Optimal Design And Optical Properties Of Rare-earth Doped Alkali Metal Fluoride Fluorescent Materials

Rare earth-doped fluorides play a crucial role in the field of rare earth luminescent inorganic materials.With continuous breakthroughs in synthesis technology,Na-based nanocrystals,typified by Na YF₄,have shown extensive application prospects in microelectronics,biomedicine,fluorescence anti-counterfeiting,and various other applications.Meanwhile,Cs-based fluorides with a（perovskite-like/double perovskite）structure have become the current research focus due to their outstanding luminescent performance.The application prospects of rare earth-doped fluorides in the field of luminescent materials are particularly broad,especially in key areas such as photoluminescence and temperature sensing.Therefore,this paper focuses on the study of Na-based fluorides,aiming to significantly enhance their luminescent and temperature sensing properties by designing inert/active core-shell structures and preparing nanocrystals with special dumbbell-shaped morphologies.Additionally,this paper delves into the luminescent properties of rare earth ions in emerging Cs₃YF₆materials,exploring their superiority in the field of temperature sensing,with the goal of achieving optical thermometers with high sensitivity.The research content of this paper is as follows:（1）Design and study of Na-based nanoparticles with spherical core-shell structures.8 types of spherical core-shell structures based on Na nanocrystals were successfully synthesized using a thermal decomposition method.The goal is to explore in depth the influence of inert and active layers on upconversion luminescence and temperature sensing performance in core-shell structure design.Firstly,through upconversion spectroscopy analysis,it was verified that efficient upconversion luminescence could be achieved using Na YF₄ inert shell layers Na YF₄:Yb³⁺,Er³⁺@Na YF₄and Na YF₄@Na YF₄:Yb³⁺,Nd³⁺,Er³⁺designs under 980 nm/808 nm laser excitation,respectively.Additionally,the introduction of Na Yb F₄ active shell layers significantly improved the relative sensitivity in ratio fluorescence thermometry design,and the highest relative sensitivity in similar structures was obtained in the Na YF₄:Yb³⁺,Nd³⁺,Er³⁺@Na Yb F₄structure at 1.12%K^-1.This work demonstrates that inert layers contribute to nanocrystal upconversion luminescence,while active shell layers facilitate high-sensitivity temperature sensing in nanocrystals.（2）Study on Na-based dumbbell-shaped nanocrystals.Currently,nanocrystals are generally in the form of spheres,rods,and flakes,with defects such as low specific surface area and mutual interference among luminescent ions.Through an improved thermal decomposition method,we successfully synthesized a unique form of Na-based dumbbell-shaped nanocrystals resulting from lattice mismatch.These nanocrystals feature a dumbbell structure of Na YF₄:Yb³⁺/RE³⁺@Na YF₄:Yb³⁺/RE³⁺@Na Nd F₄:Yb³⁺(RE³⁺=Ho³⁺or Er³⁺),exhibiting significantly higher upconversion luminescence intensity compared to traditional spherical core-shell structures.Additionally,by employing phonon-assisted energy transfer,we designed an ultra-sensitive luminescent thermometer positioned in the near-infrared range,with a relative sensitivity of up to2.543%K^-1.Furthermore,this dumbbell-shaped thermometer demonstrates excellent penetration capability through biological tissues and lower cytotoxicity,offering potential applications in the field of biomedicine.It also expands the application scope of specific structural materials in the fields of luminescence and nanothermometry.（3）Exploration of photoluminescence and dual-mode temperature sensing in Cs₃Er F₆.Chapter four of this paper focuses on the investigation of the upconversion and downshifting luminescence mechanisms of Cs₃Er F₆ host material.Based on the spectral characteristics of Er³⁺ions,two cleverly designed LIR temperature sensing modes are proposed to meet different testing requirements.Firstly,by monitoring the two Stark levels formed by the splitting of the red emission from the ⁴F_9/2(Er³⁺)energy level,a"rapid mode"response to temperature is achieved.Secondly,an"ultra-sensitive mode"is constructed based on the non-thermally coupled energy levels 2H_11/2 and ⁴F_9/2of Er³⁺ions,obtaining a higher sensitivity optical thermometer compared to the"rapid mode".Additionally,to address the deliquesce of Cs₃Er F₆ to moisture-induced decomposition,the samples are encapsulated with silicone rubber,successfully alleviating the effects of moisture-induced decomposition and ensuring the stability of luminescence intensity.This study provides preliminary insights into the photoluminescence,moisture-induced decomposition,and temperature sensing properties of Cs₃Er F₆,offering clear directions for further research.（4）Study on upconversion luminescence mechanisms and temperature sensing performance of Cs₃YF₆:Yb³⁺,Er³⁺/Ho³⁺/Tm³⁺.This work is dedicated to exploring the upconversion luminescence mechanism of Cs₃YF₆:Yb³⁺,Er³⁺/Ho³⁺/Tm³⁺and developing high-sensitivity LIR thermometers using appropriate non-thermally coupled energy levels.The highest relative sensitivity of 0.96%was achieved in the Cs₃YF₆:Yb³⁺,Er³⁺thermometer,which utilizes the non-thermally coupled energy levels ²H_11/2 and ⁴F_9/2 of Er³⁺ions.Through a comprehensive analysis of the LIR fitting results and sensitivity values,the importance of relative sensitivity Sr in temperature sensing was validated.An optical thermometer was developed using the two characteristic emission peaks of Tm³⁺ions in Cs₃YF₆:Yb³⁺,Tm³⁺at 485 nm and 800nm,with an absolute sensitivity S_a as high as 55.87%K^-1.However,the achievement of this exceptionally high sensitivity is mainly attributed to the significantly higher emission peak intensity at 800 nm compared to that at 485 nm.To comprehensively evaluate the performance of the thermometer,the temperature uncertaintyδT at different temperatures was calculated.Interestingly,we found thatδT did not decrease with increasing S_a,but rather increased with decreasing S_r.This research further confirms that luminescence intensity significantly affects S_a,while S_r is hardly affected by external conditions,serving as a reliable standard for evaluating LIR-type thermometers.（5）Study on downconversion luminescence performance of Cs₃YF₆:Tb³⁺,Eu³⁺phosphors and their application in the anti-counterfeiting field.This work delves into the downconversion luminescence mechanism of Cs₃YF₆:Tb³⁺,Eu³⁺phosphors and explores their potential applications in the anti-counterfeiting field.Based on spectral analysis,it was determined that optimal emission can be achieved with single doping of 8%Eu³⁺or 8%Tb³⁺ions in Cs₃YF₆.In co-doped Cs₃YF₆:Tb³⁺,Eu³⁺samples,theoretical calculations identified that energy transfer between Tb³⁺and Eu³⁺primarily occurs through dipole-dipole interactions.Additionally,fluorescence lifetime measurements provided conclusive evidence for energy transfer between Tb³⁺and Eu³⁺,resulting in the successful preparation of orange phosphors.Furthermore,Cs₃YF₆exhibits high sensitivity to water,leading to the innovative design of an advanced anti-counterfeiting luminescence system.Selective fluorescence quenching was achieved through waterproof treatment of specific areas,greatly enriching the anti-counterfeiting and information storage capabilities of optical information.