Direct observation and control of non-classical crystallization pathways in binary colloidal systems

Crystallization stands as a prime example of self-assembly. Elementary building blocks converge, seemingly adhering to an intricate blueprint, orchestrating order from chaos. While classical theories have long described crystallization as a monomer-by-monomer addition (whether involving atoms, ions, molecules or colloids), non-classical crystallization pathways introduce complexity. Using microscopic charged particles as monomers, we reveal the precise mechanisms underpinning the formation of ionic colloidal crystals. Our findings indicate that these crystals primarily crystallize through a two-step process, wherein metastable amorphous `blobs' condense from the gas phase, subsequently evolving into small binary crystals. Once formed, these small crystals grow to form large faceted structures through three main processes that occur simultaneously: addition of free monomers from bulk, capture and absorption of surrounding blobs, and oriented attachment of other crystals. These complex crystallization pathways occur both in bulk and on surfaces across a range of particle sizes and interaction strengths, resulting in a diverse array of crystal types and morphologies. Harnessing our ability to tune the interaction potential through small changes in salt concentration, we developed a continuous dialysis approach that allows fine control over the interaction strength in both time and space. This method enables us to discover and characterize a wide variety of crystal structures in a single experiment, including a previously unreported low-density hollow structure and the heteroepitaxial formation of composite crystal structures.