Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I-V-VI2 Nanocrystals with Tunable Composition and Interesting Properties.

Abstract

The growth mechanism and synthetic controls for colloidal multinary metal chalcogenide nanocrystals (NCs) involving alkali metals and the pnictogen metals Sb and Bi are unknown. Sb and Bi are prone to form metallic nanocrystals that stay as impurities in the final product. Herein, we synthesize colloidal NaBi1-xSbxSe2-ySy NCs using amine-thiol-Se chemistry. We find that ternary NaBiSe2 NCs initiate with Bi0 nuclei and an amorphous intermediate nanoparticle formation that gradually transforms into NaBiSe2 upon Se addition. Furthermore, we extend our methods to substitute Sb in place of Bi and S in place of Se. Our findings show the initial quasi-cubic morphology transforms into a spherical shape upon increased Sb substitution, and the S incorporation promotes elongation along the <111> direction. We further investigate the thermoelectric transport properties of the Sb-substituted material displaying very low thermal conductivity and n-type transport behavior. Notably, the NaBi0.75Sb0.25Se2 material exhibits an ultralow thermal conductivity of 0.25 W·m-1·K-1 at 596 K with an average thermal conductivity of 0.35 W·m-1·K-1 between 358 and 596 K and a ZTmax of 0.24.