| Property | Value |
| Material | MoSe2 - Molybdenum Selenide |
| Bulk Band Gap | Indirect 1.09 eV |
| Monolayer Band Gap | Direct 1.59 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
INTRODUCTION
Molybdenum selenide is an inorganic material of the transition metal dichalcogenides series. As well as MoSe2, this material has received quite a bit of attention in recent years due to its favourable optical, electrical, magnetic, and mechanic properties. Hence, there are reports of this material being used in sensors, optoelectronics, lubricants, and solar cells. As a result of Se atoms having an intrinsic metallic nature, this material has a higher electrical conductivity than MoS2, making it more suitable for battery and electrochemical applications. It undergoes the same changes from direct to indirect band gap as the sulphide, by transitioning from bulk to monolayer. In contrast, MoSe2 has a narrower band gap than sulphide, which has been used to catalytically generate hydrogen.
ELECTRONIC PROPERTIES OF MoSe2
In bulk, MoSe2 is a semiconductor with an indirect band gap of about 1,09 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 1,59 eV.
RAMAN SPECTRUM OF MoSe2
Molybdenum selenide, as well as other transition metal dichalcogenide materials, has been widely studied by Raman spectroscopy. MoSe₃ has four active modes in Raman spectroscopy denominated A1g, E1g, E2g 1 , E2g 2. The A1g mode is related to an out-of-plane vibration while the other modes are associated with in-plane transitions. Both A1g and E2g 1 are present in monolayer and bulk structures with values around 239 and 287 cm-1, respectively. The A1g vibration shows small to none change during the transition from bulk to monolayer, while the in-plane mode (E2g 1) exhibits a modification in maximum intensity and position in the bulk-to-monolayer transition, but there is not a significant change in the range from 2 to 5 layers. In addition, in the range 2-5 layers there is a new signal around 350cm-1 that is associated with the A2u 2 vibration, which should be Raman inactive but due to a break of symmetry in the few-layers limit it can be observed in the spectra. Nonetheless, its maximum intensity and position do not change significantly for 2+ layers. Thus, the presence of MoSe2 monolayers can be determined by the presence of different signals in the Raman spectrum, but the determination of 2+ layers is rather difficult. Therefore, researchers focused on other Raman signals with low wavenumbers, known as shear or layer breathing modes. These correspond to layer−layer vibrations where each layer moves nearly rigidly as a whole unit. For MoSe2 , there are two bands of shear vibrations in the range 19-24 and 14-40 cm-1, and with these bands it is possible to determine thickness very accurately.
Phonon Dispersion of MoSe2
The phonon dispersion of MoSe2 is shown above. As published in “Role of Optical Phonons in Bulk Molybdenum Diselenide Thermal Properties Probed by Advanced Raman Spectroscopy”, Dong et al, 2020.
References
Review on 2D Molybdenum Diselenide (MoSe2) and Its Hybrids for Green Hydrogen (H2) Generation Applications. Wazir et al, ACS Omega, 2022. The use of 2D MoSe2 as a catalyst for hydrogen generation, discussing recent advancements and future trends.
Molybdenum diselenide (MoSe2) for energy storage, catalysis, and optoelectronics. Eftekhari, Applied Materials Today, 2017. Theoretical paper discussing MoSe2 having promising potential for various applications in electrochemical, photocatalytic, and optoelectronic systems due to its layered structure and superior electrocatalytic activity.
Precise and reversible band gap tuning in single-layer MoSe2 by uniaxial strain. Island et al, Royal Society of Chemistry, 2016. A new clamping and bending method enables reversible tuning of the electronic band gap of single-layer MoSe2.
Band structure of MoS 2 , MoSe 2 , and α − MoTe 2 : Angle-resolved photoelectron spectroscopy and ab initio calculations. Böker et al, Physical Review B, 2001. The complete valence-band structure of the molybdenum dichalcogenides MoS2, MoSe2, and α−MoTe2 is presented and discussed in comparison.