Studies On Red And Near Infrared Upconversion Luminescence Properties Of Rare Earth Ions Er³⁺/Yb³⁺

Rare earth ions have significant application values in white light illumination,color display,solid-state laser,biological imaging,optical communication,functional detection,medical treatment and other fields owing to their abundant energy levels and unique electronic configurations.Especially,Er³⁺,Tm³⁺and Ho³⁺ions have irreplaceable positions in upconversion luminescence due to their ladder-like energy level structures and long lived intermediate energy levels.The luminescence properties of rare earth ions doped materials may be closely affected by the type of ions,concentration,host materials and so on.The type and formation of materials as well as the component and concentration of ions should be controlled artificially for satisfying various application demands.However,the application and development of rare earth upconversion materials are restricetd due to low efficiency,limited excitation and emission wavelengths as well as insufficient analysis of photodynamic processes.Therefore,it is of great practical significance and research value to find new materials with high efficiency and to analyze the related physical processes in depth for promoting the application of rare earth upconversion and exploring new luminescence materials.The rare earth luminescence will have a broader future by comprehensive analysis of the characteristic of optics,thermology and mechanics.This dissertation focuses on upconversion luminescence features of Er³⁺and Yb³⁺ions.The dual-color scheme complementary optical temperature behavior in Ba₃Y₄O₉ is investigated.Then,?-NaYF₄?Ba₅Gd₈Zn₄O₂₁1 and Y₂O₃ are selected as host materials for the compared studies on phonon energy dependent energy transfer upconversion.Besides,the Yb³⁺induced enhancement of near infrared upconversion luminescence dynamic process is discussed.At last,we also try to prepare sesquidxides transparent ceramic and obtain initial results.The specific research contents and results are as follows:1.Bright red upconversion emission phosphors Ba₃Y₄O₉:Er³⁺/Yb³⁺have been first obtained by traditional solid-state reaction method and maximum phonon energy of the host is obtained to be about 585 cm^-1 through Raman spectrum.The red upconversion emission integrated intensity of Ba₃Y₄O₉ is 6.8-fold and 5.9-fold higher than that of Y₂O₃ and?-NaYF₄ with their optimal concentration,respectively,under980nm laser diode excitation at a low density.A broad absorption band of Ba₃Y₄O₉:Er³⁺/Yb³⁺around 976 nm is observed by diffuse reflectance spectra,which is beneficial for practical applications.The upward transition from Er³⁺⁴I_13/2 to ⁴F_9/2/2 by energy transfer is verified to be the major excitation mechanism for the red upconversion emission under low excitation densities.The optical thermal sensing features of thermally coupled green levels and thermally coupled red levels were detected and analyzed.Besides,the maximum sensitivities are 0.00248 K^-1 at 563 K and 0.00371 K^-1 at 143 K for green and red respectively,in our metrical temperature range.The green-and red-based thermometers are complementary and should be properly selected for different temperature detection ranges to maintain a relatively high sensitivity in a wide temperature range.Moreover,the radiative lifetime of Er³⁺⁴F_9/2/2 state in Ba₃Y₄O₉ host is determined to be 879?s for the first time through a new method.Accordingly,the?₄ of Ba₃Y₄O₉ can reach 2.154%at optimal composition,which is a little higher than that of the famous Y₂O₃.These results suggest that Ba₃Y₄O₉:Er³⁺/Yb³⁺is feasible to be a candidate for dual-color optical thermometry and red UC displays.2.Three kinds of efficient upconversion phosphors?-NaYF₄,Ba₅Gd₈Zn₄O₂₁ and Y₂O₃ were synthesized by solid state reaction.In the condition of the same doping concentration,they show the same excitation mechanism but different output color under low densities excitation.A spectroscopic method for evaluating the relative second step energy transfer coefficient is proposed.It is revealed that the transfer coefficient in Y₂O₃ is around 4.6-fold and 408-fold larger than that in Ba₅Gd₈Zn₄O₂₁and?-NaYF₄ respectively,indicating a continuously enhanced excitation of the red emission on increasing phonon energy.The phonon energy dependent behavior essentially reflects the phonon-assisted energy transfer excitation mechanism.This work declares that effective excitation of the red upconversion requires high phonon energy to make up a large energy mismatch(¹500 cm^-1)for minimizing the phonon number.However,with consideration of high phonon energy induced fast nonradiative decay of the red emitting state,selecting material with appropriate phonon energy is essential for both effective excitation and efficient emission of red level to obtain outstanding red upconversion phosphors based on Er³⁺/Yb³⁺system.Furthermore,the Er³⁺⁴F_9/2 radiative lifetime in Ba₅Gd₈Zn₄O₂₁ matrix is evaluated to be 353?s for the first time.The research means utilized in this work can be widely applicable rather than specific to these three hosts and we also believe that this spectroscopic method might be extended to Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺system.3.Er³⁺singly doped and Er³⁺/Yb³⁺co-doped Y₂O₃ upconversion phosphors are synthesized by calcining precursor powders.For singly doped sample with the optimal Er³⁺concentration,the upconversion emission bands of 560nm,680nm,820nm and 1000nm can be monitored.The near infrared upconversion intensity at1?m exceeds 99%of the total conversion intensity.The feasibility of the scheme is analyzed by combining solar spectrum,enhanced solar cell efficiency structural schematic diagram,absorption spectrum and upconversion spectrum.Co-doped 1%Yb³⁺ions,through the energy transfer of Er³⁺to Yb³⁺,make it a new luminescent center,and eventually the near infrared upconversion strength is enhanced by².4times.We believe that the enhancement of near infrared upconversion luminescence is due to the replacement of some inefficient luminescence centers(Er³⁺)by high efficient luminescence centers(Yb³⁺),thus alleviating the energy loss caused by the multiphonon relaxation process.This method is helpful to enhance the near-infrared upconversion intensity of matrix materials with large phonon energies.The power dependence and time evolution curves of upconversion luminescence show that the main layout of upconversion luminescence in near infrared region is energy transfer upconversion.The effect of ion doping concentration on up-conversion luminescence kinetics indicates that the competitive mechanism of energy transfer upconversion and concentration quenching limits the further increase of doping concentration.4.Based on the research foundation of our research group,using the commercial oxide powder as the raw material,the transparent ceramic with high transmittance in the near infrared region have been obtained by the solid phase method.The doping of La³⁺and Zr⁴⁺promote the transparency of Y₂O₃:Eu³⁺but have little effects on the crystal structure and the shape of emission spectra.Suitable discharge temperature and hot isostatic pressing sintering mode are beneficial to enhance the transmittance of Lu₂O₃:Tm³⁺ceramic.By optimizing the process parameters and controlling the preparation process,the Y₂O₃:Eu³⁺transparent ceramic sheet with thickness of 1mm is finally obtained,and its linear transmittance can reach 81.1%at 1500nm.And the transparent Lu₂O₃:Tm³⁺ceramic column with a length of 1cm has a linear transmittance of 80.74%at 2100nm.