Research status of fluorescent materials

1. Doping modification: Other ions are added to the traditional fluorescent materials to increase the conversion efficiency and enhance the fluorescence intensity. For example, add the sensitized Yb3+ ions to the fluorescent material. Yb3+ ion has a strong absorption cross section near 980nm, and only has two energy levels, so it will not lose the excitation light energy due to the up-conversion process. Therefore, it is an ideal sensitizer. In particular, lithium ions enhance the fluorescence of rare earth materials. In 1997 found that Li + in the zinc oxide doped with rare earth ions can significantly enhance the fluorescence intensity of rare earth ions under conversion, found that Li + is a kind of very good fluorescence enhancement rare earth ions, only in the process of the preparation of materials need to be mixed with a small amount of Li +, the fluorescence intensity of rare earth ions can be greatly enhanced, with its vast potential application value.
2.2 Surface modification: Surface modification includes core-shell structure and thin film structure, which is basically consistent with the principle of various structures to enhance the luminescence of rare earth ions. The core-shell structure can greatly enhance the up-conversion luminescence intensity. There are many explanations for the fluorescence enhancement. One is that there are a large number of surface defects on the surface of nanoparticles, and the surface defects with large vibration energy will greatly increase the non-radiative relaxation probability between energy levels, thus reducing the luminous efficiency of up-conversion. Another understanding is that coating a shell with low phonon energy can effectively reduce the probability of multi-phonon relaxation of luminescent ions located on the surface of nanoparticles, and fluorescence will be enhanced at this time. It is also believed that the rare earth ions on the surface of the nanoparticles, which do not initially emit light, will become the new luminescence center and emit fluorescence after the coating of the shell and the crystal field of the core, so the fluorescence can be enhanced. In a word, the radiation coefficient of the rare-earth ion energy level increases after surface modification, which enhances the fluorescence intensity and the conversion luminous efficiency.
2.3. Rare earth complexation: Due to the weak absorption of rare earth ions, it is possible to obtain the characteristic fluorescence of strong rare earth ions only when they are combined with organic ligands with strong absorption coefficients and through effective intra-molecular energy transfer. The absorption coefficient of rare earth ions in the uV-visible region is very small, so the fluorescence emission intensity of their direct excitation is very weak. However, when rare earth ions form complexes with appropriate ligands, they can emit strong characteristic fluorescence of rare earth ions (especially Eu3+ and Tb3+) under ultraviolet and visible light irradiation. Rare earth ions with inert structure such as La, Gd, Lu and Y affect the luminescence performance of the ketone complex, extend the fluorescence life of the ligand, and cause the so-called “co-luminescence phenomenon”. The preparation of rare earth complexes has great application value.
2.4 preparation process: The properties of the materials are closely related to the synthesis methods. Therefore, it is of great importance to study various new methods that are more effective, energy-saving and economical for the synthesis of rare earth luminescent materials. Sedimentation is one of the most widely used methods for chemical synthesis of luminescent materials in liquid phase. It includes direct precipitation, homogeneous precipitation and coprecipitation. However, irreversible agglomeration affects particle size and light conversion performance. Hydrothermal method refers to an effective method to prepare materials under high temperature and high pressure by heating the reaction system to the critical temperature in a sealed pressure vessel with water as the reaction solvent. Rare earth nanoparticles were prepared by hydrothermal method with high purity, good crystal shape, monodisperse, controllable morphology and easy operation. In addition, organic solvent can be used instead of water to prepare nanomaterials by solvent thermal reaction. The materials obtained have good properties. Sol-gel method, refers to the easy hydrolysis of metal organic or inorganic compounds through hydrolysis and condensation gradually gelation, and then through the corresponding drying, sintering and other processing to get the required materials method, material uniformity can reach the sub-micron level nanoscale and even the molecular level; The disadvantages are that the raw materials of the reaction are expensive and sometimes difficult to prepare, and the reaction operation is complex and the cycle is long. Microemulsion phase is insoluble in water nonpolar substances as dispersion medium, the aqueous solution with different reactants as dispersed phase, the choice of appropriate surfactant as emulsifier, form a water-in-oil (W/O) or type oil-in-water (O/W) emulsion, make the effect of reactant space is limited to internal microemulsion drops, resulting in uniform shape, narrow particle size distribution of nanoparticles, it is adopted by the preparation of nanoparticles in recent years, a novel method. In short, through various preparation processes particles as small as possible powder, preferably nanoparticles. Because nanomaterials have (1) surface and interface effects; (2) Small size effect; (3) Quantum size effect; (4) Macro quantum tunneling effect. Nanomaterials have better properties than ordinary micron materials.