Defects and Surface Structural Stability of MoTe2 Under Vacuum Annealing. 
Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation.  In this work, the structural stability of 2H-MoTe2 with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy.  On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe2.  Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature.  Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a "wagon wheel" morphology after 400 °C annealing for 15 min.  Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain centers.  To prevent extensive Te loss at higher temperatures, where Mo6Te6 nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe2 is tested.  With this passivation strategy, the structural stability of MoTe2 is greatly enhanced up to 500 °C without apparent structural defects.