In a study published in Science, researchers used silver nanoparticles to assemble a theorized, but never-before observed crystal metallic structure.
PROVIDENCE, R.I. [Brown University] - Using finely tuned nanoscale building blocks, researchers from Brown University and the University of Michigan College of Engineering have stabilized a fleeting structural phase of matter that had been predicted theoretically but never before stabilized in a physical material.
The new nanoparticle superlattice, described in the journal Science, freezes an elusive intermediate state between two of nature's most common crystal metallic arrangements. Beyond describing new details about how this transition works, the new structure exhibits extraordinary optical properties that could be useful in quantum computing or other quantum information systems.
More broadly, the work provides a new recipe for using custom-shaped nanoparticles to engineer entirely new classes of materials with tailored properties.
"Our work is a little bit like kids playing with LEGO blocks," said Ou Chen, an associate professor of chemistry at Brown and a corresponding author of the research. "We synthesize unique nanoscale building blocks and stack them into interesting structures. In this case, we were able to stabilize these theorized transitional structures and demonstrate important quantum optical properties."
The crystal structures of many metals fall into one of two categories: face-centered cubic (FCC) or body-centered cubic (BCC). FCC is the tightest packing arrangement for spherical particles. When stacked together, spherical particles tend to arrange themselves into a repeating cubic pattern, with on particle at each corner and one particle in the center of each cube face. BCC is somewhat less tightly packed: Particles are present in each corner of a cube, with one particle at the center of the cube's body (rather than on each face). Loosely speaking, these are the arrangements that atoms form in metallic crystals.
With heating, some metals transition between the two structures. Iron, for example, goes from BCC to FCC when heated to 912 degrees Celsius. There are several pathways that have been proposed for how this transition works. One of those, the Nishiyama-Wassermann pathway, proposes a set of transition phases between FCC and BCC that are more ephemeral due to their lower symmetry. It's those fleeting "in-between" phases that this new research was able to recreate using silver nanoparticles.
"Materials scientists have cared about how to control the amount of FCC and BCC in their metals for a long time, but the transitions between these phases have been hard to study because they are so unstable," said Tim Moore, a study co-author and an assistant research scientist working in Sharon Glotzer's lab at the University of Michigan. "Being able to observe these structures is a fundamental breakthrough in materials science, and it gives us greater control over nanomaterial engineering."
To do it, Chen and his colleagues synthesized silver nanoparticles shaped like truncated octahedra, or "mecons" - a diamond shape with each of the vertices cut off to form solid with 14 sides. The shape is interesting, Chen says, because it's an intermediate between a cube and a sphere, which have distinct packing behaviors. By changing the heat involved in synthesizing the particles, Chen's team, led by Yasutaka Nagaoka, senior research scientist and lead author of the study, created a range of mecon-shapes on a continuum from more round to more cubic, coating them with long, sticky molecules that help bind the particles together. They then allowed the particles of each shape to self-assemble into nanoparticle superlattices to test how they behaved.
Using physical observations and precise computer simulations in collaboration with a team led by Glotzer's team at the University of Michigan, the researchers showed that the sticky molecules were essential for the particles that assembled themselves in configurations that matched the transient states predicted by the Nishiyama-Wassermann pathway.
"You can kind of picture them like hairy particles," said Moore. "The hairs are flexible enough that the particles have more freedom to shift, but they also fit together nicely, which allows the particles to mesh together."
Through light illumination, these silver nanoparticle superlattices show the hallmarks of deep-strong light-matter coupling, when electrons in the silver particles vibrate with light waves in perfect unison and become quantum mechanically entangled. These types of quantum optical interactions are often observed at very low temperatures, but this new structure appears to exhibit the behavior at room temperature. That, the researchers say, could provide a blueprint for making new materials for use in quantum computing or sensing.
"Anytime you're able to identify a new phase of matter, new applications are going to emerge," Chen said.
The research was supported by multiple grants from the National Science Foundation (DMR-1943930, CHE-2203700, EAR−2223273, CBET-2230729, CBET-2230891, 2243104, DMR 140129, 2138259, 2138286, 2138307, 2137603, 2138296) and the Department of Energy (DE-SC0012704, DOE-NNSA, DE-NA-0003975).
