Anomalous oxidation and its effect on electrical transport originating from surface chemical instability in...

Two-dimensional (Mo,W)Te2 films have recently attracted significant research interest as electronic device channel materials, topological insulators and Weyl semimetals. However, one critical concern that can hamper their diverse applications is surface chemical instability due to weak Mo(W)-Te bond energy reflected in the small electronegativity difference between Mo(W) and Te, which fundamentally induces unpredictable surface oxidation and remarkably affects the film electrical transport. Here, for the first time, we clarify an anomalous oxidation featuring an unbalanced oxidation process in large-area, few-layer 1T'-MoTe2, which originates from the surface chemical instability. We identify the oxidation temperature, oxygen flow rate, structural polymorphism, and atomic chemical bond electronegativity that dominate preferential surface oxidation, which can be monitored by the appearance and decomposition of Raman-active Te metalloids. Importantly, we verify the formation of an ultrathin natural amorphous MoO3-TeO2 surface layer with an approximate self-limiting thickness that significantly affects the transport properties of the underlying few-layer 1T'-MoTe2 film. We also reveal a similar oxidation tendency in few-layer 2H-MoTe2 and 1T'-WTe2 but with a higher resistance to oxidation than 1T'-MoTe2 due to their inherent phase stability. Our findings not only represent a strong advancement in understanding surface chemical instability of atomically thin 2D TMDC materials, but also highlight technically essential importance of constructing ultrathin natural oxide dielectrics/TMDC interfaces with a controllable surface oxidation process for atomically thin TMDC-based devices.