Highly-Sensitive Temperature Sensing Investigation Based On Rare Earth Doped Materials

The research of thermometry based on the optical properties of the doped rare-earth ions have reaches its maturity.Many temperature sensing methods and materials have emerged in recent years.The rare-earth doped temperature sensor belongs to semi-contact temperature sensor,which is more suitable to apply to environments such as living organism tissue,moving liquid and sealed chambers,comparing with traditional contact thermometers.The reported methods for temperature sensing include fluorescence intensity ratio(FIR)method,decay lifetime method,rising edge method and absolute intensity method.Almost all the rare earth ions could be applied to at least one of these method.Each rare earth ions and each method have their own suitable operating environment.In order to expand rare-earth-ions doped optical thermometer research,two aspect should be focused on,namely finding new thermosensing materials and improving original materials and methods.Among many temperature sensing methods,the most frequently studied method,and also universal for rare earth ions,is fluorescence intensity ratio(FIR)methods.The main idea of FIR methods is that the intensity ratio of two fluorescence band reduced the influence of the environment that affects both bands thus the results are more independent on the environmental influence.Under ideal conditions,the FIR values should be only related to the temperature.Emission bands that originating from thermally coupled energy levels(TCELs)of rare earth ions is commonly used for FIR methods.The rare earth ions provides adequate TCELs due to its ladder like energy levels.According to the nature of the TCELs,the FIR value of the TCELs are only related to the environmental temperature.Furthermore,this type of FIR method is very convenient and applicable for all rare earth ions.Because of its less dependency on detecting environment,developed theory and good sensitivity,FIR methods using TCELs are widely studied.Fluorescence decay lifetime methods is another frequently studied method for thermometry.The detection range varies due to the influence of the host lattice.The decay lifetime usually changes very fast with temperature change.In this paper we mainly focus on study of high sensitivity thermometry using FIR and decay lifetime methods.In chapter one,we firstly introduced the basic features of rare earth ions,mechanism of its luminescence and phenomenon originated from the special structure of the rare earth ions energy level,such as energy transfer,upconversion and charge transfer,etc.Secondly,we introduced the principle and application prospect of rare earth ions doped non-contact optical thermometer.The theory and application of FIR and decay lifetime methods were emphasized in this chapter.The chapter two show the study of rare earth ions doped nanoparticles and its performance in temperature sensing.The BaYFs:Yb3+,Er3+ and BaYF5:Dy3+nanoparticles were successfully synthesized through solvothermal method.The crystal structure and morphology of as-prepared samples were characterized using X-Ray Diffraction(XRD)patter and Transmission Electron Microscope(TEM).Under 980 nm laser excitation,the emission spectra of the BaYFs:Yb3+,Er3+sample was obtained.The temperature dependent FIR study of TCELs(4G11/2 and 2H9/2)was carried on and compared with the commonly used TCELs of the Er3+ ion(2H11/2 and 4S3/2).The abnormal behavior of the result at the low temperature range was explained by the analysis of the two emission's decay lif.etime.Under 355 nm laser excitation,the emission spectra and temperature dependent spectra of the TCELs(4I15/2 and 4F9/2)were obtained.By analyzing two sets of temperature dependent FIR,the reason of the energy gap difference and their sensitivity were achieved.Chapter 3 is the emphasis of this paper.The optical properties of SrB407:Sm2+sample were fully investigated and discussed.The SrB4O7:5%Sm2+ sample was synthesized by high temperature solid state method under reducing atmosphere.The crystal structure was characterized by XRD pattern.The sample's temperature dependent spectra,FIR,decay lifetime and fluorescence color were fully studied under excitation of 355 nm laser.Sm2+ have the same configuration as Eu3+ ion.But its 4f5d configuration is lower than Eu3+.This feature enables the interaction between 5DJ(J=0,1,2)energy levels and 4f5d energy levels.At relatively high temperature,these energy levels could reach thermal equilibrium,although the energy difference between 5D0 and 4f5d energy levels are much larger than 2000 cm-].For the TCELs,the bigger energy difference means larger sensitivity.Therefore,using 5D0 and 4f5d energy levels for FIR method have larger sensitivity than traditional materials.At 500 K,the sensitivity is 2.16%K-1.The transition originating from 4f5d energy levels are allowed transition,which means it's much faster than the 4f-4f transition.When 5D0 and 4f5d energy levels reaches thermal equilibrium,the decay lifetime of TCELs are the same and decreases dramatically as temperature elevating.Due to the difference of mechanism,the sensitivity of the decay lifetime method doesn't decrease.After 500 K,the sensitivity of decay lifetime method remains around 3.36%K-1.The emission spectra of the sample span a very wide range,thus the luminescence color changes with temperature.We build a simple temperature field based on this feature.By coating the sample on the surface of the device,the temperature distribution could be demonstrated by fluorescence color and could be observed by naked eyes.The spectra and decay lifetime were also measured to evaluate the temperature of the measured spots to examine its performance in application.The experimental errors were also discussed.The high sensitivity of the sample in both methods indicates that SrB4O7:5%Sm2+ have a great potential in thermometry application.The chapter 4 focuses on the upconversion spectra of the KYb2F7:Er-3+and KYb3F10Er3+ nanoparticles.The samples were synthesized using solvothermal methods.The crystal structure and morphology of the sample were characterized by XRD pattern and TEM image.The emission spectra of the samples were obtained under 980 nm excitation.By analyzing the distance of the Er3+ and Yb3+ in both samples,the reason of the difference in emission spectra were explained.The monochromaticity of two samples were useful in white LED and bio-imaging application.