Preparation And Properties Of Rare-earth Doped Up/Down Conversion Thermometric Materials

Compared to traditional contact temperature measurement method,optical thermometer has been applied in industrial production,bio-medicine and scientific investigation due to the advantages of non-contact,fast response and high sensitivity.Among these optical thermometers,the fluorescence intensity ratio?FIR?technology in rare earth doped materials can provide more accurate temperature detecting without the effect of spectrum losses and fluctuations,which has become a research hotspot in the field of accurate measurement of temperature.In addition,due to their good chemical stability,long lifetime,wide range of emission wavelength and low cell toxicity,rare earth doped luminescent materials are widely applied in illumination,display,bio-imaging,information security and fingerprints identification.In this work,the oxides are selected as matrix,and Er³⁺,Tm³⁺,Yb³⁺,Tb³⁺,Eu³⁺and Bi³⁺as doped ions,a series of upconversion?UC?or down conversion?DC?temperature measuring materials based on thermally coupled levels?TCLs?or non-thermally coupled?Non-TCLs?of FIR method are prepared by wet chemical method,and their fluorescent properties,temperature sensing mechanism and other applications are also investigated in detail.The main contents are concluded as follows:?1?In order to investigate the luminescent mechanism and temperature sensing performance of UC materials,the BaGd₂ZnO₅:Tm³⁺,Yb³⁺,Bi_2.84W_0.16O_6.24:Tm³⁺,Yb³⁺and La₂O₃:Er³⁺,Yb³⁺UC fluorescent materials with low phonon energy,small particle size and strong fluorescent intensity are synthesized by sol-gel,co-precipitation and solvothermal methods,respectively.Under 980 nm excitation,it is clear that the characteristic emissions of Tm³⁺include blue(⁴78 nm:¹G_4?2??³H₆;⁴85 nm:¹G_4?1??³H₆),red?⁶55 nm,¹G₄?³F₄?and near-infrared?⁸00 nm:³H₄?³H₆?and UC emission intensity is significantly enhanced by introducing Li⁺or Mg²⁺due to the change of local field symmetry around the rare-earth ions in the Tm³⁺and Yb³⁺co-doped systems.Er³⁺and Yb³⁺doped La₂O₃ phosphors exhibit green(²H_11/2/⁴S_3/2?⁴I_15/2)and red emission(⁴F_9/2?⁴I_15/2).Moreover,corresponding UC luminescence mechanism and the performance of temperature sensing based on TCLs(¹G_4?2?/¹G_4?1?,²H⁴_11/2/S_3/2/2 and ³H_4?1?/³H_4?2?)are also researched.Results show that UC materials have great advantages in temperature measuring.?2?In order to assess the thermometric ability of DC materials,the Bi³⁺/Tb³⁺/Eu³⁺co-doped garnet type Ca₃Sc₂Si₃O₁₂2 phosphors are prepared by sol-gel method.Under370 nm excitation,the samples show blue,green and red emission by energy transfer from Bi³⁺to Tb³⁺and finally to Eu³⁺,simultaneously.Non-TCLs are constructed for the first time based on ³P₁(Bi³⁺)/⁵D₀(Eu³⁺)and ⁵D₄(Tb³⁺)/⁵D₀(Eu³⁺)by measuring fluorescent intensities at different temperatures.Thus,a novel self-calibrated non-invasion optical thermometer with high absolute/relative temperature sensitivities and good signal discrimination can be achieved by taking advantage of FIR of such Non-TCLs.This work exhibits a novel strategy to design high-performance optical thermometer.?3?For gaining the double mode thermosensing materials,the Yb³⁺/Tb³⁺/Eu³⁺triple-doped BaGd₂ZnO₅ DC materials are synthesized by sol-gel method.Under 980nm excitation,the samples achieve green and red UC emission through the process of Yb³⁺?Tb³⁺?Eu³⁺energy transfer.Under the excitation of ultraviolet?UV?light,the energy transfer process of Gd³⁺?Tb³?Eu³⁺lead to the characteristic DC emission of Tb³⁺and Eu³⁺.The UC/DC emission spectrum of the samples in the range of303K-573K is tested.The FIR method based on Non-TCLs(Tb³⁺?⁵D₄?⁷F₅?/Eu³⁺?⁵D₀?⁷F₂?achieves high sensitive temperature measurement.In addition,under 254 nm UV lamp excitation,BaGd₂ZnO₅:Eu³⁺phosphors emit strong red light,which can be used to clearly exhibit the patterns of fingerprints.This work shows that the rare earth doped luminescent materials have potential application in fingerprints identification.?4?Taking the thermal stability of luminescent materials as research content,the Bi³⁺or Eu³⁺single-doped Ca₃Sc₂Si₃O₁₂,Y₃Al₅O₁₂2 and Y₂O₃ are synthesized by sol-gel method.The conclusion,the lattic energy as a new physical parameter can assess thermal quenching behavior of luminescent materials,is proposed.The temperature-dependent emission intensities of these materials are measured,the thermal stability is Ca₃Sc₂Si₃O₁₂>Y₃Al₅O₁₂>Y₂O₃ by calculating activation energy of them.According to the stability of crystal structure depended on lattice energy,we calculated the lattice energy of three phosphors by the dielectric theory of the chemical bond for complex crystals.In a word,the Ca₃Sc₂Si₃O₁₂ phosphors possess better thermal stability property with high lattice energy.Moreover,it is known that the human eyes are more sensitive to red light,Ca₃Sc₂Si₃O_12:Eu³⁺phosphors with high thermal stability provide a rapid,high sensitivity and low toxicity of visualization techniques in the applictions of fingerprints identification and anti-counterfeiting,indicating that phosphors possess great potenial applications in information storage.