Reversible semimetal–semiconductor phase transition in CVD-grown monolayer MoTe₂

Abstract Monolayer molybdenum ditelluride (MoTe 2 ) attracted intensive scientific interest due to the small energy difference between its semiconducting (1H) and semimetallic (1T’) phases. Understanding MoTe 2 polymorphism phenomena and developing pathways to induce reversible phase transformations is of great scientific and practical importance to develop semiconductor–semimetal phase change devices. In this paper, we show how thermal annealing induces phase transition in both 1H and 1T’ phases of chemical vapor deposition (CVD) grown MoTe 2 . We also show that depending on the temperature, those transformations are reversible. The material is kept stable by encapsulating it with CVD-grown graphene and the thermal treatments are performed in ultra-high vacuum to prevent oxidation. MoTe 2 is characterized in its different phases via Raman spectroscopy and transmission electron microscopy. We report a 1H to 1T’ transition temperature of ∼1090 °C and observe reversion (i.e. 1T’ to 1H transition) at ∼900 °C. Density functional theory simulations are performed to gain insight on the experimentally measured 1H-1T’ critical transition temperatures. These findings are relevant for fundamental understanding of phase transition phenomena in monolayer MoTe 2 that find applications in memories, transistors and semimetal–semiconductor junctions.