Electronic properties of WS$_2$ on epitaxial graphene on SiC(0001)

TL;DR

This study investigates the electronic structure of WS$_2$ grown on epitaxial graphene on SiC(0001), revealing a large spin-orbit splitting and band alignment suitable for opto-spintronic applications.

Contribution

It provides detailed experimental and theoretical analysis of WS$_2$/graphene heterostructure's electronic properties, highlighting its potential for optoelectronic devices.

Findings

Spin-orbit splitting of 462 meV in WS$_2$ on graphene

Work function of WS$_2$/EG determined as 4.5 eV

Confirmation of van der Waals interface coupling

Abstract

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulphide WS$_2$ on epitaxial graphene (EG) on SiC(0001). The WS$_2$ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS$_2$/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS$_2$ on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS$_2$/EG to be 4.5$\pm$0.1 eV. A large shift of the WS$_2$ VB maximum is observed as well , due to the lowering of the WS$_2$ work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS$_2$ crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.