Bernal stacking-assisted shear exfoliation of nanoplate bilayers

Nanoplates such as graphene and MoS₂ are promising materials due to their excellent electronic and mechanical properties. The preparation of such nanoplates is, however, still challenging due to the large free energy barrier that multilayer nanoplates need to overcome during exfoliation. In the case of a Bernal-stacked bilayer graphene, the binding energy between two graphene layers is about 17.8 meV per atom such that harsh chemical and/or mechanical treatment is usually necessary. In this paper, we perform extensive molecular dynamics simulations for a generic model of nanoplates and illustrate that when the shear is applied to the nanoplate bilayer solution, the nanoplate bilayer may exfoliate readily. In our simulations, the free energy barrier that two nanoplate layers need to overcome reaches up to 21.8k_BT, where k_B and T denote the Boltzmann constant and temperature, respectively. This implies that without external stimuli, the nanoplate bilayer would hardly exfoliate. Upon the application of shear, however, the transition between different Bernal stacked conformations occurs, which provides multiple intermediate states for exfoliation and facilitates the shear exfoliation. We also find that if one were to increase the affinity between the solvent and nanoplates slightly, the free energy barrier would be decreased significantly.