Shaping graphene with optical forging: from a single blister to complex 3D structures

Properties of graphene, such as electrical conduction and rigidity can be tuned by introducing local strain or defects into its lattice. We used optical forging, a direct laser writing method, under an inert gas atmosphere, to produce complex 3D patterns of single layer graphene. We observed bulging of graphene out of the plane due to defect induced lattice expansion. By applying low peak fluences, we obtained a 3D-shaped graphene surface without either ablating it or deforming the underlying Si/SiO₂ substrate. We used micromachining theory to estimate the single-pulse modification threshold fluence of graphene, which was 8.3 mJ cm^-2, being an order of magnitude lower than the threshold for ablation. The control of exposure parameters allowed the preparation of blisters with various topographies. The optically forged structures were studied with atomic force microscopy and Raman spectroscopy. Optically forged blisters act as building blocks in the formation of more complex structures. We found a simple geometric rule that helps to predict the shape of complex patterns which are created by the overlapping multiple exposures. Optical forging enables writing of extended patterns with diffraction unlimited features, which makes this method promising in the production of nanodevices with locally induced surface modifications.