Internal Heterostructure of Anion-Exchanged Cesium Lead Halide Nanocubes

All-inorganic cesium lead halide (CsPbX₃; X = Cl, Br, and I) perovskite nanocubes (NCs) exhibit fascinating optical and optoelectronic properties. Postsynthesis anion exchange by mixing NCs with reactive anion species has emerged as a unique strategy to control their composition and band gap. For example, we started with CsPbBr₃ NCs with intense green emission, and then anion exchange with iodide ions yields CsPb(Br/I)₃ mixed halides and CsPbI₃ with emission color systematically varying in the green-red region. However, the internal structure of the anion-exchanged perovskite NCs is not probed. It is believed that the NCs possess a homogeneous alloyed composition, but X-ray diffraction pattern could not give evidence for such alloy formation, because the crystal structure also varies with anion composition. Here, we elucidate the internal heterostructure of anion-exchanged NCs using variable energy hard X-ray photoelectron spectroscopy. The results show that, in contrast to a homogeneous alloy, there is a significant inhomogeneity in the composition across the radius of NCs. The surface of CsPb(Br/I)₃ NCs is rich with exchanged iodide ions, whereas the core is rich with native bromide ions. Even CsPbI₃ NCs obtained after assumed complete anion exchange show a small amount of bromide ions in the core. This finding of gradient internal heterostructure inside the anion-exchanged NCs will be important for future understanding of electronic properties and stability-related issues of CsPbX₃ NCs.