Glassy Dynamics and Anomalous Diffusion in Self-Assembled Nanoparticle Monolayers

We experimentally investigate the structure and dynamics of iron oxide nanoparticle thin films self-assembled at the liquid-air interface. Upon deposition on a water surface and subsequent lateral compression, iron oxide nanoparticles coated with oleic acid ligands self-assemble into a morphologically uniform quasi-2D monolayer. We examined the in-plane structure of these self-assembled films using Grazing-Incidence X-Ray Diffraction (GIXD) and investigated the interparticle dynamics using X-Ray Photon Correlation Spectroscopy (XPCS). The logarithmic relaxation of the surface pressure of the films post-compression suggests the presence of glassy dynamics in the system. Autocorrelation functions derived from XPCS measurements quantify the characteristic timescale of such dynamics and have been fit using the Kohlrausch-Williams-Watts (KWW) model to extract the degree of glassiness. Finally, the q-dependence of the interparticle dynamics in the films is supportive of an anomalous diffusion regime, $\langle x^{2}\rangle \propto t^{n}$, with $n>1$. I will discuss these results and their implications with regards to the nanoscale interactions involved in thin film self-assembly and rearrangement.