a transition-metal dichalcogenide

Picture a molecular sandwich: a filling of metal atoms with a slice of non-metal bread above and below, the whole sandwich just three atoms thick — and then imagine countless such sandwiches stacked loosely on top of each other. That is a transition-metal dichalcogenide, written MX2. The M is a transition metal such as molybdenum or tungsten; the two X's are chalcogen atoms — sulphur, selenium, or tellurium. Molybdenum disulphide, MoS2, the grey solid in dry lubricants, is the classic example. Each three-atom slab is a two-dimensional material in its own right.

Within one slab the metal and chalcogen atoms are held by strong bonds, and the metal sits at the centre of a cage of six chalcogen atoms — either a trigonal prism (the common 2H form) or an octahedron (the 1T form), two polytypes that stack the same slabs differently. Crucially, one slab is bonded to the next only by weak van der Waals forces, so the crystal cleaves easily between slabs — which is exactly why MoS2 is slippery and why single slabs can be peeled off, just like graphene. A monolayer is one S-Mo-S sandwich, roughly 0.65 nm thick.

What makes the dichalcogenides more than graphene's understudy is that many are genuine semiconductors, and thinning them changes the structure's electronic consequence: bulk MoS2 has an indirect band gap, but a single monolayer has a direct gap and glows under light. So here structure at three levels — which atoms (chemistry), how they are caged (2H versus 1T polytype), and how many layers (one versus many) — all steer the behaviour. Dichalcogenides are studied as atomically thin transistors, light emitters, and catalysts, and as partner sheets in van der Waals heterostructures.