Dynamics of gold nanoparticle clusters observed with liquid-phase electron microscopy.

The dynamics of processes of nanoparticles such as diffusion, attraction and repulsion, and self-assembly of structures of nanoparticles at the solid-liquid interfaces differ significantly from those occurring for bulk conditions and their fundamental physical rules are still unknown. Here, we used liquid phase scanning transmission electron microscopy (LP-STEM) to study several aspects of nanoparticle dynamics of colloidal chitosan coated gold nanoparticle (TCHIT-AuNP) clusters in a liquid layer enclosed between two SiN membranes. We found that upon beam irradiation using an electron flux of 0.9 e/sÅ, the AuNPs assembled in clusters that shifted and rotated with time. The newly formed clusters could join and form larger clusters via a mechanism of oriented attachment. By increasing the electron flux to 6.2 e/sÅ, we observed the fragmentation of some of the clusters and TCHIT-AuNPs were exchanged between clusters. At the highest electron flux studied 25 e/sÅ, we observed AuNPs moving at a very slow speed compared to Brownian motion in liquid even though they were not directly attached or pinned to the liquid-enclosing membrane. Experiments using branched polyethylenimine (BPEI) coated AuNPs were carried out for comparison.