Preparation Of Multiple Ions Doping Nanofiber Mats Via Eletrospinning For White LEDs

White light-emitting diodes?LEDs?have attracted increasing attention as a light source for the next-generation of general illumination and automotive lighting applications due to the low energy consumption,high efficiency,long lasting and positive environmental effects.At present,the fluorescence conversion of LED is the main way to obtain white LED.However,the phosphor layer playing an important role in the fluorescence conversion of LED greatly influences the of white light,such as correlated color temperature?CCT?,color rendering index?CRI?,luminous efficiency and operating life.Thus the control of multiple-peaks'emission and layer morphology has been regarded as an effective method to improve the optical performance.In this work,nanofiber phosphor mats doped with rare-earth ions which have controllable fiber morphology in microcosm and complete membrane at macro level were prepared at relatively low temperature.The full-spectrum emission in a single host was also achieved by the dual-doping and three-doping of rare-earth ions.A new kind of Ca₄Si₂O₇F₂:Ce³⁺,Tb³⁺,Sm³⁺fluorescent fiber mats were obtained through electrospinning process and heat treatment.The fluorescent fiber mats have banded structure with a width of 2?m and a thickness of 200 nm.XRD result shows that pure Ca₄Si₂O₇F₂ phase can be obtained at a low temperature around 900°C for 4 h,which is reduced by at least 100°C compared to usual solid reaction.Single peak emission of red,green and blue of fluorescent materials based on Ca₄Si₂O₇F₂ was achieved by single doping of different ions.Ca₄Si₂O₇F₂:Ce³⁺,Ca₄Si₂O₇F₂:Tb³⁺,and Ca₄Si₂O₇F₂:Sm³⁺exhibit characteristic emissions of Ce³⁺?4f⁷?4f⁶5d¹,blue?,Tb³⁺?4f⁸?4f⁷5d,green?,and Sm³⁺?4f⁶?4f⁶5d red?under the excitation of near-UV light,respectively.At the same time,white light emission of the fluorescent fiber film under UV irradiation was achieved by mixing a variety of rare-earth ions.Eu²⁺doped CaSi₂O₂N₂ fiber mats for white LED were prepared through electrospinning combined with gas reduction nitridation.By adjusting the content of polymer and solvent ratio in the spinning solution,different microstructures of CaSi₂O₂N₂:Eu fluorescence fiber membranes were obtained,including ordinary nanofiber mats,micron belt mats,and hollow fiber mats.The as-obtained samples not only keeps the different fiber structure but also assemble to a film.XRD patterns reveal that the sample nitrided under 1300°C for 5 h are crystallized well and maintained CaSi₂O₂N₂ phase after Eu²⁺doping.The CaSi₂O₂N₂:Eu²⁺fluorescent fiber film has a broad emission peak near 550 nm under the 400 nm excitation light,corresponding to the transition of Eu²⁺ion?4f⁶5d?4F⁷?.Compared with CaSi₂O₂N₂:Eu²⁺phosphor powders,the CaSi₂O₂N₂:Eu²⁺phosphor fiber film has stronger emission intensity in PL measurement.CaSi₂O₂N₂:RE(RE=Ce³⁺/Tb³⁺,Ce³⁺/Eu²⁺,Ce³⁺/Mn²⁺)phosphor nanofiber mats were prepared via the electrospinning process and further gas reduction nitridation.Diameters of fiber in the fiber mats are generally in the range of 400⁶00 nm,and the sample is in the state of film which exhibits smooth and uniform surface morphology.XRD patterns reveal that the sample nitrided under 1300°C for 5 h crystallized well and maintained CaSi₂O₂N₂ phase.Owing to the energy transfer of Ce³⁺-Tb³⁺,emitting color of the phosphor mats can be changed from blue to green when irradiated by near ultraviolet light.Meanwhile,we also found that the switching of emitting colors from blue to red and blue to yellow can be achieved via the energy transfer of Ce³⁺-Mn²⁺and Ce³⁺-Eu²⁺,respectively.At the same time,the emission of different light colors can be realized by encapsulating three fluorescent fiber films in the LED chip excited by near ultraviolet radiation.A double layer fluorescent fiber film?CaSi₂O₂N₂:Ce/Tb,Eu?for white LED was prepared by step-electrospinning combined with gas phase reduction nitridation.The fiber film keeps the fiber structure on the micro level,the transition between layers is smooth,the fiber structure is similar,and the film state appears macroscopically.XRD patterns reveal that the sample nitrided under1300°C for 1 h crystallized well and maintained CaSi₂O₂N₂ phase.The difference of luminescence caused by ion doping in double layer fluorescent fiber film is obvious.Under the near ultraviolet irradiation,sample with surface dopimg of Ce/Tb ions shows blue-green emission characteristics,and sample with surface doping of Eu ion shows yellow light emission characteristics.The fluorescence spectra shows that the quantum efficiency of Eu ion doped is 6%higher than that of Ce/Tb ions doped when exposure to excitation light.Finally,white light emission is successfully achieved by encapsulating double layer fluorescent fiber film with ultraviolet irradiation of Eu ion doped surface in the LED chip.