Interactions of Nanomaterials with Liquid Media

Two-dimensional materials are next-generation candidates for optoelectronic devices, sensors, and membranes because of their superior optical, electronic, mechanical, and thermal properties. Experimental characterization of 2D nanomaterials, such as graphene, MoS2, hBN, and WS2, offers glimpses of future applications in high-performance devices. Compared to micromechanical cleavage and chemical vapor deposition, liquid-phase exfoliation provides a scalable manufacturing route for high-quality layers, which is beneficial for synthesizing printable inks and functionalizing the materials in solution phase. Liquid-phase exfoliation also enables deposition on arbitrary substrates. However, precise guidelines for the selection of a good exfoliating agent (for example, solvents, surfactants, or polymers) for a given nanomaterial remain elusive. There is also a lack of understanding of how different exfoliation media interact with nanomaterials, leading to different flake sizes and thicknesses during exfoliation.

Figure 1: A schematic of the liquid phase exfoliation process, showing the solvent molecules (water in this case) penetrating between the sheets of bulk molybdenum disulfide (MoS2).

Our group uses molecular dynamics (MD) simulations as a tool to garner mechanistic insights into the organization of molecules of commonly employed solvents and surfactants around nanomaterials. We focus on the correlations between the structural features of these exfoliation media and their effect on the energy barrier hindering the aggregation of 2D material sheets. The combination of MD-based computation of the potential of mean force (PMF) between pairs of sheets and the application of theories of colloid aggregation offer a detailed picture of the mechanics underlying the stability of these solutions. Additionally, our theories allow the prediction of the temporal lifetimes of these dispersions.

Figure 2: The use of the potential of mean force (PMF) calculations (left) for graphene sheets immersed in various solvents to predict and compare the temporal stability of graphene solutions (right).

We are also interested in the wetting properties of 2D materials and their composites. We use a mix of continuum theories and molecular dynamics simulations to uncover the unique wetting behavior of nanoscale and nanostructured coatings. These coatings play an important role in the use of 2D materials in membranes, sensors, and microfluidic devices. 

Figure 3: (Left) Experimental contact angle on graphene coated substrate and (Right) Equilibrated molecular dynamics (MD) snapshot of a water droplet on an unsupported graphene sheet. The use of a continuum theory allowed us to uncover the “wetting translucency” of graphene.