Graphene transfer: a stamp for all substrates.

Owing to recent advances in chemical vapour deposition, graphene sheets can now be made in quantities that are large enough for essentially all research and development purposes1–3. For practical applications, however, new methods are required to transfer the graphene from the growth substrate to a target substrate without damaging either the graphene sheet or the target surface. Various transfer methods have previously been devised, but these either introduce defects into the honeycomb structure of the graphene or damage the target substrate, especially ‘soft’ substrates such as self-assembled monolayers2,4,5. Writing in Nature Nanotechnology, Lay-Lay Chua and co-workers at the National University of Singapore and the Defence Science Organization National Laboratories, Singapore, have now shown that a sacrificial polymer layer can be used to transfer pristine graphene sheets onto virtually any substrate including soft ones6. In general, graphene is transferred from one substrate to another using either the carrier method or the stamp method1–5,7 (Fig. 1). In the carrier method, graphene and its growth substrate are first attached to a thick poly(methyl methacrylate) film. The growth substrate is then etched away and the resulting graphene/carrier film is placed on top of the target substrate. Finally, the carrier film is removed by a chemical or thermal treatment. In the stamp method, graphene is picked up by an elastomeric material such as polydimethylsiloxane and then stamped onto a target substrate. In both methods, the transfer polymer film acts as a mechanical support for the graphene sheet and as a release agent, and should, in principle, provide a smooth and defect-free release. In practice, however, this has proved difficult. In the carrier method, for instance, aggressive chemical treatments are typically used to remove the transfer polymer layer. These treatments leave residues that require additional cleaning steps to remove, which inevitably damage the graphene and the target substrate. The stamp method, on the other hand, works only if the adhesion energy between the graphene and the target substrate is strong, restricting its usefulness to flat and hydrophilic substrates; furthermore, the mechanical stress induced during stamping usually generates cracks. The technique developed by Chua and colleagues is a modified stamp method in which a second polymer layer — termed the self-release layer — makes the stamping process easier (Fig. 1). The self-release layer is inserted between the graphene sheet and the elastomer stamp. The stamp acts as a mechanical support for the graphene and, after stamping, can be safely detached with no deterioration of the graphene layer because the graphene never comes into direct contact with the stamp. Finally, the self-release layer is readily removed by solvent dissolution under mild conditions in a graphene-friendly process. For the successful transfer of graphene to fragile substrates, the self-release layer should adhere more weakly to the GRAPHENE TRANSFER