Synthesis And Photoluminescent Characteristics Of Mn⁴⁺-doped Red Phosphors For Warm W-LED

White light-emitting diodes?WLEDs?have been extensively used in various areas because of their high performance,environmental friendliness,and long-term stability.Given their high correlation temperature?CCT>4500 K?and low color rendering index?Ra<80?resulting from a lack of red components,the use of these WLEDs is considerably limited.In recent years,an increasing number of rare earth doping,red-emitting phosphors,such as Eu3+-and Sm3+-doped phosphors,have been identified and developed.However,the costly raw materials and remarkably broad emission bands result in low color purity and restrict the lighting application.In recent years,Mn⁴⁺-doped phosphors are attracting increased attention because of their good properties,such as its low cost and simple preparation process.The ²Eg-⁴A_2g transition of Mn⁴⁺ in the octahedral environment of the emitting host leads to the sharp peaks of the phosphor in the deep red region,which can meet the application for warm WLEDs.A serious of Mn⁴⁺-doped phosphor with high efficiency and good properties have been synthesized through the conventional solid-state reaction.X ray diffraction?XRD?,electron scanning microscopy?SEM?,ultraviolet visible absorption spectroscopy?UV-vis?and luminescence spectrum analysis were used to studied the morphology,luminescence mechanism and fluorescence quenching mechanism of the material.The fabricated red-emitting LED revealed its potential application in WLEDs.The main research content is as follows:?1?A novel phosphor,Sr₂LaNbO₆:Mn⁴⁺,is synthesized by conventional solid-state reaction.The phosphor can be excited at 300-500 nm and emit a deep-red light.An emission band peak of 694 nm is observed from the spin-forbidden transition ²E?⁴_A2 of Mn⁴⁺ ions.Energy transfer between the Mn⁴⁺-Mn⁴⁺ pairs decreases fluorescence decay time from 0.815 ms to 0.512 ms.The resistance of luminescence against thermal impact is also investigated by emission spectroscopy at heating temperatures from 300 K to 500 K.The calculated activation energy ?E??0.345 eV?demonstrate good thermal stability of the phosphors.All these results indicate that the phosphor can be applied in white-light-emitting diodes.?2?A novel red-emitting phosphor Mg₂Al₄Si₅O₁₈:Mn⁴⁺ is successfully synthesized using a solid-state reaction method.Aluminosilicate Mg₂Al₄Si₅O₁₈ of low-symmetry orthorhombic phase contains abundant octahedrons and is suitable for Mn⁴⁺ doping.A broad absorption band ranging from 240 nm to 550 nm in the ultraviolet-visible spectra corresponds to the photoluminescence excitation spectrum.The emission spectrum from 650 nm to 750 nm exhibits a strong emission peak at 680 nm because of the ²Eg?⁴A_2g transition of Mn⁴⁺ ion.The critical distance Rc for concentration quenching is?33.3 A,and the energy transfer mechanism is determined to be dipole-dipole interaction.Luminescence mechanism is analyzed using the energy level diagram of Mn⁴⁺ ion.Racah parameters B and C are evaluated and used to discuss the nephelauxetic effect for the Mn⁴⁺ ions in the Mg₂Al₄Si₅O₁₈.Temperature-dependent fluorescence test demonstrates the good thermal optical properties of the phosphor.An assembled light-emitting diode?LED?device with efficient luminescence demonstrates its practical use.?3?A novel red phosphor Ba₂GdNbO₆:Mn⁴⁺was synthesized through high-temperature solid-state reaction.The host Ba₂GdNbO₆ with double perovskite structure was investigated.Scanning electron microscopy and thermogravimetric analysis were performed to evaluate the structure and thermal stability of the phosphor,respectively.The PL spectra exhibit a emission peak at 676 nm,which are attributed to the spin-forbidden transition of Mn⁴⁺:²Eg?⁴A_2g.Moreover,crystal field strength Dq and Racah parameters were calculated to estimate the nephelauxetic effect of Mn⁴⁺ on the Ba₂GdNbO₆ host lattice.Thermal quenching characteristics were also analyzed.The fabrication of red-emitting LED revealed its potential application in WLEDs.?4?The Ba₂YNbO₆:Mn⁴⁺ phosphor is successfully synthesized through conventional solid-state reaction.The crystal structure and doping site were investigated using X-ray diffraction?XRD?.Bond energy and the purity of the samples were characterized by using Fourier transform infrared?FT-IR?and Raman spectra,respectively.UV-Vis absorption spectra with a broad band ranging from 275 nm to 600 nm demonstrate that it can be efficiently excited by UV/blue light.The photoluminescence spectra exhibit a strong narrow red emission band peaking at 695 nm.The luminescence mechanism has been investigated by Tanabe-Sugano diagram.Nephelauxetic ratio,?1,crystal field strength,Dq,and Racah parameters,B and C,have been evaluated.Thermal characteristics in high and low temperatures have been studied.The quenching effect and the increment of the anti-Stokes emission intensities have been investigated.The values of the activation energy,E,have been calculated to explain thermal quenching.The fabricated red-emitting LED have good color rendering index and low color temperature.All these results show that the phosphor can be used as a fluorescent material for the warm WLEDs.