(362g) Understanding the Mechanisms of Crystallization and Reconfiguration of Anisotropic Nanoparticle Superlattices

Crystallization is a universal phenomenon in the physical sciences. Despite its importance in wide-ranging technologies, there is no single theoretical framework that accurately describes diverse crystallization phenomena. Recent breakthroughs in liquid cell transmission electron microscopy (LCTEM) have enabled direct, in situ visualization of nanoparticle systems, including their self-organization into crystalline superlattices. LCTEM studies have revealed the crystallization pathways of isotropic (i.e., spherical) and anisotropic nanoparticles in aqueous solutions; however, direct visualization of crystallization pathways of anisotropic nanoparticles in nonaqueous solution remains unexplored.

In this work, we elucidate the complex phase behavior and corresponding crystallization pathways of polymer-grafted gold nanocubes into highly ordered superlattices using experimental, computational, and theoretical techniques. LCTEM experiments reveal a solvent-dependent self-assembly phase behavior, where a series of hexagonal rotator, rhombic, and square-like phases are observed with increasing solvent polarity. To explain these observations, we develop a computational model that reproduces the experimental phase behavior and show that the variable charge screening by the solvent drives the assembly of the nanocubes into different phases. Interestingly, the self-assembly of the different phases follow distinct kinetic pathways; in particular, the assembly of the rhombic phase proceeds via a decoupling of translational and orientational order. Moreover, we demonstrate a reversible transition between the square-like and rhombic phases, and find a hexagonal rotator-like intermediate phase along the transition pathway. These findings open the door for understanding—and hence manipulating—complex microscopic crystallization pathways and phase transition kinetics of anisotropic nanoparticles.