| dc.description.abstract | Two dimensional materials, such as graphene, MoS2 and hexagonal boron nitride, show wide ranging electronic and mechanical properties which make applications such as transistors, pressure sensors and protective coatings possible. Solvents are an essential element in the production and processing of these two-dimensional (2D) materials. Liquid phase exfoliation (LPE) is one such solution-processing methods capable of industrial scale production. Here, layers are separated by the application of external force. The solvent molecules then prevent the resulting monolayers from reaggregation, stabilizing the layers in the solvent. It is generally assumed that these solvents do not interact strongly with the layer and so their effects can be neglected. Yet experimental evidence has suggested that explicit atomic-scale interactions between the solvent and layered material may play a crucial role in exfoliation and cause unintended electronic changes in the layer.
Here we use modern computational tools such as density functional theory (DFT), a powerful first-principles method, to study the role of solvent molecules in the process of liquid phase exfoliation. Molecular dynamics simulations are then used to calculate the dynamical properties of the system.
We show using DFT that the interaction between graphene or MoS2 with individual solvent molecules is van der Waals (vdW) in nature, with negligible charge transferred in between them. We use MD calculations to show that, when graphene is immersed in a solvent, distinct solvation layers are formed irrespective of the type of solvent molecule (i.e., whether polar or non polar) due to these vdW interactions. We show that the formation free energies of these solvation shells is favorable for all the molecules considered. However, energetic considerations such as these cannot explain the experimental solvent-dependence. Instead, kinetic effects can dominate. We find that interfacial solvent molecules with high diffusion coefficients parallel to the graphene layer result in the lowest experimental concentration of graphene in solution. This can be explained by the enhanced ease of reaggregation in the high diffusion regime. Solvents with smaller diffusion coefficients correspond to higher experimental graphene concentrations. In the low diffusion limit however, this relationship breaks down. We suggest that here the concentration of graphene in solution depends primarily on the separation efficiency of the initial LPE step.
On intercalating group-1 metal ions into the layers, we show that the spontaneous exfoliation and stabilization of separated layers is due to the enhanced solvation energy of charged layers of graphene. When the similar ion intercalation procedure for exfoliation of Group-VI transition metal dichalcogenides (TMDs, MX2, where M=Mo, W and X = S, Se), they are known to undergo charge induced transitions from semi-conducting H phases to metallic T phases. However, it is difficult to experimentally decouple the effect of composition-dependent phase transition barriers from indirect effects related to the exfoliation process. Here, we study the energetics of transition between the different structural polytypes of four group-VI TMDs upon lithium adsorption. We find that both the activation barrier from the H phase to the metallic phase in charged monolayers and the reverse barrier in neutral monolayers are required to explain experimental results.
The solution processing in the presence of the ions can result in co-intercalation, i.e. the intercalation of both the solvent molecule and the ions simultaneously. We develop workflows to determine the most stable configuration when both molecules and potassium ions interact with bilayer graphene. This work flow can be further used to filter molecules which enhance molecule—ion cointercalation in bilayer graphene.
The edges of 2D materials are highly reactive. We show that polar molecules align in a bistable orientation along a graphene edge, and their orientation can be switched using an external field. Experimental data shows that this effect can be used to tune and switch the graphene resistance, and depends on the type of molecule and the graphene termination. Using parameters extracted from DFT, we use an Ising-like model and Monte Carlo simulations to determine how intermolecular van der Waals interactions, dipolar interactions, molecule - graphene interactions and the coupling of the molecular dipole to the external field can be used to explain the experimental results. | en |
