Tailoring the dielectric screening in WS$_2$-graphene heterostructures

TL;DR

This study demonstrates how the dielectric environment, especially graphene doping levels, can be used to tune the electronic properties of WS$_2$ in heterostructures, with implications for material design and sensing.

Contribution

It provides experimental evidence that dielectric screening in WS$_2$ can be precisely controlled by layer spacing and graphene doping, revealing a distance-dependent and electrically tunable screening effect.

Findings

Band gap and exciton binding energy follow a 1/distance dependence.

Screening effects persist at several nanometers despite electrical isolation.

Graphene doping modulates Coulomb screening by approximately 20% at room temperature.

Abstract

The environment contributes to the screening of Coulomb interactions in two-dimensional semiconductors. This can potentially be exploited to tailor material properties as well as for sensing applications. Here, we investigate the tuning of the band gap and the exciton binding energy in the two-dimensional semiconductor WS$_2$ via the external dielectric screening. Embedding WS$_2$ in van der Waals heterostructures with graphene and hBN spacers of thicknesses between one and 16 atomic layers, we experimentally determine both energies as a function of the \WS-to-graphene interlayer distance. We find that the modification to the band gap as well as the exciton binding energy are well described by a one-over-distance dependence, with a significant effect remaining at several nm distance, at which the two layers are electrically well isolated. This observation is explained by a screening arising from an image charge induced by the graphene layer. Furthermore, we find that the effectiveness of graphene to screen Coulomb interactions in nearby WS$_2$ depends on its doping level and can therefore be controlled via the electric field effect. We determine that, at room temperature, it is modified by approximately 20\% for charge carrier densities of $2\times10^{12}$ cm$^{-2}$.