From Nonluminescent to Blue‐Emitting Cs4PbBr6 Nanocrystals: Tailoring the Insulator Bandgap of 0D Perovskite through Sn Cation Doping

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic performance such as tunable bandgaps over the entire visible spectral region. However, compared to well‐developed 3D CsPbX₃ perovskites (X = Cl, Br, and I), the bandgap tuning in 0D Cs₄PbX₆ perovskite NCs remains an arduous task. Herein, a simple but valid strategy is proposed to tailor the insulator bandgap (≈3.96 eV) of Cs₄PbBr₆ NCs to the blue spectral region by changing the local coordination environment of isolated PbBr₆]^4? octahedra in the Cs₄PbBr₆ crystal through Sn cation doping. Benefitting from the unique Pb²⁺‐poor and Br^?‐rich reaction environment, the Sn cation is successfully introduced into the Cs₄PbBr₆ NCs, forming coexisting point defects comprising substitutional Sn_Pb and interstitial Br_i, thereby endowing these theoretically nonluminescent Cs₄PbBr₆ NCs with an ultranarrow blue emission at ≈437 nm (full width at half maximum, ≈12 nm). By combining the experimental results with first‐principles calculations, an unusual electronic dual‐bandgap structure, comprising the newly emerged semiconducting bandgap of ≈2.87 eV and original insulator bandgap of ≈3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.