This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, physicists have sought a practical two-dimensional topological insulator: an ultrathin material whose interior blocks electricity but whose edges conduct it. In ordinary wires, electrons constantly bounce off imperfections, losing energy.
But along the edges of a 2D topological insulator, electrons with opposite spins travel in opposite directions, like cars in one-way lanes, so they rarely scatter and lose energy. Now, researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) have shown that ultrathin films of bismuth telluride are such a material. The work, published in ACS Nano, used a new approach to produce a uniform-thickness bismuth telluride film and confirm theoretical predictions about the material's properties.
"In a strict scientific sense, we are the first to demonstrate conclusive spectroscopic evidence that this is indeed a 2D topological insulator," said Shuolong Yang, assistant professor of molecular engineering at UChicago PME and senior author of the new work. "When people created these films in the past, they were all mixed up with one, two, three or four layers, and the topological electronic properties weren't resolved." Bismuth telluride has long been known as a three-dimensional topological insulator, which conducts electricity only on its surfaces. Theorists predicted that if it were cut down to just two of its quintuple layers—two nanometers thick in total—it would become a 2D topological insulator.
"When you go from 3D to 2D, suddenly all the physical principles have to be reconsidered," Yang said. In principle, such a system, sometimes called a quantum spin Hall system, could be nearly lossless. "If we use this quantum spin Hall system, ideally we can achieve conducting edge channels with quantized conductance for sending energy or information without loss," said Woojoo Lee, first author of the paper and a former postdoctoral scholar in Yang's lab, now a professor at Gachon University in South Korea.
But making 2D bismuth telluride has proven difficult. Its electronic properties change dramatically depending on its thickness, so a film must have a consistent thickness throughout to draw firm conclusions. Previous attempts produced films that piled up into tiny pyramids of varying thickness.
To get something more uniform, Lee and Yang used molecular beam epitaxy, a technique that, in a vacuum, deposits atoms one layer at a time onto a crystal surface. After growing hundreds of samples and fine-tuning the growth conditions, the researchers found a way to make the films grow in what they call "carpet mode," in which the film drapes continuously over tiny atomic steps in the underlying crystal. "It is really like a carpet lying on a staircase," Yang said.
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