Modeling and Experimental Studies of the Interactions of 2D Materials with Solvents and Surfactants: Exfoliation, Self-Assembly of Composites and Wetting

Two-dimensional (2D) materials such as graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2) are poised to usher in the next generation of digital electronics, sensors, and membranes due to their unique and exotic electronic, optical, and mechanical properties. However, the commercialization of technologies based on these materials is impeded by the absence of cost-effective, large-scale production techniques. One potentially inexpensive, scalable manufacturing route consists of immersing the corresponding bulk material (for example, graphite in the case of graphene) in a liquid and rapidly stirring the entire system. This separates individual monolayers from the bulk material in the same way that a food processor slices vegetables. The efficiency of this "exfoliation" process depends on the interactions between the solvent molecules and the material. Consequently, understanding these interactions at a molecular scale is essential in order to determine the optimal exfoliating solvent for a given material. To shed light on these interactions, the PIs propose to use computer simulations to study the behavior of a variety of solvent molecules around sheets of several promising 2D materials. The insights obtained from these computer simulations can also assist in designing solvents that encourage the spontaneous formation of mixed stacks of several 2D materials (such as graphene on top of h-BN on top of MoS2) that could have very special properties and applications.

Researchers seek to understand and model the molecular interactions operating between solvent / surfactant molecules and various 2D materials in a solution phase. This will enable the prediction of the thermodynamic and kinetic stability of colloidal dispersions of various new 2D materials and their composites, which are becoming increasingly important for modulating the band gap in electronic and optoelectronic applications. For this purpose, a classical molecular dynamics (MD) simulation approach retains atomic detail of the system, without sacrificing on the length and time scales involved. Although several good models for liquids and materials are currently available, much less effort has been devoted to modeling the interaction forces between liquids and materials. To this end, the PIs propose to develop force fields to model the interfacial phenomena involving 2D materials and liquid media. The predictions made by the models developed will be compared with experiments, thereby allowing transferability of parameters across different interfacial systems. The insights obtained from such modeling efforts will enable the precise control of the size, shape, number distributions, and stacking order of layers in suspensions of 2D materials to obtain desired electronic and optical properties. Furthermore, the force fields will be used to predict wetting properties, which are critical to the use of these materials in membranes, microfluidic devices, battery electrodes, and other electrochemical devices.

The modeling and experimental advances made will be incorporated into courses and workshops at MIT that will expose a larger scientific audience to the fundamentals of 2D material dispersion and stabilization in liquid phases, wetting behavior of 2D materials, as well as to modeling these phenomena at the molecular level. The students involved in the proposed research, at both the graduate and undergraduate levels, will gain intellectually and professionally from the integrated modeling / experimental research proposed here.