Nature holds a few secrets. While there are plenty of structures with low symmetry in nature, scientists have been limited to highly symmetrical designs when synthesizing colloidal crystals, a valuable type of nanomaterial used for chemical and biological sensing, and optoelectronic devices.
Now, research from Northwestern University and the University of Michigan has drawn back the curtain, showing for the first time how low-symmetrical colloidal crystals can be made – including a phase for which there is no known natural equivalent.
“We have discovered something fundamental about the system of making new materials,” said Northwestern’s Chad A. Mirkin. “This strategy of breaking symmetry rewrites the rules of material design and synthesis.”
The research was published today (January 13) in the journal Natural materials.
Mirkin is George B. Rathmann Professor of Chemistry at the Weinberg College of Arts and Sciences; a professor of chemical and biological engineering, biomedical engineering, and materials science and engineering at the McCormick School of Engineering; and Professor of Medicine at the Feinberg School of Medicine. He is also the founding director of the International Institute for Nanotechnology.
The research was led by Mirkin and Sharon C. Glotzer, Anthony C. Lembke’s chair of the Department of Chemical Engineering at the University of Michigan.
Nanoparticles can be programmed and assembled into ordered arrays known as colloidal crystals, which can be designed for applications from light sensors and lasers for communication and data processing.
“Using large and small nanoparticles, where the smaller ones move around like electrons in a crystal of metal atoms, is a whole new approach to building complex colloidal crystal structures,” Glotzer said.
In this research, metal nanoparticles whose surfaces were coated with designer DNA were used to create the crystals. The DNA acted as a codable binding material and transformed them into what are called programmable atomic equivalents (PAEs). This approach provides unique control over the shape and parameters of the crystal lattices, as the nanoparticles can be ‘programmed’ to arrange themselves in specified ways, following a set of rules previously developed by Mirkin and his colleagues.
But to this point, scientists have not had a way to prepare lattices with certain crystal symmetries. Because many PAEs are isotropic – meaning their structures are uniform in all directions – they tend to be arranged in highly symmetrical assemblies, and it is difficult to create low-symmetrical grids. This has limited the kind of structures that can be synthesized and therefore the optical properties that can be realized with them.
The breakthrough came through a new approach to controlling valence. In chemistry, valence is related to the arrangement of electrons around an atom. It determines the number of bonds the atom can form and the geometry it assumes. Based on a recent discovery that small PAEs can behave like electron equivalents, roaming through and stabilizing the lattice of larger PAEs, Northwestern and Michigan researchers changed the valence of their electron equivalents by adjusting the density of DNA the strings grafted to their surfaces.
Next, they used advanced electron microscopy to observe how changing the valence of the electron equivalents affected their spatial distribution among the PAEs and therefore the resulting grids. They also investigated the effects of changing temperatures and changing the ratio of PAEs to electron equivalents.
“We explored more complex structures where control over the number of neighbors around each particle produced additional symmetry breaks,” Glotzer said. “Our computer simulations helped to decipher the intricate patterns and reveal the mechanisms that enabled the nanoparticles to create them.”
This approach set the stage for three new, never before synthesized crystalline phases. One, a triple double-gyroid structure, has no known natural equivalent.
These low-symmetrical 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 revealed here are only the beginning of the possibilities now that the conditions for breaking symmetry 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.”
The study, “Emergence of valence in colloidal crystals through electron equivalents”, was primarily supported by the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences (award DE-SC0000989) and also by the Air Force Office of Scientific Research (award FA9550-17-1-0348) and the Sherman Fairchild Foundation.
Glotzer is also John Werner Cahn Distinguished University Professor of Engineering, Stuart W. Churchill Collegiate Professor of Chemical Engineering and Professor of Materials Science and Engineering, Macromolecular Science and Engineering and Physics at the University of Michigan. Byeongdu Lee from the Argonne National Laboratory is a similar author with Mirkin and Glotzer.
The emergence of valence in colloidal crystals through electron equivalents
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