Gate and Temperature Driven Phase Transitions in Few-Layer MoTe2

Abstract

MoTe2 has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T′) and orthorhombic (Td) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties. Here we make extensive Raman measurements as a function of layer thickness, temperature, and electrostatic doping on few layer 2H-MoTe2 and also on 1T′-MoTe2 and Td-WTe2. Recent work in MoTe2 has raised the possibility of a 2H-1T′ transition through technology compatible pathways. It has been claimed that such a transition, of promise for device applications, is activated by electrostatic gating. We investigate this claim and find that few-layer tellurides are characterized by high mobility of Te ions, even in ambient conditions and especially through the variation of external parameters like electric field or temperature. These can generate Te clusters, vacancies at crystalline sites, and facilitate structural transitions. We however find that the purported 2H-1T′ transition in MoTe2 cannot be obtained by a pure electrostatic field.