The new crystal synthesis method leads researchers to an entirely new phase of metal, which may be useful for new technologies. The results were confirmed using the Advanced Photon Source.
A research team led by Northwestern University and the University of Michigan has developed a new method to assemble particles into colloidal crystals, a valuable type of material used for chemical and biological light sensing and sensing devices. Using this method, the team showed for the first time how these crystals can be engineered in ways not found in nature.
The team used the Advanced Photon Source (APS), a US Department of Energy (DOE) Office of Science User Facility at the DOE’s Argonne National Laboratory, to confirm their crucial discovery.
“A powerful X-ray beam enables the high-resolution measurements you need to study this type of assembly. APS is an ideal facility to conduct this research. — Byeongdu Lee, Argonne National Laboratory
“We discovered something fundamental about the system for making new materials,” said Chad A. Mirkin, George B. Rathmann Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences. “This symmetry breaking strategy rewrites the rules for material design and synthesis.”
The research was led by Mirkin and Sharon C. Glotzer, chair of the Anthony C. Lembke Department of Chemical Engineering at the University of Michigan, and was published in the journal Natural materials.
Colloidal crystals are very small particles with other smaller particles (called nanoparticles) arranged inside them in an orderly or symmetrical fashion. They can be designed for applications ranging from light sensors and lasers to communications and computing. For this research, scientists attempted to break nature’s natural symmetry, which tends to order tiny particles in the most symmetrical way.
“Imagine stacking basketballs in a box,” said Byeongdu Lee of Argonne, APS group leader and paper author. “You would have a specific way of doing it that would get the most value out of the space. That’s how nature does it.”
However, says Lee, if the balls are deflated to some degree, you can stack them in a different pattern. The research team, he said, is trying to do the same with nanomaterials, teaching them to self-assemble into new patterns.
For this research, the scientists used DNA, the molecule inside cells that carries genetic information. Scientists have learned enough about DNA to be able to program it to follow specific instructions. This research team used DNA to teach metallic nanoparticles to assemble into new configurations. The researchers attached DNA molecules to the surfaces of nanoparticles of varying sizes and found that the smaller particles moved around the larger ones in the spaces between them, while binding the particles together into a new material.
“The use of large and small nanoparticles, where the smaller ones move like electrons in a crystal of metal atoms, is a whole new approach to building complex colloidal crystal structures,” said Glotzer.
By adjusting this DNA, the scientists changed the parameters of the small electronic equivalent particles and thus changed the resulting crystals.
“We explored more complex structures where controlling the number of neighbors around each particle produced additional symmetry breaking,” Glotzer said. “Our computer simulations helped decipher the complicated patterns and reveal the mechanisms that enabled the nanoparticles to create them.”
This approach paved the way for three new, never-synthesized crystalline phases, one of which has no known natural equivalent.
“Assemblies of colloidal particles always have some analogy in the natural atomic system,” Lee said. “This time, the structure we found is completely new. The way it fits together, we haven’t seen metals, metal alloys or other materials naturally fit together that way.
“We don’t know the physical properties of the material yet,” Lee said. “Now we’re turning it over to materials scientists to create this material and study it.”
The team used the ultra-bright X-ray beams from APS to confirm the new structure of their crystals. They used the high-resolution small-angle X-ray scattering instruments on the 5-ID and 12-ID beamlines to create precise images of the arrangement of the particles they had created.
“A powerful X-ray beam enables the high-resolution measurements you need to study this type of assembly,” Lee said. “APS is an ideal facility to conduct this research.”
The APS is currently undergoing a massive upgrade, which Lee says will allow scientists to determine even more complex structures in the future. The 12-ID’s instruments are also being upgraded to take full advantage of the brighter X-ray beams that will be available.
These low symmetry colloidal crystals have optical properties that cannot be achieved with other crystal structures and can be used in a wide range of technologies. Their catalytic properties are also different. But the new structures unveiled here are just the beginning of the possibilities now that symmetry breaking conditions are understood.
“We are in the midst of an unprecedented era of material synthesis and discovery,” Mirkin said. “This is another step forward in bringing new, unexplored materials out of the sketchbook and into applications that can take advantage of their rare and unusual properties.”
Reference: “The emergence of valence in colloidal crystals by electron equivalents” by Shunzhi Wang, Sangmin Lee, Jingshan S. Du, Benjamin E. Partridge, Ho Fung Cheng, Wenjie Zhou, Vinayak P. Dravid, Byeongdu Lee, Sharon C. Glotzer, and Chad A. Mirkin, January 13, 2022, Natural materials.