Transmission electron microscopy (TEM)
Electrons are fired through a sample thin enough to be transparent to them, forming an image that can show individual atoms and which element each one is. A 2D crystal is the ideal specimen, because it is already thin enough.
Imaging with transmitted electrons, either parallel-beam (TEM) or scanned-probe (STEM), where annular dark-field contrast scales roughly with Z2 and identifies single atoms chemically. It resolves point defects, grain boundaries, stacking and moiré patterns directly, at the cost of knock-on damage – which is why 2D samples are imaged at 60–80 kV – and of a clean, suspended specimen.
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Doping
Adding a small amount of extra electrons to a material, or taking some away to leave ‘holes’ that act as positive charges, to change how well it conducts. In silicon this is done by mixing in foreign atoms; in 2D materials it can also be done with a nearby voltage, molecules on the surface, or the material underneath.
Control of carrier type and density by substitutional impurities, surface charge transfer, electrostatic gating or the dielectric environment. Stable, spatially localised substitutional doping remains difficult in 2D semiconductors, and surface-transfer doping is often unstable in air.
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Charge carrier mobility
How easily electrons, or the gaps they leave behind (holes), move through a material when pushed by a voltage. Higher mobility means faster, more efficient electronics – but it is easily ruined by dirt, defects and a poor substrate.
Drift velocity per unit electric field, usually quoted in cm2 /V·s. In 2D materials it is limited intrinsically by phonon scattering and extrinsically by charged impurities, roughness, remote phonons and disorder; field-effect values extracted from two-terminal devices can be distorted by contact resistance.
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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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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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Exfoliation
Peeling thin layers off a bulk crystal. The famous version uses sticky tape: press it on a crystal, pull it away, repeat, and some flakes end up only one layer thick. Other versions do the same in a liquid to make large amounts of flakes at once.
Separation of layers from a layered bulk crystal by overcoming interlayer van der Waals bonding – mechanically with adhesive tape or stamps, which gives the highest-quality flakes but low yield, or in liquids by sonication, shear or electrochemical intercalation, which scales but yields smaller, more defective nanosheets with broad size distributions.
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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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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.
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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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Atomic force microscopy (AFM)
A tip on a tiny springy arm is moved across a surface and its bending is measured, mapping height point by point – much as a fingertip reads Braille. It works on anything, conducting or not, which is why it is the routine way to check how thick a flake is.
Force microscopy in contact, tapping or non-contact mode, used for thickness, roughness, bubbles and polymer residue, with variants for surface potential (KPFM) and local conductivity (c-AFM). Apparent height is not the crystallographic thickness: the tip interacts differently with flake and substrate, so a monolayer commonly reads between 0.4 and 1 nm. Quote the mode, and calibrate against a step of known height.
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Scanning tunnelling microscopy (STM)
A needle sharpened to a single atom is brought so close to a surface that electrons tunnel across the gap – a quantum effect by which they pass through a barrier they could never climb over. The current maps the surface atom by atom, and how it changes with voltage tells you which energies electrons are allowed to have at that exact spot.
Tunnelling between a sharp tip and a conducting surface, giving atomic-resolution topography and, in spectroscopy mode, the local density of states against energy. On 2D materials it resolves defect levels, moiré potentials, edge modes and quasiparticle gaps site by site. The sample has to be clean and conducting, so it is run on epitaxial films or on flakes placed on graphite, in vacuum and often at cryogenic temperature.
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Molecular beam epitaxy (MBE)
Growth in ultrahigh vacuum by aiming beams of atoms at a heated surface, one element at a time and slowly enough that they settle into place layer by layer. It is the slowest and cleanest way to build a film, and the surface can be watched while it grows.
Ultrahigh-vacuum growth from effusion or cracker sources with in-situ RHEED monitoring, usually at chalcogen-rich flux ratios for TMDCs. It gives coverage over a whole wafer on weakly interacting substrates, atomically abrupt interfaces and access to metastable phases and doping profiles that CVD cannot reach; domains stay small and mirror twin boundaries are common, so grain structure belongs in any report of an MBE film.
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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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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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Encapsulation
Sealing a delicate 2D material between protective layers so that air, water and dirt cannot reach it. Wrapping it in boron nitride, itself a 2D material, is the standard way to get the cleanest results.
Enclosure of a 2D layer between barrier layers – most often hexagonal boron nitride for the highest quality, or deposited oxides and polymers for larger areas – to suppress oxidation, adsorbates, charge disorder and substrate roughness. It also seals in whatever contamination was present when it was applied.
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