The University of Michigan, Ann Arbor, reports that its researchers have developed a thallium-doped bismuth telluride topological insulator, a material that conducts electricity on its surface, but not in its interior. With their new approach, the researchers can tweak the properties of the bulk of their material from a so-called “p-type” semiconductor, which conducts electricity because of a deficiency of electrons, to an “n-type,” which conducts because of an excess of electrons.
“By doping with thallium, we can add electrons to the system and take it across the full spectrum of carrier densities,” says Ctirad Uher, the C. Wilbur Peters Collegiate Professor of Physics in the College of Literature, Science and the Arts. Topological insulators could enable advanced electronics that harness not only the charge of electrons, but also their spin and magnetic properties.
At thallium concentrations up to 1%, the bulk of the material was conductive. At between 1.6 and 2.4% thallium, the interior transformed into an insulator, whose unique resistivity profile seemed to indicate that the surface state dominates electrical conduction at temperatures below ‒369ºF. The surface state is detectable at higher temperatures, and Prof. Uher believes it is likely present at room temperature too. But it’s difficult to measure as the temperature rises because of interference from the vibration of the molecules in the solid.
