Nature has a few mysteries of her own. While there are many low-symmetry structures in nature, scientists have been limited to high-symmetry designs for creating colloidal crystals, a useful kind of nanomaterial utilized for chemical and biological sensing as well as optoelectronic devices. Now, researchers from Northwestern University and the University of Michigan have pulled back the veil, revealing for the first time how to make low-symmetry colloidal crystals, including one phase with no known natural analog.
We've learned something essential about the method for creating new materials, according to Northwestern's Chad A. Mirkin. This symmetry-breaking technique rewrites the rules of material design and synthesis. The findings were published in the journal Nature Materials today (Jan. 13).
Mirkin is a professor of chemical and biological engineering, biomedical engineering, and materials science and engineering at the McCormick School of Engineering, as well as a professor of medicine at the Feinberg School of Medicine. He is also the founder and president of the International Institute for Nanotechnology. Mirkin and Sharon C. Glotzer, the Anthony C. Lembke Department Chair of Chemical Engineering at the University of Michigan, led the study. Nanoparticles may be programmed and constructed into ordered arrays known as colloidal crystals, which can be created for a variety of applications ranging from light sensors and lasers to communications and computation.
"Using big and tiny nanoparticles, where the smaller ones move around like electrons in a crystal of metal atoms," Glotzer explained, "is a totally new method to generating complicated colloidal crystal formations."
Metal nanoparticles containing designer DNA on their surfaces were employed to make the crystals in this study. The DNA served as an encodable bonding medium, converting them into programmable atom equivalents (PAEs). Because the nanoparticles can be "programmed" to organize themselves in certain ways, using a set of rules previously devised by Mirkin and his colleagues, this technique provides remarkable control over the structure and characteristics of the crystal lattices. However, scientists have not yet developed a method for creating lattices with certain crystal symmetries. Because many PAEs are isotropic, meaning their structures are uniform in all directions, they prefer to assemble into highly symmetric assemblies, making low-symmetry lattices challenging to produce. As a result, the types of structures that can be synthesized, and hence the optical characteristics that can be achieved with them, have been constrained.
The breakthrough was made possible through a novel method of valency control. Valency in chemistry refers to the arrangement of electrons around an atom. It determines the number of bonds an atom may make as well as the shape it can adopt. The Northwestern and Michigan researchers altered the valency of their electron equivalents by adjusting the density of the strands of DNA grafted to their surfaces, building on a recent discovery that small PAEs can behave as electron equivalents, roaming through and stabilizing lattices of larger PAEs. The researchers next employed sophisticated electron microscopy to investigate how modifying the valency of the electron equivalents influenced their spatial distribution among the PAEs and therefore the resultant lattices. They also investigated the impacts of varying temperatures and adjusted the PAE-to-electron equivalent ratio.
"We investigated more sophisticated structures in which we had control over the number of neighbors around each particle, which resulted in even greater symmetry breakdown," Glotzer explained. "Our computer simulations aided in deciphering the complex patterns and revealing the mechanics that allowed the nanoparticles to produce them."
This method paved the way for the creation of three novels, never-before-synthesized crystalline phases. One structure, a triple double-gyroid, has no known natural counterpart. These low-symmetry colloidal crystals offer optical features that conventional crystal forms cannot match and may find applications in a variety of technologies. Their catalytic characteristics are also distinct. However, the novel structures shown here are simply the tip of the iceberg in terms of the possibilities now that the requirements for breaking symmetry are known.
According to Mirkin, we are living in an unparalleled period of material synthesis and discovery. This is another step toward taking novel, unknown materials from the drawing board and into applications that can benefit from their unique and odd features.
Glotzer is also a material science and engineering, macromolecular science and engineering, and physics professor at the University of Michigan, as well as the John Werner Cahn Distinguished University Professor of Engineering and the Stuart W. Churchill Collegiate Professor of Chemical Engineering. Argonne National Laboratory's Byeongdu Lee, along with Mirkin and Glotzer, is a corresponding author.
Journal Information: Shunzhi Wang et al, The emergence of valency in colloidal crystals through electron equivalents, Nature Materials (2022). DOI: 10.1038/s41563-021-01170-5