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.