The band gap of few layered MoS2 can be tuned between the semiconducting and the metallic regime just by applying various types of strain (in-plane tensile or compressive, and out of plane compressive strain). A team led by Abhishek K Singh at the Materials Research Centre of the Indian Institute of Science, Bangalore explore the origin of this semiconductor to metal (S-M) transition. They do this under different strains and with increasing layer numbers of MoS2. Their study explains the mechanism behind this electronic phase transition.

Determining the number of layers in MoS2

They demonstrate that the inter-layer interaction between out of plane d (dz2) orbitals of Mo and p (pz) orbitals of S causes the S-M transition under normal compressive strain. However, S-M transition under bilayer compressive and tensile strain is different. It is caused by the strong hybridization of the in-plane Mo-d and S-p orbitals and the out of plane Mo-d and S-p orbitals, respectively within the same layer. Moreover the strain at which this transition occurs increases with an increase in number of layers for normal strain. The value of the threshold strain thus provides a means to determine the number of layers in an experimentally grown multilayer MoS2.

Enhancing thermoelectric properties

This change in electronic properties has a positive influence on thermoelectric properties. The application of strain on MoS2 enhances the thermoelectric properties by three-fold. This shows its potential for future low-dimension thermoelectric devices. Owing to the similar crystal structure and electronic properties, this proposed S-M transition mechanism can be extended to all other semiconducting transition metal dichalcogenides (TMDs) under various types of strains. It is possible to tune both the electronic and thermoelectric properties by applying various combinations of strain types and number of layers. This makes this family of semiconducting TMDs useful in building very low dimensional devices for smaller, more powerful, energy efficient and cheaper technologies than those we currently use.

The team is now investigating the effect of strain on other 2D materials such as metallic TMDs, graphene, and mixed hybrid structures. An optimization of the thermoelectric figure of merit by designing new 2D materials is another challenge taken by this group. A successful accomplishment of this work can show a new direction in the area of renewable energy.

More information about this research can be found in the journal Nanotechnology 25 465701.

Further reading

Tweaking the magnetism of molybdenum sulphide nanoribbons (2014)
Putting a twist in molybdenum diselenide (August 2014)
Layered semiconductors: thickness changes conductivity (September 2014)