| Property | Value |
| Material | WS2 - Tungsten Sulfide |
| Bulk Band Gap | Indirect 1.35 eV |
| Monolayer Band Gap | Direct 1.8 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
| Property | Value |
| Material | WS2 - Tungsten Sulfide |
| Bulk Band Gap | Indirect 1.35 eV |
| Monolayer Band Gap | Direct 1.8 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
INTRODUCTION
Tungsten sulfide is an inorganic material of the transition metal dichalcogenides series. As a non-zero-bandgap alternative to graphene, this material and its 2D version have attracted a lot of attention. WS2 has favourable magnetic, optical, electrical, mechanical, and thermal properties. As a result, WS2 can be used in nanoelectronics devices. In addition, the monolayers present a large direct band gap that allows their use in the production of hydrogen from water electrolysis. This band gap can be modified by doping or applying strains to the material, which allows versatility and it’s critical for some electrical applications at the nanoscale. Furthermore, there are commercial WS2 field effect transistors, together with several nanostructures, including nanotubes and nanosheets. Moreover, WS2 can be used as a dry lubricant.
ELECTRONIC PROPERTIES OF WS2
In bulk, WS2 is a semiconductor with an indirect band gap of about 1,35 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 1,8 eV.
RAMAN SPECTRUM OF WS2
Tungsten sulfide is a semiconductor from the transition metal dichalcogenide series. In terms of its Raman spectrum, there are three Raman-active transitions: LA(M) at about 174 cm-1, E2g 1(Γ) at about 357 cm-1, and A1g (Γ) at about 420 cm-1. These vibrations correspond to a longitudinal acoustic mode at M symmetry, while in the Γ symmetry point the E2g 1 corresponds to an in-plane mode, while the A1g is an out-of-plane mode. The absolute intensity of both in-plane and out-of-plane vibration increases with the addition of layers. By increasing the number of layers the A1g (Γ) mode presents a blue-shift, while the E2g 1 (Γ) and LA(M) modes exhibit a red-shift. Therefore, the wave number difference between the two modes in the Γ symmetry point is systematically increasing with the number of WS2 layers. In this case, the blue-shift is due to the hardening of vibration. This is consistent with the increasing restoring force caused by the Van der Waals interaction between the sheets. This, in turn, implies a higher Van der Waals interaction, and therefore a higher vibrational energy requirement. Furthermore, different spectra have been observed for WS2 bulk and monolayer structures when a different excitation wavelength is used. For instance few-layer structures at 514.5 nm present more peaks, including those associated with second order vibrations. In other words, at this excitation condition the Raman spectra shows drastically between double- and single-layered films, providing an accurate fingerprint for monolayer WS2 . The latter can be explained as a double resonance process, which is only active in the monolayer case. Therefore, both frequency shifts and changes in relative intensity can provide an unambiguous, non-destructive identification of monolayer WS2 .
Phonon Dispersion of WS2
The phonon dispersion of WS2 is shown above. As published in “Phonons in single-layer and few-layer MoS2 and WS2”, Molina-Sánchez et al, 2011.
References
Electronic properties of WS2 and WSe2 monolayers with biaxial strain: A first-principles study. Muoi et al, Elsevier, 2019. The electronic properties of WS2 and WSe2 monolayers under biaxial strain, revealing their sensitivity to compression strain and direct-indirect band gap transitions.
Influence of quantum confinement on the electronic structure of the transition metal sulfide T S 2. Kuc et al, Physical Review B, 2011. Theoretical paper discussing Bulk MoS2 is an indirect band gap semiconductor, but a monolayer of MoS2 becomes a direct band semiconductor due to quantum confinement.