Subatomic microscopy key to building new classes of materials

Wednesday, August 31, 2016

Researchers at Penn State and the Molecular Foundry at Lawrence Berkeley National Laboratory are pushing the limits of electron microscopy into the tens of picometer scale, a fraction of the size of a hydrogen atom.

The ability to see at this subatomic level is crucial for designing new materials with unprecedented properties, such as materials that transition from metals to semiconductors or that exhibit superconductivity. The researchers' work describing the first atomic scale evidence for strain-induced ferroelectricity in a layered oxide appears online today, (Aug. 31), in Nature Communications.

 "This paper is important because it highlights our ability to design new classes of materials that can be tuned, one atomic layer at a time, to get interesting new properties such as high-frequency tunable dielectrics, which are of interest to the semiconductor industry," said first author Greg Stone, a former Penn State post-doctoral scholar now at the U.S. Army Research, Development, and Engineering Center.

Designing new materials with potentially useful properties requires the close collaboration of theory, synthesis and characterization – the first to build the mathematical models needed, the second to create the material in the lab, and the third to visualize and measure the material's properties and provide feedback to tweak theories and improve synthesis.

This study builds on previous theoretical work by coauthors Turan Birol and Craig Fennie of Cornell University and experimental work by coauthors Venkatraman Gopalan of Penn State and Darrell Schlom, formerly at Penn State and now at Cornell, and their students. Gopalan and Nasim Alem, professors of materials science and engineering at Penn State, led the current study. 

"The material we are looking at is a form of strontium titanate called a layered oxide," said Gopalan. "This study brings together electron microscopy and density functional theory on a 5 to 10 picometer length scale to show why these materials are such good tunable dielectrics. The key is phase competition, and for the first time, we show that many polar phases with similar energies compete in this material on the atomic scale, just as theory predicted, which gives it large tunability under a voltage."

Complex oxides are materials that form by combining negatively charged oxygen and two other positively charged ions. In this instance, the team examined strontium titanate with a structure called Ruddlesden-Popper (RP), after the two scientists who discovered it. The structure looks like a brick and mortar wall, with the bricks made of the strontium titanate and the thin mortar between the bricks made up of strontium oxide. When the bricks are layered in this fashion, new properties emerge that would not appear in a single brick.