Understanding Phase Transition of Epitaxial Two Layer Graphene during AFM Nanoindentation

Phase transition of epitaxial multilayer graphene is systematically studied by combining atomic force microscopy (AFM) detection and density functional theory (DFT) calculation. Experimental results reveal that mechanical response of graphene to AFM indentation is dependent on layer numbers. The stiffness of bilayer graphene is extremely large and is comparable to diamond, and that of multilayer graphene (layer number larger than 3) is much smaller than SiC substrate. DFT simulations demonstrate that the ultrahardness two layer graphene comes from graphene/diamond film phase transition under nanoindentations. Similar evolution process of multilayer graphene is impeded by stacking patterns and high energy barriers. Graphene/SiC substrate interface greatly impacts phase transition process by inducing certain layer deformation of graphene, and by increasing carbon layer interactions. Surface dangling bonds saturation is prerequisite for diamond film formation. It not only determines phase transition barriers, but also stabilizes final diamond film.