Superlattice structures formed by nanoparticle (NP) self-assembly have attracted increasing attention due to their potential as a class of nanomaterials with enhanced physicochemical properties tailored by the assembly structure. However, many key questions remain regarding the correlation between the dynamics of individual NPs and emerging superlattice patterns. Here we investigated the self-assembly of gold NPs by employing in situ transmission electron microscopy equipped with direct detection camera capabilities, which enabled us to track the rapid motion of individual nanoparticles in real time. By calculating the contributions of Brownian, van der Waals, hydrodynamic, and steric hindrance forces, we obtained a quantitative evaluation of the competitive interactions that drive the assembly process. Such competition between forces over various separations is critical for the kinetics of cluster growth, leading to the superlattice formation. Brownian motion resulted in the formation of small-sized clusters, whose growth dynamics was characterized as reaction-limited aggregation. Subsequently, at relative short-range particle separations, van der Waals force overrode the Brownian force and dominantly drove the assembly process. When the particles were in close proximity, a delicate balance between van der Waals and steric hindrance forces led to an unexpected dynamic nature of the assembled superlattice. Our study provides a fundamental understanding of coupling energetics and dynamics of NPs involved in the assembly process, enabling the control and design of the structure of nanoparticle superlattices.
Keywords: Brownian force; and steric hindrance force; cluster growth; dynamics of self-assembly; hydrodynamic drag force; in situ liquid cell TEM; van der Waals force.