Bi_1-xSb_x alloy nanocrystals: Colloidal synthesis, charge transport, and thermoelectric properties

Nanostructured Bi_1-xSb_x alloys constitute a convenient system to study charge transport in a nanostructured narrow-gap semiconductor with promising thermoelectric properties. In this work, we developed the colloidal synthesis of monodisperse sub-10 nm Bi _1-xSb_x alloy nanocrystals (NCs) with controllable size and compositions. The surface chemistry of Bi_1-xSb_x NCs was tailored with inorganic ligands to improve the interparticle charge transport as well as to control the carrier concentration. Temperature-dependent (10-300 K) electrical measurements were performed on the Bi_1-xSb_x NC based pellets to investigate the effect of surface chemistry and grain size (∼10-40 nm) on their charge transport properties. The Hall effect measurements revealed that the temperature dependence of carrier mobility and concentration strongly depended on the grain size and the surface chemistry, which was different from the reported bulk behavior. At low temperatures, electron mobility in nanostructured Bi_1-xSb_x was directly proportional to the average grain size, while the concentration of free carriers was inversely proportional to the grain size. We propose a model explaining such behavior. Preliminary measurements of thermoelectric properties showed a ZT value comparable to those of bulk Bi_1-xSb_x alloys at 300 K, suggesting a potential of Bi_1-xSb_x NCs for low-temperature thermoelectric applications.