Fig. 4. Indentation tests with different numbers of indenting cycles on the hillock-like structure surface. (a) A hillock-like structure on the supported monolayer graphene surface before the indentation tests; (b), (c), and (d) show the topography variation of the hillock-like structure with the number of indentation cycles of 10, 50, and 100 times, respectively. The applied normal load Fn during such tests was 1 μN. The scale bars in the AFM images were 50 nm.
