Crystal Structure of Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs) are semiconductors of the type MX2, where M is a transition metal atom such as molybdenum (Mo) or tungsten (W), and X is the chalcogen atoms (typically Sulphur (S), selenium(Se) or tellurium (Te) [1-4] .

TMDCs exists in Two Crystal structures
a. 2H-phase trigonal prismatic, with semiconductors properties
b. 1T phase octahedral with Metallic properties
They have strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into 2D layers of single unit cell thickness.

Synthesis of Transition Metal Dichalcogenides (TMDCs)

There are two methods for making 2D materials:
1. Bottom-up methods
Start with atomic ingredients and assemble them together. Such as (i) Chemical vapour deposition (CVD) and (ii) Hydrothermal method.
i . Chemical vapour deposition (CVD)
These methods use different solid precursors heated to high temperatures. In this case, sulphur powder and MoO3 powder vaporized and co-deposited onto a substrate.
(ii) Hydrothermal method.
The growth of single crystals from an aqueous solution in an autoclave at high temperature and pressure).

2. Top-down methods
Start with a bulk (3D) material and make it thinner
Mechanical exfoliation
Thin flakes(2D) of TMDCs can be peeled from their parent bulk (3D) crystals by micromechanical cleavage using adhesive tape. This method is also known “Scotch-tape method”

References
1. Manzeli, S., et al., 2D transition metal dichalcogenides. Nature Reviews Materials, 2017. 2(8): p. 17033.
2. Splendiani, A., et al., Emerging Photoluminescence in Monolayer MoS2. Nano Letters, 2010. 10(4): p. 1271-1275.
3. Wang, Q.H., et al., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotechnology, 2012. 7(11): p. 699-712.
4. Radisavljevic, B., et al., Single-layer MoS2 transistors. Nature Nanotechnology, 2011. 6(3): p. 147-150.