'Carpet' Growth Method Makes Millimeter-Wide Film of Long-Sought 2D Topological Insulator
University of Chicago researchers grew bismuth telluride just two layers thick, thousands of times wider than earlier patches, though electrical tests remain to be done.
Physicists have spent decades looking for a practical two-dimensional topological insulator, an ultrathin material whose interior blocks electricity while its edges conduct it. A team at the University of Chicago reports it has grown one in films a millimeter wide, thousands of times larger than earlier patches.
In ordinary wires, electrons constantly bounce off imperfections and lose energy. 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. The effect is also called the quantum spin Hall state, and it could lead to electronics that waste less energy.
The team worked with bismuth telluride, known as a 3D topological insulator that conducts only on its surfaces. Theorists predicted that cutting it to two quintuple layers, about 2 nanometers in total, would turn it into a 2D topological insulator. Earlier attempts produced films that piled up into pyramids of varying thickness, a problem because the electronic properties depend strongly on thickness.
Shuolong Yang, an assistant professor at the UChicago Pritzker School of Molecular Engineering, led the work. First author Woojoo Lee, a former postdoctoral scholar in Yang's lab who is now a professor at Gachon University in South Korea, grew hundreds of samples while tuning conditions. They used molecular beam epitaxy, which deposits atoms one layer at a time in a vacuum, and found a regime they call "carpet mode," in which the film drapes continuously over atomic steps in the crystal. The films could also be transferred onto other surfaces.
Three kinds of measurements pointed to a topological state. Angle-resolved photoemission spectroscopy mapped the band structure, which changed with each added layer in close agreement with theory. Ultrafast laser pulses set the atoms vibrating in unison, and the resulting shifts in electron energy resembled patterns expected for a topological state. Scanning tunneling spectroscopy, done in Mark Hersam's lab at Northwestern University, probed the edge states and also pointed to a 2D topological material. Pinshane Huang's group at the University of Illinois Urbana-Champaign contributed atomic-scale electron microscopy, and groups led by Chong Liu and Shirley Meng at UChicago also took part.
The team also converted the top layer to manganese bismuth telluride, which widened the band gap. They said that could potentially allow operation near room temperature, a major hurdle for the field.
The study, published in ACS Nano under the title "Millimeter-Scale, Atomically Controlled 2D Topological Insulators Revealed by Multimodal Spectroscopy," is spectroscopic. Electrical resistance measurements, which would give a final confirmation of edge conduction, have not yet been done. Yang described that as "clearly the next goal."
