Reversible Phase Transition and Lattice Direction Switching in Anisotropic Two-Dimensional Transition Metal Dichalcogenides

Description

The polymorphism (spanning from semiconductive to metallic phases) in the two-dimensional transition metal dichalcogenides (TMDs) has been found effective in modulating the electrical and optical properties, which has great potential for future smart electronic applications especially for memory devices. There have been a few strategies developed for the phase control, e.g., the electronic doping (either by voltage or exotic donor/acceptor), atomic doping, strains, defects, etc.However, owing to the high surface ratio and instability nature of the 2D materials, to achieve a reversible, complete and atomically-precise phase transition in monolayer 2D TMDs is challenging. It is also necessary to explore more polymorphic 2D materials/structures with more facile phase transition control, in particular, with anisotropic crystal lattices such as (tetragonal) ReS2 or MoTe2 (attached Figure 1a). The anisotropic lattices in distorted tetragonal phases (such as ReS2 and MoTe2) provide one more degree of freedom in the lattice direction switching (attached Figure 1b), e.g., under certain strain conditions, providing controllable anisotropic electrical and optical properties along different lattice directions, promising for the novel sensor applications and changeable electronic/optoelectronic devices/circuits, etc.Therefore, in this project we plan to systematically study the phase transition and lattice switching mechanisms in these 2D anisotropic TMDs and TMD alloys. We will use the optical, mechanical, electrical approaches to control and investigate the phase transition/lattice switching mechanisms down to the atomic scale. The in situ scanning probe microscopy (atomic force microscopy (AFM), conductive-AFM (CAFM), Kelvin force microscopy (KFM)), in situ polarized Raman/PL spectroscopy and in situ transmission electron microscopy (TEM) will be mainly employed to search for the optimized 2D TMD structures/compositions and the best external control strategies which can achieve reversible, facile and quick phase transition/lattice switching.The phase transition atomic mechanisms related to the nucleation of secondary phases/lattice directions, the structures of the phase boundaries or grain boundaries, and the changes in electrical/optical properties after transitions will be investigated in detail, to support the phase control development. Furthermore, the optimized approaches will be applied in applications such as electronic devices with memory functions.