Research On Ratiometric Temperature Sensing Based On Thermally Enhanced NIR Luminescence Of Rare Earth

Temperature plays a prominent role in many physical,chemical,and biological processes.Therefore,the achievement of quick and accurate temperature determination is critical for industrial production,biomedical diagnosis and scientific research.And it has always been the goal of scientific researchers in various fields.Compared with conventional thermometric methods,the rare-earth（RE）doped luminescent materials have attracted great attention in various areas owing to the rapid thermal response,high spatial resolution,excellent sensitivity,and the non-contact feature.In particular,the fluorescence intensity ratio（FIR）technology has been widely used because the interference of external factors,for example,the spectrum losses and the pumping source fluctuations,can be suppressed to a great extent.Although many successes have been achieved for such thermometry method,there are still some bottleneck problems to be solved.The strong luminescence thermal quenching of RE doped materials leads to the sharp reduction of signal-to-noise ratio,which limits their application in thermal mapping within high temperature region.And the FIR-based optical thermometry mostly utilizes the thermally coupled energy levels（TCL）of single RE ion,the further improvement of thermal sensitivity is greatly hindered as a result of the limitation of energy difference between TCL.This thesis aims to explore the highly sensitive and accurate thermometric strategy via FIR technology,and is committed to solving these key problems.The main work of this thesis is as follows:Nd³⁺singly doped GdVO₄,NaY（WO₄）₂,and CaAl₂Si₂O₈phosphors,which have different phonon energy,were prepared via the high temperature solid-state method.Under the 1064 nm laser excitation,the near infrared（NIR）emission mechanisms of Nd³⁺and the temperature dependence in three samples were studied.As the temperature rises from303 K to 873 K,the NIR luminescence from Nd³⁺exhibited significant enhancement.The thermally enhanced emission mechanism and the effect of host matrix on the NIR luminescence enhancement of Nd³⁺were analyzed.The temperature sensing ability based on the TCL(⁴F_7/2/⁴F_5/2,⁴F_5/2/⁴F_3/2,⁴F_7/2/⁴F_3/2)was further evaluated.Especially,a much high relative sensitivity of about 1.9%K^-1has been achieved by utilizing the ⁴F_7/2/⁴F_3/2,which is higher than the most reported TCL-based thermometers.Additionally,the pump power dependence of NIR emission intensities for three samples were studied at room temperature and 873 K.By calculating the FIR between ⁴F_5/2/⁴F_3/2,it is proved that the proposed thermometric strategy can effectively avoid the laser-induced heating effect,so as to ensure the accuracy of temperature detection.In order to break through the limitation of TCL,further improve the thermal sensitivity of FIR-based thermometry,and achieve the extraction of thermal information at the nanoscale such as biological tissue,Y₂O₃:Nd³⁺-Yb³⁺nanocrystals were synthesized by coprecipitation method.The temperature dependence of emissions from Nd³⁺and Yb³⁺was investigated.When excited by 980 nm laser,the luminescence of Nd³⁺and Yb³⁺exhibited opposite thermal response,subsequently the variation law of the FIR of Nd³⁺and Yb³⁺ions with temperature was analyzed.The multizone thermometry was performed owing to the different dependence of energy transfer on the temperature.The maximum relative sensitivities achieved in the temperature ranges of 303-333 K and 423-773 K were both approximately 2.3%K^-1.In addition,the impact of laser-induced heating on measurement sensitivity was studied via modulating the excitation mode（square and continuous wavelength）.Under the excitation of 980 nm square wave laser,the relative sensitivity was enhanced from 2.3%K^-1to 2.9%K^-1,together with a measurement uncertainty of about 0.17 K.In order to discover the influence mechanism of crystal field environment on the thermometry based on the FIR of NIR emissions from Nd³⁺and Yb³⁺,and optimize the temperature sensing capability,Ca Al₂O₄:Nd³⁺-Yb³⁺phosphors and a series of Nd³⁺-Yb³⁺co-doped phosphors containing different Si O₂concentrations were prepared via high temperature solid-state reaction method.The influence of Si O₂on the host structure and the luminescence behaviors of Nd³⁺and Yb³⁺was systematically investigated.The results demonstrated that the luminescence and the subsequent thermal sensing properties of Nd³⁺-Yb³⁺ion pair were strongly dependent on the host.As the host structure varied from aluminate to aluminosilicate,the spontaneous radiative transition and the energy transfer for RE ions were clearly improved,whereas the multiphonon nonradiative relaxations were weakened.The Nd³⁺-Yb³⁺codoped aluminosilicate phosphors synthesised by adding100 mol%Si O₂exhibited much high sensitivity(0.7%-4.6%K^-1)at temperatures ranging from 303 to 773 K,accompanied with a measurement uncertainty of about 0.11 K.Combined with the thermally enhanced NIR luminescence of Nd³⁺from Nd³⁺-Yb³⁺codoped system,the NIR and red emissions from Nd³⁺and Er³⁺ions were studied for FIR thermometry through modulating the luminescence behaviors of RE ions with designing nano core-shell structure.A serious of nanoparticles with different structures,which were activated by Nd³⁺-Yb³⁺and Er³⁺-Yb³⁺organizations,were synthesized by solvothermal method.The luminescence behaviors of Nd³⁺and Er³⁺in present samples have been systematically investigated upon 980 nm laser irradiation.The contrary thermal response for emissions from Nd³⁺and Er³⁺was achieved in Na YF₄:Nd³⁺-Yb³⁺/Na YF₄/Na YF₄:Er³⁺-Yb³⁺core-shell-shell structure nanoparticles,and the mechanisms were theoretically and experimentally investigated.The dependence of the FIR on temperature was further analyzed in the region of 303-423 K,obtaining the optimal relative sensitivity up to 3.8%K^-1at 303 K.Moreover,by enhancing the thermal quenching effect of red luminescence,a much higher thermal sensitivity up to 4.3%K^-1was achieved,accompanied with a low measurement uncertainty of about 0.12 K.