Preparation And Internal And External Environment Of Rare Earth Doped Nanocrystals Fluorescence Spectroscopy

Rare earth(RE) doped nanocrystal is one kind of luminescence material that has been developed rapidly in recent years.It has both advantangeous of nanocrystals and RE ions.Researches about RE doped nanocrystals mainly focused on two aspects:one is the preparation of RE ion doped nanocrystals with certain luminescence properties with chemical or physical methods;another one is the investigation of luminescence properties and their potential applications by high resolution spectroscopic methods.In cureent thesis,we present a new developed method on the preparation of Tm³⁺ and Eu³⁺ doped nanocrystals,and the researches on the influence of local crystal symmetry and the surrounding media of nanoparticles on their spectroscopic properties.There are three parts included in the thesis.In the first part,a newly developed method which is named hydrothermal-sintering method is presented.With this method,we successfully prepared LaOF:Eu³⁺ red fluorescent nanocrystals.In the second part,we discussed the influence of local anisotropies on the fluorescence emission and relaxation of Eu³⁺ doped tetragonal and hexagonal LaOF nanocrystals. In the last part,we discussed the influence of surrounding media on the local electric field of Tm³⁺ in LaF₃:Tm³⁺ nanocrystals.Part one:A newly developed method for synthesis of tetragonal LaOF:Eu³⁺ nanocrystalsBased on the hydrothermal method,a new method,which is named hydrothermal-sintering method by us is developed.This method is simple to conduct and has a lower requirement on the synthesizing conditions.The characterization results of XRD and TEM indicated that the LaOF:Eu³⁺ red phosphor sample has tetragonal crystal structure.The doped concentration for the strongest red fluorescence emission is about 9.0 mol%.Comparing with other red fluorescence materials like Y₂O₂S:Eu³⁺ and ERT₄E-002 red phosphors,we found that our sample have showed higher red fluorescence intensity under the same excitation condition.Part two:Influence of the crystal local symmetry on fluorescence emission of LaOF:Eu³⁺ nanocrystalsThe optical transitions of Eu³⁺ in LaOF:Eu³⁺ nanocrystals with tetragonal and hexagonal crystal structures were studied with laser spectroscopic mehod.Excited with 532 nm pulsed laser, fluorescence emissions from ⁵D₁ and ⁵D₀ excited states were observed.It was found that the intensity of red fluorescence emission,which resulted from the ⁵D₀→⁷F₂ forced electric-dipole transition,is obviously stronger in tetragonal crystal structure than that in the hexagonal structure.The difference in the number of anions that directly interact with Eu³⁺ and the length of La-O and La-F bonds in tetragonal and hexagonal structures were considered to be the main reasons.Increase on the unsymmetry of local crystal field resulted in a bigger probability of forced electric-dipole transition, and thus a stronger fluorescence emission in the LaOF:Eu³⁺ namocrystals with tetragonal crystal structure.Part three:Influence of surrounding media on fluorescence dynamics of LaF₃:Tm³⁺ nanocrystalsThe influence of surrounding media on dynamic process of fluorescence emission in LaF₃:Tm³⁺ nanocrystals was studied by observing the the relation of surrounding media and the fluorescence relaxation.We found that the fluorescence relaxation of Tm³⁺ had the second-order-exponential decay nature.The quicker decay part is owned to Tm³⁺ ions at the surface of nanocrystals,and the slower decay part is due to Tm³⁺ ions inside the nanocrystals.Fitting the experimental results with theoretical descriptions,we found that the real-cavity model explained our observations well.