Optical Properties Of Te⁴⁺/Er³⁺doped Cs₂SnCl₆ Low-dimensional Double Perovskite

Lead based perovskites have attracted wide attention in display,lighting,photovoltaic,detection,scintillation and other fields due to their excellent optoelectronic properties,such as high photoluminescence quantum yield,adjustable luminescence spectrum and high defect tolerance.However,the toxicity of lead and the poor stability of this kind of material greatly hinder its practical applications.Therefore,as an environmentally-benign and highly stable alternative of lead halide perovskites,Cs₂SnCl₆ double perovskite is deeply studied because of the similar properties between B-site elements Snand Pb.However,the explored luminescent behavior of Cs₂SnCl₆double perovskite is still limited to the visible region under UV excitation.Studies on the near infrared（NIR）luminescence of Cs₂SnCl₆ and its optical properties under X-ray excitation have not been carried out yet.In addition,the hydrothermal method is the main preparation method of Cs₂SnCl₆ micro crystals,which not only has a long experimental period but also is not conducive to low-cost batch preparation.In this thesis,a facile and low-cost route for synthesizing Cs₂SnCl₆ double perovskites were developed based on a coprecipitation method.Yellow light emission was realized in the obtained Cs₂SnCl₆ microcrystals via Te ion doping,which were then loaded into PDMS to produce a large-area flexible screen.The applications in X-ray imaging were explored.Furthermore,a high-efficiency NIR luminescence was achieved after the co-doping of Te ion and rare earth Er ion in Cs₂SnCl₆.The obtained results are as follows:1.Te⁴⁺doped Cs₂SnCl₆ double perovskites with a relative uniform size distribution of around 3μm were prepared through a coprecipitation method.Under 391 nm excitation,the doping of Te⁴⁺in Cs₂SnCl₆ displayed an intense yellow emission at～577nm and a Stokes shift larger than 100 nm,in sharp contrast to the no emission for the pristine Cs₂SnCl₆ microcrystals.Temperature-dependent PL spectra results demonstrated the existence of strong electron-phonon coupling in Cs₂SnCl₆ with soft lattice,supporting the easy occurrence of self-trapped excitons（STEs）upon photogeneration.Furthermore,the still strong yellow emission（96%of initial level）and unchanged structure of the Cs₂SnCl₆ microcrystals were discovered after being immersed in deionized water for 2 hours,illustrating the excellent stability against water.2.Benefitting from its special STEs luminescence mechanism,the scintillation properties of Cs₂SnCl₆:Te4+were investigated under X-ray excitation.The results show that the Cs₂SnCl₆ double perovskite exhibits excellent X-ray radiation luminescence properties due to the fact that its special zero-dimensional（0D）structure facilitates the formation of spatially separated STEs.The prepared Cs₂SnCl₆:1.4%Te⁴⁺microcrystals achieved a light yield of 9336 photons/Me V and a low detection limit of132 n Gyair/s under X-ray irradiation.Furthermore,Cs₂SnCl₆:1.4%Te⁴⁺microcrystals were mixed with PDMS to produce a flexible scintillation screen with a large area of10 cm×10 cm.High-quality imaging of large objects can be achieved on the scintillation screen with a spatial resolution of 6 lp/mm estimated via a simple camera（Sonyα7）.3.In Te⁴⁺/Er³⁺co-doped Cs₂SnCl₆ microcrystals,excitation energy was transferred from STEs to the well-matched ²H_11/2 energy level of Er³⁺ions,resulting in the enhanced 1540 nm NIR emissions.The varying NIR luminescence intensity of Er³⁺with the doping concentration of Te⁴⁺and Er³⁺reveals the sensitization of Te⁴⁺on the luminescence of Er³⁺.The highest NIR emission intensity in Cs₂SnCl₆:Te⁴⁺/Er³⁺was achieved at the co-doping level of 10%Er³⁺and 1.4%Te⁴⁺.In addition,the Te⁴⁺/Er³⁺co-doped Cs₂SnCl₆ showed superior stability of NIR luminescence in both air and water.