Transition metal dichalcogenide (TMDC)

Also called TMD

Everyday term

In plain words

A family of layered materials in which a sheet of metal atoms, such as molybdenum or tungsten, sits between two sheets of sulfur, selenium or tellurium. Molybdenum disulfide (MoS₂) is the best-known member – as a dark grey mineral it has long been used as a , because its layers slide so easily. The family ranges from to metals and .

Going deeper

Two things decide what a TMDC does: how the chalcogens surround the metal – a trigonal prism (H) or an octahedron (T) – and how many d electrons the metal has left over. The same simple count explains why MoS₂ is a semiconductor while NbSe₂, one column to the left, is a metal.

One formula, several structures

Every layer is a plane of metal atoms between two planes of atoms, bonded strongly within the layer and held to its neighbours only by . Each metal atom sits among six chalcogens, arranged either as a (the H forms) or as an octahedron (the T forms). A distorted octahedral form, in which metal atoms pair into zigzag chains, is called , or Td in the bulk stacking of WTe₂.

Stacking adds a second label: 2H has two layers per repeat, 3R three, 1T one. One compound can exist in more than one form. MoS₂ is 2H in most natural crystals, 3R in some, and can be converted into 1T by inserting lithium between its layers.

Why the metal’s group decides

In a simple picture the chalcogen p states form a filled band, and the metal d states sit above it, split into sub-bands by the coordination. What matters is how many d electrons remain to fill them. Group 4 metals give up all of theirs (d⁰), so ZrS₂ and HfS₂ are semiconductors. Group 5 metals keep one (d¹), which half-fills a band: NbSe₂ and TaS₂ are metals, prone to and superconductivity. Group 6 metals keep two (d²), exactly enough to fill the lowest band of the trigonal prism, so 2H-MoS₂ and WSe₂ are semiconductors.

Change the coordination and the result changes too: distorted MoTe₂ and WTe₂ are semimetals. Further along, ReS₂ distorts into metal chains and stays semiconducting, and PtSe₂ is a semiconductor as a but a semimetal in bulk.

What changes in a single layer

The semiconducting group 6 compounds change most. Their in bulk becomes a direct gap in the monolayer, at the corners of the Brillouin zone, so a single layer glows where the bulk barely does. A 2H monolayer has no centre of inversion, so the two at K and K′ become distinguishable by circularly polarised light, and splits the valence band by about 150 meV in MoS₂ and about 450 meV in WSe₂. Screening is weak in one layer, so bind with energies of hundreds of millielectronvolts and dominate the optics even at room temperature.

The usual cautions follow from the chemistry: are the most frequently reported defect, although metal and antisites dominate in some crystals, and tellurides and many selenides degrade in air faster than sulfides.

For specialists

Layered compounds MX₂ in which a transition-metal plane (M = Mo, W, Nb, Ta, Pt and others) is sandwiched between two chalcogen planes (X = S, Se, Te) in trigonal-prismatic (H) or octahedral (T, T′) coordination. Properties span direct-gap semiconductors (monolayer MoS₂, WSe₂), semimetals (WTe₂) and superconductors (NbSe₂).

Where this comes from

Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors Zhu, Cheng and Schwingenschlögl · Physical Review B 84, 153402 (2011) cited by 1,681
Electronics and optoelectronics of two-dimensional transition metal dichalcogenides Wang et al. · Nature Nanotechnology 7, 699 (2012) cited by 16,403

Solid lubricant

A powder or coating that makes surfaces slide over each other easily without oil. Layered crystals such as graphite, MoS₂, WS₂ and hBN are natural lubricants: their layers are strongly bonded inside but only weakly stuck to each other, so they shear apart and coat the rubbing surfaces with slippery sheets. They work where oil cannot – in a vacuum, in space or at high temperatures – and are among the oldest uses of layered materials.

Lamellar solids whose weak interlayer bonding allows easy shear: graphite (which needs adsorbed water or vapour to lubricate well), MoS₂ and WS₂ (best in vacuum and dry gas, oxidising in humid air), and hBN (stable to high temperature in air). Used as burnished or sputtered films, bonded coatings and additives in oils and greases, they reach friction coefficients of a few hundredths, and in space mechanisms MoS₂ is a standard. At the nanoscale, incommensurate contact between layers can make friction nearly vanish – structural superlubricity, seen when graphite flakes are rotated out of register and in MoS₂ – while friction on atomically thin sheets rises as they get thinner because the sheet puckers ahead of the tip.

Full explanation

Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

Coordination polyhedron

The small shape traced by the atoms that surround a metal atom in a crystal. In many layered materials each metal sits inside an octahedron – six neighbours, three above and three below, turned against each other – or a trigonal prism, where the upper three sit right above the lower three. That one turn can make the same compound a semiconductor or a metal.

The polyhedron formed by an atom’s nearest neighbours; their number is its coordination number. In MX₂ layers the metal is either trigonal prismatic (D3h, as in 2H-MoS₂) or octahedral (D3d, as in 1T-TaS₂, and distorted in 1T′-WTe₂), and the ligand-field splitting of the d levels in each geometry, together with the d-electron count, decides whether the layer is a semiconductor or a metal. Halides and many oxides build layers from MX₆ octahedra sharing edges (CrI₃, RuCl₃) or corners (layered perovskites), and that connectivity sets the lattice – honeycomb, triangular or square – that the metal ions form.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Point defect

A flaw in a crystal that involves a single atomic site: an atom missing (a vacancy), an atom of the wrong kind in a site (a substitution, or an antisite when the crystal’s own two kinds of atom swap places), or an extra atom squeezed in between (an interstitial). Every real crystal has some. In a sheet a few atoms thick each one sits at the surface, so a handful of them can change how the material conducts, glows or reacts far more than in a thick crystal.

A defect confined to one or a few lattice sites – vacancy, interstitial, substitutional impurity, antisite or adatom – or a small complex of them. What it does is set by the levels it introduces: shallow levels dope, deep levels trap carriers and act as non-radiative recombination centres, and an isolated deep level in a wide gap can emit single photons. Formation energies and charge states depend on the chemical conditions of growth and on the Fermi level. In 2D materials every defect is a surface defect, less screened and more exposed to the environment, and densities of 10¹² to 10¹³ per cm² are typical of TMDC monolayers.

Full explanation