Dispersions and inks

In plain words

of a 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.

Going deeper

Stirred or sonicated in the right liquid, a layered crystal comes apart into flakes that stay suspended. Spinning at increasing speeds sorts them, thick and large first, thin and small last. Printed or sprayed, they form a network of overlapping flakes in which current must hop from one to the next.

From crystal to liquid

A layered crystal stirred or sonicated in the right liquid comes apart, because the energy needed to separate the layers is partly repaid by the liquid clinging to the new surfaces. Solvents whose surface energy matches that of the crystal work best; water works when a surfactant coats the flakes and keeps them apart. Graphene oxide and are easier still: their surfaces carry oxygen-containing groups that make them disperse in water on their own.

The result is a mixture of flakes of every size and thickness. Spinning it in a centrifuge at increasing speeds separates them – thick, large flakes settle first, thin, small ones last – into fractions with narrower distributions, whose thickness and size can then be read from their optical spectra.

Printing with flakes

Given the right viscosity and drying behaviour, a dispersion becomes an ink that can be printed by inkjet, screen or spray, or coated onto plastic, paper or textiles. Printed graphene makes conductive tracks and electrodes, printed MoS₂ or WSe₂ semiconducting channels and printed hBN layers – and all-printed combining the three have been made from flake networks.

A printed film is a network of overlapping flakes, so current must hop from flake to flake, and the junctions, voids and leftover additives keep its conductivity and far below those of a . That matters little for electrodes, , antennas or heaters, where cheap processing over large areas counts for more than peak performance.

Specifying what you buy

Material sold as graphene ranges from to finely ground graphite, often without the buyer being able to tell. The technical specification ISO/TS 21356-1 sets out how the structure of graphene powders and dispersions should be characterised – thickness distribution, lateral size, specific surface area – and ISO/TS 23359 their chemistry, including oxygen content, so that a datasheet means something. Dispersions also age: flakes restack, and MXenes and black phosphorus oxidise within days unless kept cold, dark and free of oxygen; solvents and surfactants left in the final film affect its properties, so storage and washing belong in the specification too.

For specialists

Colloidal suspensions of nanosheets made by 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.

Where this comes from

High-yield production of graphene by liquid-phase exfoliation of graphite Hernandez et al. · Nature Nanotechnology 3, 563 (2008) cited by 6,295
Two-dimensional nanosheets produced by liquid exfoliation of layered materials Coleman et al. · Science 331, 568 (2011) cited by 7,283
Production of highly monolayer enriched dispersions of liquid-exfoliated nanosheets by liquid cascade centrifugation Backes et al. · ACS Nano 10, 1589 (2016)
All-printed thin-film transistors from networks of liquid-exfoliated nanosheets Kelly et al. · Science 356, 69 (2017)

Dielectric

An electrical insulator used for what it does in an electric field rather than for carrying current – the thin layer inside a capacitor, or the insulator that separates a transistor’s gate from its channel. A good dielectric blocks current completely, withstands a strong field without breaking down and, for gates, has a high permittivity, so that a voltage on the gate pulls many charges into the channel. For 2D transistors the dielectric is as hard to get right as the 2D layer itself.

An insulator characterised by its permittivity κ, band gap and band offsets to the channel, breakdown field, leakage, and density of fixed charge and traps at the interface and in the bulk (border traps). Silicon technology moved from SiO₂ (κ ≈ 3.9) to hafnium-based high-κ oxides (κ ≈ 20–25) to keep gate control at sub-nanometre equivalent oxide thickness. For 2D channels, hBN gives clean, trap-poor interfaces but a low κ of about 3–4 and is hard to grow at wafer scale; ALD oxides nucleate poorly on basal planes; alternatives include crystalline CaF₂, native oxides such as Bi₂SeO₅ on Bi₂O₂Se, and molecular crystals. Reliability – hysteresis, bias-temperature instability and time-dependent breakdown – is the yardstick for any dielectric meant for products.

Full explanation

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, so physics experiments and the best devices 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.

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