| Property | Value |
| Material | WSe2 - Tungsten Selenide |
| Bulk Band Gap | Indirect 1.2 eV |
| Monolayer Band Gap | Direct 1.64 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
INTRODUCTION
Tungsten selenide is an inorganic material of the transition metal dichalcogenides series. As well as WS2, this 2D material has favourable magnetic, mechanical, thermal, optical, and electrical properties. 2D WSe2 is known to be a flexible, nearly transparent, high-strength and direct-bandgap semiconductor, capable of being both n- and p- doped. Therefore, it has several possible applications in electronics, detectors, and photovoltaics. For example, this material has been used in different kinds of transistors, diodes, and integrated circuits. Furthermore, WSe2 has an enhanced spin–orbit coupling (SOC) compared to other 2D materials. In addition, WSe2 monolayers have been used as transparent photovoltaic materials with LED properties and more stable electrodes for electrochemical solar cells. Like most 2D inorganic materials, WSe2 undergoes an indirect-to-direct band gap transition, switching from bulk to monolayer scales.
ELECTRONIC PROPERTIES OF WSE2
In bulk, WSe2 is a semiconductor with an indirect band gap of about 1,2 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 1,64 eV.
RAMAN SPECTRUM OF WSE2
Tungsten selenide is a semiconductor from the transition metal dichalcogenide series. It has four active Raman modes, as well as other hexagonal 2D materials, denominated E1g (175 cm-1), E2g 1 (248 cm-1), A1g (251 cm-1), and A2u (308 cm-1). WSe2 consists of slabs of trilayers with ionic-covalent metal-chalcogenide bonds within each trilayer. In general, the most predominant bands in the Raman spectra are related to the E2g 1 and A1g modes, which in the few-layer limit present a small difference of less than 5 cm-1. Therefore, in most of the literature only one strong and broad vibrational mode around 250 cm-1 is observed, and this peak normally presents a blue shift with decreasing the thickness of the WSe2 samples. Moreover, by analysing the phonon dispersion and electronic structure of this material, these two modes are degenerated in energy. Nevertheless, these peaks can be resolved by using polarized Raman scattering, where one peak is observable in one polarization configuration and the other in the opposite one. The latter, due to the nature of each mode, where the E2g 1 mode is associated with in-plane opposing motions of Se and W atoms while the A1g mode originates from out-of-plane relative motions of Se atoms. In polarized Raman spectra, these peaks show opposite shifts with the increase of layer thickness: the in-plane mode exhibits red shifts, while the out-of-plane mode exhibits blue shifts. Regarding the A2u2 vibration, this mode should be Raman inactive but due to a break of symmetry in the few-layers limit it can be observed in the spectra.
Raman Spectrum of WSe2
PHONON DISPERSION OF WSE2
Phonon Dispersion of WSe2
The phonon dispersion of WSe2 is shown above. As published in “Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2”, Huang et al, 2014.
References
Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide. Britton et al, Nature Nanotechnology, 2014. Ambipolar monolayer WSe2 devices with electrically tunable p–n junctions have been developed, demonstrating both p–n and n–p diodes with promising photodetection responsivity and photovoltaic power generation.
Crystal structures of tungsten disulfide and diselenide. Schutte et al, Elsevier, 1987. Theoretical paper discussing the crystal structures of the 2H- and 3R-forms of WS2 from single-crystal data.