Mechanisms of Au and Ag nanoparticle array evolution studied by in-situ TEM and molecular dynamics simulation

Understanding the formation and evolution of arrays of metallic nanoparticles is a very important task as they are increasingly used in various devices. In this work, we used HRTEM mode for continuous in-situ observations of the evolution of Ag and Au nanoparticle arrays with an average size of ∼4 nm, formed on the surface of amorphous carbon by vacuum-thermal evaporation, under the influence of a microscope electron beam without any other energetic influence. Our studies show that electron beam exposure induces a process of nanoparticle coalescence for Au and simultaneous coalescence and vaporization for Ag. As a result, the evolution of Ag and Au nanoparticle arrays has different mechanisms. In the case of Ag, the aggregation of nanoparticles occurs through the gas phase by the Ostwald ripening mechanism. In the case of Au, it was found that the aggregation of nanoparticles depends on their mutual crystalline orientation, resulting in the realization of mechanisms through bridge formation or through jumping. The influence of the crystalline orientation of the nanoparticles on the coalescence mechanism was confirmed by molecular dynamics simulations. MD simulations revealed that it is most favorable for coalescence if the densely packed planes of neighboring nanoparticles have a perpendicular arrangement in space.