Upconversion Luminescence Characteristics And Applications Of Rare-earth Doped β-Ba₂ScAlO₅

Upconversion luminescence（UCL）of rare-earth-doped materials has great potential in various applications such as color display,biological imaging,white light illumination,biological probe,medical treatment and optical thermometry.The upconversion red and near-infrared emissions of the materials always has been a research hotspot for many researchers.In general,the type and concentration of rare earth ions and host play an important role in the optical properties of upconversion luminescent materials.Therefore,it is necessary to realize various luminescence by modulating the concentration of rare earth ions and selecting suitable hosts to meet current application needs.However,the application and development of upconversion red emission of rare-earth doped materials are restricted by the poor activator,low luminous efficiency,few types of excitation light sources and insufficient research on luminescence dynamics.Therefore,it is important to find new and strongly red upconversion luminescent materials and carry out in-depth research on the related mechanism.The research in this dissertation mainly takes a novelβ-Ba₂Sc Al O₅ as the host.Single phaseβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺was successfully prepared by high temperature solid state method and tested their optical properties.The main researches focus on the effect of oxygen defect state on upconversion luminescence,enhancing upconversion luminescence by intermediate band through Ca²⁺doping and modulation upconversion red and green emission by the clipping effect of Ce³⁺ions.The single upconversion red emission ofβ-Ba₂Sc Al O₅:Yb³⁺/Er~3 is realized.And it is explored its application in gas sensing,optical thermometry and temperature sensing in vitro.The main research contents and results of this dissertation are as follows:1.β-Ba₂Sc Al O₅:Yb³⁺/Er³⁺with red upconversion emission was successfully prepared by high temperature solid state method.Using Ba CO₃、Sc₂O₃、Al（OH）₃、Yb₂O₃、Er₂O₃ as raw materials,single phaseβ-Ba₂Sc Al O₅:Yb³⁺/Er~3was prepared by sintering at1700℃in the hydrogen furnace.β-Ba₂Sc Al O₅:Yb³⁺/Er~3 material has the same crystal structure as Ba₂In Al O₅ with space group P6₃/mmc.The results of refined XRD ofβ-Ba₂Sc Al O₅:Yb³⁺/Er~3 show that the Yb and Er ions occupy the position of Sc ion in the cell.The doping amount（Yb and Er）reaches 73%,and the cell parameters increase with the increase of Yb and Er content.They emit weak green emission（525 nm and 565 nm）and strong red emission（662 nm）under 980 nm NIR laser irradiation.The optimal composition for the UCL emission intensity is 0.3 Yb³⁺and 0.03 Er³⁺.Its red-green integrated intensity ratio can reach up to 82.5.The overall emission profile of upconversion luminescence is insensitive to the dopant concentration and pump power.Based on the analyses of the luminescence spectrum,Upon absorbing one photon of 980nm to excite the sensitizer Yb³⁺from ~2F_7/2 to ~2F_5/2 level,resonance energy transfer brings the system from ~2F_5/2 level of Yb³⁺to the ~4I_11/2 level of Er³⁺.After non-radiative multi-phonon relaxation（MPR）from ~4I_11/2 to ~4I_13/2,the energy transfer will further promote Er³⁺onto the level of ~4F_9/2,which undergoes radiative transition to the ~4I_15/2 level to give red emission（662 nm）.The upward transition of Yb³⁺/Er³⁺co-dopedβ-Ba₂Sc Al O₅phosphor[Yb³⁺(~2F_5/2)+Er³⁺(~4I_13/2)-Yb³⁺(~2F_7/2)+Er³⁺(~4F_9/2)]is verified to be the major excitation mechanism for the red emission.2.After annealed in air,the upconversion fluorescence quenching phenomenon was observed in theβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺samples.Through experimental characterization and first principle calculation,it is found that the main reason for the fluorescence quenching of theβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺sample is that the interstitial oxygen defect produced by annealing in air atmosphere absorbs the energy of Yb³⁺in the excited state.The phosphors with oxygen vacancy synthesized in hydrogen atmosphere at 1700℃exhibit a strong red emission and weak green emission from Er³⁺ions.After annealing at800℃in muffle furnace,the sample hardly emits light.After annealing in air,the phase of the sample does not change,and the cell parameters increase.The results of Rietveld structure refinement,thermogravimetric analysis and electron spin resonance show that the oxygen vacancy of the sample is reduced after annealing in air.First principles calculations revealed that the O interstitial defects are energetically favored to form in theβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺sample annealed in air.A new near-infrared absorption band（0.93 e V above the Fermi level）formed near the unoccupied Yb 4f bands,resulting in the almost full quenching of the upconversion luminescence of the sample.3.Giant enhancement in upconversion luminescence ofβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺phosphor by intermediate band through Ca²⁺doping.When the Ca²⁺doping concentration is x=0.25 inβ-Ba_2-xCa_xSc Al O₅:0.3 Yb³⁺,0.03 Er³⁺,the sample can maintain single phase.A graphical refinement result of the x=0.2 sample is shown,Ca²⁺replaces the sites of the 2 a and 4 f sites of Ba²⁺and the 4 e site of Al³⁺.The partial replacement of bivalent calcium for trivalent aluminum causes the increase in the oxygen vacancy,the occurrence of valence reduction of ytterbium and more defects to maintain electroneutrality.When x=0.2,the red upconversion luminescence intensity of the sample reaches the strongest.Compared with the phosphor without Ca²⁺doping,the red emissions ofβ-Ba₂Sc Al O₅:Yb³⁺/Er~3,0.2 Ca²⁺are enhanced by a factor of 45.2.Based on down conversion luminescence spectrum and absorption spectrum and other experimental results,the physical mechanism of enhancing red upconversion luminescence is attributed to the VB-IB transition to excite abundant electrons from the host into the Er³⁺ions.4.Ce³⁺dopedβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺achieve enhancement of red emission and single-band red UCL emission.The optimal doping concentration of Ce³⁺ions is 0.008 inβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺sample,and a 4.1-fold red emission intensity enhancement was achieved than that ofβ-Ba₂Sc Al O₅:Yb³⁺/Er³⁺sample without Ce³⁺ions.The red to green integrated intensity ratio reaches up to 430.9.Ce³⁺ions dopants are beneficial for the depopulation of green emitting while the population of red emitting to improve the red to green integrated intensity ratio.5.β-Ba₂Sc Al O₅:Yb³⁺/Er³⁺sample with strong upconversion red has potential applications in optical thermometry,gas sensor and temperature sensor in vitro.In the temperature range of 298 K to 523 K,the red light intensity ofβ-Ba₂Sc Al O₅:0.3 Yb³⁺,0.03 Er³⁺has a linear relationship with temperature.At the same time,the maximum sensitivity is 2.1%K^-1 at 523 K.Acicular nanoβ-Ba₂Sc Al O₅:0.3 Yb³⁺,0.03 Er³⁺was obtained by ultrasound.The intensity of upconvertion red emission is used as the signal of the gas sensor.The sensitivity is 10.2 times higher than that of the reported upconversion gas sensor.Finally,the excellent upconversion red emission ofβ-Ba_1.8Ca_0.2Sc Al O₅:0.3 Yb³⁺,0.03 Er³⁺was used as the signal to study the performance of depth temperature sensing under fish fillet tissue.A good linear relationship is obtained,and the maximum sensitivity is 1.35%K^-1 at 358 K.At the same time,the maximum sensing depth of red upconversion emission can reach 7 mm under the irradiation of near-infrared 980 nm laser with 300 m W/cm~2 low power density.