Fig. 1.

(a) Structure of a single molybdenum disulphide molecule and an elemental unit of molybdenum disulphide film. Reprinted with permission from Ref. [13]. Copyright 2019, Journal of Physics D: Applied Physics. (b) Three dimensional representation of the structure of MoS₂. Reprinted with permission from Ref. [14]. Copyright 2011, Nature Nanotechnology. (c) Schematics of the structural polytypes: 1T (tetragonal symmetry, one layer per repeat unit, octahedral coordination), 2H (hexagonal symmetry, two layers per repeat unit, trigonal prismatic coordination) and 3R (rhombohedral symmetry, three layers per repeat unit, trigonal prismatic coordination). Reprinted with permission from Ref. [15]. Copyright 2012, Nature Nanotechnology.