The Luminescence Characteristics Of Tunable Silicate Single-phased Phosphors

With the implementation of the green lighting project,the fourth generation of lighting source represent by LED is gradually recognized and accepted by people.Long service life,higher efficiency,color variety,easy to control,and the advantages of safe and reliable,can not only satisfy people’s requirements in green,energy-saving,environmental protection of lighting equipment,but also have broad prospects for development in many fields.As an important factors which can influence the performance like luminance,color index,color temperature and luminous life of LED,the quality of phosphor is of crucial importance.Currently,the most widely used method to achieve white light is fabricate a blue InGaN chip with yellow YAG:Ce³⁺phosphor.However,due to the lack of the red component,that method suffers a low color rendering index（CRI;usually less than 75）and high correlated color temperatures（CCT;usually higher than 5500 K）.To solve this problem,researchers are focusing on the near ultraviolet（near-UV）or UV LED chips coated with a single-phase phosphor with tri-color emission to obtain white light-emitting.In this paper,tunable single-phased silicate phosphors with different rare-earth ions doped have prepared by the high temperature solid state method.The XRD pattern,photoluminescence properties,decay curves and quantum efficiencies were used to characterize these samples.The crystal structures,luminescence properties and the energy transfer mechanism are investigated systematically.The main studies and results are list as follows:（1）A series of solid solution phosphors Ba₂Gd_2-xY_xSi₄O₁₃:Eu²⁺（x=0.0,0.2,0.4,0.6,0.8 and 1.0）have been synthesized by a high temperature solid state method.The rietveld refinement confirms that the sample is a single phase.Upon 365 nm excitation,all the samples emitted green light and peaked in 503 nm.With the increase of Y³⁺concentration,the emission peak was shifted to shorter wavelength（blue-shift）.Simultaneously,the color coordinate is tuned from（0.2000,0.4525）for Ba₂Gd₂Si₄O₁₃:Eu²⁺to（0.1869,0.3827）for Ba₂GdYSi₄O₁₃:Eu²⁺.The temperature-dependent photoluminescence analysis optimized the solid solution phosphor and got the stability decrease with increasing Y³⁺concentration.However,those phosphors exhibit excellent thermally stable properties.（2）A series of novel Ce³⁺/Eu²⁺single and co-doped phosphor K₂Ba₇Si₁₆O₄₀ have been synthesized via a high-temperature solid-state.The crystal structure was confirmed by its XRD pattern,the position of doped ions take up in the matrix was also discussed.The optical properties and optimum doped concentration can be obtained by its fluorescence spectra.The energy transfer from Ce³⁺to Eu²⁺has been detected in Ce³⁺and Eu²⁺codoped K₂Ba₇Si₁₆O₄₀.The critical distance was calculated to be 25.06?according to the Dexter’s theory and the energy transfer mechanism was proved to be dipole-dipole interaction.The emission color can be tuned from deep blue（0.16,0.05）to bluish-green（0.15,0.27）by adjusting Eu²⁺concentrations due to the energy transfer from Ce³⁺to Eu²⁺ions.The thermal quenching was also investigated and the activation energyΔE is obtained to be 0.26 eV.（3）Ce³⁺/Tb³⁺/Eu³⁺single and co-doped MgY₄Si₃O₁₃ phosphors were prepared by the high temperature solid state method successfully.The structure of MgY₄Si₃O₁₃was present obviously in the text and two sites for the luminescence centers Ce³⁺/Tb³⁺/Eu³⁺to substitute.The Y³⁺sites was high proportion than Mg²⁺for Ce³⁺/Tb³⁺/Eu³⁺to substitute in MgY₄Si₃O₁₃ considering the ionic radii and charge balance.Emitting color of Ce³⁺,Tb³⁺,Eu³⁺single doped MgY₄Si₃O₁₃ is blue,green and red,respectively.Under 355 nm excitation,energy transfer process from Ce³⁺to Tb³⁺and Tb³⁺to Eu³⁺can been detected in the emission spectra of MgY₄Si₃O₁₃:0.01Ce³⁺,x Tb³⁺and MgY₄Si₃O₁₃:0.01 Ce³⁺,0.10 Tb³⁺,y Eu³⁺and further demonstrate by the decay curves of Ce³⁺and Tb³⁺.The energy transfer mechanism from Ce³⁺to Tb³⁺and Tb³⁺to Eu³⁺in phosphors has been calculated to be a resonant type via the electric dipole-quadrupole and dipole-dipole interaction.According to the CIE coordinates,the emission color can be tuned from blue to green region in MgY₄Si₃O₁₃:0.01 Ce³⁺,x Tb³⁺and eventually white.Expecially in MgY₄Si₃O₁₃:0.01 Ce³⁺,0.10Tb³⁺,0.11 Eu³⁺,its CIE coordinate（0.332,0.364）,is very close to the standard white light（0.33,0.33）.The phosphors synthesized in this paper all own good luminescence characteristics,possess certain application prospect and certain reference effect on the design and investigate of the whole color tunable single-phase phosphors.