Transition-metal dichalcogenides with type-II Dirac fermions: Surface properties and application capabilities

Among the various layered materials "beyond graphene", the class of transition-metal dichalcogenides MTe2 (M= Ni, Pd, Pt) is particularly interesting, due to the existence of bulk type-II Dirac fermions, arising from a tilted Dirac cone. The Dirac cone in these materials is located in the bulk, with inherently superior robustness to surface modifications compared to other Dirac materials, among which graphene, topological insulator and silicene. In addition, MTe2 also displays application capabilities in optoelectronics and catalysis. Here, with surface-science experiments and theory, we assess the surface properties of MTe2, including i) ambient stability, ii) chemical reactivity and iii) aging mechanisms. Remarkably, MTe2 shows outstanding tolerance to CO and stability in water environment. We also demonstrate that passivation in ambient atmosphere is achieved in less than 30 minutes with the TeO2 skin having a sub-nanometric thickness even alter one year in the air. The existence of Te vacancies leads to the enhancement of the surface chemical reactivity. These results pave the way toward the exploitation of this class of Dirac materials in optoelectronics and catalysis.