Phase-Selective Solution Synthesis of Perovskite-Related Cesium Cadmium Chloride Nanoparticles

All-inorganic metal halide perovskite-related phases are semiconducting materials that are of significant interest for a wide range of applications. Nanoparticles of these materials are particularly useful because they permit solution processing while offering unique and tunable properties. Of the many metal halide systems that have been studied extensively, cesium cadmium chlorides remain underexplored, and synthetic routes to access them as nanoscale materials have not been established. Here we demonstrate that a simple solution-phase reaction involving the injection of a cesium oleate solution into a cadmium chloride solution produces three distinct cesium cadmium chlorides: hexagonal CsCdCl₃ and the Ruddlesden-Popper layered perovskites Cs₂CdCl₄ and Cs₃Cd₂Cl₇. The phase-selective synthesis emerges from differences in reagent concentrations, temperature, and injection rates. A key variable is the rate at which the cesium oleate solution is injected into the cadmium chloride solution, which is believed to influence the local Cs:Cd concentration during precipitation, leading to control over the phase that forms. Band structure calculations indicate that hexagonal CsCdCl₃ is a direct band gap semiconductor while Cs₂CdCl₄ and Cs₃Cd₂Cl₇ have indirect band gaps. The experimentally determined band gap values for CsCdCl₃, Cs₂CdCl₄, and Cs₃Cd₂Cl₇ are 5.13, 4.91, and 4.70 eV, respectively, which places them in a rare category of ultrawide-band-gap semiconductors.