A well-placed step can turn a high hurdle into an easier jump. The same idea applies to how nanoparticles transition into crystals, according to new research from Cornell.
Crystalline nanomaterials are valuable because their highly ordered structures give them useful properties for technologies such as data storage and optical devices. But forming nanoparticles from those orderly crystals is difficult because instead of snapping into place, the particles often get stuck in arrangements that never become the intended crystal.
In a study published Feb. 26 in the Proceedings of the National Academy of Sciences, researchers from the Cornell Duffield College of Engineering demonstrate that mesophases - states of matter that fall between fully disordered fluids and solid crystals, such as the liquid crystals used in electronic displays and sensors - can act as important steps that make crystallization faster and more reliable.
"People had suspected that mesophases might be helpful because they're en route to a crystalline state," said senior author Fernando Escobedo, the Samuel W. and M. Diane Bodman Professor in the R.F. Smith School of Chemical and Biomolecular Engineering. "What we show is that mesophases really do serve as stepping stones, providing a kind of golden path toward crystallization."
Using advanced computer simulations, Escobedo and B.P. Prakash, Ph.D. '24, studied several nanoparticle systems and tracked how they transitioned from disorder to crystalline order. Across all cases, systems that passed through a mesophase crystallized more quickly than those that attempted to in a single step. The reason, according to the research, is because crystallization requires overcoming a free-energy barrier that slows the process.
"It's like trying to jump over a 1-meter-high barrier," Escobedo said. "That's hard to do. But if you replace that 1-meter jump with two half-meter steps, then it becomes an easier, more efficient jump. That's essentially what the mesophase does."
The researchers were able to quantitatively measure both the free energy barriers and the rates of crystallization, confirming that mesophases reduce kinetic bottlenecks and in some cases, sped up crystallization by orders of magnitude. Beyond speed, mesophases also present an opportunity to improve crystal quality.
"It's a lot easier to anneal defects in a mesophase because it's a more mobile, more flexible phase," Escobedo said. "There are different ways you can ensure that it's a more homogeneous state and then when that phase crystallizes, you end up with fewer defects."
The findings provide new design rules for scientists and engineers working with nanomaterials, showing how to plan the conditions that help particles assemble in the right sequence. By predicting which intermediate steps are most helpful, the research points to ways to build material structures more reliably while avoiding dead ends where materials form the wrong crystal.
"Even when a material has not been previously seen to exhibit a mesophase, if it forms a crystal then it can often be 'persuaded' to form a mesophase by changing the external conditions or by tweaking the nanoparticle design," Escobedo said.
While the study focused on nanoparticles, the principles may extend to other systems, Escobedo said, including polymers and proteins, where intermediate states often appear during assembly. By intentionally using in-between phases, researchers may be able to more efficiently create materials with novel properties.
The research was supported by the National Science Foundation.
Syl Kacapyr is associate director of marketing and communications for Duffield Engineering.
