An artist’s illustration of the strontium titanate–calcium titanate superlattice, showing the overlapping of interface regions as layers become smaller. Image: Andrew Sproles, Oak Ridge National Laboratory.Scientists and engineers collaborating across seven universities and two national laboratories have made a fundamental discovery about the atomic structure and vibrations in multilayer nanostructures, advancing the design of materials with unique infrared and thermal properties. The researchers report their discovery in a paper in Nature.
This discovery emerged from a long-standing collaboration of microscopy, spectroscopy and theory experts spanning fields from physics to engineering to materials science. Eric Hoglund, first author of the paper and a postdoctoral researcher, and James Howe, professor of materials science and engineering, both at the University of Virginia (UVA) School of Engineering and Applied Science, employed microscopy techniques to study the atomic structure and vibrations of perovskite oxide superlattices.
These superlattices were produced by repeatedly stacking precisely controlled samples of strontium titanate and calcium titanate. “You can tune desired properties such as magnetism, conductivity and heat transport or induce emergent phenomena by changing how different oxides couple to each other, how many times the oxides are layered and the thickness of each layer,” explained Hoglund.
The samples were grown by Ramesh Ramamoorthy, professor of physics and materials science and engineering, and his group at the University of California Berkeley and Lawrence Berkeley National Laboratory, and Roman Engel-Herbert, associate professor of materials science and engineering, physics and chemistry at Penn State University.
Hoglund and Jordan Hachtel, an R&D associate in the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, were the first researchers to use a new generation of scanning transmission electron microscopes to observe the atomic structure and vibrational aspects of these superlattices.
Using atomic-resolution imaging, Hoglund showed that when the interfaces of the superlattice become very close, the atomic arrangements characteristic of the layers cease to exist and the atom positions at the interfaces appear everywhere. Hachtel then used a novel technique called monochromated electron energy-loss spectroscopy, which combines high-energy resolution with high-spatial resolution for simultaneous imaging and spectroscopy, to unravel the consequence of the rearranging atoms.
“We were able to peer inside the superlattice to see vibrations in each layer and at each interface,” Hachtel said. “This allowed us to directly connect the motion of microscopic heat-carrying vibrations to the atomic structure of the superlattice. Experiments like ours enable the rational design of infrared materials with tailored photonic and phononic properties.”
Sokrates Pantelides, professor of physics and engineering at Vanderbilt University, provided the theoretical underpinnings for the detailed analysis of the experimental findings. “Theory enables diverse observations to be combined into a coherent whole. In this case, the results enable the design of nanostructures with desired thermal and infrared properties,” Pantelides said.
Postdoctoral scholar De-Liang Bao and research assistant professor Andrew O’Hara in Pantelides’ group performed extensive quantum calculations to produce the detailed analysis and identify the precise vibrations of each atom in the collective modes that Hoglund and Hachtel had observed.
The changing landscape of atomic vibrations affects infrared optical properties in the entire superlattice. This was shown in experiments performed by graduate student Joseph Matson, a member of Vanderbilt University’s Nanophotonic Materials and Devices Lab, led by Joshua Caldwell, associate professor of mechanical engineering and electrical engineering, and experiments conducted at Sandia National Laboratories by teammates in the Specere lab at Purdue University, led by Thomas Beechem, associate professor of mechanical engineering.
UVA’s ExSiTE research group, led by Patrick Hopkins, professor of nuclear engineering and professor of mechanical and aerospace engineering, provided the final proof. Senior scientist John Tomko and PhD student Sara Makarem used ultrafast spectroscopy to demonstrate that interfaces in the superlattices control non-linear optical properties and the lifetime of thermal vibrations.
“I think this will enable advanced materials discovery,” Hopkins said. “Scientists and engineers working with other classes of materials may now look for similar properties in their own studies.”
The long-standing collaboration continues. Hoglund is in his second year as a postdoctoral researcher, working with both Howe and Hopkins. Together with Pantelides, Hachtel and Ramamoorthy, he expects they will have new and exciting atomic structure-vibration ideas to share in the near future.
This story is adapted from material from the University of Virginia School of Engineering and Applied Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.