The crystal structure of barium stannate (left) and an atomic-resolution scanning transmission electron microscopy image of barium stannate showing the irregular arrangement of atoms identified as the metallic line defect core. Image: Mkhoyan Group, University of Minnesota.
The crystal structure of barium stannate (left) and an atomic-resolution scanning transmission electron microscopy image of barium stannate showing the irregular arrangement of atoms identified as the metallic line defect core. Image: Mkhoyan Group, University of Minnesota.

In ground-breaking materials research, a team led by the University of Minnesota's Andre Mkhoyan has made a discovery that blends the best of two sought-after qualities for touchscreens and smart windows – transparency and conductivity.

As reported in a paper in Science Advances, the team is the first to observe metallic lines in a perovskite crystal, specifically barium stannate (BaSnO3). Perovskites have not been studied extensively for metallic properties because of the prevalence of more conductive materials like metals or semiconductors. This finding was made using advanced transmission electron microscopy (TEM), a technique that can form images with magnifications of up to 10 million.

"The conductive nature and preferential direction of these metallic line defects mean we can make a material that is transparent like glass and at the same time very nicely directionally conductive like a metal," said Mkhoyan, a TEM expert and a professor in the Department of Chemical Engineering and Materials Science. "This gives us the best of two worlds. We can make windows or new types of touch screens transparent and at the same time conductive. This is very exciting."

Defects, or imperfections, are common in crystals. One example is line defects, where a row of atoms deviate from the normal order, with dislocations being the most common type of line defect. Because dislocations have the same composition of elements as the host crystal, the electronic band structure at the dislocation core is often only slightly different to that of the host. This meant the researchers needed to look outside the dislocations to find the metallic line defect, where defect composition and the resulting atomic structure are vastly different.

"We easily spotted these line defects in the high-resolution scanning transmission electron microscopy images of these BaSnO3 thin films because of their unique atomic configuration and we only saw them in the plan view," said Hwanhui Yun, a graduate student in the Department of Chemical Engineering and Materials Science, and a lead author of the paper.

For this study, BaSnO3 films were grown by molecular beam epitaxy (MBE) – a technique for fabricating high-quality crystals – in a lab at the University of Minnesota Twin Cities. Because the metallic line defects observed in these BaSnO3 films propagate along the film growth direction, researchers can potentially control how and where they appear. This could allow them to engineer these defects as needed in touchscreens, smart windows and other future technologies that demand a combination of transparency and conductivity.

"We had to be creative to grow high-quality BaSnO3 thin films using MBE. It was exciting when these new line defects came into light in the microscope," said Bharat Jalan, associate professor in the Department of Chemical Engineering and Materials Science, who heads up the lab that grows a variety of perovskite oxide films by MBE.

Perovskite crystals contain three different elements in the unit cell, which allows for structural alterations in its composition and crystal symmetry, and gives it the ability to host a variety of defects. Because the atoms in a line defect core have different coordination and bonding angles, new electronic states are introduced and the electronic band structure is modified locally in such a dramatic way that it turns the line defect into a metal.

"It was fascinating how theory and experiment agreed with each other here," said Turan Birol, assistant professor in the Department of Chemical Engineering and Materials Science and an expert in density functional theory (DFT). "We could verify the experimental observations of the atomic structure and electronic properties of this line defect with first principles DFT calculations."

This story is adapted from material from the University of Minnesota, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.