Electronic properties of atomically thin MoS₂ layers grown by physical vapour deposition: band structure and energy level alignment at layer/substrate interfaces

Abstract

We present an analysis of the electronic properties of an MoS 2 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 2 single crystal domains. In particular, the thickness dependent direct-to-indirect band gap transition previously reported only for MoS 2 layers obtained by exfoliation or via epitaxial growth processes, was found to be also accessible in our PVD grown MoS 2 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 2 /substrate interface.

Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer

“…The peaks at 230.1 eV (162.9 eV) and 233.2 eV (164.1 eV) in pristine ML (Figure a,d) can be attributed to the Mo 3d 5/2 (S 2p 3/2 ) and Mo 3d 3/2 (S 2p 1/2 ) spin–orbit components, respectively, in good agreement with reported binding energies in MoS 2 layers. , In the Mo 3d region, the S 2s core level (227.2 eV) is also clearly resolved. , …”

“…In order to fully simulate the Mo 3d spectra of sputtered ML, an additional low binding energy doublet needs to be introduced (Figure b) at ∼0.9 eV from the main Mo 3d components. Comparable energy separations were reported in previous XPS measurements on MoS 2 layers and the low binding energy Mo 3d doublet attributed to undercoordinated Mo atoms in a substoichiometric MoS 2– x phase, originating from S-vacancies. , …”

Exciton g factors of van der Waals heterostructures from first-principles calculations

“…However, the effect is so small that it has a negligible influence on the g factor [53]. In TMD HB such calculations also show highly spin polarized band edge states at the K points [54]. Therefore, taking n,±K = ±1 for those states is indeed a reasonable approximation.…”

Electronic band dispersion determination in azimuthally disordered transition-metal dichalcogenide monolayers

“…mm 2 range), which is still challenging for TMDC monolayers and thus limits obtaining the desired information on the electronic band structure for many interesting systems. Specifically, TMDC monolayers fabricated by various methods generally comprise flakes (typical lengths ranging from a few 10 nm to several 10 µm) with azimuthal (φ) disorder, i.e., samples are two-dimensional (2D) powders [16][17][18][19][20] . Accordingly, the electronic band dispersion of each azimuthally rotated flakes contributes to the ARPES spectra, generally prohibiting band dispersion observation due to the angular averaging.…”