Preparation And Photoluminence Properties Of Lanthanide Ions Activated Novel Phosphate Phosphors For White LED

Energy saving is more beneficial solution for the development of society and the great significance of science and technology has attracted wide attention.Compared with traditional lighting sources?incandescent lamps,fluorescent lamps?,the solid state lighting?SSL?technology particularly phosphor–converted white light emitting diodes?pc-WLEDs?paying more attention owing to their merits of energy saving,luminescence,high efficiency,long life,rugged,mercury-free and easy to maintain and so on.The commercial WLED was encapsulated using the yellow emitting YAG:Ce³⁺phosphor and blue emiting InGaN chip.In this type of WLED led to poor color rendering index?CRI?and high correlated color temperature?CCT?due to the lack of red component in the visible spectral region.Nowadays,researchers are mainly focusing on novel phosphors with their excellent performance of high CRI,low CCT,and which can be efficiently excited by near-UV chip?350⁴10 nm?or blue?450⁴70 nm?chip.At present,phosphate based phosphors are widely used for solid state lighting applications due to their easy synthesis,low-cost,high thermal and chemical stabilities over a wide range of temperatures.Therefore,after a lot of literature investigation and consideration of actual experimental conditions,we finally decided to adopt the high-temperature solid phase method to synthesize several phosphate based phosphors.Furthermore,X-ray powder diffraction was used to determine the structural properties of synthetic phosphors.The photoluminescence mechanism of phosphors was given from the various aspects of excitation spectrum,emission spectrum,doping concentration,color purity analysis and fluorescence lifetime.Main contributions of this work are as follows:?1?The Na₃Sc₂?PO₄?₃:xEu³⁺?x=0.10,0.25,0.30,0.35,0.45?phosphors were synthesized by a conventional high-temperature solid-state reaction technique.Crystal structure of as-synthesized Na₃Sc₂?PO₄?₃:xEu³⁺phosphor was characterized by using the XRD data.The morphology of the sample was analyzed through SEM.The Photoluminescence excitation spectrum?PLE?of Na₃Sc₂?PO₄?₃:xEu³⁺phosphor is composed of charge transfer band?CTB?of O^2-?Eu³⁺and several sharp peaks,which corresponds to the f-f transitions of Eu³⁺ions.The PLE spectrum of Na₃Sc₂?PO₄?₃:Eu³⁺phosphor has a strong excitation peak at 394 nm.The Photoluminescence?PL?spectrum of Na₃Sc₂?PO₄?₃:Eu³⁺phosphor consisted of several peaks,corresponding to the ⁵D₀?⁷F_J?J=0,1,2,3,4?transitions of Eu³⁺ions.The Na₃Sc₂?PO₄?₃:Eu³⁺phosphor exhibited a strong red emission peak at 613 nm?⁵D₀?⁷F₂?.Eu³⁺ions have distinct concentration quenching characteristics in the Na₃Sc₂?PO₄?₃ host,and the optimum doping concentration was found to be 35 mol%.The lifetime of Na₃Sc₂?PO₄?₃:Eu³⁺phosphors were determined in the order of milliseconds.The luminous brightness of Na₃Sc₂?PO₄?₃:0.35Eu³⁺phosphor is 6.17 times higher than that of commercial Y₂O₂S:Eu³⁺red phosphor under the excitation at 394 nm.The color coordinates?0.642,0.353?and quantum efficiency?49%?were estimated for the optimized Na₃Sc₂?PO₄?₃:0.35Eu³⁺phosphor at?_ex=394 nm.These results indicated that the Na₃Sc₂?PO₄?₃:0.35Eu³⁺red emitting phosphor can be used as a potential candidate for near-UV excited white light-emitting diodes.?2?The investigation of Na₃Sc₂?PO₄?₃:Re(Re=Ce³⁺/Tb³⁺/Ce³⁺,Tb³⁺)blue and green phosphors were synthesized by conventional high temperature?1100??solid state reaction technique.The Na₃Sc₂?PO₄?₃:Ce³⁺phosphor showed a blue-purple emission at?_ex=320 nm.Upon excitation at 378 nm,the Tb³⁺ions-doped Na₃Sc₂?PO₄?₃ phosphors exhibited a strong green emission centered at 542 nm due to the ⁵D₄?⁷F₅ transition.The optimum doping concentration of Tb³⁺ions in Na₃Sc₂?PO₄?₃:xTb³⁺was 30 mol%.Under the excitation of 320 nm,the Na₃Sc₂?PO₄?₃:Ce³⁺,Tb³⁺phosphors can be tuned by adjusting the proportion of Ce³⁺,Tb³⁺ions.In addition,the energy transfer efficiency from Ce³⁺to Tb³⁺ions in Ce³⁺and Tb³⁺co-doped Na₃Sc₂?PO₄?₃ phosphors were also studied.The Na₃Sc₂?PO₄?₃:Ce³⁺,Tb³⁺phosphors had high activation energy?0.245 eV?and better energy transfer efficiency?76.63%?.The results showed that the as-prepared Ce³⁺/Tb³⁺co-doped Na₃Sc₂?PO₄?₃ blue-green phosphors can be used as a potential candidate for near-UV-based WLEDs.?3?A series of red-emitting phosphors NLPM:xEu³⁺were prepared by a low-temperature solid-state method.Under the 396 nm near-UV excitation,all the samples gave rise to bright red emissions at around 616 nm corresponding to the⁵D₀?⁷F₂ transition of Eu³⁺ions.The emission intensity of NLPM:xEu³⁺phosphors was greatly dependent on the Eu³⁺ion concentration and the optimal doping concentration was found to be 60 mol%.Meanwhile,the CIE chromaticity coordinate of NLPM:0.6Eu³⁺was determined to be?0.660,0.338?and the IQE was 73.1%,which are both better than that of commercial red phosphor Y₂O₂S:Eu³⁺and Y₂O₃:Eu³⁺.More importantly,the colour purity of the NLPM:0.6Eu³⁺phosphor was up to 93.5%.All these outstanding characteristics make the Eu³⁺-activated Na₂Lu?MoO₄??PO₄?phosphors promising candidates for white LEDs as red-emitting phosphors.At last,we discuss about the summary of the investigated results in a brief manner.The future scope of this dissertation work has also been discussed in detail.