Wavelength Regulation Mechanism Of Ce³⁺-doped Fluorescent Materials

In recent years,white LEDs have been widely used in lighting and display fields due to their excellent properties such as energy saving,environmental protection,small size,and long luminescent life.Key performance such as light-emitting brightness,luminescent quality,and stability of lighting devices largely depend on the performance of phosphors.The current white LED is mainly realized by using blue chip to excite yellow YAG:Ce³⁺phosphor,due to lack of sufficient long-wave red light ingredients in the spectrum,the light source typically has a problem of poor color performance,which is difficult to meet the market for high quality white LED illumination demand.Therefore,the design of long-wave fluorescent materials is especially important for high quality white LED lighting.In this paper,the related electronic structures of Ce³⁺-doped Y（Gd）AG,Y-Si-N-O systems and La₃Si₆N₁₁fluorescent materials are calculated by first-principle calculation,respectively.The electronic energy band structures of the three systems,and structure and wavelength regulation mechanism of related rare earth materials are clarified.The specific research contents are as follows:（1）In the YAG:Ce system,the density functional theory calculations show that when the Y atom is replaced by Gd atom,the band gap between the VBM and CBM decreases from the 4.57 e V to 4.51 e V.The co-doping of Gd-Ce ions do not seriously affect the partial density of states（PDOS）of Y,O and Al atoms,but the PDOS of Ce³⁺atoms changes;this indicates that the emission wavelength of Ce³⁺ions is affected by Gd ions.In order to verify the calculation results,this work also successfully synthesized Gd YAG:Ce fluorescent materials doped with different Gd³⁺concentrations by high-temperature solid-phase method.When Gd:Y=1:2,the luminescence intensity reaches the maximum,and the luminescence wavelength of the material also undergoes a red shift with the increase of Gd ion concentration.On this basis,Gd YAG:Ce fluorescent glass films were prepared,and the effects of synthesis temperature and the ratio of phosphor powder to glass powder on the luminescence properties and transmittance of the samples were studied in detail.Under laser excitation,the luminescence saturation threshold of the Gd YAG:Ce fluorescent glass film can reach 6.5 W/mm²,and the luminous flux at the saturation threshold point is 487.2 lm.（2）Research on the Y-Si-N-O system:by studying crystal and electronic structure characteristics and experimental test results of Y₂Si₃N₄O₃:Ce³⁺,Y₄Si₂N₂O₇:Ce³⁺and Y₃Si₅N₉O:Ce³⁺,the intrinsic relationship between crystal and electronic structure and Ce³⁺luminescence is analyzed.The calculated results of the electronic structure show that the calculated band gaps of Y₂Si₃N₄O₃ and Y₄Si₂N₂O₇ are 3.48 e V and 3.80 e V,respectively,and the energy difference between the 4f and 5d energy levels of the Ce³⁺-doped samples are 2.54 e V and 2.61 e V,respectively,showing the 470 nm and 441 nm short-wave emissions.Whereas the calculated band gap of Y₃Si₅N₉O is 2.3 e V,and Ce³⁺ions have the opportunities occupy two sites of Y1（Ce1）and Y2（Ce2）in the Y₃Si₅N₉O structure,respectively.The difference between the two coordination environments leads to the 4f and 5d energy levels difference of Ce1 and Ce2 doping are 1.24 e V and 2.04 e V,respectively,making the Ce1 and Ce2 ions exhibit long-wave orange emission at 620 nm and short-wave blue emission at 474 nm,respectively.Compared with the Y₂Si₃N₄O₃ and Y₄Si₂N₂O₇ structures,the Y₃Si₅N₉O structure has a smaller band gap,which makes it possible to reduce the energy difference between the 4f and 5d energy levels of Ce³⁺ions,while the high concentration of N coordination structure and the shorter Ce-N coordination bond length are beneficial to reduce the energy difference between the 4f and 5d energy levels spilling of Ce³⁺ions,so as to realize the long-wave emission of Ce³⁺ions.By comparing the emission spectra of the three materials and the coordination structure of Ce³⁺ions,it can be founded that Ce³⁺ions tend to emit light at long wavelengths in the high-concentration N coordination structure.By comparing the Ce1 and Ce2 ion coordination structures and emission wavelengths in the Y₃Si₅N₉O structure,it can be founded that the shorter Ce-N coordination bond length and lower coordination symmetry are beneficial to reduce the energy levels of Ce³⁺ions 4f-5d energy level difference,so as to obtain long-wave luminescence.（3）The crystal and electronic structure characteristics of La₃Si₆N₁₁:Ce fluorescent material were studied by three different methods.The calculation results of the electronic structures show that the Ce³⁺ion has two substitution sites in the La₃Si₆N₁₁ structure.Both sites are coordinated with eight N ions,of which the 2a site has a shorter Ce-N average bond length,while the 4c site doping has a lower point symmetry.The difference of the two coordination environments results in the energy difference between the 4f and 5d energy levels of Ce_2a and Ce_4c doping,which are 2.15e V and 2.78 e V,respectively,making Ce_2a site doping exhibit long-wavelength luminescence compared with Ce_4c site doping.Replacing part of Si-N bonds in nitride fluorescent materials by Al-O bonds has been widely used in the emission spectrum regulation of fluorescent materials.Therefore,this work also studies the influence of the introduction of Al-O bonds on the electronic structure of La₃Si₆N₁₁ materials.The calculation results show that after replacing part of Si/N atoms with Al/O atoms in the La₃Si₆N₁₁ structure model,the doping of 2a and 4c sites both show indirect band gaps,and the band gap value is about 2.57 e V,which is much smaller than that of the La₃Si₆N₁₁:Ce³⁺sample.The calculated energy band and density of states spectra show that the introduction of Al-O bonds in the material has a certain regulatory effect on the luminescence spectrum of fluorescent material of the system,so the substitution of Al-O bonds can be used to initially realize the wavelength regulation of nitride fluorescent materials.