Researchers from Brown University and the University of Michigan have achieved something that scientists had only imagined until now. By carefully arranging tiny particles of silver into custom-built structures, they created and stabilized a previously elusive state of matter that had existed only in theoretical models.
The work, published in Science, captures an intermediate structural state that appears during a transformation between two common crystal arrangements found in metals. In addition to revealing new details about how these transformations occur, the newly created material displays unusual optical behavior that could eventually be useful for quantum computing and other quantum information technologies.
More broadly, the research demonstrates a new strategy for designing materials from the bottom up by assembling specially engineered nanoparticles into entirely new structures with customized 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.”
Capturing a Missing Step in Crystal Transformations
Many metallic materials naturally organize their atoms into one of two crystal arrangements known as face-centered cubic (FCC) and body-centered cubic (BCC).
In an FCC structure, particles are packed as tightly as possible. They occupy each corner of a cube as well as the center of every face. A BCC structure is slightly less densely packed, with particles located at the cube’s corners and a single particle at the center of the cube itself.
Some metals can switch between these arrangements when heated. Iron, for example, changes from a BCC structure to an FCC structure at 912 degrees Celsius.
Scientists have proposed several explanations for how this transformation takes place. One leading model, known as the Nishiyama-Wassermann pathway, predicts a series of short-lived intermediate structures that form during the transition. Because these intermediate phases are highly unstable, they have been extremely difficult to observe directly.
This new study succeeded in recreating and stabilizing those fleeting structural states 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.”
Building New Materials From Custom Nanoparticles
To create the new structures, the researchers synthesized silver nanoparticles shaped like truncated octahedra, which they call “mecons.” These particles resemble a diamond-like shape with their corners cut off, creating a 14-sided geometry.
According to Chen, the shape is especially useful because it falls between a sphere and a cube, two forms that naturally pack together in different ways.
The team, led by senior research scientist and study lead author Yasutaka Nagaoka, adjusted the heating conditions during synthesis to produce mecons with varying degrees of roundness and cubelike features. They then coated the particles with long molecular chains that acted like sticky connectors and allowed them to assemble into larger, ordered structures known as nanoparticle superlattices.
Combining laboratory observations with detailed computer simulations performed in collaboration with Glotzer’s group at the University of Michigan, the researchers found that these molecular coatings played a critical role in stabilizing arrangements that matched the transitional structures 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.”
Room-Temperature Quantum Optical Effects
The newly assembled silver superlattices exhibited another remarkable property when exposed to light.
The researchers observed signs of deep-strong light-matter coupling, a phenomenon in which electrons inside the silver nanoparticles oscillate in perfect synchrony with light waves and become quantum mechanically entangled.
These kinds of quantum optical effects are often associated with extremely low temperatures. However, the new material appears to display this behavior at room temperature.
The finding could provide a foundation for developing future materials used in quantum computing, sensing technologies, and other advanced quantum systems.
“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).