Control Of The Rare Earth Fluoride Nanocrystal Fluorescence

Lanthanide (Ln)-doped nanostructure materials have drawn consistent attention of researches due to their unique optical, electrical and magnetic properties. The narrow band luminescence emission and very low background give them special advantages in the applications of biological sensing and nanodevices. Meanwhile, the small absorption cross-section of excitation light also limits the fluorescence efficiency. Therefore, enhancing the fluorescence emission has been one of the most important issues for developing the application of Ln³⁺ doped nanomaterials (NMs). The main challenge in this area is how to control the sizes, compositions and crystal structures precisely in nanoscaled materials, which may lead to the development of a powerful tool for tailoring the optical properties of materials. In current dissertation, carefully designed investigations have been carried out to obtain the luminescence enhancement in Ln³⁺ doped lanthanumfluoride NMs, which include the adjustment of the crystal phase, the shape and size of nanocrystals (NCs), and the dopant concentration. It was found that the local symmetry and local environment could be adjusted effectively through introducing the codopant ions or changing the crystal structure of the matrix material, which led to the fluorescence enhancement and spectral improvement. The main results are summarized as follow:(1) The optical characteristics of Tm³⁺/Ln³⁺(Ln³⁺=Yb³⁺, Er³⁺, Pr³⁺, Ho³⁺, Eu³⁺) ions co-doped in LaF3 and LaOF nanoparticles are investigated systematically with laser spectroscopic methods. The possible ways of obtaining visible light, especially blue and green light with different pumping conditions are explored. The dependences of photoluminescence on the matrices and the co-doped ions are also investigated. It is shown that by codoping other Ln³⁺ ions (Ln³⁺=Ho³⁺, Pr³⁺, Er³⁺, Eu³⁺) to Tm³⁺ doped nanoparticles, one can not only get the visible light effectively, but also can obtain the significant increase on the luminescence quantum yield, which is very important for broadening method of obtaining blue-green luminescence.(2) Luminescence enhancing (LE) or quenching (LQ) for lanthanide doped nanocrystals was obtained by a second Ln³⁺ ion doping method. Singly or doubly doped LaOF, LaF3 and NaYF4 nanocrystals have been studied in detail under the selective or two-colour excitations. The underlying reason for LE by codoping is revealed, and a mechanism of the enhancement based on low local point symmetry effect of the matrix is proposed. It is found that the modification of local environment induced by dopant ions can result in the LE if the non-radiative relaxation probability can be ignored. The observations reported here should be useful for improving the quality of the Ln³⁺ doped nanomaterials.(3) Efficient spectral conversions from 325-550 nm to 570-710 nm have been obtained in LaOF: Eu³⁺ NCs. When any level above 5D0 of Eu³⁺ is optically excited, strong emissions arising from 5D0 level are obtained in the range of 570-710 nm that is the highly efficient work range for organic solar cells. The influences of ambient temperature, particle size, dopant concentration and codoped ions on the luminescence intensity of Eu³⁺ have been discussed in detail. When the light reflecting and scattering are ignored, the photon conversion efficiency from 350-550 nm up to 3.91% in Tm³⁺ codoped LaOF:Eu³⁺ NCs.(4) Efficient up and down frequency conversions in Tm³⁺ and Ho³⁺ doped LaOF tetragonal nanocrystals have been investigated. Bright luminescence emissions are obtained in co-doped Tm³⁺/Ho³⁺:LaOF tetragonal nanocrystals through UV and infrared excitations. Green florescence from Ho³⁺ ions, which can be clearly seen with bared eyes, is obtained when Tm³⁺ ion is excited. Specific mechanism of the cross relaxation between doped ions is explored through spectroscopic measurements in time and frequency domains. When local environmental symmetry and ion distribution were modified by introduction sensitization, we revised the calculation process of energy transfer efficiency. About 83.1% energy transfer efficiency is obtained when the weak radiative and nonradiative relaxations are neglected.(5) Luminescence emission of Tm³⁺ doped transparent oxyfluoride glass ceramics containing LaF3 nanocrystals was investigated with selective excitation at different local environments. The effects of SiO2 contented in the matrix material and the temperature on up-conversion fluorescence intensity and decay process depend on local environments of Tm³⁺. It was found that influence of the content of SiO2 and the temperature on fluorescence intensity and decay process were much stronger for the ions in the crystal phase than those in the glass phase. The spectrum of phonon energy was adjusted by varying the content of SiO2 and the temperature resulting in modification of luminescence characteristic. The reported here should be useful for discriminating the orderly degree of the local environments.