Electronic and optical properties of transition metal dichalcogenides under symmetric and asymmetric field-effect doping

Abstract

Doping via electrostatic gating is a powerful and widely used technique to tune the electron densities in layered materials. The microscopic details of how these doping strategies affect the layered material are, however, subtle and call for careful theoretical treatments. The external gates do not just increase the Fermi level in the system, but also generate external electric fields which affect the layered material as well. As a result, the electron densities within the system can redistribute and might thereby affect the electronic band structure in a non-trivial way. Theoretical descriptions via rigid shifts of the Fermi level can, therefore, be highly inaccurate. Using semiconducting monolayers of transition metal dichalcogenides (TMDs) as prototypical systems affected by electrostatic gating, we show that the electronic and optical properties change indeed dramatically when the gating geometry is properly taken into account. This effect is implemented by a self-consistent calculation of the Coulomb interaction between the charges in different sub-layers within the tight-binding approximation. Thereby we consider both single- and double-sided gating. Our results show that, at low doping levels of 10 ^13 cm ^−2 , the electronic bands of monolayer TMDs shift rigidly for both types of gating, and subsequently undergo a Lifshitz transition. When approaching doping levels of 10 ^14 cm ^−2 , the band structure changes dramatically, especially in the case of single-sided gating where we find that monolayer MoS _2 and WS _2 become indirect gap semiconductors. The optical conductivities calculated within linear response theory also show clear signatures of these doping-induced band structure renormalizations. Our numerical results based on light-weighted tight-binding models indicate the importance of charge screening in doped layered structures, and pave the way for further understanding gated super-lattice structures formed by multilayers with extended Moiré patterns.

Keywords

• transition metal dichalcogenides
• tight binding model
• self-consistent approach