Peeling thin layers off a bulk crystal. The famous version uses sticky tape: press it on a crystal, pull it away, repeat, and some end up only one layer thick. Other versions do the same in a liquid to make large amounts of flakes at once.
Going deeper
Mechanical exfoliation in four steps. Tape lifts thin stacks off a layered crystal, repeated peeling thins them, and pressing the tape onto an oxidised silicon wafer leaves flakes behind. Thin-film interference in the oxide makes flakes of different thickness look different, so even a single layer can be found by eye before being confirmed spectroscopically.
Why layers come apart at all
Exfoliation works because layered crystals are strongly in their bonding: within a layer the atoms are held by or ionic bonds, while neighbouring layers are held by attraction that is roughly a hundred times weaker. A force that is modest on the scale of chemical bonds – the adhesion of tape, the shear of a flowing liquid, a gold film pressed onto the surface – is enough to separate layers without breaking the layers themselves. Materials whose layers are bonded to each other chemically, such as most oxides and metals, cannot be exfoliated this way.
Mechanical exfoliation
The adhesive-tape method that isolated graphene in 2004 is still the source of the highest-quality flakes, because nothing touches the crystal except tape and the target . Its limits are yield and size: are a rare fraction of what lands on the , typically micrometres to tens of micrometres across, and finding them takes patient searching. Variants trade some cleanliness for scale – pressing the crystal onto a freshly deposited gold film, whose strong affinity for sulfur or selenium can pull off monolayers far larger than tape does, or heating the substrate during transfer to improve adhesion.
Exfoliation in liquids
For large quantities the crystal is exfoliated in a liquid instead. Sonication or high-shear mixing supplies the energy, and a solvent or surfactant solution with a well-matched surface energy keeps the separated sheets from restacking. The product is a dispersion of nanosheets rather than a few pristine flakes: smaller, more defective at the edges and spread over a range of thicknesses, but available by the gram. -assisted routes, in which ions or molecules are first driven between the layers, loosen the crystal further and can raise the proportion of monolayers considerably.
For specialists
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.