Optical Properties Of Alkali Metal And Rare Earth Ions Doped Double Perovskite Crystals

In recent years,all-inorganic perovskite materials have been widely studied due to their excellent photoelectric properties,but it should not be neglected that perovskite has lead toxicity and poor environmental stability.Lead-free double perovskite is a new material which replaces two bivalent lead ions with one univalent ion and one trivalent ion heterovalently.It avoids lead toxicity effectively and has high environmental stability.The original double perovskite has optical forbidden transition,and the absorption and emission are very weak.It needs to be doped with other ions such as transition metal ions and rare earth ions to break the symmetry and improve its optical properties.In addition,most of the current research on double perovskite focuses on nanocrystals and microcrystalline powders,and there are few studies on single crystals with high transparency.In this thesis,two common double perovskite single crystals,Cs₂Ag In Cl₆ and Cs₂NaInCl₆ were synthesized by hydrothermal method,and the effects of doping on the optical properties of double perovskite and the internal energy transfer process were investigated by doping alkali metal ions and rare earth ions into them.In the first chapter,the research progress and existing problems of lead-free double perovskite were introduced.Lead-free double perovskite was a three-dimensional perovskite structure with abundant doping sites.Synthesis of double perovskite crystals with sizes ranging from nanometer to centimeter has been reported by different synthesis methods.Long afterglow luminescent materials have been reported,but the synthesis of traditional long afterglow materials has high energy consumption,and the synthesized long afterglow powders scatter seriously,which reduces the utilization rate of light.High transparency perovskite afterglow crystals are expected to achieve more practical applications.In addition,lanthanide ions,as a commonly used dopant,can introduce a new upconversion luminescence mode into double perovskite matrix,which gives double perovskite dual-mode emission performance and improves the optical properties of double perovskite.In the second chapter,a single crystal of Cs₂Ag In Cl₆ co-doped with Li⁺/K⁺and Mn²⁺was grown by a hydrothermal method.The unirradiated crystal was colorless and transparent,and its color changed to purple after ultraviolet lamp irradiation.Under the excitation of 365 nm UV lamp,the Li⁺or K⁺doped single crystals showed red emission,the steady-state spectra showed that the emission peak position was not affected by changing the type and doping amount of alkali metal ions,indicating that doping does not affect the emission of luminescent center.By measuring the temperature dependent spectra and temperature dependent lifetime of single crystal,the internal exciton dynamics were analyzed,and the energy transfer process between the self-trapped exciton and Mn ion was revealed.After turning off the UV lamp,the crystals showed a bright red afterglow,the spectral shape of the afterglow curve was adjusted by changing the doping amount of alkali metal ions,and the optimal afterglow efficiency was calculated.The synthesized Cs₂（Li/K）_xAg_1-xIn Cl₆:Mn²⁺single crystals was different from the doped alkali metal ion Na⁺in that it introduced abundant low temperature traps,and the trap depth can be tuned by changing the doping amount of Li⁺/K⁺.Finally,combined with the thermoluminescence test results,the possible afterglow mechanism was proposed.In the third chapter,Cs₂NaInCl₆ double perovskite was selected as the substrate,and Yb ions and Er ions were added to obtain a single crystal with the size of millimeter,which was colorless and transparent under daylight.Under UV excitation,Cs₂NaInCl₆ matrix showed a weak blue emission.After the addition of lanthanide ions,the green emission band of Er³⁺was introduced,and the luminescence intensity was enhanced.With the increased of Er³⁺doping concentration,the luminous color of single crystals changed from blue to green,orange and red,achieving multicolor tuning.The energy transfer from self-trapped exciton to Er³⁺was proved by time-resolved fluorescence spectroscopy.Under the excitation of 980 nm laser,the synthesized single crystals showed strong green emission and the intensity of red emission band was very weak.The power-dependent upconversion spectra of single crystals were tested,and the mechanism of pure green emission was further explored and explained.The two green thermal coupled energy levels of Cs₂NaInCl₆:Er³⁺,Yb³⁺single crystals were used for optical temperature measurement,showing excellent sensitivity in the low temperature region,which provided the possibility for the design of fluorescence temperature sensor.Finally,the fourth chapter made a comprehensive summary of the work done in this thesis.Combined with the current research status of doped double perovskite,the future research direction was prospected.