Electrical tuning of vibrational modes in transition metal dichalcogenides*

Understanding frictional phenomena at nanoscale in layered materials is an important stage in order to control layer sliding, occurring in nanoelectromechanical devices.
To this aim, we studied the atomic scale tribological properties of transition metal dichalcogenides (TMDs) by using ab initio techniques.
We considered 6 prototypical MX₂ TMDs (M=Mo, W; X=S, Se, Te) with hexagonal P6₃/mmc symmetry, focusing on how some specific phonon modes contribute to the intrinsic friction. We described the exchange-correlation interaction energy within the DFT framework, including long range dispersion interactions in the Grimme formulation.
We identified and disentangled the electro-structural features that determine the intra- and inter-layer motions affecting the intrinsic friction by means of electro-structural descriptors such as orbital polarization, bond covalency and cophonicity.
We show how the phonon modes affecting the intrinsic friction can be adjusted by means of an external electrostatic field, the latter then acting as a knob to control intrinsic friction in layered materials.
The presented outcomes are a step forward in the development of TMD-based nanoelectromechanical systems.