Mn⁴⁺ Activated Oxide Red Fluorescent Materials:Synthesis,Luminescence And Mechanism

As the latest generation of light sources,the Light-emitting-diode?LED?has the advantages of low energy consumption,high luminous efficiency,fast response speed,environmental friendliness,and long lifetime.Through the combination of LED chips with yellow phosphors,the phosphor-converted white LEDs?pc-WLEDs?are widely applied.However,due to the lack of red light,commercially produced pc-WLEDs have low color rendering index and high color temperature,which makes it difficult to be applied in indoor and dining places.In the preparation of warm WLEDs,red phosphors are generally added,which can reduce the correlated color temperature of white light and improve the color rendering index.Moreover,the phosphor-converted red LEDs combined with red phosphors and LED chips can be applied to plant cultivation to increase plant yield and regulate photomorphogenesis.However,the existing red phosphors are still far from the practical standards.On one hand,the red phosphors with better performance are basically phosphors with rare earth metal ions as active ions and a small part of Mn⁴⁺doped fluoride phosphors.The cost of rare earth metals is relatively high,and the reserves are low.The hydrofluoric acid used in the preparation of fluoride is harmful to the environment,which severely limits the application potential of red phosphors.On the other hand,the quantum yield and luminescence stability of the red phosphors cannot meet the complex requirements of various practical applications.Therefore,it is of great significance to explore the red phosphor with high luminous efficiency,good stability,and low cost.In view of the above problems,this study has explored phosphors with Mn⁴⁺,the low-cost and abundant non-rare earth metal ions,as the activator and the oxides as the matrixes.At the same time,various characterization and testing methods are used to study the luminescence mechanism of Mn⁴⁺in different matrixes,and the law of lattice adjusting and valance regulation of Mn⁴⁺luminescence has been explored from the microscopic molecular and atomic levels.Finally,red/far-red phosphors with excellent luminous performance are obtained,and the application potentials of the phosphors in the fields of far-red LEDs for plant cultivation,white LEDs for general lighting,and optical temperature sensing are explored.The specific contents are as follows:?1?Modulating the local environment around the emitting ions with component screening to increase the quantum yield and thermal stability is an effective and promising strategy for the design of high-performance fluorescence materials.In this work,smaller Lu³⁺is introduced into the La³⁺site in a Mn⁴⁺-activated LaAlO₃ phosphor with the expectation of improving the luminescence properties via lattice contraction induced by cation substitution.Finally,La1-_xLu_xAlO₃:Mn⁴⁺?x=0-0.04?perovskite phosphors with a high quantum yield of86.0%and satisfactory thermal stability was achieved,and the emission peak at 729 nm well matches with the strongest absorption peak of the phytochrome P_FR.The favorable performances could be attributed to the suppressed cell volume and superior lattice rigidity after the substitution of Lu³⁺.This work obtains a highly efficient La1-_x Lu_xAlO₃:Mn⁴⁺?x=0.02?phosphor,which holds great potential for application in plant-cultivation light-emitting diodes.?2?Transition metal ions Mn²⁺or Mn⁴⁺activated fluorescent materials exhibit an increasingly important role in solid-state lighting applications.Herein,Li⁺as an efficient charge regulator is used to modulate the valence and photoluminescence of Mn²⁺/Mn⁴⁺in ZnAl₂O₄.In-depth density functional theory calculations and detailed experimental results manifest that the incorporation of Li⁺could decrease negative charge density and suppress the self-reduction of Mn ions,which greatly facilitate the formation and stabilization of Mn⁴⁺.Accordingly,the red-emitting ZnAl₂O₄:Mn⁴⁺affords an impressive quantum yield of 69.9%,and a warm WLED prototype comprised of this red phosphor shows favorable properties?R_a=80.2,CCT=4385 K?.Furthermore,dual-emitting ZnAl₂O₄:Mn²⁺/Mn⁴⁺peaking at 511 and651 nm obtained by adding the appropriate amount of Li⁺,can be applied as a thermochromic FIR temperature sensing probe with the maximum relative sensitivity of 2.85%K^-1 at 423 K.The present work not only offers an in-depth elucidation of Mn valance regulation but also provides a new avenue toward controlled design and fabrication of multifunctional Mn²⁺/Mn⁴⁺activated optical materials.