| White light-emitting diodes(WLEDs)have been attracting more and more attention from researchers all over the world and used extensively in many fields nowadays because their high energy efficiency,long-term reliability,environment friendliness.There are many researchers called LED as the next general lighting source.In general,typical WLEDs is packaged by yellow phosphor Y3Al5O12:Ce3+(YAG)and a blue GaN chip,which lacks high efficiency red-emitting components.The light out of those WLEDs suffers from a high correlated color temperature and a low color rendering index.Hence,it is necessary to explore suitable red phosphors to improve the CCT and Ra quality of WLEDs.In all kinds of red phosphors,Mn4+doped fluoride phosphors have been widely investigated due to their simple synthesis strategy and excellent luminescence properties.Owing to their broadband absorption in the blue region and intense emission in red region,Mn4+doped fluoride phosphors have been known as excellent red-emitting fluoride phosphors for WLEDs.In this paper,we synthesized a series of ternary-alkaline fluoride red phosphors via co-precipitation method and hydrothermal method.Furthermore,a series of efficient red phosphor K2SiF6:Mn4+(KSFM)particles with uniform morphology were successfully prepared by a two-step chemical co-precipitation method in surfactant-and template-free process at room temperature.The crystal structure,phase purity,morphology,composition and electronic band structure of these phosphors were investigated in detail using X-ray diffraction(XRD),environmental scanning electron microscopy(ESEM),UV-Vis-NIR spectrophotometer,and DFT calculation.The main points are summarized as follows:1.A novel ternary-alkaline red emitting fluoride phosphor,K2NaGaF6:Mn4+,has been successfully synthesized via co-precipitation method at room-temperature.From the structure diagram of K2NaGaF6,we can see that K2NaGaF6 has the cubic unit cell with the space group Fm3m.The electronic structure of K2NaGaF6 was calculated with the CASTEP module of the Materials Studio package.Obviously,the indirect band gap of K2NaGaF6 is approximately 5.92 eV,that indicates K2NaGaF6 is a good luminescent host due to its wide band gap.It can be found that the PLE spectra contain two bands,centered at?360 nm and?465 nm when monitored at 630 nm.Under a 466 nm excitation,the phosphor emits a series of narrow emission lines located at 598,607,613,622,629,633 and 646 nm can be assigned to the transitions of anti-Stokes V3(t1u),V4(t1u),and v6(t2u),zero phonon line(ZPL),and Stokes v6(t2u),V4(t1u)and V3(t1u)vibronic modes,respectively.The optimal reaction conditions to obtain the red light K2NaGaF6:Mn4+about 4 h aging time with 1.7 mol%Mn4+.Finally,we fabricated a white LED using a blue LED chip combined with commercial yellow YAG:Ce3+phosphor and synthesized K2NaGaF6:Mn4+red phosphor.The Ra and CCT of this WLED are 81.6 and 3643 K.All these results indicate that K2NaGaF6:Mn4+phosphor may be a suitable candidate for indoor lighting applications.2.A series Rb2NaGaF6:Mn4+red phosphors has been successfully synthesized via hydrothermal method.From the structure diagram of Rb2NaGaF6 we can see it is similar to the structure of K2NaGaF6.The effective radius of Ga3+ion and Mn4+ion at a CN=6(CN=coordination number)are 0.62 A and 0.53 A,respectively.Being of the similar valence state and ionic radius between Ga3+and Mn4+,Mn4+might occupy the site of Ga3+ion in this fluoride phosphor.Under a 466 nm excitation,Rb2NaGaF6:Mn4+red phosphors emit a series of narrow shape line lights around 590?650 nm attributed to the 2Eg?4A2g transition.The optimal doping concentration is 1.0 mol%Mn4+.we fabricated a white LED using a blue LED chip combined with commercial yellow YAG:Ce3+phosphor and synthesized Rb2NaGaF6:Mn4+red phosphor and results indicate that Rb2NaGaF6:Mn4+phosphor may be a suitable candidate for W-LED.3.A series of efficient red phosphor K2SiF6:Mn4+(KSFM)particles with uniform morphology were successfully prepared by a two-step chemical co-precipitation method in surfactant-and template-free process at room temperature.The KF-HF precipitator adding speed plays an important role in forming KSFM particles with different morphology.From the viewpoint of thermodynamics,the morphology of the lowest surface energy of(111)facet appeared as octahedron when the KF-HF precipitator at the lower concentration.The fast KF/HF precipitator adding speed(1.0 ml/s),that is to say,higher concentration of precipitant in the crystal surface which could promoted chemical reaction resulted in the KSFM particles being surrounded by higher surface energy lattice planes(100).Finally,(100)facet existed and displayed cube morphology when the precipitator adding speed at 1.0 ml/s.Moreover,the Photo-luminescence,Decay curves and Raman spectra dependence of its morphology were investigated,respectively.White-LED devices made of a blue GaN chip merged with yellow phosphor(YAG:Ce3+)and the obtained red phosphor(KSFM)could produce white light,which is much warmer than that only merged with yellow(YAG:Ce3+)component. |
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