Electron Beam-Induced Interaction of Metal Nanoparticles on an Ionic Liquid Surface*

The motions of ~10-~200-nm metallic nanoparticles on vacuum-ionic liquid interfaces were observed over several minutes by in-situ scanning electron microscopy, and regardless of metal and ligand chemistry, the particles slowly migrated to the center of the area imaged, typically 12 μm x 10 μm. As the image center was approached, motion became less directed but the arriving particles eventually formed a multi-particle aggregate. A particle approaching an existing aggregate was drawn toward the aggregate’s protuberances, and so, the aggregate was initially loose, although subsequent rearrangements made the object denser. This behavior was monitored under different experimental conditions, including varying particle size and beam current. We suggest that the directed particle motion reflects an interplay between the in-plane electric field created by charging of the liquid interface and the polarization of the metallic particles. Our observations underscore electron beam radiation as means to assemble nano-sized metallic objects into 2D hierarchical structures.