Study On The Luminescence Mechanism And Its Regulation Of Bi-based And In-based Zero-dimensional Metal Halides

Organic-inorganic hybrid metal halide has the advantages of an adjustable band gap,high quantum yield,strong photoluminescence,and long carrier lifetime.The simple synthesis method and easy availability of these materials render them potential candidates for lighting,display devices,solar cells,and photodetectors.Among them,low-dimensional metal halides have a larger exciton binding energy and high radiative recombination rate due to the decreasing connectivity of metal polyhedrons and the gradual effect of dielectric and structural limiting effects caused by the large difference of dielectric constants between organic and inorganic parts.Usually,the excitons produced in these materials are more easily localized and hence bring about high efficiency in luminescence.The current work explores the factors that contribute to the enhancement of luminescence efficiency of lead-free metal halides,the suppression of non-radiative recombination is found to play an important role in boosting luminescence efficiency.In addition,the construction of intermolecular weak interactions which helps improve the rigidity of molecules is a reliable means to decrease non-radiative recombination.Based on these findings,several zero-dimensional lead-free organic-inorganic hybrid metal halides were synthesized,and the effect of intermolecular weak interactions on the luminescence performance was studied.Also,we investigated the luminescence mechanism.Our results can be summarized as follows:（1）Rational Design of a Super-Alkali Compound with Reversible PhotoluminescenceA novel 0D organic-inorganic hybrid bismuth halide single crystal（H₅O₂）(C₄H₁₄N₂S₂)₂Bi Cl₈:Sb³⁺was synthesized using a program-controlled temperature reduction crystallization method,which exhibited bright yellow emission under UV light.Vacuum heat treatment of the crystals resulted in the escape of the volatile substance H₅O₂⁺Cl^-from the lattice,yielding the single crystal(C₄H₁₄N₂S₂)₂Bi Cl_7:Sb³⁺,which exhibits orange-red light emission under a UV lamp.We also found that the former was able to be reobtained after the dropwise addition of trace amounts of hydrochloric acid to the latter.Through a series of optical characterizations and theoretical calculations,the underlying cause of this phenomenon is the large distortion of the original[Sb Cl₆]^3-octahedron after the removal of H₅O₂⁺Cl^-,which significantly enhances the Huang-Rhys factor and reduces the exciton binding energy at the same time.The changes of the two parameters resulted in the red-shift of emission peak and the reduction of PLQY from the original 24.18%to 2.07%.Since(C₄H₁₄N₂S₂)₂Bi Cl_7:Sb³⁺has obvious luminescence color change before and after the dropwise addition of hydrochloric acid,and the response time is fast.it can be used as the trace detection of hydrochloric acid.（2）Research on the luminescent properties of 0D indium metal halides with afterglow phenomenon0D organic-inorganic hybrid indium halide single crystals(C₁₃H₁₆N₂)In Cl₅·H₂O were prepared using a hydrothermal method.After combining the organic cation C₁₃H₁₆N₂²⁺（weak in both the fluorescence and phosphorescence）with the emission-free inorganic unit[In Cl₅OH₂]^2-,the resulting single crystals exhibit a blue-white fluorescence centered at 415 nm with a PLQY of 21%.A green afterglow with a duration of about 2 s was also observed after turning off the UV lamp.Relevant characterization and theoretical calculations confirm that the periodically arranged C₁₃H₁₆N₂²⁺possesses a strongπ-πstacking,and the introduction of the inorganic unit[In Cl₅OH₂]^2-promotes the construction of a huge hydrogen bonding network inside the crystal,thus the rigidity of the molecule is enhanced,a phenomenon that would effectively suppress the nonradiative recombination,thus enhancing the PL and phosphorescence intensity.Our work provides a new strategy for improving material optics through weak intramolecular interactions.