Strain regulation of two-dimensional transition metal dichalcogenides

Abstract

As layered materials beyond graphene began to be discovered after 2004, two-dimensional (2D) transitional metal dichalcogenides (TMDs) became vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as their diverse electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet the increasing demand for programmable and function-integrated devices, property modulation has been considered an essential strategy for employing the useful properties of 2D TMDs. This can be achieved through strain regulation. In comparison to conventional external electrical field induction, strain regulation is much more efficient. In detail, external induction through the electric field leads to electron delocalization along the field direction, which then induces the transformation of band structures. Yet the electric field exhibits only a small modulation for monolayer TMDs. On the contrary, strain regulation exhibits excellent tuning efficiency for continuous and reversible modulation. Based on lattice transformation, strain regulation of 2D TMDs can lead to different overlaps of the d and p orbitals of metal and chalcogen atoms, which then affects the electronic structures of 2D TMDs. As a result, strain regulation has become a commonly used strategy for the property-tuning of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices, and piezoelectronics. The strategies for introducing strain to 2D TMDs can be classified into lattice induction, local deformation, macroscopic regulation, and so on. Lattice induction is attributable to structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter demonstrates 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 thus induce strain. However, strain introduced through lattice induction is fixed, thus making it difficult to achieve reversible modulation. The second classification for introducing strains, local deformation, refers to 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. In general, large but non-uniform strain can be introduced to 2D TMDs through local deformation, resulting in funnel effects, which then induces large property variations. The third classification, macroscopic regulation, introduces strains to lattices through deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell), and thermal expansion coefficient mismatch. Macroscopic regulation could be compatible with industrial manufacture in order to achieve strain regulation on an extremely large scale in the future. Apart from these three classifications, there are still other ways to achieve strain regulation, such as through the design of special stack structures or the induction of an external electric field. After summarizing the methods of introducing strain to 2D TMDs, we presented possible applications of strain regulation, such as field effect transistors, flexible photodetectors, and strain sensors. Finally, we pointed out further developments and challenges to the strain regulation of 2D TMDs.