Chalcogenide, halide and pnictide

Everyday term

In plain words

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.

Going deeper

The three families are named for columns of the periodic table. A compound is a chalcogenide, halide or pnictide after the element in it that takes the negative charge, and going down a column – sulfur to tellurium, chlorine to iodine – the anion gets bigger and holds its electrons less tightly, so the same metal gives a smaller gap and stronger spin–orbit effects.

Named for the anion

In a compound of a metal with a non-metal, the non-metal usually carries the negative charge – it is the anion – and chemistry names the compound after it: oxides, sulfides, chlorides. Anions with similar chemistry get a collective name from their column of the periodic table. The chalcogens are the column headed by oxygen – sulfur, selenium and tellurium below it – and their compounds other than oxides are chalcogenides. The halogens, fluorine to iodine, make halides; the word means salt-former. The pnictogens, the column headed by nitrogen – phosphorus, arsenic, antimony and bismuth – make pnictides.

The layered materials in the catalogue fall largely along these lines. MoS₂, WSe₂, Bi₂Se₃ and In₂Se₃ are chalcogenides; CrI₃, RuCl₃, NiI₂ and PbI₂ are halides; CsV₃Sb₅, EuSn₂As₂, PtBi₂ and GeAs are pnictides. Some layers use two anions at once – Bi₂O₂Se, CrSBr, FeOCl, SbSI – and are called mixed-anion compounds; layers put a different chalcogen on each face.

What the anion does to a layer

Down each column the anion gets bigger and holds its outer electrons less tightly. Bonds with the metal become more , bands broaden and gaps shrink, and the heavier nucleus strengthens . The molybdenum dichalcogenides show the trend cleanly: a of MoS₂ has a gap near 1.9 eV, MoSe₂ about 1.55 eV and MoTe₂ about 1.1 eV, and in the tellurides the semiconducting and are so close in energy that or can switch between them.

The families also differ in character. Halides are more ionic, so the metal keeps its d electrons on the atom, and many layered halides are magnetic insulators – CrI₃ and CrBr₃ , NiI₂ a , RuCl₃ a candidate – with the heavy iodides giving the strongest . Pnictides bond more like metals and include many metals and semimetals, among them the and the parent compounds of the iron-based superconductors discovered in 2008.

Stability and handling follow the chemistry

The same trends decide how a material has to be handled. Sulfides are generally the most robust; selenides and especially tellurides oxidise faster, and thin WTe₂ or MoTe₂ degrades in air within hours to days. Many halides draw water from the air, and CrI₃ decompose within minutes of exposure, so they are exfoliated and in a . Black phosphorus, the element itself rather than a pnictide, is the classic case of a pnictogen layer that oxidises in air unless protected.

The anion also sets the typical defect. Chalcogenides lose chalcogen atoms easily, leaving that dope the layer and seed oxidation; halides lose halogen under an electron beam; and in mixed-anion compounds the two anions can swap sites. A composition measured on the finished flake, rather than assumed from the starting materials, is the safe basis for any claim that depends on it.

For specialists

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 and metals, halides mostly ionic, often magnetic insulators built from octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Where this comes from

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets Chhowalla et al. · Nature Chemistry 5, 263 (2013) cited by 10,029
Crystal and magnetic structures in layered, transition metal dihalides and trihalides McGuire · Crystals 7, 121 (2017) cited by 480
Iron-based layered superconductor La[O1-xFx]FeAs (x = 0.05–0.12) with Tc = 26 K Kamihara et al. · Journal of the American Chemical Society 130, 3296 (2008) cited by 7,886

Janus 2D material

A single layer whose two faces are made of different atoms – selenium on top and sulfur underneath, for example – named after the two-faced Roman god. Because top and bottom differ, the sheet carries a built-in electric field from one face to the other, which a symmetric layer cannot have. That lopsidedness allows effects a symmetric sheet forbids, such as producing a voltage when it is squeezed from above.

A monolayer in which the two outer atomic planes are different elements, as in MoSSe, where one chalcogen sheet is S and the other Se. Mirror symmetry through the metal plane is broken (D₃h → C₃v for the 1H structure), giving an intrinsic out-of-plane dipole, out-of-plane piezoelectricity, Rashba-type spin splitting and an out-of-plane second-harmonic response. Most are made by replacing one face of a parent monolayer; RhSeCl is a rare bulk crystal whose layers are Janus by nature.

Full explanation

Chemical bond

The glue that holds atoms together. In a covalent bond two atoms share electrons, as within a sheet of graphene; in an ionic bond one atom hands electrons to another and the resulting charges attract, as in table salt; in a metal the outer electrons are shared by all the atoms at once. Much weaker van der Waals forces hold molecules and the layers of a layered crystal together. A 2D material is a crystal with strong bonds within its layers and only weak ones between them – which is why it can be peeled.

The attractive interactions that hold atoms in molecules and solids, classified by how electrons are shared: covalent bonds, from overlapping orbitals holding shared electron pairs, strong and directional (sp² carbon, Mo–S in TMDCs); ionic bonds, from electron transfer and electrostatic attraction (halides, oxides); metallic bonding, with electrons delocalised in a partly filled band; and the weak, non-directional van der Waals interaction from correlated charge fluctuations, besides hydrogen bonds. Real bonds are mixtures, characterised by electronegativity differences and charge analyses. Layered crystals combine in-plane bonds of several electronvolts with interlayer binding of only about 20 meV per Å² (some 0.3 J/m²), the criterion high-throughput screens use to identify exfoliable compounds.

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

Gating

Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an insulator. The voltage pulls electrons into the material or pushes them out, as in every transistor. Because a 2D material is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or superconductivity switched on. Two gates, above and below, can also apply an electric field across the layer.

Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 10¹³ cm⁻² with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 10¹⁴–10¹⁵ cm⁻², enough to induce superconductivity in MoS₂ or raise the Curie temperature of Fe₃GeTe₂, but can intercalate or react electrochemically and work only while the ions are mobile. Hysteresis from traps, contact effects and quantum capacitance are the usual caveats.

Full explanation

Multiferroic

A material that is magnetic and electrically polarised at the same time, so a magnetic field can shift its charge and an electric field can turn its magnetism. The combination is rare, because the two kinds of order usually want incompatible atoms.

A material carrying more than one ferroic order – usually ferroelectric together with (anti)ferromagnetic – plus magnetoelectric coupling between them. Type-I multiferroics have independent origins and weak coupling; in type-II the polarisation is produced by the magnetic order itself, typically a spiral, and the coupling is strong. NiI₂ has been reported to keep a type-II order, with spiral magnetism and polarisation appearing together, down to the single layer – although the optical evidence is disputed.

Full explanation

Quantum spin liquid

A magnet whose atomic magnets never settle into a pattern, even at absolute zero. Quantum effects keep them fluctuating and entangled – linked so that none has a direction of its own – so instead of freezing into order the material stays restless, like a liquid that never turns solid. Disturbances in it can behave like fractions of an electron, carrying its magnetism but not its charge.

A ground state of a frustrated magnet with no symmetry breaking down to zero temperature, marked by long-range entanglement, an emergent gauge structure and fractionalised excitations such as spinons or Majorana fermions. Honeycomb materials with bond-dependent Kitaev exchange – α-RuCl₃ above all – are the leading layered candidates, but the evidence is indirect: a scattering continuum and thermal Hall signals, against residual magnetic order in zero field.

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

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