Rare-earth Ion Activated Luminescent Materials:Optical Thermometry And First-principles Calculations

As rare-earth ions have unique energy-level structures and unique spectroscopic properties,the rare earth(RE)ion-doped luminescent materials are widely used in various fields.At present,the dominant applications of RE doped luminescent materials are as labeling and thermometry materials,as phosphors and scintillators,and as laser crystals.The author have carried out experimental investigations or first-principles calculations in these fields.In order to search new materials and explore new mechanisms for optical thermometry,experimental studies on RE doped luminescent materials were carried out.With the development of the theoretical calculation methods and the improvement of computational capacity,first-principles calculations have become powerful tools in predicting and analyzing the properties of luminescent materials.In the second part of this dissertation,the author presents a first-principles calculation scheme to predict the electronic structure properties of phosphors or scintillators,and also presents a series first-principles calculations on site occupation preference of RE ions in laser crystal to find ways to alleviate the quenching effect caused by the aggregation of RE ion.In addition to the introduction chapter(the first chapter),the dissertation consists of two parts of research on RE ion activated luminescent materials:One part is about the optical thermometry studies and the other is about the first-principles calculations.In the first chapter,the author briefly introduces the fundamentals of luminescence,the basic knowledge of RE elements,the electronic structures of RE ions,first-principles calculation and optical thermometry methods based on the fluorescence intensity ratio and on the fluorescence decay curves.In the second chapter,the upconversion luminescence and optical thermometry properties of the transparent glass ceramics containing CaF2:Yb3+/Er3+ nanocrystals are reported,and the results indicate that the glass ceramics samples have good sensitivity,low phonon energy and high thermal stability,which are promising for optical fiber temperature sensing.The samples were fabricated via high temperature melt quenching method.And it is indicated that well-crystallized transparent CaF2:Yb3+/Er3+ glass ceramics are formed,and the nanocrystals are uniformly distributed in glass matrix.The author studied the energy transfer from Yb3+to Er3+ by the upconversion emission spectra under the excitation of 980 nm laser,the intensities of the upconversion emissions show an upward trend as the Yb3+ concentration and the thermal treatment temperature increase,and reach their maxima for the sample with 25 mol%Yb3+ and thermally-treated at 680 ℃.The pump power dependence of the emission intensity of 25%Yb3+/2%Er3+ doped glass ceramics sample was measured and it is indicated that the population of the 4S3/2 and 4F9/2 levels are two-photon upconversion process,while the transition from 4I15/2 to 2H9/2 is a three-photon upconversion process.An output power of 59 mW(which causes negligible heating effect)was chosen to study the temperature dependence of the fluorescence intensity ratio between the 2H11/2 and 4S3/2 thermally coupled levels of Er3+,and the results show that the absolute temperature sensitivity value increases with temperature and reaches its maximum at 633 K,and the relative temperature sensitivity value is 1.4%K-1 at 300 K.In the third chapter,the author reported a temperature sensing scheme based on the temperature dependence of the thermal population of the low-lying excited energy levels,and experimental realization was carried out on(Sm0.01Gd0.99)VO4 sample.The results indicate that high sensitive temperature sensing over the broad range of 180-800 K can be achieved with the combination of this scheme together with the traditional thermal quenching scheme.The main advantage of this scheme is effective elimination of the background scattering noise and the heating effect caused by the excitation laser.The 6HJ(J=5/2,7/2,9/2,and 11/2)multiplets of Sm3+ is staircaselike with energy gaps between neighboring levels of about 1000 cm-1.To realize this scheme in(Sm0.01Gd0.99)VO4,the author excited Sm3+ to the 4G5/2 level from the 6H7/2 level(Process A)and from the 6Hg/2 level(Process B)with excitation wavelengths 601.6 nm and 644.0 nm respectively,and then detected the emission intensity from 4G5/2 to 6H5/2.For Process A,the sensitivity is 1267 K/T2 in 183-413 K.For Process B,the sensitivity is 2600 K/T2 in 393-603 K.In higher temperature range 600-800 K,the relative temperature sensitivity increases from 0.52%K-1 at 640 K to maximum of 3.23%K-1 at around 750 K achieved by Process C based on the temperature dependence of the luminescent decay lifetime.Furthermore,by taking into account the temperature-dependent quantum efficiency of the luminescent level and the broadening of the thermally activated energy levels,the author successfully explored the affecting factors of the temperature dependence of the luminescent intensities in this temperature sensing scheme.In the fourth chapter,the author reported a systematic calculation scheme to predict the relative position of the localized energy level of Ce3+ in the host band,the 4f→5d absorptions,the 4f→5d emissions and the Stokes shifts.The calculation results on Ce3+-doped M2B5O9Cl(M=Ca,Sr)charge-compensated by Na+ are verified by experimental results.It is expected that the scheme have important applications in screening potential lanthanide-doped phosphors and scintillators from minimal information about the host crystal structure.Reliable predictions on the optical properties are of paramount importance for RE-doped phosphors and scintillators.The calculation sheme combines the hybrid density functional calculation and the constrained occupancy approach.The relative energies of ground and excited states to the band edges obtained with hybrid density functional are related to hole capture or electron ionization.Together with the the absorption energy,the emission energy and the Stokes shifts achieved with constrained occupancy approach,the results achieved with hybrid density functional calculations indicate that this calculation scheme is applicable to reasonably predict the the optical properties of RE activated phosphors and scintillators.In the fifth chapter,systematic first-principles calculations were conducted to study the site occupation preference and the aggregation mechanism of the doped trivalent rare earth ion(RE3+)in Ca/Sr/BaF2,which is a class of promising super power laser crystals.Particularly,the author analyzed the site occupation preference of RE3+in monomer.It is indicated that reasonable analyzation and prediction on the stable ground-state geometric structure of rare-earth ion doped materials can be achieved from first-principles calculations,which provides applicable simulation method in further improving the properties of rare-earth doped laser materials through chemical design.During the last few decades,it has been realized that RE3+ aggregation caused quenching effect is quite severe even at a low doping concentration in these crystals.Not only the obtained variation stability trend,that C4v monomer becomes less stable with lanthanide(Ln)contraction while that of C3v monomer shows a countertrend,but also the calculated dipolar monomers’ reorientation energies are in qualitative consistent with experiment in every available case.The author further quantitatively analyzed the formation energies of monomers by a series self-consistent fittings with RE3+-Fi-Coulomb energy and the mismatch energy.The mismatch energy,which is introduced to simplify our discussion,is proportional to the effect of lattice deformation on the formation of monomer.The results show that in addition to the Coulomb effect between RE3+ and Fi-the mismatch between isolated RE3+ and isolated Fi-is also an important factor affecting the formation of monomer in these crystals.