Effect of the External Velocity on the Exfoliation Properties of Graphene from Amorphous SiO2 Surface

External action has a significant influence on the formation of high-quality graphene and the adhesion of graphene on the surface of the MEMS/NEMS device. The atomic-scale simulation and calculation can further study the exfoliation process of graphene by external actions. In multilayer graphene systems where graphene layers were simulated weakly contacted with SiO2 substrate, a constant vertical upward velocity (Vup) was applied to the topmost layer. Then two critical velocities were found, and three kinds of distinct exfoliation processes determined by critical upward velocities were observed in multilayer graphene systems. The first critical velocities are in the range of 0.5 Å/ps–3.18 Å/ps, and the second critical velocities are in the range of 9.5 Å/ps–12.1 Å/ps. When the Vup is less than the first critical velocity, all graphene layers will not be exfoliated. When Vup is between the first and second critical Vup, all layers can be exfoliated almost synchronously at last. When Vup is larger than the second critical Vup, the topmost layer can be exfoliated alone, transferring energy to the underlying layers, and the underlying layers are slowly exfoliated. The maximum exfoliation force to exfoliate the topmost layer of graphene is 3200 times larger than that of all graphene layers. Moreover, it is required 149.26 mJ/m2 to get monolayer graphene from multilayers, while peeling off all layers without effort. This study explains the difficulty to get monolayer graphene and why graphene falls off easily during the transfer process.