Two-dimensional (2D) material
A crystal so thin that it is only one or a few atoms thick – a sheet rather than a lump. Graphene, a single layer of carbon atoms, is the best-known example: about 300,000 of its layers stacked up would be as thick as one sheet of paper. At that thickness a material can conduct, glow or respond to magnetism quite differently from the same substance in bulk.
A crystalline material whose thickness is one or a few unit cells, so that electrons, phonons and other excitations are confined in one direction. Most are obtained from layered bulk crystals in which strong in-plane bonds coexist with weak van der Waals bonding between layers.
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Wafer and wafer-scale growth
A wafer is the thin, round slice of ultra-pure silicon – today 300 millimetres across – on which computer chips are made, hundreds at a time. Wafer-scale growth means making a 2D material evenly over a whole wafer rather than as scattered flakes, which is what any factory needs. For the best 2D materials that is still hard: a sheet one atom thick has to have the same thickness, quality and orientation from one edge of a dinner-plate-sized disc to the other.
The substrate format of the semiconductor industry – 200 and 300 mm silicon wafers, with sapphire, SiC and glass for particular processes – and the scale at which a material must be grown, transferred and processed to enter manufacturing. Wafer-scale growth of 2D materials uses CVD and MOCVD of TMDCs, CVD of graphene and hBN, and MBE, with targets of uniform layer number, low defect and grain-boundary density and, increasingly, a single orientation on vicinal sapphire or Cu(111). The obstacles are the growth temperature against the thermal budget of a finished wafer (growth on the back end of line needs to stay below about 400 to 450 °C), the transfer from growth substrate to device wafer, and metrology that confirms thickness and quality across the wafer at production speed.
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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 (D3 h → C3 v 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.
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Chemical vapour deposition (CVD)
A way of growing thin films from gases. The ingredients arrive as vapour, react on a hot surface and leave a solid layer behind. It is how large sheets of graphene and other 2D materials are made for industry.
Growth of films from gas-phase precursors that react or decompose on a heated substrate. For 2D materials this includes graphene on copper foil, powder-source CVD of TMDCs from metal oxides and chalcogens, and metal-organic CVD (MOCVD) for wafer-scale TMDC films; grain size, nucleation density and residues are the usual quality limits.
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Grain boundary
The seam where two crystal grains grown in different orientations meet. A film made of many grains is only as good as its seams: they scatter electrons, weaken the sheet and are where damage usually begins.
A line defect joining two domains of different orientation, built in 2D from rows of non-hexagonal rings – pentagon–heptagon pairs in graphene, dislocation cores of four- to eight-membered rings in TMDCs, with the mirror twin boundary as a special case. Its effect depends on misorientation and core structure more than on grain size: some boundaries barely scatter carriers while others block transport, which is why wafer-scale growth aims for aligned single-orientation domains rather than merely larger ones.
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Sensor
A device that turns something in its surroundings – a gas, a magnetic field, a pressure, a molecule in blood – into an electrical signal. 2D materials make unusually sensitive sensors because every atom is on the surface: a few molecules landing on a single layer change how well it conducts. Graphene also makes very good magnetic-field sensors. The hard part is not sensitivity but selectivity – responding to one thing and ignoring everything else – and staying stable for years.
Transducers in which a stimulus changes an electrical property of a 2D layer: chemiresistive and field-effect gas and humidity sensors, in which adsorbates transfer charge and shift the conductance (graphene registered single NO2 molecules in 2007); Hall sensors, in which graphene’s low carrier density and high mobility give a large, temperature-stable Hall voltage; biosensors with receptors on a graphene or MoS2 channel that bind target molecules; and strain, pressure and light sensors. Sensitivity comes from the all-surface geometry and the low density of states; the limits are selectivity, drift and hysteresis from humidity and contamination, recovery time, device-to-device variation and calibration, addressed by functionalisation, encapsulation and arrays read by pattern recognition.
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Photodetector
A device that turns light into an electrical signal. A single layer absorbs only a few percent of the light that hits it. In return it responds quickly, can be tuned by a gate voltage and can be placed on surfaces an ordinary detector cannot reach, such as a flexible sheet or an optical chip.
A device converting incident photons into current or voltage. A 2D channel trades absorption per pass for gate tunability, short transit times and transfer onto arbitrary substrates or waveguides; the response may be photoconductive with gain, photovoltaic at a junction, or bolometric. Responsivity alone says little – it is only comparable alongside the mechanism, the bandwidth and the illuminated area.
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Single crystal
A piece of material in which the atoms are lined up in one unbroken pattern from one side to the other, with every row pointing the same way. Most solids are not like that: they are polycrystalline, made of many small crystals stuck together at random angles, like a pavement of tiles laid every which way. The boundaries between those grains scatter electrons, weaken the material and blur measurements. Physics experiments and the best devices therefore want single crystals: a centimetre-sized block to peel flakes from, or a sheet grown across a whole wafer in one orientation.
A solid whose lattice is continuous and uniformly oriented throughout, without grain boundaries – unlike polycrystalline material, made of misoriented grains, or amorphous solids without long-range order. Bulk single crystals of layered compounds are grown by chemical vapour transport, flux or Bridgman methods and are the usual source of exfoliated flakes; their quality is judged by X-ray rocking-curve widths, the residual resistivity ratio and the absence of twins and stacking faults. Large single-crystal films grow either from one nucleus or from aligned nuclei that merge without boundaries, as for graphene on Cu(111) or Ge(110) and MoS2 or hBN on vicinal sapphire or copper, where substrate steps fix one orientation and remove the twin boundaries that two antiparallel orientations would form.
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Monolayer
One single layer of a layered material. Two stacked layers are a bilayer; a handful are called few-layer. Many properties change between one, two and several layers, so the exact count matters.
A single structural layer of a layered crystal – one atom thick for graphene and hBN, three atomic planes for a TMDC such as MoS2 . Band structure, screening and symmetry depend on layer number (monolayer 2H-MoS2 lacks the inversion symmetry of the bilayer), so monolayer, bilayer and few-layer samples are distinct systems.
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Angle-resolved photoemission spectroscopy (ARPES)
A measurement that shines light on a crystal, catches the electrons it knocks out and records the direction and energy of each one. From that it reconstructs how the electrons were moving inside – the closest thing there is to a photograph of a material’s electronic structure.
Photoemission with energy and momentum resolution, giving the occupied band structure, Fermi surface and self-energy directly. It needs a clean, flat, conducting surface in ultrahigh vacuum; focused micro- and nano-ARPES beamlines bring the spot down to the size of an exfoliated flake, which is what makes single-domain twisted and few-layer samples measurable at all.
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Semiconductor
A material whose ability to carry electricity can be switched on and off, for example by an applied voltage. That switching is what every transistor in a computer chip relies on. Silicon is the classic example; several 2D materials, such as MoS2 , are semiconductors too.
A material with a band gap of a few electronvolts or less, whose conductivity can be tuned over many orders of magnitude by doping, gating, temperature or light. 2D semiconductors attract interest because their sub-nanometre thickness preserves electrostatic gate control in very short channels.
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Raman spectroscopy
Shining a laser on a material and measuring the tiny fraction of light that comes back at slightly different colours. The shifts reveal how the atoms vibrate, which tells researchers how many layers a flake has and whether it is strained or damaged.
Inelastic light scattering that measures phonon energies. In 2D materials peak positions, widths and intensity ratios – graphene’s G and 2D bands, or the E′ and A′1 modes of TMDCs – report layer number, strain, doping and defect density; laser heating and substrate interference must be controlled.
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Photoluminescence (PL)
Light given off by a material after it has absorbed light – what a highlighter pen does under a black light. In 2D semiconductors the colour and brightness of that glow report how many layers there are, how clean the sample is and whether it is carrying extra charge.
Radiative recombination following optical excitation. In monolayer TMDCs the spectrum is dominated by excitons and trions rather than free carriers, so peak energy, linewidth and quantum yield track layer number, strain, dielectric environment, doping and defect density. The indirect-to-direct crossover makes the monolayer far brighter than the bilayer, and chemical treatment of defects can bring the yield close to unity.
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X-ray photoelectron spectroscopy (XPS)
X-rays knock electrons out of the atoms at a surface, and the energies those electrons carry say which elements are present and what they are bonded to. It is the standard check that a film really is the compound you wanted and has not oxidised.
Core-level photoemission giving elemental composition and chemical state from the top few nanometres. On 2D films it fixes stoichiometry, detects oxidation and substitutional impurities, and follows doping through rigid core-level shifts. Binding-energy referencing is the usual source of error – adventitious carbon at 284.8 eV is not a reliable standard on a semiconductor – and charging has to be controlled on insulating substrates.
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Alloy and solid solution
A crystal in which two similar kinds of atom share the same sites at random – some molybdenum sites taken by tungsten, say, or some sulfur sites by selenium – so that one crystal structure holds a mixture. By choosing the mix, its properties can be set almost anywhere between those of the two pure compounds, much as mixing two paints gives any shade between them. Chemists call it a solid solution; alloy is the everyday word.
A single-phase crystal in which two or more species share a sublattice at random, such as Mo1−x Wx S2 or MoS2(1−x) Se2x , with lattice constants close to Vegard’s law and band gaps that vary continuously with x, often with a small bowing. Complete miscibility needs similar radii, the same structure type and favourable mixing energetics; otherwise the mixture separates into phases or orders. Random disorder adds alloy scattering and inhomogeneous broadening.
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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.
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FEOL and BEOL
The two halves of making a chip: the front end builds the transistors in the silicon itself, and the back end adds the layers of metal wiring on top – at lower temperatures, so nothing underneath is damaged.
Front end of line forms the transistors in the silicon; back end of line builds the metal interconnect layers above them at lower temperatures.
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Thermal budget
How much heat, and for how long, a chip can take at a given production step before the parts already built are damaged. New materials often have to be grown or added within that limit.
The maximum temperature–time exposure a process step may impose without damaging structures already on the wafer.
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Memristor and neuromorphic computing
A memristor is a device whose resistance remembers the voltage it has seen, so the same spot both stores a number and multiplies by it. Chips built from arrays of them work more like a brain than like a processor with memory bolted on the side – which is what neuromorphic means.
Non-volatile resistive switching used as memory and as an analogue synaptic weight, so that a crossbar performs multiply–accumulate where the data already sits. Two-dimensional materials contribute switching layers under a nanometre thick – vacancy or metal-atom migration in a monolayer TMDC gives non-volatile switching – along with gate-tunable synaptic transistors and back-end-compatible integration above logic.
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Van der Waals force
A weak attraction between neighbouring atoms and molecules – strong enough, added up over millions of tiny hairs, to let a gecko walk up a pane of glass. In layered crystals it is what holds the layers together, and it is weak enough that a single layer can be peeled off, which is how many 2D materials are made from ordinary crystals.
The weak, non-directional attraction arising from correlated charge fluctuations (dispersion) and related dipolar terms. In layered crystals it binds adjacent layers with energies of tens of meV per atom, roughly two orders of magnitude below in-plane covalent bonds, which enables exfoliation and the free stacking of dissimilar layers.
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Nanosheet
A general word for a very thin sheet of material, often just a few atoms thick and up to a few micrometres – thousandths of a millimetre – across. It is used especially for flakes made in large quantities in liquids.
A particle whose lateral dimensions far exceed its nanometre-scale thickness; the term is used especially for liquid-exfoliated and chemically derived dispersions, whose lateral-size and thickness distributions must be specified rather than assumed. In transistor engineering ‘nanosheet’ also denotes the stacked silicon channel sheets of gate-all-around devices.
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Van der Waals heterostructure
A stack of different 2D materials placed on top of each other, like a sandwich built one atom-thin slice at a time. Because the layers only stick together weakly, almost any combination can be stacked, which lets researchers build materials that do not exist in nature.
A vertical assembly of dissimilar 2D layers bound by van der Waals forces, so no lattice matching is required. Interfaces can be atomically sharp, and twist angle, stacking order and dielectric environment become design parameters. Stacks are built by dry transfer or by sequential growth.
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Photocatalysis and electrocatalysis
Using light, or an applied voltage, to drive a chemical reaction at a surface – most often splitting water to make hydrogen. Thin materials are natural candidates because nearly every atom is a surface atom, and because their edges are usually far more active than their flat faces.
Surface reactions driven by photons (photocatalysis) or by electrode potential (electrocatalysis), judged by overpotential, Tafel slope and turnover per site rather than by raw current. In MoS2 the basal plane is close to inert while the edge sites carry the hydrogen-evolution activity – the result that made layered chalcogenides catalyst candidates – so activity scales with edge length, defect density and phase, and any comparison needs an active-site count rather than a geometric area.
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