Understanding the self-assembly dynamics of 2D materials at fluid-fluid interfaces

Abstract

Two-dimensional (2D) materials represent a class of scientifically fascinating and technologically relevant particles that have atomic-scale thicknesses (<100 nm) but colloidal-scale lateral dimensions (~102 – 104 nm). 2D materials can preferentially adsorb at fluid-fluid interfaces, yielding a “2D colloidal system”, and there is growing interest in the assembly and deposition of 2D materials from a fluid-fluid interface for uses in next-generation, thin-film applications. However, the 2D nature of a fluid-fluid interface produces a different interaction environment for interfacially adsorbed particles than when the same particles are in a bulk fluid, and it is not understood how nanoscale or colloidal forces dictate the interfacial self-assembly dynamics of 2D materials. In this thesis, our findings suggest a balance of capillary, thermal, entropic, and van der Waals (vdW) forces, each of which manifests from physical particle parameters, govern the dynamics of graphene particles at planar fluid-fluid interfaces. A major contribution of this thesis was the development of a concerted methodology for fabrication, transfer, and in situ visualization of 2D materials with controlled thickness, shape, and size at fluid interfaces in order to study fundamental interactions. A combination of indirect and direct techniques were used to investigate the dynamics of graphene particles at an air-water interface as a function of particle thickness, lateral dimension, and shape. We found 2D particle thickness could tune capillary interactions across a breadth of interaction energies (~10-1 to >102 kBT), the dynamics of small (<25 m lateral dimension) monolayer graphene particles were strongly influenced by thermal forces, and particle shape induced structural order through entropy maximization at high particle area densities. Additionally, the attractive edge-to-edge vdW force was found to be short-ranged but significant, enabling preservation of self-assembled morphology after deposition. Finally, the stacking kinetics of monolayer graphene particles were found to be dependent on interparticle contact length. The work in this thesis highlights substantial advancements made in observing and understanding the fundamental dynamics governing the morphology of 2D material-based films at fluid interfaces, and will have a significant impact on the fabrication of films of 2D materials with morphologies engineered for next-generation applications.