Binding Free Energy Calculations of Bilayer Graphenes Using Molecular Dynamics

Abstract

Bilayer graphenes are dimeric assemblies of single graphene layers bound together by π-complexation interactions. Controlling these assemblies can be complicated, as the layered compounds disperse in solvents or aggregate into higher columnar configurations and clusters. One way to assess the interactions that contribute to the stability of the layered compounds is to use molecular simulation. We perform pulling molecular dynamics on bilayer graphenes with different sizes and obtain the normal and shear force profiles of dissociation. We generate pathways of dissociation along the two directions and calculate the binding free energies of the structures with umbrella sampling simulations. We show that the dissociation process is direction-dependent. Along the shear direction, we compute the same free energy for the different samples, which validates the consistency of our simulations. We notice that the dissociation is less adiabatic on the normal than the shear direction, having an entropic contribution to the Gibbs energy. This contribution is more enhanced for the larger bilayer graphenes.

On the Consistency of the Exfoliation Free Energy of Graphenes by Molecular Simulations

“…For the sake of comparison, we plot the exfoliation free energies of the bilayer graphenes computed with umbrella sampling simulations, in the right-hand panel of Figure 4 . The umbrella simulations are detailed in a previous work [ 40 ]. The exfoliated graphene is pulled on the shear direction relevant to the initial plane of the bilayer.…”

“…F AB is the exfoliation free energy expressed by (F A − F C ) − (F B − F C ). F u AB is the exfoliation free energy for the corresponding bilayer graphene samples computed using umbrella sampling in a previous work [40]. The energies are normalized by the area of the graphene layers, expressed in kJ mol −1 nm −2 .…”

“…A different technique to compute the exfoliation free energy of bilayer graphenes is to use umbrella sampling simulations. The umbrella sampling method was discussed in detail in a previous work [ 40 ]. There, we employed umbrella sampling to compute the binding free energies of the same, as in this work, three bilayer graphene samples.…”

“…The energies are shown as a function of the interlayer distance as the one layer is pulled away from the other on the shear direction. The umbrella sampling simulations are performed in a previous work [ 40 ].…”

“…In a previous work, we employed one method to compute the exfoliation free energy of graphenes [ 40 ]. In brief, we considered a normal and a shear reaction coordinate to exfoliate (dissociate) a graphene layer from a bilayer configuration in an aqueous solution.…”