Moving an atomically thin layer from where it was made to where it is needed – from the copper foil it grew on to a silicon , or from a on tape onto another flake. The layer is too thin to pick up on its own, so it is carried on a polymer film or a soft stamp, put down, and the carrier removed. Each step risks tears, wrinkles, trapped bubbles and a film of leftover polymer, which is why transfer is often the dirtiest step in making a 2D device.
Going deeper
Wet transfer floats a polymer-coated layer off its growth substrate and scoops it onto the target; it scales to large films but traps water and residue. Dry transfer moves flakes with a soft stamp and builds stacks in which only the top layer touches polymer. Either way, bubbles, wrinkles and residue are what a transferred layer has to live with.
Wet and dry
In wet transfer, used above all for graphene, the layer is coated with a thin film of PMMA, the copper beneath is away or separated by hydrogen bubbles formed at it as an electrode, and the floating film is scooped onto the target. The PMMA is then dissolved. The method scales to large areas – graphene sheets tens of centimetres across have been transferred roll-to-roll – but water and etchant get trapped under the layer, and the polymer never comes off completely.
Dry transfer avoids liquids. A soft stamp of PDMS, or a polymer film such as polycarbonate on a stamp, picks up an exfoliated flake by adhesion and puts it down where it is wanted, aligned under a microscope; warming releases it. Stacks are built by picking up one flake with another, so that only the top layer ever touches polymer – and are refinements of this.
What transfer leaves behind
Polymer residue is the most common trace: a film of hydrocarbons, often a nanometre or more thick, that dopes the layer, scatters carriers, adds and spoils surface measurements. Annealing in a hydrogen–argon mixture or in vacuum, or sweeping the surface with the tip of an atomic force microscope, reduces it, but rarely to nothing. Water and air trapped between layers gather into bubbles, and contamination pushed ahead of the stamp collects in them, leaving clean regions in between – so stacking slowly at a raised temperature gives larger clean areas. Wrinkles, folds and tears add and break the continuity of the film.
A device on a transferred layer carries the history of that transfer, so the method and the cleaning belong in the report along with the results.
A problem of scale
In a laboratory, transfer is a skill; in a factory, it is a question of yield. Every transfer step succeeds with a probability below one and the probabilities multiply, so a device that needs several transferred layers quickly becomes impossible to make in volume. Hence the interest in wafer-to-wafer transfer with , in growing layers directly on the final wafer where the allows, and in products that need no transfer at all, such as graphene grown directly on their wafers.
For specialists
The step that moves a 2D layer from its growth or source to a target substrate or stack. Wet transfer coats the layer with a supporting polymer, usually PMMA, detaches it by etching the growth substrate or by electrochemical bubbling, and floats it onto the target before the polymer is dissolved. Dry transfer picks layers up with a viscoelastic PDMS stamp or a polymer film such as polycarbonate, often using the adhesion of a top flake, and releases them by heating. The costs are cracks and wrinkles, trapped water, hydrocarbons and polymer residue, strain and , and – at wafer scale – yield that compounds with every transfer, which is why growth directly on the target and transfer-free routes are pursued.