Silver chlorobromide nanocubes with significantly improved uniformity: synthesis and assembly into photonic crystals

Abstract

Silver chlorobromide (AgCl_xBr_1−x, 0 < x < 1) nanocubes with a highly uniform size, morphology, and crystallinity have been successfully synthesized through a co-precipitation of Ag⁺ ions with both Cl⁻ and Br⁻ ions in ethylene glycol containing polyvinyl pyrrolidone at mild temperatures. Compositions of the synthesized nanocubes can be easily tuned by controlling the molar ratio of Cl⁻ to Br⁻ ions in the reaction solutions. The size of the nanocubes is determined by varying a number of parameters including the molar ratio of Cl⁻ to Br⁻ ions, injection rate of Ag⁺ ions, and reaction temperature. The real-time formation of colloidal AgCl_xBr_1−x nanocubes has been monitored, for the first time, by in situ high-energy synchrotron X-ray diffraction. The time-resolved results reveal that a fast injection rate of Ag⁺ ions is critical for the formation of AgCl_xBr_1−x nanocubes with a highly pure face-centered cubic crystalline phase. The improved uniformity of the AgCl_xBr_1−x nanocubes is beneficial for assembling them into order superlattices (e.g., photonic crystals) even by simply applying centrifugation forces. The stop band of the resulting photonic crystals can be easily tuned from the ultraviolet to the infrared region by using AgCl_xBr_1−x nanocubes with different sizes. The variation of the dielectric constant of AgCl_xBr_1−x associated with the change of the relative concentration of halide ions provides an additional knob to tune the optical properties of photonic crystals.