All-Inorganic Luminescent Ternary Cadmium Halide Cs₇Cd₃Br₁₃ with Two Types of Cd-Centered Polyhedrons

All-inorganic metal halides comprise a class of semiconductor material with a wide range of applications. In these materials, cadmium-related materials have excellent optical properties in the field of binary quantum dots. However, ternary cadmium halides have not attracted much attention until now. Here, we successfully synthesize a ternary all-inorganic cesium cadmium bromide, Cs₇Cd₃Br₁₃, and obtained its acicular single crystals through a simple solvent evaporation method. Cadmium atoms are found to be coordinated with bromine atoms in two different polyhedrons including isolated [CdBr₄]^2- tetrahedrons and corner-shared [CdBr₆]^4- octahedral chains in the unit cells of Cs₇Cd₃Br₁₃. Optical properties are then measured and the needlelike single crystals are found to have an orange emission at 630 nm upon 365 nm UV light with a photoluminescence quantum yield (PLQY) of 9.85%. The orange luminescence with large Stokes shift (255 nm) and broad full width at half-maximum (fwhm = 147 nm) is derived from self-trapped excitons, and it is further evidenced by temperature-variable photoluminescence spectra. The PL decay spectrum shows the lifetime of Cs₇Cd₃Br₁₃ is 1.26 μs. Finally, DFT calculation reveals that Cs₇Cd₃Br₁₃ has a direct band gap with a value of 3.09 eV.