Strain regulation of two-dimensional transition metal dichalcogenides

As layered materials beyond graphene discovered after 2004, two-dimensional (2D) transition metal dichalcogenides (TMDs) are vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as diversity of the electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet with the increasing requirements of programmable and function-integrated devices, property modulation could be concerned as one of the most essential strategies for 2D TMDs. In comparison with conventional external electrical field induction, strain regulation could be much more efficient. In detail, external induction through the electrical field leads to the electron delocalization along the field direction and then induces the transformation of band structures, but electrical field exhibits small modulation for monolayer TMDs. On the contrary, the strain regulation shows excellent tuning efficiency for continuous and reversible modulation. As a result, the strain regulation has become a commonly-used strategy for property tuning of 2D TMDs. On the basis of the lattice transformation, the strain regulation of 2D TMDs can lead to different overlapping of d and p orbits of metal and chalcogen atoms, and then affect the electronic structures of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices and piezoelectronics. The strategies of introducing strains to 2D TMDs are classified into lattice induction, local deformation, macroscopic regulation and so on. Lattice induction is attributable to the structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former one demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter one means that the lattice mismatches between two materials (between two different TMDs in a heterostructure or between a TMD and the substrate) can result in lattice distortion and then induce the strain. However, strain introduced through lattice induction is fixed and difficult to achieve the reversible modulation. Local deformation refers to the morphological transformation at the scale of several micrometers, which can be induced by the bubbles and wrinkles of 2D TMDs, external forces of tips, as well as patterned substrates. Generally, large but nonuniform strain can be introduced to 2D TMDs through the local deformation, which results in the funnel effects and then induces large property variation. Furthermore, macroscopic regulation introduces the strains to the lattices, including the deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell) and thermal expansion coefficient mismatch. Macroscopic regulation would be compatible with industrial manufacture to achieve strain regulation on an extremely large scale in the future. There are some other ways to achieve strain regulation such as the design of special stack structures and the induction of the external electrical field. After summarizing the methods of introducing strain to 2D TMDs, we presented the applications based on strain regulation, such as field effect transistors, flexible photodetectors and strain sensors. Finally, we pointed out the further development and challenges of strain regulation of 2D TMDs.