Luminescence And Temperature Sensing Properties Of Rare-earth Ion Doped Molybdate Phosphors

Recently,rare earth ion-doped upconversion luminescent materials have shown promising application prospects in the field of optical temperature sensing for their rich radiation band,long fluorescence life,no photobleaching,and anti-electromagnetic interference.However,the quantum efficiency and luminescence intensity of most rare earth doped upconversion luminescent materials are currently low,and the temperature measurement sensitivity still needs to be further improved.Meanwhile,the influence of the excitation power fluctuation on the temperature measurement results has not been studied.Based on the above consideration,this article selects scheelite-structured alkaline earth molybdate（AMo O₄,A=Ca,Sr,Ba）with excellent optical properties,stable physicochemical properties,and non-hygroscopicity as the matrix material,and mainly studies the different rare earth ions doped concentration and excitation power on the upconversion luminescence characteristics and optical temperature sensing properties of alkaline earth molybdate phosphors are as follows:（1）The optical properties and temperature sensing characteristics of Ba Mo O₄phosphors doped with RE³⁺/Yb³⁺（RE=Er,Tm,Ho）are systematically studied.Firstly,Ba Mo O₄ phosphors with different Er³⁺/Yb³⁺doping concentrations are prepared by the hydrothermal method.Under the pumping of 980nm laser,the prepared phosphors emit strong green emission（536nm and 558nm）and weak red emission（658nm）.When the fluorescence intensity ratio（FIR）of thermal coupled energy levels ²H_11/2 and ⁴S_3/2 are used to measure the temperature,the temperature calibration curve and temperature sensitivity are basically unchanged in the power range of 0～300m W,which indicats that the change of excitation power in a small range will not affect the temperature measurement results and sensing performance.Conversely,when non-thermally coupled energy levels ⁴F_9/2 and ⁴F_9/2 are used for temperature measurement,the excitation power fluctuation has a significant effect on the FIR value,temperature calibration curve and sensitivity,which will lead to the temperature measurement errors caused by the pump power fluctuation.On the other hand,Ba Mo O₄ phosphors with different Tm³⁺/Yb³⁺doping concentrations are prepared using the hydrothermal method.It is found that when the fluorescence intensity ratio of ¹G₄→³F₄（652nm）and ³F_2,3→³H₆（691nm）transition based on Tm³⁺ions is used to measure the temperature,optical temperature measurement of FIR signals in the first biological window（650nm-950nm）can be realized;Meanwhile,its FIR value and temperature calibration curve are not affected by the changes of excitation power,which is conducive to accurate temperature monitoring in practice.In addition,the best absolute sensitivity（S_a）,relative sensitivity（S_r）,and temperature resolution（δT）reach 1034×10^-4 K^-1,1.36%K^-1,and～0.36 K,which are better than most the temperature measurement performance of rare-earth ion-doped luminescent materials.It indicates that this material has great application potential in the field of optical temperature sensing.Finally,Ba Mo O₄ phosphors with different Ho³⁺/Yb³⁺concentrations are prepared by gel-combustion method.Under the excitation of 980nm,0.5%Ho³⁺/7%Yb³⁺:Ba Mo O₄ phosphor exhibits the strongest up-conversion luminescence intensity.When temperature sensing is conducted based on the fluorescence intensity ratio of ⁵F₅→⁵I₈（666nm）and ⁵F₄/⁵S₂→⁵I₈（547nm）transitions of Ho³⁺ions,its temperature calibration curve and temperature measurement sensitivity will change with the fluctuation of excitation power.This is mainly because the cooperative energy transfer process（CET）at high power is beneficial to the population of the ⁵F₄/⁵S₂ energy levels rather than the ⁵F₅ energy levels.Moreover,the intrinsic optical bistable phenomenon（IOB）has been found in Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺doped Ba Mo O₄phosphors,which indicates that besides the optical sensing,they also have potential applications in optical storage devices,optical switches and other devices.（2）A series of Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺ion-doped Sr Mo O₄phosphors are synthesized by hydrothermal method.For Er³⁺/Yb³⁺doped Sr Mo O₄samples,relatively high sensitivity and insensitive temperature monitoring are achieved based on the fluorescence intensity ratio(I₅₃₀/I₅₅₁)of ²H_11/2 and ⁴S_3/2 under the 980nm excitation.On the contrary,FIR technique for temperature measurement based on the ²H_11/2 and ⁴F_9/2 energy levels of Er³⁺ions may be affected by excitation power fluctuations,causing unnecessary temperature measurement errors and complicating the calibration process.For Tm³⁺/Yb³⁺co-doping Sr Mo O₄,under 980nm excitation,the luminescence intensity of the blue(I₄₇₇),red(I₆₅₅)and deep red(I₆₉₂)all show abnormal enhancement with the increase of temperature,and this anti-thermal quenching effect is beneficial to avoid the signal attenuation problem at high temperature.Besides,FIR(I₆₉₂/I₄₇₇)value based on Tm³⁺ion used for temperature measurement can obtain ultra-high sensitivity(S_a-max=1750×10^-4 K^-1,S_r-max=2.33%K^-1),which is higher than most reported rare earth doped up conversion temperature measurement material system,so it has promising application potential.For the Ho³⁺/Yb³⁺co-doped Sr Mo O₄ sample,the FIR data between the red(I₆₆₆)and green(I₅₄₇)emission of Ho³⁺ion remains stable to the excitation power variation,which is beneficial to reduce unnecessary measurement errors.While,the temperature sensitivity is low(S_a-max=190.1×10^-4 K^-1,S_r-max=0.20%K^-1).（3）A series of Ca Mo O₄ phosphors doped with Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺are prepared by high temperature solid phase method,and their optical properties and temperature sensing characteristics are studied.Under the 980nm excitation pumped,Er³⁺/Yb³⁺:Ca Mo O₄ phosphors emit strong green emission（537nm and 560nm）and weak red emission（665nm）.Based on the FIR of ²H_11/2 and⁴S_3/2 energy levels,the obtained temperature fitting curve will not be changed by the excitation power fluctuation.The Tm³⁺/Yb³⁺co-doped Ca Mo O₄ phosphors are studied,and it is found that it emits strong blue emission（473nm）and weak red emission（648nm and 688nm）.473nm and 648nm luminescence intensity will decrease with increasing temperature,while 688nm luminescence intensity is the opposite.When the FIR(I₆₈₈/I₄₇₃)is used for temperature measurement,the FIR data is also sensitive to the excitation power.Therefore,fluctuations of the excitation power should be avoided.At the same time,the FIR value(I₆₈₈/I₄₇₃)shows a high temperature sensitivity(S_r-max=3.76%K^-1).For Ho³⁺/Yb³⁺co-doped Ca Mo O₄ samples,weak green and strong red upconversion luminescence with wavelength of 548nm and651nm are emitted.Based on the FIR of I₆₅₁/I₅₄₈,both temperature calibration curve and temperature measurement sensitivity will be deviated due to the fluctuation of excitation power,and its maximum absolute and relative sensitivity are 206.6×10^-4 K^-1and 0.54%K^-1,respectively.