Alloy CsCdxPb1-xBr3 Perovskite Nanocrystals: The Role of Surface Passivation in Preserving Composition and Blue Emission

Various strategies have been proposed to engineer the band gap of metal halide perovskite nanocrystals (NCs) while preserving their structure and composition and thus ensuring spectral stability of the emission color. An aspect that has only been marginally investigated is how the type of surface passivation influences the structural/color stability of AMX₃ perovskite NCs composed of two different M²⁺ cations. Here, we report the synthesis of blue-emitting Cs-oleate capped CsCd_xPb_1-xBr₃ NCs, which exhibit a cubic perovskite phase containing also Cd-rich domains of Ruddlesden-Popper phases (RP- phases). The RP domains spontaneously transforms into pure orthorhombic perovskite ones upon NC ageing and the emission color of the NCs shifts from blue to green over days. On the other hand, post-synthesis ligand exchange with various Cs-carboxylate or ammonium bromide salts, right after NC synthesis, provides monocrystalline NCs with cubic phase, highlighting the metastability of the RP domains. When the NCs are treated with Cs-carboxylates (including Cs-oleate), most of the Cd²⁺ ions are expelled from the NCs, the NCs’ phase evolves from cubic to orthorhombic and their emission color changes from blue to green. Instead, when the NCs are coated with ammonium bromides, the loss of Cd²⁺ ions is suppressed and the NCs tend to retain their blue emission (both in colloidal dispersions and in electroluminescent devices), as well as their cubic phase, over time. The improved compositional and structural stability in these latter cases is ascribed to the saturation of surface vacancies, which may act as channels for the expulsion of Cd²⁺ ions from the NCs.