Enhancing nanoscale charged colloid crystallization near a metastable liquid binodal

TL;DR

This study demonstrates that controlling electrostatic interactions near a metastable liquid binodal significantly enhances the rate and order of nanoscale colloid crystallization, providing a kinetic strategy for improved superlattice assembly.

Contribution

It introduces a method to control nanocrystal self-assembly via electrostatics near a metastable liquid phase, revealing new pathways and kinetics for ordered superlattice formation.

Findings

Metastable liquid phase accelerates superlattice formation.

Self-assembly rate varies over three orders of magnitude.

Order increases with faster assembly rates.

Abstract

Bottom-up assembly of nanocrystals (NCs) into ordered arrays, or superlattices (SLs), is a promising route to design materials with new functionalities, but the degree of control over assembly into functional structures remains challenging. Using electrostatics, rather than density, to tune the interactions between semiconductor NCs, we watch self-assembly proceeding through a metastable liquid phase. We systematically investigate the phase behavior as a function of quench conditions in situ and in real time using small angle X-ray scattering (SAXS). Through quantitative fitting to colloid, liquid, and SL models, we extract the time evolution of each phase and the system phase diagram, which we find to be consistent with short-range attractive interactions. Using the phase diagram's predictive power, we establish control of the self-assembly rate over three orders of magnitude, and identify one- and two-step self-assembly regimes, with only the latter implicating the metastable liquid as an intermediate. Importantly, the presence of the metastable liquid increases SL formation rates relative to the equivalent one-step pathway, and SL order counterintuitively increases with the rate, revealing a highly desirable and generalizable kinetic strategy to promote and enhance ordered assembly.