Nanosheet

Also called flake

Everyday term

In plain words

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.

Going deeper

Nanosheets are made in bulk rather than one at a time: a layered crystal is broken apart in a liquid, giving a dispersion of many small, thin flakes. The result is a distribution – of thickness, as sketched here, and of lateral size – rather than a single well-defined flake, and centrifugation is used to narrow it.

Nanosheet versus monolayer

The two words describe different ways of working with thin crystals. A is a single, characterised flake, usually peeled off with tape and studied individually. Nanosheets are produced in quantity, as a dispersion or powder containing billions of flakes, typically a few layers thick and from tens of nanometres to a few micrometres across. What they give up in individual perfection they gain in quantity: they can be printed, sprayed, filtered into or mixed into , which is what most large-scale applications of use.

How they are made

The most general route is : a layered crystal is sonicated or sheared in a solvent or surfactant solution whose surface energy is close to that of the crystal, so that the separated layers stay dispersed instead of restacking. The same approach works for graphene, hBN, MoS₂ and many other layered materials. Chemical routes go further for particular families – intercalating lithium or other ions to push layers apart, the A layer out of to make MXenes, or swapping ions in layered oxides and hydroxides until single sheets delaminate.

Sorting and measuring a population

A freshly exfoliated dispersion contains everything from unexfoliated chunks to monolayers. Centrifugation at stepwise increasing speeds separates it into fractions of different average size and thickness, and those averages can be estimated from the optical absorption spectrum once it has been calibrated against microscopy. Reporting a nanosheet sample therefore means reporting distributions – mean thickness and lateral size with their spread – rather than a single number. The persistent practical problem is restacking: when a dispersion is dried into a film or electrode, the sheets tend to stack back together and lose much of the surface they were made for.

For specialists

A particle whose lateral dimensions far exceed its nanometre-scale thickness; the term is used especially for liquid-exfoliated and chemically derived , whose lateral-size and thickness distributions must be specified rather than assumed. In engineering ‘nanosheet’ also denotes the stacked silicon channel sheets of devices.

Where this comes from

Two-dimensional nanosheets produced by liquid exfoliation of layered materials Coleman et al. · Science 331, 568 (2011) cited by 7,283

2D membrane

A sheet that lets some molecules through and holds others back. A perfect layer of graphene is so tight that not even helium gets through, so useful membranes need openings: tiny holes made in a single layer, or the narrow gaps between flakes stacked like a pack of cards. Because such a membrane is only atoms thick, water or gas can pass far faster than through today’s plastic membranes, which could make filtering, purifying water or separating gases cheaper.

Separation membranes made from 2D materials, in two designs: nanoporous single layers (graphene, hBN, MoS₂ with pores opened by ion irradiation, oxidative etching or defects), whose pore size sets the selectivity, and laminates of stacked flakes (graphene oxide, MXenes, COF and MOF nanosheets), whose interlayer channels – often swelling in water – act as sieves. Pristine graphene is impermeable to gases including helium, yet transmits protons; atomic thickness promises high permeance, and restraining the laminate spacing can exclude hydrated ions. Against mature polyamide reverse-osmosis membranes the obstacles are large defect-free areas, swelling and stability in water, fouling and cost. The word also names any freestanding thin film, as in oxide membranes lifted off a sacrificial layer.

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

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

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