Watching Materials Crystallize

Researchers from Northwestern University and the University of Illinois have made it possible to observe and simulate the self-assembly of crystalline materials at a much higher resolution than ever before.

Using computer modeling and an imaging technique called liquid-phase electron microscopy, the team pinpointed the individual motions of tiny nanoscale particles as they orient themselves into crystal lattices.

The work confirms that synthetic nanoparticles — the fundamental building blocks of many synthetic and biological materials — can assemble in ways far more complex than larger particles. The finding paves the way to more general applications for mineralization, pharmaceuticals, optics, and electronics.

“Imaging and modeling are routinely performed for particles about 1 micrometer in size,” said Northwestern Engineering’s Erik Luijten, who led the computation modeling portion of the study. “Here, we have newly developed techniques that can do this for particles that are 100 nanometers in size — 10 times smaller than before.”

The study was published October 28 in the journal Nature Materials. Luijten is chair and professor of materials science and engineering in the McCormick School of Engineering. He co-led the research with Qian Chen, a professor of materials science and engineering at University of Illinois, Urbana-Champaign.

Because nanoparticles are very small and interact in liquid solutions, verifying their crystallization pathways through direct observation was not possible before liquid-phase electron microscopy, said Chen, who led the experimental portion of the study.

Chen’s team performed laboratory experiments using tiny gold prisms in a fluid, watching closely as the particles began to interact with each other. The particles first stacked together in columns before packing tightly into ordered crystals.

“What we have observed is an intermediate amorphous phase that occurs along the crystallization pathway for nanoparticles — something not witnessed before this work,” Chen said.

However, there are details about crystallization pathways that cannot be measured by imaging alone, the researchers said. 

“Our computer simulations, developed by Northwestern University graduate student Ziwei Wang, allow us to sort out the details of the fundamental driving forces behind nanoparticle motion and crystallization,” Luijten said. “It turns out that randomness in the orientation of the particles leads to a different type of crystallization on larger-length scales. That is a notion that was suggested by the experimental data, but it really required simulations to confirm this principle.”

The researchers envision a wide range of applications for this development, from understanding how proteins self-assemble to the nanoscale physics behind new battery materials, for example.

Luijten also has appointments in the departments of chemistry and of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences. 

The study, “Kinetic Pathways of Crystallization at the Nanoscale,” was supported by the US Department of Energy and the National Science Foundation.