Achieving Excitation Wavelength Dependence of Cesium Cadmium Halogen Quantum Dots with Multi-Excitonic Emission Center

Excitation wavelength-dependent emission or multiexcitonic emission in metal halide perovskite crystal is observed and has demonstrated broad application in the fields of imaging and lighting. However, these two interesting luminescence phenomena in all-inorganic lead-free halide perovskite quantum dots (QDs) are largely unexplored. Here, we have successfully synthesized CsCdCl_3-xBr_x (0 ≤ x ≤ 1.5) QDs with a uniform size distribution that present excitation wavelength-dependent emission caused by surface defect states and two other different emissions including the intrinsic host self-trapped excitons and Br-induced extrinsic self-trapped excitons, respectively. Structural characterizations and the calculated distortion index confirm that Br^- ions partially occupy the sites of Cl^- ions of the [CdCl₆]^4- octahedron with both C_3v and D_3d symmetry, which induces the local lattice distortion of CsCdCl₃ QDs and promotes the formation of multiexcitonic emission. Meanwhile, the crystal structures of pure and Br-activated CsCdCl₃ QDs are demonstrated by element mapping and surface states. Combined with the theory calculations, temperature-dependent photoluminescence measurements are performed to clarify the multiexcitonic emission mechanism and further verify the broad green emission comes from [CdCl_6-nBr_n]^4- in the D_3d and C_3v symmetries. These findings put forward an effective strategy to design the novel excitation wavelength-dependent or multiexcitonic emissive perovskite and provide exciting opportunities for the application in X-ray images.