Preparation And Properties Of Rare Earth Doped Ca₂GeB₂O₇ Phosphors With Tunable Emission Color

The fourth generation solid state lighting source represented by white light emitting diode(WLED)has exhibited a broad application prospect in the fields of display and lighting,due to the advantages of high luminous efficiency,energy saving and environmental protection.The 4f inner electronic structure makes rare earth ions produce unique luminescent properties when doped into stable borate lattices.Therefore,it is of great theoretical and practical significance to study the rare earth doped borate luminescent materials.Ca₂GeB₂O₇ can be selected as an ideal borate luminescent matrix material,due to its advantages of good thermal stability,easy synthesis,and the similar radius of Ca2+to those of trivalent rare earth ions.To obtain the phosphors with adjustable luminous color,Ca2GeB207 matrix was doped with single,double and trible rare earth ions through high-temperature solid-state method,respectively.The composition and structure of the samples were characterized by XRD,EDS and SEM.The fluorescence properties and luminescent thermal stability of the samples were studied by fluorescence spectra(excitation and emission),quantum yield,fluorescence lifetime and CIE color coordinates.Finally,the single-phase white light emission was successfully realized.1.Single-doped Ca₂GeB₂O₇:Ln3+(Ln3+=Tb3+,Eu3+,Ce3+,Sm3+,Dy3+)phosphors with different luminescent colors were prepared by high-temperature solid-state method.Ca₂GeB₂O₇:3%Tb3+exhibited the best green emission with quantum yield of 42.3%.Ca₂GeB₂O₇:1%Eu3+exhibited the best red emission with quantum yield up to 98.5%.Ca₂GeB₂O₇:0.5%Ce3+shown the best emission of ultraviolet light and visible blue light at the same time.Ca₂GeB₂O₇:3%Sm3+achieved the best orange-red emission with quantum yield of 19.7%.Under the excitation of 350 nm,Ca2GeB207:1%Dy3+was successfully realized cold white light emission with a quantum yield of 60.1%and excellent thermal stability,which makes it a candidate material for WLED single-phase white light phosphor.2.A series of rare earth ions co-doped Ca₂GeB₂O₇:Dy3+/Sm3+,Dy3+/Ce3+and Tb3+/Eu3+phosphors were prepared by high temperature solid state method.Ca₂GeB₂O₇:Dy3+/Sm3+series samples were all realized warm white light emission.The CIE color coordinate of Ca₂GeB₂O₇:l%Dy3+/0.5%Sm3+sample was close to that of positive white light.So it could be used as WLED single-phase white light phosphor for indoor lighting,due to the advantages of low color temperature,high quantum yield and good thermal stability.Ca₂GeB₂O₇:Dy3+/Ce3+series samples were realized to emit blue-white light or white light under different excitation wavelength.By optimizing the doping concentration of Eu3+ and excitation wavelength,the luminous colors of Ca₂GeB₂O₇:Tb3+/Eu3+series samples were changed,and the white light emissions with adjustable color temperature was achieved,which would guide for regulating the fluorescence properties of white phosphors.The CIE color coordinate of white light of Ca₂GeB₂O₇:3%Tb3-/0.5%Eu3+sample excited by 310 nm was close to that of positive white light,and it also exhibited good thermal stability.3.Rare earth ions tri-doped Ca₂GeB₂O₇:Tb3+/Eu3+/Dy3+and Ca₂GeB₂O₇:Tb3+/Eu3+/Ce3+phosphors were prepared by high temperature solid state method.The luminous color of Ca₂GeB₂O₇:Tb3+/Eu3+/Dy3+sample was adjusted by changing the excitation wavelength or relative doping concentration.Ca₂GeB₂O₇:3%Tb3+/0.5%Eu3+/0.5%Dy3+sample was realized white light emission,and it exhibited good thermal stability and high quantum yield,which realized the purpose of improving the quantum yield of white light emission by doping Dy3+.The white light emissions of Ca₂GeB₂O₇:Tb3+/Eu3+/Ce3+ samples with adjustable color temperature was realized by changing the doping concentration of Ce3+.When the doping concentration of Ce3+is 0.2%,the phosphor is expected to be a single-phase white phosphor with excellent thermal stability.