Abstract
We present an analysis of the electronic properties of an MoS2 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 MoS2 single crystal domains. In particular, the thickness dependent direct-to-indirect band gap transition previously reported only for MoS2 layers obtained by exfoliation or via epitaxial growth processes, was found to be also accessible in our PVD grown MoS2 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 MoS2/substrate interface.
Highlights
Transition-metal dichalcogenides (TMDCs; MX2 where M 1⁄4 Mo or W and X 1⁄4 S, Se, or Te), are a wide class of layered semiconducting materials with promising functionalities for optoelectronic applications.[1]
Angle resolved photoemission spectroscopy (ARPES) studies have already proven to be useful for elucidating the electronic band structures of both exfoliated[12] and large scale growth TMDC single crystal layers.[13,14,15,16]
We report on the electronic properties of multidomain MoS2 monolayer (ML) and bilayer (BL) grown by physical vapour deposition (PVD) on highly ordered pyrolytic graphite (HOPG) substrate using a labbased angle-resolved photoemission spectroscopy (ARPES) system
Summary
Transition-metal dichalcogenides (TMDCs; MX2 where M 1⁄4 Mo or W and X 1⁄4 S, Se, or Te), are a wide class of layered semiconducting materials with promising functionalities for optoelectronic applications.[1]. To meet the low cost wafer-scale fabrication requirements for industry adoption, large-scale deposition methods based on physical vapour deposition (PVD) and chemical vapour deposition (CVD) techniques were alternatively developed.[10,11] A detailed characterization of the electronic properties of the asgrown TMDC layers at variance of the deposition conditions is critical for the device optimization, as they control the charge transport in the layer and at its interface with conductive electrodes In this context, angle resolved photoemission spectroscopy (ARPES) studies have already proven to be useful for elucidating the electronic band structures of both exfoliated[12] and large scale growth TMDC single crystal layers.[13,14,15,16] Synchrotron-based ARPES facilities can achieve excellent energy and spatial resolutions due high intensity and collimated photon sources ((100 mm of spot size17,18) but in situ 2D TMDCs growth cannot be generally provided which is detrimental for a proper optimization of the deposition process. The complex interplay between the defect related electronic gap states, and the interlayer coupling in determining the energy level alignment at MoS2/substrate interface were discussed
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