| White light-emitting diodes (LEDs) have attracted increasing attention as a light source for the next-generation general illumination and the automotive lighting applications due to the energy savings and positive environmental effects. There are several methods to generate white light, such as combining three semiconductor LED dice of red. green and blue, however, the blue LED coupled with phosphors is the most efficient solution to realize fixed chromaticity white LED lamps in low cost. (Oxy)nitride phosphors are newly emerged family for white LEDs, which have various crystal structures, high stability, excellent luminescence properties. They can perfectly overcome the drawback of conventional phosphors, such as poor color rendering properties and low thermal stability.N doped M2SiO4:Eu (M=Ba, Sr) phosphors were synthesized by a solid state reaction in NH3 atmosphere. XRD patterns showed that the N doped Ba2SiO4:Eu powders remained the Ba2SiO4 phase after nitridation. The emission spectra were tuned in the range of 500～510 nm by changing the nitridation time. Meanwhile, as the nitridation time increased, the red-shift and peak-broaden were observed in the excitation spectra, and the excitation in the range of 390～400 nm was efficiently enhanced, which showed the advantages in the application with the UV-LED chips. Moreover, red-shift in the emission spectra of Sr2SiO4:Eu has also been achieved, and the excitation band has been tuned to fit the blue LED chips simultaneously by doping N into the host. The N contents of the samples were calculated from the weight loss data, indicating that N had been doped into the lattice. XRD results showed that the unit cell volume markedly increased after nitridation, which can be attributed to the substitute of Si-O bonds by Si-N bonds.CaSiO2N2:Eu phosphors were prepared by solid-state reaction, and the effects of heat-treatment conditions and the overall composition of host lattice on the optical properties have been discussed. CaSi2O2N2:Eu phosphors displayed a single broad emission band peak at 540 nm, which could be assigned to the allowed transition of Eu2+ from the lowest crystal field component of 4f65d to 4f7 ground-state level. The excitation band of samples, extending from UV to blue, is extremely wide, so the phosphors are suitable for white LED lamps in combination with a UV or blue LED dies. The highest PL intensity is found for the sample sintered at 1400℃. The emission intensity decreases when N partially replaces O. A red shift of emission wavelength did not occur with increasing of the N content. Moreover, we have also shown that the optical properties depended greatly on the Eu2+ concentration. A systematic red shift of the emission band was observed as the Eu2+ concentration increased. The results indicated the photoluminescence properties can be tailored by controlling the Eu2+ concentration. Starting at low Eu2+ concentration, the emission intensity rised to a maximum at a Eu2+ concentration of x=0.06 and fell again when the Eu2+ concentration continued to increase. This decrease in emission intensity beyond a critical concentration can be explained by concentration quenching, which is mainly caused by the energy transfer between Eu2+ ions. Yttrium doped CaSi2O2N2:Eu2+ phosphors were prepared by the solid state reaction method. XRD and XPS results revealed that the solubility of Eu2+ in the lattice could be increased by doping Y3+ ions. This assistant effect of Y3+ ions could be explained by a mechanism similar to Vegard’s law. Moreover, PL spectra showed that doping Y3+ ions can result in the enhanced emission of CaSi2O2N2:Eu2+ phosphors. Especially, the enhancement became more obvious when the Eu2+ content approaches to the quenching concentration. It is suggested that these novel Y3+ doped CaSi2O2N2:Eu2+ phosphors could be applied in white LEDs.CaSi2O2N2:Eu phosphor with porous spherical morphology was synthesized from a novel (Ca,Eu)CO3@SiO2 core-shell template precursor, using NH3 gas as a reduction-nitridation agent. SEM images show that the morphology of the reactive template was imprinted in the resulting material, and pores were formed during the nitridation process. TGA and nitrogen sorption measurements confirmed that the decomposition of the carbonates promoted the formation of the porous structures which supplied sufficient interface with the NH3 gas. Thus, the desired nitridosilicate phosphors were obtained at relatively low temperatures of around 1300℃. and the reaction time was significantly shortened to 1 h. The phosphors prepared by this route exhibited better photoluminescence properties compared to those of the conventionally prepared powder. Moreover, rod-like, uniform, and poorly aggregated BaSi2O2N2:Eu powders with tunable luminescence for white LEDs have been synthesized by direct gas-reduction nitridation from the core-shell oxide (Ba,Eu)CO3@SiO2. The maximization of the contact surface between the reactants and the minimization of diffusion distances improved the efficiency of the nitridation. Thus, the desired oxynitride phosphors were obtained at relatively low temperatures around 1100～1200℃. Under these conditions, the (Ba,Eu)CO3 cores melted into small seeds which decreased the particle size and promoted the oriented growth of BaSi2O2N2:Eu. The nucleation and subsequent crystal growth in (Ba,Eu)CO3@SiO2 were further discussed in the samples with different Ba/Si ratios. By well controlling the nitridation process, the tunable luminescence was achieved in the range of 490～530 nm. The low sedimentation rate of the phosphors from this route shows additional advantages in the LED packaging.Oxynitride phosphor film consist of CaSi2O2N2:Eu nanofibers has been prepared by a simple and cost-effective electrospinning process. The Ca-Eu-Si-O precursor was smooth and uniform with a length of several tens to hundreds of micrometers. After nitridation at 1300℃, the precursor morphology was well remained and the X-ray diffraction results confirmed that the obtained nanofibers were composed of pure CaSi2O2N2 phase. Compared with the phosphor film consist of CaSiO2N2:Eu powders,the phosphor film consist of CaSi2O2N2:Eu fibers had better transmittance and stronger emission intensity under the improved PL measurement,which can be attributed to the regular structure of the fiber film. The white LED with colour render index as high as 83.7 can be facilely prepared by pasting the CaSi2O2N2:Eu fiber film to the blue LED chip. |
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