Research On 808 Nm Infrared Light Triggered Oxide Up-conversion Luminescence Materials And Their Fluorescence Enhancement

Rare-earth ions(RE³⁺)doped up-conversion luminescence（UCL）materials possess great application prospects in display and anti-fake,temperature sensing,bio-medicine,etc.However,the traditional 980 nm excitating resource overlaps the absorption peak of H₂O molecule,which restricts the application of UC phosphors in biological medicine and other rich-water environments.Besides,the low light conversion efficiency also limits the development of UC phosphors.Therefore,808 nm near-infrared light locating in the small absorption waveband of H₂O molecules was chosen as excitating resource,oxide(MIn₂O₄（M=Ca,Sr,Ba）,Lu₂O₃,SrTiO_3.)with good stability and relative lower phonon energy were employed as matrix materials and Nd³⁺/Yb³⁺/Er³⁺were used as doping ions,a series of UCL materials with excellent luminescence properties were prepared by liquid phase synthesis techniques aiming to achieve the application in biological medicine and optical thermometry field.In order to improve the light conversion efficiency of the phosphors,starting from the selection of matrix materials,the luminescence performances of the samples were optimized gradually through the co-doping of sensitizer and the contents regulation of doping RE³⁺ions.The energy transfer process among doped RE³⁺ions were focused and the optical thermometry performances based on the fluorescence intensity ratio of the thermal coupled energy levels of Er³⁺were studied.Moreover,the UCL enhancement were realized under 808nm excitation by co-doping metal ions and the luminescence enhancement mechanisms were analyzed systematically.The main research contents of this paper are as follows:The MIn₂O₄:Nd³⁺/Yb³⁺/Er³⁺（M=Ca,Sr,Ba）phosphors were prepared by a sol-gel method.Under 808 nm excitation,CaIn₂O₄:Nd³⁺/Yb³⁺/Er³⁺samples exhibit strong up-conversion green（524 and 551 nm）and red（657 nm）emissions.The optimized UCL performances and adjustable luminescence color outputs（from green to orange and finally pure red）of the samples were obtained by changing the doping contents of sensitizers.The energy transfer of Nd³⁺→Yb³⁺→Er³⁺were proved by fluorescence decay curve fitting analysis.Based on the two thermal coupled energy levels of Er³⁺(²H_11/21/2 and ⁴S_3/2),the optical thermometry properties of MIn₂O₄:Nd³⁺/Yb³⁺/Er³⁺（M=Ca,Sr,Ba）samples were studied at 290⁴90 K by using the fluorescence intensity ratio technique（FIR）.The thermometry sensitivity of these samples always follow the order:S_A（BaIn₂O₄）>S_A（CaIn₂O₄）>S_A（SrIn₂O₄）.The distinction of thermometry performances of these samples were studied deeply by analyzing the lattice structure of matrix materials,the lattice symmetry of M²⁺ions and the characteristics of M-O bond（including bond length,coordination number,covalency,etc.）.The results indicate that the doping RE³⁺ions entering into the matrix lattice with high covalency or with long bond and more coordination numbers are more likely to bring excellent optical thermometry performance.Employing urea as precipitant,Lu₂O₃:Nd³⁺/Yb³⁺/Er³⁺and Lu₂O₃:Nd³⁺/Yb³⁺/Er³⁺/Li⁺nanospheres were prepared by a co-precipitation method.Under 808 nm excitation,the UC green（521 nm）and red（660 nm）emissions were obtained,the doping contents of Nd³⁺and Yb³⁺were optimized.Furthermore,the modulation of particle size and luminescence performances of the samples were achieved by co-doping Li⁺ion.The results indicate that the generated oxygen vacancy in samples under low Li⁺doping contents and the expanded matrix lattice in sample with large Li⁺doping contents lead to increased particle size.The enlarged particle size reduces the quenching centers(CO₃^2-,OH^-)on the surface of the samples,which brings increased green to red ratios and luminescence intensity.The influence of changed energy transfer efficiency among Nd³⁺/Yb³⁺/Er³⁺ions and that of local crystal field symmetry variation around RE³⁺ions on UCL enhancement after Li⁺doping were analyzed systematically.The results indicate:the substitution occupation of Li⁺can effective promote the energy transfer of Nd³⁺→Yb³⁺→Er³⁺and decrease the local crystal field symmetry around RE³⁺,and both of them can also bring enhanced UC emission.It is proposed that the substitution doping of Li⁺ion is more likely to enhance UCL than interstitial occupation.SrTiO₃:Er³⁺/Zn²⁺UC phosphors were synthesized by a hydrothermal synthesis method.The doping contents of Er³⁺,Zn²⁺,Yb³⁺,and Nd³⁺were optimized,and the UC luminescence mechanism in Er³⁺single doped SrTiO₃ matrix were studied.The amplified XRD indicates that Sr²⁺ion were replaced by Zn²⁺in SrTiO₃ samples under low doping contents.The green and red UC emissions of SrTiO₃:Er³⁺phosphors were enhanced about 6.3 and 3 times,respectively,after Zn²⁺doping.The near-infrared fluorescence decay curve test and the fluorescence probe role of Eu³⁺ions indicate that the enhanced absorption for excitation resource and the reduced local crystal field symmetry around Er³⁺ion at low Zn²⁺ion doping contents can lead to the major luminescence enhancement.