Synthesis And Upconversion Luminescent Properties Of Rare-earth Doped Oxide Phosphors

Oxide upconversion materials exhibit excellent stability of physical,chemical,and thermal properties,which can well satisfy some applications in special conditions,such as in the environment with high temperature,humidity,and corrosivity.In this thesis,we selected the Gd₃Al₅O₁₂（GAG）and Re₂TeO₆（Re=Lu,Gd,La）with relatively low cut-off phonon energy as host matrices.The rare earth ions of(Yb³⁺,Ho³⁺),(Yb³⁺,Tm³⁺,Ho³⁺),and(Yb³⁺,Er³⁺)ions were respectively incorporated for realizing high-performance upconversion luminescence.The upconversion materials were fabricated by different methods.The influence of preparation process parameters on phase structure and morphology were investigated.By changing the doping concentrations of rare earth ions,the upconversion luminescence properties of materials were well regulated,and the mechanisms of upconversion energy transfer were analyzed in detail.The specific research contents include the following aspects:（1）Exploring the controllable synthesis of materials.（1）The GAG:Yb³⁺,Ho³⁺nanophosphor was prepared by sol-gel method.The effects of different drying methods,calcinating temperature,pH values of precursor solution on crystalline,morphology and crystal particle size distribution were studied.It was found that an intermediate phase of H-GAP was occurred before the formation of GAG garnet crystal phase.In addition,the incorporation of 10 at%Yb³⁺can well stabilize the crystal structure of GAG at high temperature.（2）Re₂TeO₆:Yb³⁺,Tm³⁺,Ho³⁺were prepared by solid phase reaction.The effect of sintering temperature on the phase purities of the desired products were studied,and the lowest sintering temperatures of the solid state reactions for Re₂TeO₆upconversion phosphors was determined.（3）Furthermore,the Lu₂TeO₆:Yb³⁺,Er³⁺nanocrystals were prepared by hydrothermal method.The effects of different reaction conditions and the heat treatment temperature on the desired phase were studied.Some intermediate phases were formed and finally transformed into the desired crystalline phase of Lu₂TeO₆ during heat treatment.The lowest sintering temperature was explored.（2）Carrying out XRD structural refinement of materials.The structural parameters of GAG and Lu₂TeO₆ were refined based on the Rietveld method.The high resolution XRD patterns of related materials were collected.By using the FullProf software,the structural parameters of cell parameters,point groups of rare earth lattices,and the nearest interionic distances,etc,which are closely related to the upconversion performance were analyzed.（3）Tuning upconversion properties by changing rare earth concentrations.（1）Under 980 nm NIR excitation,the yellow-green upconversion luminescence was yield by 10%Yb³⁺and 1%Ho³⁺co-doped GAG nanophosphors,and the incorporated rare earth ions played an important role in stabilizing the GAG crystalline structure at high temperature.（2）The tunable white upconversion luminescence were realized in the Re₂TeO₆ by regulating the doping concentrations of Tm³⁺.Moreover,the Yb³⁺concentration for the best performance of green upconversion in Lu₂TeO₆ was optimized.（4）Discussing the possible energy transfer mechanisms.The possible energy transfer mechanisms were discussed based on the relation between the upconversion intensity upon different pump powers.Moreover,based on the different energy transfer mechanisms of blue emission and green/red emission,the tenability of light color of upconversion luminescence for Re₂TeO₆ can be realized by changing pump power.（5）Evaluating optical temperature sensing ability of Lu₂TeO₆:Yb³⁺,Er³⁺.Based on the FIR technique,the upconversion emissions of two thermal coupled levels of ²H_11/2and ⁴S_3/2 of Er³⁺in the Lu₂TeO₆ were used for temperature sensing.The max sensitivity S=0.0103 K^-1 was obtain at 623 K.The above upconversion materials have potential applications in lighting,flat panel display,and temperature sensing.Our research on these materials also provides a reference for revealing the structure-activity relationship of rare earth oxide upconversion materials and exploring new upconversion materials and their applications.