Grad student advancing silicon inverse opal research

Jun Ma

Jun Ma, a Ph.D. student in the Department of Mechanical Science and Engineering, was recently published in the scientific journal Nano Letters.

Nano Letters is the second most-cited journal in the fields of nanoscience and nanotechnology and is a publication of the American Chemical Society.

The paper, “Coherent Phonon-Grain Boundary Scattering in Silicon Inverse Opals,” is the result of a two-year project under the guidance of MechSE assistant professor Sanjiv Sinha and MatSE professor Paul Braun. Two other members of the Sinha research group, MechSE Ph.D. students Marc Ghossoub and Jyothi Sadhu, also contributed to important parts of the project, as did Bibek Parajuli and Agustin Mihi from Braun's research group.

The project studied the thermal properties of a material called a silicon inverse opal. An opal is a material that looks like it is made of spheres placed in rows and columns to make a three-dimensional periodic structure. To get the inverse of this structure, the space between the spheres is filled, while the spheres themselves are removed.

"The material, fabricated by our collaborators in Materials Science, is interesting for its 3D periodic structure," Sinha said, "and our measurement of heat flow in these kinds of structures provides new and fundamental insight into the physics of heat conduction at the nanoscale."

Measuring the heat flow of the material is complicated by the fact that it is a nanostructure and that it is three-dimensional. The heat flow is very different in a two-dimensional periodic artificial crystal, and that is what had been studied in the past. So not only did the experiment have to invent a way to accurately measure the heat flow but also how to explain the trends that were found.

"Jun has been able to explain this trend of thermal conductivity versus temperature in terms of a theory that goes back to 1955," Sinha said. "It has never been verified prior to this work. And it's about how phonons, which are the carriers of heat in this structure, are able to carry heat across grain boundaries coherently."

The grains in the structure are of silicon. They are crystals but are disoriented with respect to each other, with no defined pattern. How quickly heat is transferred across each boundary from one grain to another is what limits heat transport through the material. The material was found to have a very low thermal conductivity, but the surprising result was more fundamental: coherence in phonon transport.

"This is relatively new," Sinha said. "Coherence in phonon transport is unexpected and remarkable. It's not something we were looking for; it's just something that we found."

A phonon is an excitation or vibration in a crystalline solid. Coherence in this case means that these excitations are "in phase" across the grain; they are not random or uncoordinated. This phenomenon controls the thermal conductivity of the material, which is critical in photonics and thermoelectrics, and is an important discovery in the study of nanostructures like inverse opals.

"Jun is a very smart and dedicated student," Sinha said. "He basically built this measurement capability from scratch. And he did all this in the space of two years, while contributing to other projects. He’s extremely sincere in what he does; he's very quick and very motivated."