Structure, Dynamics, and Viscoelasticity of Nanoparticle Thin Films at the Liquid-Air Interface

We experimentally probe the structure and inter-particle dynamics of iron oxide nanoparticle thin films self-assembled at the liquid-air interface. We find that upon deposition on a water substrate, iron oxide nanocrystals coated in oleic acid ligands spontaneously arrange themselves into a hexagonally close-packed configuration. At low particle concentrations, this close-packing results in isolated islands of particles distributed across the liquid surface. Compression in a Langmuir-Blodgett trough and the corresponding increase in surface pressure results in the formation of a uniform quasi-2D monolayer. Using X-Ray Reflectivity (XR) measurements, we were able to quantify the overall change in surface-normal film structure due to an increase in surface pressure. Utilizing X-Ray Photon Correlation Spectroscopy (XPCS), we have measured the characteristic timescale of in-plane particle dynamics. I will discuss these results and their relation to viscoelasticity in quasi-2D self-assembled monolayers.