CPM Seminar

Investigations in graphene morphology: conformal adhesion, wrinkling instability and adhesion transitions

William G. Cullen

University of Maryland

In addition to remarkable electronic and chemical properties, graphene exhibits remarkable mechanical properties as well. The graphene monolayer uniquely combines the strongest 2D in-plane elastic modulus with a modest bending energy yielding an effective mechanical thickness well below 1 Å. This exceptional mechanical thinness allows graphene to conform to nanometer-scale substrate roughness. However, the structure of graphene on SiO₂ (the standard substrate for graphene exfoliation) remained controversial due to conflicting measurements of its topography and that of the bare SiO₂ substrate, leading some to conclude that substrate-supported graphene could possess its own intrinsic roughness. Our high-resolution noncontact atomic force microscopy of SiO₂ reveals previously unresolved roughness at the few-nm length scale, and scanning tunneling microscopy of graphene on SiO₂ shows graphene to be slightly smoother than the supporting SiO₂ substrate. A quantitative energetic analysis explains the observed roughness of graphene on SiO₂ as extrinsic, and a natural result of highly conformal adhesion [1].

Novel and complex behavior arises when graphene is exfoliated onto a substrate with more pronounced asperities. We have carried out a systematic study of the wrinkling instability of graphene membranes supported on SiO₂ substrates with randomly placed topographic perturbations, produced by SiO₂ nanoparticles of diameter ~ 7 nm. At small nanoparticle density, monolayer graphene adheres to the substrate and is highly conformal over the nanoparticles. With increasing nanoparticle density, and decreasing nanoparticle separation to ~ 100 nm, graphene's elastic response dominates substrate adhesion, and elastic stretching energy is reduced by the formation of wrinkles which connect protrusions. Above a critical nanoparticle density, the wrinkles form a percolating network through the sample. As the graphene membrane is made thicker, delamination from the substrate is observed. Since the wrinkling instability acts to remove inhomogeneous in-plane elastic strains through out-of-plane buckling, our results can be used to place limits on the possible in-plane strain magnitudes that may be created in graphene to realized strain-engineered electronic structures [2].

Finally, we address the question of roughness on the chemical reactivity of graphene by comparing oxidative reactivity of graphene on the following substrates: SiO₂, hexagonal boron nitride, mica, and nanoparticle-decorated SiO₂. This combination of substrates allows us to separate the effects of surface roughness from charge disorder (inhomogeneity in surface potential), and we find that it is the latter which determines the enhanced oxygen reactivity [3] of monolayer graphene previously observed on SiO₂.

[1] W. G. Cullen et al., "High-fidelity conformation of graphene to SiO₂ topographic features", Phys. Rev. Lett. 105, 215504 (2010).
[2] M. Yamamoto et al., "Princess and the Pea at the nanoscale: Wrinkling and unbinding of graphene on nanoparticles", arXiv:1201.5667 (2012).
[3] M. Yamamoto et al., "Charge Inhomogeneity Determines Oxidative Reactivity of Graphene on Substrates", to appear in ACS Nano (2012).