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 NaBi_1-xSb_xSe_2-yS_y 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 NaBi_0.75Sb_0.25Se₂ 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 ZT_max of 0.24.