Nature and origin of unusual properties in chemically exfoliated 2D MoS2

Debasmita Pariari,D D Sarma

doi:10.1063/5.0005413

Abstract

MoS2 in its two-dimensional (2D) form is known to exhibit many fundamentally interesting and technologically important properties. One of the most popular routes to form an extensive amount of such 2D samples is the chemical exfoliation route. However, the nature and origin of the specific polymorph of MoS2 primarily responsible for such spectacular properties have remained controversial with claims of both T and T′ phases and metallic and semiconducting natures. We show that a comprehensive scrutiny of the available literature data of Raman spectra from such samples allows little scope for such ambiguities, providing overwhelming evidence for the formation of the T′ phase as the dominant metastable state in all such samples. We also explain that this small bandgap T′ phase may attain substantial conductivity due to thermal and chemical doping of charge-carriers, explaining the contradictory claims of the metallic and semiconducting nature of such samples, thereby attaining a consistent view of all reports available so far.

Highlights

The discovery of an atomically thin layer of graphene from a three-dimensional graphite crystal by Novoselov et al in 20041 opened up a new avenue of research in two-dimensional (2D) layered materials
Investigations on MoS2 can be traced through decades due to its natural abundance in the earth’s crust, important catalytic properties,[2,3,4] and extensive usage as a solid state lubricant,[5,6,7] it has seen an exponential increase in the number of publications recently.[8]
Each layer of MoS2 is three atomic layer thick with a thickness of 6.2 Å9 in which the planes of Mo atoms are sandwiched between two atomic layers of S with strong in-plane covalent bonding and between Mo and S planes, while such layers of MoS2 with three atomic planes are vertically stacked via weak van der Waals interactions