MXene

Everyday term

In plain words

A large family of 2D carbides and nitrides – thin sheets of metal atoms bonded to carbon or nitrogen. They are made by chemically one kind of atom out of a bulk crystal, and they conduct electricity well, which makes them useful for , and .

Going deeper

MXenes are made by dissolving one element out of a bulk carbide, not by peeling a van der Waals crystal. The etch always leaves chemical groups behind on the new surfaces, and those groups set conductivity, wetting, storage behaviour and stability.

Made by etching, not peeling

MAX phases are layered ceramics of formula M_n+1AX_n, where M is an early transition metal, A is a main-group element such as aluminium and X is carbon or nitrogen. The M–X bonds are strong, the M–A bonds weaker but still chemical, so the layers cannot simply be peeled apart.

In 2011 it was shown that hydrofluoric acid dissolves the aluminium out of Ti₃AlC₂ and leaves Ti₃C₂ sheets behind, which can then be separated into . The family has grown to dozens of compositions – carbides, nitrides and carbonitrides, with one to four metal layers, and ordered double-metal variants – collectively M_n+1X_nT_x, where T_x stands for whatever terminates the freshly exposed surfaces.

The terminations are part of the material

Etching never leaves a bare metal-carbide surface. It leaves a mixture of −O, −OH and, with fluoride-based routes, −F; molten-salt routes give −Cl or −Br instead. These groups are not a contaminant to be removed but a component that sets the properties: they fix the over a range of more than an electronvolt, decide whether the sheet is or opens a small gap, make the surface hydrophilic enough for water-based processing, and provide the redox-active sites behind charge storage.

This is also why reported properties scatter. Two samples of “Ti₃C₂T_x” made by different etches are chemically different materials, and comparisons need the synthesis route, the termination mixture and the storage history.

What they are used for, and their weakness

Ti₃C₂T_x conducts like a metal, disperses in water, and forms films by spraying, -coating or filtration. That combination has made MXenes strong candidates for electrodes, where volumetric capacitance around 900 F per cm³ has been reported for clay-like films; for electromagnetic shielding, where thin films block radiation extremely effectively; and for battery electrodes, antennas, and printed conductors.

The weakness is stability. MXene sheets oxidise in water and air, especially as dilute and at elevated temperature, turning into titanium oxide and carbon over days to weeks. Storage cold, dry, dark and deoxygenated, or as films rather than dispersions, is standard practice, and any performance claim should say how old the material was.

For specialists

Two-dimensional transition-metal carbides, nitrides and carbonitrides M_n+1X_nT_x, obtained by selectively etching the A-element layers from MAX phases, commonly in fluoride-containing acids. Surface terminations T_x (–O, –OH, –F, –Cl) set their properties; Ti₃C₂T_x is the most studied, with metallic conductivity and hydrophilic surfaces that allow water-based processing.

Where this comes from

Two-dimensional nanocrystals produced by exfoliation of Ti₃AlC₂ Naguib et al. · Advanced Materials 23, 4248 (2011) cited by 11,886

Etching

Removing material on purpose, with a chemical or a plasma. In chip-making it carves patterns: a stencil of light-sensitive polymer protects some areas while a plasma eats away the rest. For 2D materials etching has three jobs: cutting flakes into shaped devices, thinning a crystal one layer at a time, and – for MXenes – making the material in the first place, by dissolving one kind of atom out of a layered ceramic so that the remaining sheets come apart.

Material removal by chemical reaction in a liquid (wet etching) or by reactive and physical processes in a plasma (dry etching, including reactive ion etching with oxygen-, fluorine- or chlorine-based chemistries, and atomic layer etching). For 2D materials it patterns channels and Hall bars through resist masks – oxygen plasma for graphene, fluorine-based plasmas for hBN and TMDCs – exposes edges in encapsulated stacks for edge contacts, thins crystals layer by layer, and selectively removes the A element, usually aluminium, from MAX phases with hydrofluoric acid or fluoride-containing acids to make MXenes. Its side effects are edge roughness and damage, residues, doping and plasma-induced defects in neighbouring layers, so the etched edge and its electronic states have to be considered in narrow devices.

Full explanation

Battery electrode

A battery stores energy by moving ions – in phones and cars, lithium ions – between two electrodes, the anode and the cathode. Charging pushes the ions into the anode, where they slip between the layers of a material such as graphite; using the battery lets them flow back. Layered and 2D materials matter because ions can enter and leave the gaps between layers, and because thin conductive flakes can help electrons reach every grain of an electrode.

A lithium- or sodium-ion cell shuttles ions through an electrolyte between a negative electrode (the anode on discharge) and a positive one (the cathode), while electrons pass through the external circuit. Graphite, the standard anode, intercalates lithium between its layers up to LiC₆, about 372 mAh/g; layered oxides such as LiCoO₂ and NMC dominate cathodes. 2D materials enter as conductive additives (graphene flakes replacing part of the carbon black), as higher-capacity anodes that alloy or convert (phosphorus, antimony, SnS₂, SnO), as hosts for larger ions such as sodium, which graphite takes up poorly, and as MXene electrodes. Their weaknesses are large volume change, irreversible first-cycle loss from the high surface area, low packing density and cost, so gains at cell level are usually far smaller than those claimed for the material.

Full explanation

Composites and coatings

The biggest market for 2D materials so far: flakes of graphene or other layered materials mixed in small amounts into plastics, rubber, concrete, paints or battery electrodes, to make them stiffer, tougher, more conductive or less permeable. Sold by the tonne, these products use cheap flakes made in bulk rather than perfect single layers. Whether a few percent of flakes really helps depends on how well they are spread out, how they line up and how firmly they hold on to the material around them.

Bulk uses in which exfoliated or chemically derived flakes – graphene nanoplatelets, reduced graphene oxide, hBN, MXenes, clays – are dispersed in a matrix or applied as a film. In polymer composites a few weight percent can raise stiffness and toughness, cross the percolation threshold for electrical conduction – as low as about 0.1 percent by volume for thin, wide flakes – and raise thermal conductivity; aligned flakes in barrier coatings lengthen the diffusion path of water, oxygen and ions and slow corrosion; flakes used as conductive additives replace part of the carbon black in battery electrodes; MXene films give strong electromagnetic shielding. Performance depends on flake size and thickness, dispersion without restacking, alignment and bonding to the matrix, and on powders whose properties vary widely between suppliers.

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

Spin

A built-in property of every electron that makes it a tiny magnet. Measure it along any direction and you only ever find one of two answers, ‘up’ or ‘down’. Despite the name, nothing is actually spinning – the word stuck from an early picture – but the magnetism is real: countless electron spins lined up are what make a fridge magnet stick. Spintronics tries to carry information in spin rather than in charge.

The intrinsic angular momentum of the electron, ħ/2, with a magnetic moment of almost exactly one Bohr magneton; a measurement along any axis yields one of two values. In solids spin couples to orbital motion through spin–orbit coupling and to other spins through exchange; in 2D materials it sets magnetic order, spin–valley locking and how long a spin survives as a carrier of information.

Full explanation

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.

Full explanation

Dispersions and inks

Flakes of a 2D material floating in a liquid, and that liquid used like paint or ink. Stirring or shaking a layered crystal in the right liquid splits it into flakes that stay suspended; spinning away the thick ones leaves a dispersion of thin flakes, which can be sprayed, printed or coated onto almost anything. It is the cheapest way to make 2D materials by the kilogram, but the flakes are small and overlap in a film, so they never match a perfect continuous sheet.

Colloidal suspensions of nanosheets made by liquid-phase exfoliation of layered crystals in solvents whose surface energy matches the crystal (NMP, DMF, cyclohexanone) or in water with surfactants, by electrochemical exfoliation, or – for graphene oxide and MXenes – by chemical routes that leave hydrophilic surfaces. Centrifugation sorts flakes by size and thickness (liquid cascade centrifugation), and optical spectra give mean thickness and lateral size in the liquid. Formulated as inks with tuned viscosity and surface tension, they are inkjet-, screen- or spray-printed or coated into films limited by flake–flake junctions, porosity, residual solvent or surfactant, and oxidation in storage. Powders and dispersions sold as graphene range from monolayers to graphite, which ISO/TS 21356-1 addresses by defining how they are characterised.

Full explanation