Researchers at Brown University and the University of Michigan have achieved something that scientists could only imagine before. By carefully arranging tiny particles of silver into a custom structure, they created and stabilized a previously elusive state of matter that existed only in theoretical models.
The works published in sciencecaptures intermediate structural states that appear during transformations between two common crystalline arrangements found in metals. In addition to revealing new details about how these transformations occur, the newly created materials exhibit unusual optical behavior that could ultimately be useful for quantum computing and other quantum information technologies.
More broadly, this study demonstrates a new strategy for designing materials from the bottom up by assembling specially engineered nanoparticles into entirely new structures with customized properties.
“Our job is a bit like children playing with Lego blocks,” says Oh Chen, associate professor of chemistry at Brown University and corresponding author of the study. “We synthesize unique nanoscale building blocks and stack them together to create interesting structures. In this case, we were able to stabilize these theorized transition structures and demonstrate important quantum optical properties.”
Capture missing steps in crystal conversion
Many metallic materials naturally organize their atoms into one of two crystalline arrangements known as face-centered cubic (FCC) and body-centered cubic (BCC).
In an FCC structure, the particles are packed as tightly as possible. They occupy each corner of the cube and the center of all faces. The BCC structure has a slightly lower density, with particles located at the corners of the cube and a single particle in the center of the cube itself.
Some metals can switch between these configurations when heated. For example, iron changes from a BCC structure to an FCC structure at 912 degrees Celsius.
Scientists have proposed several explanations for how this change occurs. One leading model, known as the Nishiyama-Wasserman pathway, predicts a series of short-lived intermediate structures that form during the transition. These mesophases are very unstable, so it has been very difficult to observe them directly.
This new research successfully used silver nanoparticles to reproduce and stabilize their temporary structural state.
“Materials scientists have long looked at ways to control the amount of FCC and BCC in metals, but the transition between these phases has been difficult to study because they are so unstable,” said study co-author Tim Moore, a research assistant in Sharon Glotzer’s lab at the University of Michigan. “Being able to observe these structures is a fundamental advance in materials science and allows for greater control over nanomaterials engineering.”
Build new materials from custom nanoparticles
To create the new structure, the researchers synthesized silver nanoparticles shaped like truncated octahedrons, called ‘mekons.’ These particles resemble a diamond-like shape with truncated corners, creating a 14-sided geometry.
According to Chen, this shape is particularly useful because it falls between a sphere and a cube, two shapes that naturally fit together in different ways.
The team, led by senior researcher and study lead author Yasutaka Nagaoka, adjusted the heating conditions during synthesis to produce mekon with varying degrees of roundness and cubic-like features. They then coated the particles with long molecular chains that act like sticky connectors, allowing them to assemble into larger, ordered structures known as nanoparticle superlattices.
Combining laboratory observations with detailed computer simulations conducted in collaboration with Glotzer’s group at the University of Michigan, the researchers found that these molecular coatings play a key role in stabilizing sequences consistent with the transition structure predicted by the Nishiyama-Wasserman pathway.
“You can imagine them like hairy particles,” Moore says. “The hair is flexible enough that the particles can move more freely, but at the same time it fits well and allows the particles to interlock.”
Quantum optical effects at room temperature
The newly assembled silver superlattice exhibited another remarkable property when exposed to light.
The researchers observed signs of deep optical-matter coupling, a phenomenon in which electrons within silver nanoparticles vibrate in perfect synchrony with light waves and become quantum mechanically entangled.
This type of quantum optical effect is often associated with extremely low temperatures. However, new materials appear to exhibit this behavior even at room temperature.
The discovery could provide a foundation for developing future materials used in quantum computing, sensing technologies, and other advanced quantum systems.
“If we can identify new levels of matter, new applications will emerge,” Chen said.
This 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 Department of Energy (DE-SC0012704, DOE-NNSA, DE-NA-0003975).
