Origin of Rapid Coalescence and Active Unstable Fluctuation of Au Nanoparticles under TEM Observation: Electron Bombardment Versus Charge Buildup

Nanoparticles (NPs) in contact with each other observed in a transmission electron microscope (TEM) tend to undergo rapid coalescence. Similarly, isolated NPs show unstable fluctuations and changes in orientation due to active vibration and rotation; these behaviors are accompanied by a rapid diffusion of surface atoms and are likely related to the rapid coalescence of NPs, which has been widely attributed to bombardment by high-energy electrons. However, indirect experimental results have suggested that these phenomena are due to the built-up charge. Here, we used TEM to examine the behavior of gold NPs between insulating and conducting membranes to discern between electron bombardment and built-up charge effects. Our observations indicated that the rapid coalescence and active motion of NPs originate mainly from the built-up charge. This novel finding of charge-enhanced atomic diffusion in NPs helps clarify the mechanisms of many phenomena, such as the almost liquid-like rapid coalescence of NPs that produces smooth, single-crystalline nanowires. In contrast, NPs of the same size may show very sluggish coalescence, producing pearl-like chains, which may be attributed to the absence of charge.