Electronic properties of atomically thin MoS layers grown by physical vapour deposition: band structure and energy level alignment at layer/substrate interfaces.

We present an analysis of the electronic properties of an MoS monolayer (ML) and bilayer (BL) as-grown on a highly ordered pyrolytic graphite (HOPG) substrate by physical vapour deposition (PVD), using lab-based angle-resolved photoemission spectroscopy (ARPES) supported by scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) for morphology and elemental assessments, respectively. Despite the presence of multiple domains (causing in-plane rotational disorder) and structural defects, electronic band dispersions were clearly observed, reflecting the high density of electronic states along the high symmetry directions of MoS single crystal domains. In particular, the thickness dependent direct-to-indirect band gap transition previously reported only for MoS layers obtained by exfoliation or epitaxial growth processes, was found to be also accessible in our PVD grown MoS samples. At the same time, electronic gap states were detected, and attributed mainly to structural defects in the 2D layers. Finally, we discuss and clarify the role of the electronic gap states and the interlayer coupling in controlling the energy level alignment at the MoS/substrate interface.