As mechanical structures enter the nanoscale regime, the influence of van der Waals forces increases. Graphene is attractive for nanomechanical systems 1,2 because its Young's modulus and strength are both intrinsically high, but the mechanical behavior of graphene is also strongly influenced by the van der Waals force 3,4 . For example, this force clamps graphene samples to substrates, and also holds together the individual graphene sheets in multilayer samples. Here we use a pressurized blister test to directly measure the adhesion energy of graphene sheets with a silicon oxide substrate. We find an adhesion energy of 0.45 ± 0.02 J/m 2 for monolayer graphene and 0.31 ± 0.03 J/m 2 for samples containing 2-5 graphene sheets. These values are larger than the adhesion energies measured in typical micromechanical structures and are comparable to solid/liquid adhesion energies [5][6][7] . We attribute this to the extreme flexibility of graphene, which allows it to conform to the topography of even the smoothest substrates, thus making its interaction with the substrate more liquidlike than solid-like. Figure 1a shows optical images of the devices used for this study. Graphene sealed microcavities were fabricated by the mechanical exfoliation of graphene over predefined wells (diameter ~5 um) etched in a SiO 2 substrate (See Methods). Two exfoliated graphene flakes were used, yielding membranes with between 1 and 5 graphene layers, which were suspended over the wells and clamped to the SiO 2 substrate by the van der Waals force. After exfoliation the internal pressure in the microcavity, p int , is equal to the external pressure, p ext , which is atmospheric pressure. In this state the membrane is flat, adhered to the substrate, and it confines N gas molecules inside the microcavity.To create a pressure difference across the graphene membrane, we put the sample in a pressure chamber and use nitrogen gas to increase p ext to p 0 . Devices are left in the pressure chamber at p 0 for between 4 and 6 days in order for p int to equilibrate to p 0 (Fig. 1b). This is thought to take place through the slow diffusion of gas through the SiO 2 substrate 3 . We then remove the device from the pressure chamber, and the pressure difference (p int > p ext ) causes the membrane to bulge upwards and the volume of the cavity to increase (Fig. 1c). We use an atomic force microscope (AFM) to measure the shape of the graphene membrane, which we parameterize by its maximum deflection, δ, and its radius, a (Fig. 1d).This technique allows us to measure δ and a for different values of p 0 . Figure 1e shows a series of AFM line cuts through the center of a mono-layer membrane as p 0 is increased. At low p 0 , the membrane is clamped to the substrate by the van der Waals force and δ increases with increasing p 0 . At higher p 0 (e.g., p 0 > 2 MPa) in addition to an increased deflection, we also observe delamination of the graphene from the SiO 2 substrate which leads to an increase in a (Fig. 1e). In Fig. 2a, we plot δ vs. p 0 for all the bi...

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Several researchers have demonstrated, through experiments and analysis, that the structure and properties of nanometre-scale materials can be quite different to those of bulk materials due to the effect of surfaces. Here we use atomistic simulations to study a surface-stress-induced phase transformation in gold nanowires. The emergence of the transformation is controlled by wire size, initial orientation, boundary conditions, temperature and initial cross-sectional shape. For a <100> initial crystal orientation and wire cross-sectional area below 4 nm(2), surface stresses alone cause gold nanowires to transform from a face-centred-cubic structure to a body-centred-tetragonal structure. The transformation occurs roughly when the compressive stress caused by tensile surface-stress components in the length direction exceeds the compressive stress required to transform bulk gold to its higher energy metastable crystal structure.

Interfacial adhesion and friction are important factors in determining the performance and reliability of microelectromechanical systems. We demonstrate that the adhesion of micromachined surfaces is in a regime not considered by standard rough surface adhesion models. At small roughness values, our experiments and models show unambiguously that the adhesion is mainly due to van der Waals dispersion forces acting across extensive non-contacting areas and that it is related to 1/Dave2, where Dave is the average surface separation. These contributions must be considered because of the close proximity of the surfaces, which is a result of the planar deposition technology. At large roughness values, van der Waals forces at contacting asperities become the dominating contributor to the adhesion. In this regime our model calculations converge with standard models in which the real contact area determines the adhesion. We further suggest that topographic correlations between the upper and lower surfaces must be considered to understand adhesion completely.