Particle Size Disparity Controls Stacking Disorder in Colloidal Assemblies

Conventional wisdom in colloidal assembly holds that particle polydispersity prohibits long-range order. However, a thorough experimental investigation connecting the disorder in colloidal packing and particle size-dispersity has remained elusive, likely due to the challenges associated with fabricating colloids with specific size distributions. By functionalizing gold nanoparticle cores with monodisperse synthetic DNA strands of different lengths, anionic particles of tunable size and polydispersity can be fabricated. Using electrolytes to assemble such particles, we analyze size-dispersity effects by tuning: 1. the variance of the continuous particle size distributions and 2. the composition and mean component particle sizes of binary mixtures. Our X-ray scattering studies reveal that the assembly transforms from FCC (face centered cubic) to RHCP (random hexagonal) to random close packed (RCP) as the variance is increased, or as the mixture composition is moved toward equimolarity. Furthermore, stacking disorder continuously increases as the assembled structure moves across these phases. This work provides quantitative insights into the role of size dispersity in colloidal assembly.