Title: Electronic properties of WS$_2$ on epitaxial graphene on SiC(0001),
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.