| Property | Value |
| Material | MoTe2 - Molybdenum Telluride |
| Bulk Band Gap | Indirect 1 eV |
| Monolayer Band Gap | Direct 1.1 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
INTRODUCTION
Molybdenum telluride is an inorganic material of the transition metal dichalcogenides series. As well as MoX2 (X=S, Se), this compound has attracted attention due to its electrical, mechanical, magnetic, and optical properties. Specially, it presents a band gap in the infrared region with a transition from indirect to direct, going from the bulk to the 2D structure. Furthermore, this material can crystallize in various phases, which gives a range of exotic physical properties, as well as, quantum states that can be used for several applications, such as, circuits, lasers, electronical devices, among others. Furthermore, MoTe2 has a low energy barrier for changing phase, which can be used in photodetectors, sensors, and memory.
ELECTRONIC PROPERTIES OF MOTE2
In bulk, MoTe2 is a semiconductor with an indirect band gap of about 1 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 1,1 eV.
RAMAN SPECTRUM OF MoTe2
The literature regarding the Raman spectra of molybdenum telluride is extensive. Several phases can be identified by the Raman signature of the 2D limit, namely the number of peaks that distinguish the 2H, 1T, and 1T' phases. In the bulk phase, it has four Raman-active transitions denominated A1g , E1g, E2g 1, E2g2. These modes correspond to one out-of-plane vibration and three in-plane vibrations. MoTe2 flakes show a strong E2g 1 mode at 235 cm-1, together with a relatively weak peak related to the A1g vibration at 174 cm-1. Furthermore, the peak at 235 cm-1 splits into two lines in the 30-layer material. Those lines have been related to the E2g 1 and E1u2 modes, which are conjugated modes. Furthermore, with decreasing the number of layers from 30-layers to monolayer, there is an upshift of 1.5 cm-1 for the E2g 1, while the A1g suffers a downshift of 2 cm-1. Interlayer interactions are responsible for the latter phenomenon. In the case of in-plane vibrations the increase is due to effects of the boundary surface layers that lead to more effective forces on the MoTe2 molecules with decreasing thickness. As for the out-of-plane mode, the interlayer interactions produce a restoring force for the MoTe2 molecules. In addition, for the few-layer samples a distinct band has been observed at 291 cm-1, which is a Raman inactive mode for both monolayer and bulk structure related to the notation B2g 1. This mode is allowed for few-layer MoTe2 due to the breaking of translational symmetry and it is strongly enhanced probably due to the large polarizability of the Te atom.
Phonon Dispersion of MoTe2
The phonon dispersion of MoTe2 is shown above. As published in “Efficient method for calculating Raman spectra of solids with impurities and alloys and its application to two-dimensional transition metal dichalcogenides”, Hashemi et al, 2019.
References
MoTe2: Semiconductor or Semimetal?. Deng et al, ACS Nano, 2021. MoTe2's Janus nature, rich phases, and recent advances in material structures and emerging quantum states, offer both opportunities and challenges for device design and applications.
Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals. Ruppert et al, American Chemical Society, 2014. Theoretical paper discussing Monolayer MoTe2 as a direct-gap semiconductor, displaying strong photoluminescence and extending the range of atomically thin direct-gap materials from the visible to the near-infrared.
The electrical properties and the magnitude of the indirect gap in the semiconducting transition metal dichalcogenide layer crystals. Grant et al, IOP Science, 1975. The electrical resistivity and Hall effect measurements of p-type MoSz and n-type MoS2, MoSe2, and MoTe2 as a function of pressure, showing a decrease in resistivity due to an increase in carrier concentration and the predominance of extrinsic conduction until above room temperature.