Shape Focusing during the Anisotropic Growth of CuS Triangular Nanoprisms

Nanocrystals composed of CuS that exhibit semimetallic behavior are capable of supporting localized surface plasmon resonances in the near-infrared wavelengths. A major challenge in utilizing these nanocrystals for plasmonic applications is the ability to accurately control their nanoscale morphology and chemical composition, both of which are known to affect plasmon wavelength and amplitude. Here, we investigate the important role of halide ions in controlling the solvothermal synthesis of colloidal CuS nanocrystals. When oleylamine is used as a stabilizing surfactant, we find that the addition of halide ions to the reaction mixture instigates nanocrystal shape focusing in a process similar to Ostwald ripening, from nanodisks to faceted triangular prisms. We demonstrate that this shape focusing is likely to occur from the competition between CuS surface binding interactions with oleylamine ligands and halide ions and is the strongest for I^- ions. We also demonstrate that this shape focusing, in combination with seed-mediated growth, can be used to generate colloidal dispersions of CuS nanoprisms with narrow size distributions.