The Structural Stability and Phase Transition of MoTe2 Activated by Thermal Annealing
Among group-VIB transitional-metal dichalcogenides (TMDs), semiconducting molybdenum ditelluride (2HMoTe2) with a similar bandgap to Si (~1.1 eV for monolayer and 1.0 eV for bulk state), is a promising candidate for electronic and photovoltaic applications. Additionally, MoTe2 possesses phase transition behavior, for example, the well-known phase transition between its semiconducting 2H structure and its semimetallic, distorted octahedral 1T’ structure due to their small formation energy difference (~0.03 eV). The thermally induced structural stability
of MoTe2 needs careful evaluation for nano-electronic device applications compared to the other TMDs due to a small electronegativity difference (~0.3) between Mo and Te, which may weaken the Mo-Te bonding strength. In this work, using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM), we investigated the thermal structural stability of MoTe2 heated under high vacuum conditions and discovered an interesting decomposition or phase transition process from 2H-MoTe2 (initial) to 2H-MoTe2 surface decomposition with random Te atomic vacancies (200 °C and 300 °C) to semiperiodic, “wagon wheel” patterns of 60° inversion domain boundaries (MoTe1.5 at boundaries, 400 °C) to one
dimensional, metallic Mo6Te6 nanowires (NWs, 450 °C). Particularly, the Mo6Te6 nanowires registered along the <11-20> 2H-MoTe2 crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe2 due to the favorable band-alignment. Furthermore, an atomically sharp MoTe2/Mo6Te6 interface and van der Waals gap with the 2H layers are preserved. The work highlights an alternative pathway for forming new transition metal chalcogenide phases and will enable future exploration of their intrinsic transportation properties.
This research was supported in part by the SWAN Center, a SRC center sponsored by the Nanoelectronics Research
Initiative and NIST, and the Center for Low Energy Systems Technology, one of the six SRC STARnet Centers,
sponsored by MARCO and DARPA.