Atomically thin graphene exhibits fascinating mechanical properties, althoughits hardness and transverse stiffness are inferior to those of diamond. Todate, there hasn't been any practical demonstration of the transformation ofmulti-layer graphene into diamond-like ultra-hard structures. Here we show thatat room temperature and after nano-indentation, two-layer graphene on SiC(0001)exhibits a transverse stiffness and hardness comparable to diamond, resistingto perforation with a diamond indenter, and showing a reversible drop inelectrical conductivity upon indentation. Density functional theorycalculations suggest that upon compression, the two-layer graphene filmtransforms into a diamond-like film, producing both elastic deformations andsp2-to-sp3 chemical changes. Experiments and calculations show that thisreversible phase change is not observed for a single buffer layer on SiC orgraphene films thicker than 3 to 5 layers. Indeed, calculations show thatwhereas in two-layer graphene layer-stacking configuration controls theconformation of the diamond-like film, in a multilayer film it hinders thephase transformation.
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