Growth Kinetics and Temperature-Driven Phase Selectivity of Colloidal Two-Dimensional Cesium Bismuth Bromide Nanocrystals

Abstract The ability to selectively control phase formation in metal halide perovskite nanocrystals is critical for understanding and optimizing their optoelectronic properties. Two-dimensional (2D) vacancy-ordered Cs3Bi2Br9 (CBB) nanocrystals have attracted interest in this domain, however, significant discrepancies in their reported optical properties have limited understanding of their intrinsic behavior. Here, we demonstrate that these inconsistencies can originate from temperature-dependent phase behavior. Using a systematic hot-injection approach, we investigate the formation of CBB nanocrystals over injection temperatures ranging from room temperature to 240 °C. At low synthesis temperatures, the optical response is dominated by isolated [BiBr6]3– octahedra, whereas higher synthesis temperatures promote the formation of pure 2D CBB with a characteristic absorption feature near 435 nm. Structural characterization further confirms the injection temperature-dependent structural change, and the resulting phase purity strongly influences the emission properties of the colloidal nanocrystals. Additionally, a systematic growth kinetics analysis provides the first experimental determination of the growth barrier for the 2D CBB phase, yielding an activation energy of 51.7 ± 5.6 meV. Together, these results provide a comprehensive understanding of synthesis temperature-dependent phase formation in colloidal CBB nanocrystals and demonstrate how kinetic control can be leveraged to achieve selective synthesis of 2D CBB.