Introduction

Molybdenum disulfide (MoS2), a widespread material as molybdate in nature, belongs to a class of advanced next-generation materials called transition metal dichalcogenides (TMDs). MoS2 with different structures offers strong covalent bonds between the Mo and S atoms and weak van der Waals force between the layers, allowing them to be mechanically separated and forming 2D sheets of MoS2.

Structure

Molybdenum disulfide is a class of two-dimensional transition metal dichalcogenides (TMDCs) inorganic materials analog to graphene. Each monolayer of 2D MoS2 is about 0.65 nm thick which is stacked by weak van der Waals forces to form bulk MoS2. However, graphene is a zero-bandgap semiconductor, but bulk, multi-layer, and few-layer MoS2 are indirect bandgap semiconductors. Its bandgap can be converted to direct bandgap by converting bulk to mono-layer MoS2. The single layer MoS2 comprises two hexagonal planes of the S atom separated by one plane of the Mo atom, oriented like S–Mo–S. Depending on the coordination environment and stacking order, layered bulk MoS2 has three polymorphic crystalline forms known as 1T (tetragonal), 2H (hexagonal), and 3R (rhombohedral). Where T denotes tetragonal (D3d group), H stands for hexagonal (D3h group), and R represents rhombohedral. The Mo–S coordination is octahedral or trigonal anti-prismatic in the 1T phase but trigonal prismatic in the 2H and 3R phases. The 3R and 1T phases are metastable and often found in synthetic MoS2. The 2H phase can occur spontaneously in nature and is thermodynamically stable. The trigonal prismatic coordination leads to the completely filled dz2 and empty dxy and dx2-y2 orbitals, and this leads to two structures (i.e., 2H and 3R) semiconductors in nature. The T crystal phase shows a metallic nature because the 1T phase (D3d point group) has two sets of degenerate d orbitals, which are dz,x2-y2 (e.g.) and dxy,yz,zx (t2g). Each of the two t2g orbitals has one electron, making the orbitals partially filled, resulting in the metallic character of the 1T phase. The lattice parameters for 2H, 3R, and 1T MoS2 are a = 3.15 ? and c = 12.30 ?, a = 3.17 ? and c = 18.38 ?, and a = 5.60 ? and c = 5.99 ?, respectively[1].

Uses

Due to the direct bandgap, MoS2 has a great advantage over graphene in several applications such as photodetectors, solar cells, chemical sensors, supercapacitor electrodes, Valleytronic devices, optical sensors, and field-effect transistors. 3D bulk materials may reveal similar properties corresponding to their 2D counterparts. MoS2 has a melting point of ～1185 °C and Young's modulus of 0.33 ± 0.007 TPa. Monolayer MoS2 is a typical stable existing semiconductor material with a direct bandgap of ～1.8 eV which makes it advantageous compared to graphene with zero bandgap. This specific property makes MoS2 a promising next-generation material in applications such as switching devices, optoelectronic devices, stretchable electronics, and energy conversion/storage[2].

References

[1] Pawanpreet Kour. “MoS2-based core-shell nanostructures: Highly efficient materials for energy storage and conversion applications.” Journal of energy storage 1 1 (2023).

[2] Navid Alinejadian, Inger Odnevall, Lauri Kollo . “Progress in additive manufacturing of MoS2-based structures for energy storage applications – A review.” Materials Science in Semiconductor Processing 139 (2022): Article 106331.