of degradation of many 2D materials, including transition metal dichalcogenides (TMDs), under ambient conditions. Some of the materials are more affected by oxidation than others. To elucidate the oxidation-induced degradation mechanisms in TMDs, the chemical effects in single layer MoS₂ and MoTe₂ are studied in situ in an electron microscope under controlled low-pressure oxygen environments at room temperature. MoTe₂ is found to be reactive to oxygen, leading to significant degradation above a pressure of 1 × 10−7 torr. Curiously, the common hydrocarbon contamination found on practically all surfaces accelerates the damage rate significantly, by up to a factor of forty. In contrast to MoTe₂, MoS₂ is found to be inert under oxygen environment, with all observed structural changes being caused by electron irradiation only, leading to well-defined pores with high proportion of molybdenum nanowire-terminated edges. Using density functional theory calculations, a further atomic-scale mechanism leading to the observed oxygen-related degradation in MoTe₂ is proposed, and the role of the carbon in the etching is explored. Together, the results provide an important insight into the oxygen-related deterioration of 2D materials under ambient conditions relevant in many fields.