The ability to directly image the individual building blocks makes colloidal crystallization a model platform for studies of phenomena that occur on the atomic and molecular scales. Simultaneously, the self-assembly of colloidal nanocrystals is a promising route to the fabrication of next-generation materials with properties that extend beyond those of the nanocrystal building blocks. For example, superlattices of plasmonic nanoparticles display deep strong light–matter coupling [1], which has many potential applications, e.g., in nonlinear optics, light harvesting, and controlling chemical reactions. The progress made in controlling the synthesis of nanocrystals makes them highly tunable material building blocks, where the tailorability of particle shape, size, composition, and surface chemistry enable the engineering of their assembly behavior. Building upon this progress, we have developed a synthesis protocol to control the degree of faceting of the nanocrystal shape. This method enables to the synthesis of particles with tunable shape, ranging from spherical to truncated octahedral. These particles self-assemble into superlattices with shape-dependent structure; the truncated octahedral and spherical particles self-assemble into structures with body-centered (BCC) and face-centered cubic (FCC) symmetries, respectively. Intriguingly, particles with shapes between these two limits form superlattices with structures that can be described as intermediate between BCC and FCC. The transformation between BCC and FCC, a diffusionless or martensitic transition, has important implications in metallurgy, yet remains poorly understood owing to the extremely short timescales on which the intermediate structures typically exist. The nanocrystal-based intermediate superlattices therefore present a unique opportunity to gain insight into martensitic transformations. A necessary first step in this direction is to uncover the factors that stabilize these intermediate structures.
In this work, we elucidate the important features of the interparticle interactions that lead to the stability of martensitic intermediates using a computational approach. We develop a model of shape-tunable nanocrystals that accounts for the shape of the particles as well as their ligand–ligand interactions. We find that in the hard shape limit, where ligand–ligand interactions are neglected, the intermediate structures are not stable and rapidly transition to either BCC or FCC structures. When ligand–ligand interactions are included as a soft, short-ranged repulsion between particles, the shape-dependent martensitic intermediates become stable. Additionally, systems self-assemble from disorder into the intermediate structures. We show that for a given shape of the nanocrystal core, the same structure forms independently of the initial configuration, highlighting the equilibrium—and hence reversible—nature of these intermediate structures. This work highlights the important role that capping ligands play in the self-assembly and stability of nanocrystal superlattices and motivates further work towards fine control over superlattice structure.
[1] Mueller, N.S., Okamura, Y., Vieira, B.G.M. et al. Deep strong light–matter coupling in plasmonic nanoparticle crystals. Nature 583, 780–784 (2020).
