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 (approximate to 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 approximate to 437 nm (full width at half maximum, approximate to 12 nm). By combining the experimental results with first-principles calculations, an unusual electronic dual-bandgap structure, comprising the newly emerged semiconducting bandgap of approximate to 2.87 eV and original insulator bandgap of approximate to 3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.