Among 2D dichalcogenide transition metals (TMDs), Molybdenum Ditelluride (MoTe2) has gained significant applications in the field of nanotechnology due to its fascinating optical and electronic properties. This article examines the structure and properties of MoTe2, which have led to its application in the fabrication of nanomaterials.
Two-Dimensional (2D) Materials in Nanotechnology
Nanomaterials mainly consist of 2D materials with a thickness of a few atoms or even a single atom. The thin structure of 2D nanomaterial provides them with distinct properties that primarily contribute to breakthroughs in a myriad of fields, from electronics to medicine.
The incredibly thin structure of 2D nanomaterials contributes to their remarkable mechanical strength, high surface-area-to-volume ratio, superior electronic conductivity, and extraordinary physical and chemical properties. Thus, 2D materials are of significant interest to the scientific community compared with their three-dimensional (3D) counterparts.
Properties of MoTe2
Crystal Structure
Conventionally, MoTe2 exists in four phases, hexagonal (2H) or hexagonal α, octahedral (1T), monoclinic or distorted octahedral (1T') or orthorhombic β', and orthorhombic (Td) structures or monoclinic β.
Although the 1T' semimetal phase of MoTe2 has bands that overlap near the Fermi level, the 2H semiconducting phase is more stable and synthetically accessible. The hexagonal 2H phase exhibits trigonal prismatic coordination and is a semiconductor with a band gap of ~1.0 eV.
MoTe2 has a layered structure with a perfect basal cleavage. Crystallization occurs in the D13h symmetry group. Each layer of the MoTe2 2D crystal is built in hexagonal close packing (HCP) consisting of Te atoms bridged between planes of Mo, resulting in a Te-Mo-Te stacking sequence along the c-axis of the unit cell. The layers are bonded by weak van der Waals force. Thus these layers are loosely coupled, making them interesting compounds for researchers.
Electrical Properties
The electrical behavior of MoTe2 has been published in the Journal of Crystal Growth. According to the authors, MoTe2 has anomalous electrical behavior. At room temperature, MoTe2 has an electrical conductivity of 8.25 Ω-1 cm-1 and shows maximum conductivity of 9.36 Ω-1 cm-1 at 235 K.
A further increase in temperature above 235 K resulted in a decrease in conductivity, reaching its lowest value at 705 K, beyond which conductivity increased with temperature. This anomalous behavior was attributed to the presence of impurities in the crystal lattice.
Optical Properties
Optical studies provide information about the energy bands. MoTe2 of single- and few-layers is a promising candidate for fabricating systems with narrow band gaps. The Raman spectrum of MoTe2 varies with thickness and exhibits greater phonon-limited room-temperature mobility than that of molybdenum sulfide (MoS2).
An article published in Nano Letters reported an optical band gap of 1.10 eV in monolayer MoTe2, and the intense photoluminescence indicated that it was a direct gap material. Furthermore, thicker MoTe2 layers showed an indirect gap, exhibiting behavior similar to that of MoS2.
Thermal Properties
Among the TMDs, MoTe2 stands out for its polymorphic nature at room temperature. MoTe2 exhibits a minimal energy difference between its 2H and 1T′ phases. The effect of thermal stress is more on MoTe2 behavior compared to other TMDs.
Compared to other TMDs, MoTe2 exhibits a low room-temperature thermal conductivity. These properties of MoTe2 are pivotal for its applications in optoelectronics and thermoelectrics.
Synthesis and Fabrication Techniques
The polymorphism exhibited by MoTe2 presents an opportunity to explore the role of the crystal structure in its electronic properties. Also, there is a need for controlled growth techniques to obtain controlled phase crystals.
The 2H-phase of MoTe2 can be synthesized via liquid exfoliation, mechanical exfoliation, and chemical vapor deposition (CVD). On the other hand, the 1T and 1T' phases were prepared via self-flux technology and stoichiometric reactions, respectively.
Despite extensive research, the growth of high-quality atomically thin MoTe2 with controllable proportions of 2H and 1T' phases has been a challenge because of the facile transformation between the phases owing to the low energy barrier between these two phases.
Although various TMDs have been prepared via van der Waals epitaxy (vdWE)-based self-assembly, the low electronegativity of Te makes it difficult to deposit atomically thin MoTe2 via this method. Consequently, direct tellurization CVD has been explored as a convenient alternative to vdWE to achieve the high-area growth of MoTe2.
Applications of MoTe2 in Nanotechnology
With the surge in demand for high-performance field-effect transistors (FETs), significant attention has been focused on two-dimensional (2D) heterophase homojunction with metallic electrodes and semiconducting channels.
In this regard, MoTe2’s intriguing transition between the semiconducting 2H and metallic 1T' phases has presented MoTe2 as a promising candidate for potential applications in electronics.
The direct band gap of 1.07 eV in monolayered MoTe2 facilitates the formation of MoTe2 flakes, making it applicable for visible and short-wavelength infrared (SWIR) photodetection.
Few-layered MoTe2-based FETs are used in digital and analog circuit applications. Additionally, the indirect bandgap of 0.8 eV in few-layered MoTe2 makes it suitable for near-infrared (NIR) photodetection.
MoTe2 is a low-symmetry TMD material that demonstrates multidirectional charge-to-spin conversion in semi-metallic, distorted 1T octahedral phases. Hence, MoTe2 has been used in the development of spintronic applications.
MoTe2 is used in the fabrication of wearable electronic devices because of its stability and mechanical flexibility. Moreover, MoTe2 quantum dots can be used to construct optical devices.
1T′-MoTe2 has been used for electrolysis. The Te sites adsorb hydrogen (H) atoms and exhibit a rapid and reversible activation process. Thus, the overpotential required to maintain a certain current density decreases when held at a cathodic bias.
Challenges and Future Directions
Despite the intriguing applications of MoTe2, the synthesis of high-quality MoTe2 thin films or single layers is challenging owing to the rapid transition between the semiconducting 2H and the metallic 1T' phase. Additionally, accurate control of the number of layers and their influence on the properties of MoTe2 is yet to be explored.
Conclusion
In summary, among the various TMDs, MoTe2 stands out because of its fascinating electronic, mechanical, and thermal properties. Additionally, the phase transition ability of MoTe2 is a favorable characteristic of this TMD, making it a promising candidate for optoelectronic and nanotechnology applications.
References and Further Reading
Sun, Y., et al. (2016). Low‐Temperature Solution Synthesis of Few‐Layer 1T′‐MoTe2 Nanostructures Exhibiting Lattice Compression. Angewandte Chemie, 128(8), pp. 2880-2884. doi.org/10.1002/ange.201510029.
Balakrishnan, K. & Ramasamy, P. (1994). Study of anomalous electrical behaviour of molybdenum ditelluride single crystals. Journal of crystal growth, 137(1-2), pp. 309-311. doi.org/10.1016/0022-0248(94)91291-2
Agarwal, M. K. & Capers, M. J. (1972). The measurement of the lattice parameters of molybdenum ditelluride. Journal of Applied Crystallography, 5(2), pp. 63-66. https://scripts.iucr.org/cgi-bin/paper?a09482
Ruppert, C., et al. (2014). Optical properties and band gap of single-and few-layer MoTe2 crystals. Nano letters, 14(11), pp. 6231-6236. doi.org/10.1021/nl502557g
Fraser, J. P., et al. (2020). Selective phase growth and precise-layer control in MoTe2. Communications Materials, 1(1), p. 48. https://www.nature.com/articles/s43246-020-00048-4
Guo, J. & Liu, K. (2021). Recent progress in two-dimensional MoTe2 hetero-phase homojunctions. Nanomaterials, 12(1), p. 110. doi.org/10.3390/nano12010110
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