Chemical Ordering in Bi1–xSbx Nanostructures: Alloy, Janus, or Core–Shell

Abstract

Bismuth antimonide is an important material in nanoelectronics as a topological insulator and thermoelectric material. However, some of its thermal properties are still mostly unknown at the nanoscale. The goal of this paper is to fill this gap in knowledge and provide the phase diagram of bismuth antimonide at small sizes for different types of morphologies like the sphere, rod, wire, and film. By using nanothermodynamics concepts, a surface segregation index is newly defined to easily determine which chemical element segregates preferentially to the surface. In bismuth antimonide nanostructures, bismuth is predicted to be the pnictogen element segregating naturally to the surface of the alloy. Moreover, the chemical ordering (alloy, core–shell, and Janus) of bismuth antimonide is provided as a function of size, temperature, and composition. Finally, the miscible region where bismuth and antimony forms a randomly mixed alloy is found to shrink considerably when the size of the nanostructure is reduced, whatever the considered shape is. This confirms that miscibility is not enhanced at the nanoscale but phase separation is.