Design and Study of New Phosphors for White LED Lighting and Display

Author:Zhu Ying Li

Supervisor:liang yu jun


Degree Year:2019





The development of solid state lighting,represented by the fourth generation lighting and display technology,puts forward higher requirements for the performance of phosphors.Single host phosphor materials with high color rendering index,high efficiency and high thermal stability,high efficiency,narrowband emission phosphor materials and morphologically controllable phosphor materials have become the current research hotspots.In order to overcome the shortcomings of phosphors used in white LED,several kinds of single-host multi-color emission phosphors and high-efficiency narrow-band emission phosphors were synthesized.Several methods to synthesize morphologically controllable and multi-components-colors phosphors were developed.At the same time,the crystal structure,luminescence performance and energy transfer process,particle size distribution,micro-morphology and so on of phosphors were also studied in depth.The specific research contents are as follows:(1)Ce3+/Tb3+,Ce3+/Mn2+,Ce3+/Eu2+and Eu2+/Mn2+were selected as activator combinations,by adjusting the doping ratio of sensitizer and activator ions,the control from blue light to green,white and yellow lights,from cyan light to white and yellow lights were realized in Sr3Gd2(Si3O9)2(SGSO)-based phosphors.The interaction mechanism of energy transfer between activator ions was discussed by means of fluorescence spectra and lifetimes.By introducing the sensitizer Ce3+,Tb3+and Mn2+,Eu2+all show better thermal stability than that of single doping sample.Their luminescence intensities at 423 K are 72.3%,86.1%and 85.6%in comparison to that at room temperature.In addition,Mn2+acts as a structural regulator,which regulates the luminescence properties of Eu2+and Eu3+by changing the structure and polyhedral distortion occupied by Eu2+and Eu3+in SGSO.At lower Mn2+concentration,SGSO:Eu,Mn exhibits good thermal stability and slight color shift,while at higher Mn2+concentration,the phosphor exhibits good temperature sensitivity.(2)Mg2+,Zn2+cationic ions and Gd3+-Na+ion-pair substituted composite silicate Na2Ba6(Si2O7)(SiO4)2(NBSS)phosphors were designed and synthesized by using Eu2+as activator ion.By changing the doping ratio of Mg2+,Zn2+and Gd3+-Na+,the emission color of NBSS:Eu2+can be controlled in blue and cyan regions.Combined with the changes of crystal structure and spectra of NBSS:Eu2+,xMg2+/yZn2+/z(Gd3+-Na+),it can be found that the main reason for the change of emission peak position of Eu2+is that the change of matrix composition and content changes the preferential position of Eu2+in different cation sites.In addition,the Mg2+and Zn2+substitutions can obviously improve the thermal stability of Eu2+,and Mg2+has better effect on improving the thermal stability than Zn2+and Gd3+-Na+,and the change of thermal stability of Eu2+is closely related to the distortion of crystal structure.(3)Sr2MgAl22O36(SMAO):Mn2+narrow-band green emission phosphors were synthesized by using the hexagonal and high symmetry crystal structure SMAO as matrix.Under the excitation of 450 nm,SMAO:Mn2+emits bright green light in the range of 485 nm-590 nm,with the maximum emission peak at 518 nm and a FWHM of26 nm.The lattice vibration intensity of several kinds of phosphors was studied by FTIR measurement.It can found that the narrow-band emission is closely related to the symmetry of the matrix structure.In addition,the high symmetrical coordination environment around Mn2+and there are only one kind of Mg2+site in the matrix lattice for Mn2+doping are beneficial to the narrow-band emission of Mn2+.The SMAO:20%Mn2+sample has good thermal stability,and it can still maintain 81.91%of the initial strength measured at 298 K at 473 K.When SMAO:Mn2+,K2SiF6:Mn4+and GaN-based chips are integrated into white LED devices,about 127%of the ultra-wide NTSC gamut can be displayed.In addition,by introducing Mn2+and Mn4+into Mg2+and Al3+lattices,both Mn2+and Mn4+emit narrow-band green and red lights simultaneously in the SMAO host.Under the excitation of 385 nm,SMAO:Mn2+,Mn4+samples can simultaneously produce a green emission in the range of 485 nm-590 nm and a red emission in the range of 630 nm-730 nm.The maximum emission peak of green light is about 518 nm,FWHM is 26 nm,while the maximum emission peak of red light is about 659 nm and FWHM is 42 nm.(4)By using Gd0.99Eu0.01(OH)3 nanorods as precursors,flake,flower and spherical morphologies of Gd0.99Eu0.01BO3 red phosphors were successfully synthesized.The increase of NaOH concentration in colloidal solution and the decrease of PEG(20000)content in the treatment of Gd0.99Eu0.01(OH)3 will gradually transform the morphology of Gd0.99Eu0.01BO3 from sphericall to flower-like and flake-like shape.By investigating the XRD spectra and corresponding SEM images of intermediate products obtained at different reaction time intervals,the formation of Gd0.99Eu0.01BO3 products with differentmorphologieswassummarizedasakineticallycontrolled dissolution-recrystallization mechanism.The emission spectra of Gd0.99Eu0.01BO3samples are characteristic emission peaks of Eu3+ions.Moreover,the activating agent ions obtained by this method have a large number of nano-scale surface positions,resulting in lower local environmental symmetry of Eu3+,which greatly improves the emission ratio of red light to orange light.In this work,a method of synthesizing micro-and nano-materials with controllable morphology and stable performance using mesoporous SiO2 as reaction vessel was proposed and designed.In this paper,Bi0.96Eu0.04PO4 nanospheres with high luminescent efficiency were successfully synthesized by the above methods using Bi0.96Eu0.04 nanospheres as precursors.Compared with one-step synthesis of Bi0.96Eu0.04PO4,this method not only has controllable spherical morphology,but also has higher energy transfer efficiency from matrix to Eu3+.The preparation method can also be extended to design and fabricate micro-and nano-materials with controllable morphology in other material systems.(5)Two design concepts of multi-component and multi-color phosphors were proposed.The first process includes preparation of homogeneous precursors,coating mesoporous SiO2 on the surface of precursors,treating intermediate products with metal solution and calcination.Y2Si2O7:Ce3+,Eu3+@Zn2SiO4:Mn2+series samples were successfully synthesized by using Y(OH)CO3 as precursor.XRD,SEM and TEM analysis show that the samples present hollow spheres with shell thickness about 100nm,mainly consisting of two phases:Zn2SiO4 and Y2Si2O7.This Y2Si2O7@Zn2SiO4structure can accommodate a variety of luminescent activated ions(Ce3+,Mn2+,Eu3+),and full spectrum emission under a single wavelength excitation can be achieved.In addition,in the Y2Si2O7@Zn2SiO4 structure,the emissions of Ce3+,Eu3+and Mn2+exhibit excellent thermal stability due to interface defects and energy transfer from defect level to activator at high temperature.The second process includes the preparation of phosphors with uniform morphology and the deposition of nanostructured g-C3N4 on the surface of phosphors by vapor deposition.Y2O3:Tb3+,Eu3+@g-C3N4 and Y2O3:Tb3+,Eu3+@mSiO2@g-C3N4 phosphors were successfully synthesized by using Y2O3 and Y2O3@mSiO2 microspheres as precursors.Y2O3 and Y2O3@mSiO2 microspheres were evenly coated with an 8 nm thick amorphous g-C3N4layer.Under the excitation of 368 nm,Y2O3:Eu3+,Tb3+@g-C3N4 samples contain the characteristic emission peaks of Tb3+and Eu3+,and blue emission peak of g-C3N4 at 463nm.In addition,there was an effective energy transfer process from g-C3N4 to Tb3+and Eu3+.By controlling the coating quality of melamine,the luminescent color of the phosphor can be controlled from blue light to white and yellow light.These two methods can also be extended to the design and preparation of other morphology controllable multi-component materials.