A density-ionic strength phase diagram for nanocrystals with the crystallization pathway also depicted through the intermediate phase. Courtesy the Ginsberg Group
Scientists have long dreamed of controlling how tiny particles—called nanocrystals—come together to form larger, ordered structures. If we could guide this "self-assembly" reliably, it would open doors to new materials for electronics, energy storage, and even medicine. But in practice, the process is often messy: particles clump into disordered blobs or get stuck before reaching their final form.
In a new study published in Nature Physics, 'Enhancing nanoscale charged colloid crystallization near a metastable liquid binodal,' Berkeley researchers tackle that challenge by zooming in on how nanocrystals crystallize. Instead of forcing particles together by simply packing them more tightly, the researchers used soluble ions to tune the way the nanocrystals interact. They studied lead sulfide nanocrystals suspended in solution and watched, in real time, as the particles organized into ordered, repeating lattices—much like atoms form crystals.
New design principles for perfecting crystal growth. Courtesy the Ginsberg Group
Using powerful X-ray scattering techniques, they discovered that crystallization does not always happen in a single step. Sometimes the particles first condense into a dense, liquid-like state before "snapping" into an ordered crystal. This temporary metastable liquid phase turns out to be very useful: it speeds up crystallization and produces crystals with fewer defects. In fact, by carefully adjusting salt concentration, the team could control the assembly speed over three orders of magnitude—from seconds to hours.
Crucially, they showed that crystallization pathways that involve the liquid phase as an intermediate yield faster and higher-quality crystals compared to direct crystallization. This finding provides a general strategy: if you want better nanocrystal superlattices, encourage a liquid intermediate rather than trying to crystallize in one go.
The work does not just explain how nanocrystals behave. It also lays out design principles for engineering self-assembly in many nanoscale systems, from proteins to new energy materials. By balancing attractive and repulsive forces at the nanoscale, researchers can harness metastable liquids as stepping stones toward orderly, functional structures.
Published in the 26 August issue of Nature Physics, the article features leading contributions from Christian Tanner, who recently completed his PhD, and Vivian Wall, a current PhD student. Additional contributors include Naomi Ginsberg (Physics and Chemistry) and David Limmer (Chemistry) at UC Berkeley, Teitelbaum (Arizona State University), and Talapin (University of Chicago) and their trainees. X-ray beamline staff at the SSRL and ALS facilities also contributed to the study. “This work is the culmination of a true team effort to find new and more effective mechanisms to create advanced materials that can even be printed like an ink,” says Ginsberg.