| Property | Value |
| Material | MoS2 - Molybdenum Disulfide |
| Bulk Band Gap | Indirect 1.02 eV |
| Monolayer Band Gap | Direct 1.82 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
Natural single-crystal molybdenum disulfide flakes with domain sizes of the order of centimetres. The crystals may be exfoliated with scotch tape to produce atomically thin layers of MoS2 or used in STM measurements or other experiments.
INTRODUCTION
Molybdenum disulphide is an inorganic material of the transition metal dichalcogenides series. This material, and its 2D version, have attracted much interest in the last years due to its technological applications at the nanoscale. For instance, the design of silicon-based semiconductor devices is facing problems due to quantum and tunnelling effects. In contrast, MoS2 showed favourable, tuneable, and promising mechanical, electrical, optical, and magnetic properties. For example, its large band gap changes from indirect to direct when moving from bulk to 2D structure, which allows its use in electronics. MoS2 monolayers are used in several applications, including biosensors, optical sensors, electrochemical sensors, batteries, solar cells, transistors, etc.
ELECTRONIC PROPERTIES OF MOS2
In bulk, MoS2 is a semiconductor with an indirect band gap of about 1,02 eV and hexagonal crystal structure with P6₃/mmc crystal structure symmetry. When exfoliated to a single crystal, the band structure evolves and becomes direct, with a size of 1,82 eV.
RAMAN SPECTRUM OF MOS2
Like graphene, single-layer and few-layer molybdenum disulphide (moly) has distinctive signatures in its Raman spectrum.
The Raman spectrum of bulk MoS2 has two prominent peaks: an in-plane (E2g) mode located around 383 cm-1 and an out-of-plane (A1g) mode which is located at 407 cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the Molybdenum atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.
As MoS2 becomes single-layer, these two modes evolve with thickness. The in-plane mode upshifts to 386 cm-1 and the out-of-plane downshifts to 404 cm-1 . The difference of these two modes (~18 cm-1 ) can be used as a reliable identification for monolayer MoS2.
Phonon Dispersion
The phonon dispersion of MoS2 is shown above. As published in “Chirality and Vacancy Effect on Phonon Dispersion of MoS2 with Strain”, Gan et al, Physics Letter A, 2015.
References
Anomalous lattice vibrations of single- and few-layer MoS2. Lee et al, ACS Nano, 2011. Thickness-dependent Raman spectroscopy of MoS2 flakes.
Phonons in single-layer MoS2 and WS2. Molina-Sanchez et al, Physical Review B, 2011. Theoretical paper discussing evolution of phonon dispersions, focus on the out-of-plane and in-plane mode. It discusses in detail the mechanisms of shifting of the two peaks.
From bulk to Monolayer MoS2: Evolution of Raman scattering. Li et al, Advanced Functional Materials, 2012. Multi-wavelength thickness-dependent Raman characterization of MoS2.