Charge Density Wave States in 2H-MoTe_2 Revealed by Scanning Tunneling Microscopy

2H- and 1T'-phase monolayer MoTe_2 films on highly oriented pyrolytic graphite are studied using scanning tunneling microscopy and spectroscopy (STM/STS). The phase transition of MoTe_2 can be controlled by a post-growth annealing process, and the intermediate state during the phase transition is directly observed by STM. For 2H-MoTe_2, inversion domain boundaries are presented as bright lines at high sample bias, but as dark lines at lower sample bias. The dI/dV mappings reveal the distinct distributions of electronic states between domain boundaries and interiors of domains. It should be noted that a 2\times2 periodic structure is clearly discernable inside the domains, where the STS measurement shows a small dip of size \sim150 meV at the vicinity of the Fermi level, indicating that the 2\times2 periodic structure may be an incommensurate charge density wave. Moreover, a 4\times4 periodic structure appears in 2H-MoTe_2 grown at a higher substrate temperature.