Soft hydrogen plasma induced phase transition in monolayer and few-layer MoTe₂

Phase transition from the semiconducting hexagonal (2H) phase to the metallic monoclinic (1T') phase in two-dimensional (2D) transition metal dichalcogenides like MoTe₂ is not only of great importance in fundamental study but also of technological significance for broad device applications. Here we report a universal, facile, scalable and reversible phase engineering technique (between 2H and 1T' phases) for both monolayer and few-layer MoTe₂ based on a soft hydrogen plasma treatment. The 2H → 1T' transition was confirmed by a series of characterizations including Raman spectra and mapping studies, XPS analysis and FET device measurements at varying temperatures. We attribute the phase transition to the warping of Te-Mo bonds and the lateral sliding of the top Te-layer induced by the soft hydrogen ion bombardment according to both the structural and electronic characterizations as well as the horizontal comparison with the cases of Ar or O₂ plasma treatment. We have also prepared a 2D heterostructure containing periodical 2H and 1T' MoTe₂ and showed that such phase transition can be readily reversed by post annealing. These results thus provide a robust and efficient approach for the phase engineering of monolayer and few-layer MoTe₂ and could aid the development of 2D optoelectronic, memory and reconfigurable devices.