Structural Diversity in Cesium Bismuth Halide Nanocrystals

Cesium bismuth halides (Cs-Bi-X) have recently been the subject of considerable attention as possible less-toxic alternatives to lead halide perovskites for luminescence and photovoltaics applications, but the full range of synthetically accessible Cs-Bi-X compositions has not been thoroughly explored, and some inconsistent results have appeared in the literature. Here, we have used a combination of hot-injection synthesis and post-synthetic anion exchange to prepare Cs-Bi-X nanocrystals with many structures and compositions, including several that have not previously been characterized. The structural and optical properties of Cs3BiX6 (X = Cl, Br, I) nanocrystals and Cs3Bi2X9 (X = Cl, Br, I) nanoplatelets are reported, and interconversion of these structures between different halide compositions is demonstrated through anion exchange using trimethylsilyl halides. Notably, we find that anion exchange can be used to access structural polymorphs not readily prepared through direct synthesis. In particular, a new structural modification of Cs3Bi2I9 has been accessed; whereas hot injection gives the previously reported “zero-dimensional” nonperovskite structure, anion exchange provides access to a “two-dimensional” layered, ordered-vacancy perovskite phase with a red-shifted absorption spectrum and distinctly different photoluminescence. Spectroscopic and computational characterization of these materials provides insight into structure/property relationships, including properties of the layered Cs3Bi2I9 material, that may be advantageous for optoelectronic applications.