Researchers at the University of Chicago Pritzker School of Molecular Engineering have created a bismuth telluride film just two nanometers thick, definitively demonstrating its properties as a two-dimensional topological insulator. This achievement resolves inconsistencies from previous attempts that yielded films with varying layer counts and provides confirmation of the material’s unique electronic behavior.
“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,” said Shuolong Yang, assistant professor of molecular engineering at UChicago PME and senior author of the work. Theorists predict this ultrathin film, a quantum spin Hall system, could conduct electricity with nearly lossless efficiency, requiring a reconsideration of physical laws as dimensions shrink.
“Carpet Mode” Molecular Beam Epitaxy Creates Uniform 2D Bismuth Telluride
The University of Chicago team achieved millimeter-scale bismuth telluride films with a consistent two-nanometer thickness, a critical dimension for realizing a two-dimensional topological insulator; previous attempts struggled to produce films with uniform layer counts. This precise control over thickness is essential because the material’s electronic properties shift dramatically with even slight variations, hindering conclusive analysis of its topological state.
The team’s success in creating a uniform film allowed for detailed spectroscopic analysis, confirming theoretical predictions about the material’s behavior. This alignment between experimental data and prediction was so strong, Yang admitted, “When we first started seeing these results, it was actually hard to believe.” The researchers further validated their findings using ultrafast laser pulses and scanning tunneling spectroscopy, each technique reinforcing the evidence for a 2D topological state.
The fabrication process, dubbed “carpet mode” growth, relies on molecular beam epitaxy to deposit atoms onto a crystal surface in a vacuum; this technique allowed the team to overcome the challenges of creating a uniform film. Unlike earlier methods that produced films resembling tiny, uneven pyramids, the new approach enables the material to drape continuously over atomic steps in the underlying crystal. Yang described the resulting film as “really like a carpet lying on a staircase,” highlighting the uniformity achieved.
This “carpet mode” extends beyond mere visual consistency. The film’s large area, millimeters across, is thousands of times wider than those produced in prior studies, enabling more comprehensive characterization. The team’s success is not limited to fabrication. They also demonstrated the ability to transfer the film onto other surfaces, broadening its potential applications.
Further manipulation of the material’s composition, specifically converting the top layer to manganese bismuth telluride, widened the band gap, a step that could allow the material to function effectively at room temperature, a significant improvement over materials requiring cryogenic conditions. Woojoo Lee, first author of the paper and now a professor at Gachon University in South Korea, explained the potential impact: “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.”
The theoretical basis for this work lies in the prediction that reducing bismuth telluride to two quintuple layers would transform it from a 3D to a 2D topological insulator; Yang explained the fundamental shift in physics, stating, “When you go from 3D to 2D, suddenly all the physical principles have to be reconsidered.” A 2D topological insulator promises nearly lossless edge conduction, where electrons travel along the material’s edges with minimal scattering, offering potential advantages for energy transfer and information processing.
The team acknowledges that the next step is to directly measure the electrical resistance of the film, providing final confirmation of its topological properties. “So far our study is spectroscopy, so we haven’t done that electrical resistance measurement, but we think that it should be there—the physics is there,” Yang said, outlining the immediate research focus. The findings are detailed in the paper, “Millimeter-Scale, Atomically Controlled 2D Topological Insulators Revealed by Multimodal Spectroscopy,” published in ACS Nano.
In a strict scientific sense, we are the first to demonstrate conclusive spectroscopic evidence that this is indeed a 2D topological insulator.
Shuolong Yang, assistant professor of molecular engineering at UChicago PME and senior author of the new
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