For the first time ever, researchers have watched the mesmerizing process of nanoparticles self-assembling into solid materials. In the stunning new videos, particles rain down, tumble along stairsteps and slide around before finally snapping into place to form a crystal’s signature stacked layers.
Led by Northwestern University and the University of Illinois, Urbana-Champaign, the research team says these new insights could be used to design new materials, including thin films for electronic applications.
The research was published March 30 in the journal Nature Nanotechnology.
Described by the researchers as an “experimental tour de force,” the study used a newly optimized form of liquid-phase transmission electron microscopy (TEM) to gain unprecedented insights into the self-assembly process. Before this work, researchers have used microscopy to watch micron-sized colloids — which are 10 to 100 times larger than nanoparticles — self-assemble into crystals. They also have used X-ray crystallography or electron microscopy to visualize single layers of atoms in a crystalline lattice. But they were unable to watch atoms individually move into place.
“We know that atoms use a similar scheme to assemble into crystals, but we have never seen the actual growth process,” said Northwestern’s Erik Luijten, who led the theoretical and computational work to explain the observations. “Now we see it coming together right in front of our eyes. By viewing nanoparticles, we are watching particles that are larger than atoms, but smaller than colloids. So, we have completed the whole spectrum of length scales. We are filling in the missing length.”
"Previously, our team resolved the mystery of nucleation, namely how the embryos of crystals composed of tens of nanoparticles are formed, which follows a nonclassical pathway in solution,” said Illinois’ Qian Chen, who led the experimental work. “With recent advances in liquid-phase TEM and data science, in this work, we are now able to capture and track motions of thousands of nanoparticles over time. These nanoparticles wiggle in solution and grow into crystals of various morphologies like polyhedral or wedding cake.”
Luijten is a professor of materials science and engineering at Northwestern’s McCormick School of Engineering, where he also is an associate dean. Chen is an associate professor of materials science and engineering at Illinois.