Graphene: show of adhesive strength.

G raphite is composed of layers of carbon atoms that are held together by strong covalent bonds within each graphene layer and relatively weak van der Waals forces between the layers. The mechanical properties of graphite — it is anisotropic and prone to interlayer cleavage — are closely related to the characteristics of these forces. Furthermore, according to recent work at the University of Colorado in Boulder, van der Waals forces could also be responsible for the ultrastrong adhesion between atomically thin graphene membranes and a silicon oxide substrate 1. Writing in Nature Nanotechnology, Scott Bunch and co-workers report that the adhesion energy for monolayer graphene on silicon oxide is higher than that for membranes containing between two and five layers of graphene. Although the physical origin for this difference is not fully understood, the team suggests that the extreme flexibility of the monolayer graphene may be responsible. Measurement of the interfacial adhesion energy is essential for practical applications of graphene in integrated electronic and mechanical devices. Previously, researchers at the University of Maryland estimated the graphene–SiO 2 adhesion energy to be 0.096 J m –2 , based on the interlayer van der Waals interaction in graphite 2 , and a team at Northeastern University reported an average value of 0.151 J m –2 for multilayer graphene sheets (roughly five layers) on a silicon wafer, which was obtained with a nanoparticle intercalation method 3. The values reported by Bunch and colleagues are higher — 0.31 J m –2 for membranes containing two to five layers of graphene — because they are based on independent measurements of the Young's modulus of the membranes 4 , whereas the measurements at Northeastern University used smaller values for Young's modulus. This ability to independently measure both the Young's modulus and the adhesion energy is an important advantage of the method developed by Bunch and co-workers, which is based on the pressurized blister test (also known as the bulge test). Pressurized blister tests are routinely used to measure the mechanical properties of thin films and also the interfacial adhesion energies between films and substrates 5. In a typical experiment, a free-standing window of a thin film with clamped edges is pressurized from one side, and the deflection of the film is measured (often optically) from the other side. The mechanical properties of the thin film — including Young's modulus, Poisson's ratio and residual stress — …