MoS₂ (molybdenum disulfide): structure, band gap and uses

One MoS₂ layer is three atomic planes thick: a plane of molybdenum between two planes of sulfur. Each Mo is bonded to six S atoms (Mo–S 2.42 Å) and each S to three Mo, so the layer is held together by strong bonds within itself, while neighbouring layers are held only by van der Waals forces.

In the common 2H form the sulfur atoms above and below each Mo sit directly over one another, so Mo is in a trigonal prism; in the 1T form the two sulfur sheets sit over different sites, so Mo is in an octahedron (Wang et al. 2014). 2H-MoS₂ is a semiconductor; 1T-MoS₂ is metallic (Voiry et al. 2013; Acerce et al. 2015).

Bulk 2H-MoS₂, the mineral molybdenite, is hexagonal (P6₃/mmc, a = 3.16 Å, c = 12.29 Å) with two layers per cell, so the layers repeat every 6.15 Å (from its crystal file).

1. Mechanical exfoliation. Tape peels single layers off bulk crystals; several dichalcogenides were among the first 2D crystals isolated this way (Novoselov et al. 2005).
2. Liquid exfoliation. MoS₂ disperses in common solvents into thin flakes that can be deposited individually or formed into films and composites (Coleman et al. 2011).
3. Chemical exfoliation. Chemically exfoliated nanosheets contain a high concentration of the metallic 1T phase (Acerce et al. 2015).
4. Chemical vapour deposition. Single-layer MoS₂ grows from vapour on a substrate, with grains that meet at grain boundaries (Najmaei et al. 2013); continuous monolayers have been grown at atmospheric pressure at 750 °C (Prasad & Singh 2023).

• Band gap that depends on thickness. As MoS₂ is thinned from six layers to one, its indirect gap moves up by more than 0.6 eV until, in the monolayer, the gap becomes direct (Mak et al. 2010; Splendiani et al. 2010). The monolayer gap is 1.8 eV (Radisavljevic et al. 2011).
• Light emission. Bulk MoS₂ has negligible photoluminescence; the monolayer emits strongly, with a quantum efficiency more than 10⁴ times higher (Mak et al. 2010; Splendiani et al. 2010).
• Transistor action. A single layer with a hafnium oxide gate reached a room-temperature mobility of at least 200 cm² V⁻¹ s⁻¹ and on/off ratios of 10⁸ (Radisavljevic et al. 2011); how that mobility was extracted was later debated in the same journal (Radisavljevic & Kis 2013). h-BN-encapsulated devices gave two-terminal mobilities of 33–151 cm² V⁻¹ s⁻¹ (Lee et al. 2015).
• Catalytic edges. The edges of the layers evolve hydrogen; the flat basal plane is inert (Jaramillo et al. 2007; Kibsgaard et al. 2012).
• Piezoelectric single layers. Single-atomic-layer MoS₂ is piezoelectric (Wu et al. 2014).

Graphene has no band gap, so a graphene transistor never switches fully off. Single-layer MoS₂ has a large intrinsic gap of 1.8 eV, and its transistors switched with on/off ratios of 10⁸ and very low standby power (Radisavljevic et al. 2011). Together with the discovery that the monolayer gap is direct (Mak et al. 2010; Splendiani et al. 2010), this put MoS₂ at the centre of research on 2D transition metal dichalcogenides for electronics and optoelectronics (Wang et al. 2012).

In catalysis MoS₂ showed that a cheap, earth-abundant sulfide can evolve hydrogen at a moderate overpotential (Hinnemann et al. 2005), and that the activity comes from the edges (Jaramillo et al. 2007), so MoS₂ catalysts are now engineered to expose as many edges as possible (Kibsgaard et al. 2012).

• Transistors and logic. Single-layer and few-layer MoS₂ channels (Radisavljevic et al. 2011; Wang et al. 2012).
• Optoelectronics. Light emission and absorption from the direct-gap monolayer (Mak et al. 2010; Wang et al. 2012).
• Hydrogen evolution. Edge-rich nanoparticles and double-gyroid films (Jaramillo et al. 2007; Kibsgaard et al. 2012), and conducting 1T-MoS₂ nanosheets (Voiry et al. 2013).
• Supercapacitors. 1T-rich MoS₂ nanosheets reach about 400–700 F cm⁻³ in aqueous electrolytes (Acerce et al. 2015).
• Composites and heterostructures. MoS₂ flakes reinforce polymers (Coleman et al. 2011) and are stacked with graphene and h-BN in van der Waals heterostructures (Geim & Grigorieva 2013).

For a paper, slide or poster, show a top view of the hexagonal lattice and a side view of the S–Mo–S sandwich, with the trigonal prisms of 2H-MoS₂ (or the octahedra of 1T) drawn as polyhedra if coordination matters. The figures above show MoS₂ as a catalyst and among the other 2D materials, and open as editable LabCanvas figures. The structure drawing is a high-resolution SVG of one layer seen side on that opens in the LabCanvas figure editor. The crystal structure drawing is a high-resolution SVG from the 2H-MoS₂ crystal file and opens in the figure editor, where you can change the style, the number of cells and the view. The 3D model opens in the LabCanvas 3D Studio, where you can switch between 2H and 1T, show the polyhedra and export a high-resolution image.