Luminescence Properties Of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺,Mn²⁺ And BaMg₂Al₆Si₉O₃₀:Eu²⁺,Tb³⁺,Mn²⁺ Phosphors For White L

The light-emitting diodes (LEDs)-based white light sources have receivedincreasing interest in recent years for their promising applications on illuminationswith advantages in power efficiency, reliability, long lifetime, and environmentalprotection. Generally, there are two ways to gain white light: one is combination of ablue LED (460nm) with a yellow phosphor or green and red phosphor. The secondapproach to achieve white light is combination of a near-UV(NUV) or UV LED withred, green, and blue phosphor. The blue LED can achieve high luminous efficiency,while the near-ultraviolet (UV) based white LEDs can show high color renderingindex and stability, and have more extensive application on the filed of specialillumination. At present, there are very few inorganic green phosphors for blue LEDand a single-phase full-color emitting phosphor with environmental stability andnontoxicity for NUV LEDs. In this paper, Ca8Zn(SiO4)4Cl2:Eu2+,Mn2+andBaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+are chosed and their luminescent properties, energytransfer and mechanism have been studied. The main results obtained are listed asfollows:1. Eu2+activated Ca8Zn(SiO4)4Cl2were prepared by high temperature solid statereaction with nominal composition of (2-x)CaO-xZnO-SiO2-CaCl2(0≤x≤1.1). It is observed that the material phase gradually converts from Ca₃SiO₄Cl₂:Eu²⁺toCa₈Zn(SiO₄)₄Cl₂:Eu²⁺followed by improved luminescent properties withincreasing x. The two phosphors both emit with a maximum at505nm.Significantly, Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺shows a more intense excitation band in theblue centered at450nm. Upon450nm excitation, the integrated emissionintensity of Ca₈Zn (SiO₄)₄Cl₂:Eu²⁺is1.3times that of Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺andnearly the same as that of Ca₃Sc₂Si₃O₁₂: Ce³⁺phosphors. Attempts to understandthe origins of the intense luminescence are presented on the basis of diffusereflection spectra and fluorescence decays of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺in comparisonwith related phosphors. The present results suggest that Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺could be a promising green emitting phosphor for excitation by blue light emittingdiodes.2. Luminescence and energy transfer in Eu²⁺and Mn²⁺codoped Ca₈Zn(SiO₄)₄Cl₂are investigated. The emission spectra of the phosphors show a green band at505nm of Eu²⁺and a yellow band at550nm of Mn²⁺. The ratio of the yellowemission550nm the green505nm obtained from emission spectra is consistentwith the theoretical calculation based on energy transfer and lifetimemeasurements.3. A series of single-phase full-color emitting BaMg₂Al₆Si₉O₃₀:Eu²⁺, Tb³⁺, Mn²⁺phosphors have been synthesized by solid state reaction. Energy transfer fromEu²⁺to Tb³⁺and Eu²⁺to Mn²⁺in BaMg₂Al₆Si₉O₃₀host matrix is studied byluminescence spectra, energy transfer efficiency and lifetimes. InBaMg₂Al₆Si₉O₃₀:Eu²⁺, the PL spectrum consists of two bands, located at about376nm and450nm, respectively. Results indicate that the band at376nm isassigned to Eu²⁺(I) occupying Ba2+with weak crystal field, and the other one at450nm corresponds to Eu²⁺(â…¡) occupying Mg₂+with strong crystal field. Theenergy transfer leads to the following results:(1) the energy transfer of Eu²⁺-Tb³⁺is dominated by Eu²⁺(â…¡)–Tb³⁺transfer rather than Eu²⁺(I)–Tb³⁺transfer.(2) theenergy of the red emission of Mn²⁺is considered to come from both Eu²⁺(I) andEu²⁺(â…¡).(3) the ratio of the red emission of Mn²⁺to the emission of Eu²⁺byexperiment is consistent with the theoretical calculation basing on energy transferand lifetime measurements. The wavelength-tunable white light can be realized bycoupling the emission bands centered at450nm,542nm and610nm ascribed tothe contribution from Eu²⁺and Tb³⁺and Mn²⁺, respectively. By properly tuning the relative composition of Tb³⁺/Mn²⁺, chromaticity coordinates of (0.31,0.30),high color rendering index Ra=90and correlated color temperature CCT=5374Kcan be achieved upon excitation of UV light. We have demonstrated that thevaried emitted color from blue to green or red and eventually to white can beachieved by properly tuning the relative ratio of Tb³⁺and Mn²⁺. All these resultsindicate that BaMg₂Al₆Si₉O₃₀:Eu²⁺, Tb³⁺, Mn²⁺is a promising single-compositionphosphor for application involving white light LED.4. Eu²⁺and Eu²⁺-Mn²⁺codoped (Ba, Sr) Mg₂Al₆Si₉O₃₀phosphors have beensynthesized by solid state reaction, and their luminescent properties areinvestigated. A detail analysis on the energy transfer from Eu²⁺to Mn²⁺inSrMg₂Al₆Si₉O₃₀host is presented, which indicates the energy of the red emissionof Mn²⁺is derived mainly from Eu²⁺(I). We have also demonstrated thatBaMg₂Al₆Si₉O₃₀: Eu²⁺, Mn²⁺exhibits better thermal quenching properties thanthat of SrMg₂Al₆Si₉O₃₀: Eu²⁺, Mn²⁺because of bigger activation energy.