Short review on recent progress in Mn4+ -activated oxide phosphors for indoor plant light-emitting diodes

Equivalent chemical substitution in double-double perovskite-type ALaLiTeO6:Mn4+ (A = Ba2+, Sr2+, Ca2+) phosphors enabling wide range crystal field strength regulation and efficient far-red emission

Mn⁴⁺-activated phosphors have shown wide prospective applications in phosphor-converted white light-emitting diodes (pc-WLEDs) and pc-LEDs used in illumination and indoor plant cultivation, respectively. Recently, double perovskites A₂B'B''O₆ with a tunable crystal structure and versatile octahedral sites have been extensively studied as good host matrixes for Mn⁴⁺-emitters to realize tunable far-red emissions. Herein, a series of double-double perovskite-type ALaLiTeO₆:Mn⁴⁺ (A = Ba, Ba_0.5Sr_0.5, Sr, Sr_0.5Ca_0.5, Ca) phosphors were synthesized and structurally characterized, and the correlations between their structure and luminescence were also studied systematically. With a decrease of the A-cation size, an increased distortion in the average structure and a structure symmetry lowering (I2/m → P2₁/_n) were observed for ALaLiTeO₆:Mn⁴⁺. In contrast, on the local scale, the degree of (Li/Te)O₆-octahedral distortion is positively correlated with the ΔIR value, which is the ionic radius difference between A²⁺ and La³⁺. The local structural changes were found to be irrelevant to the significant improvements in photoluminescence properties. In combination with careful spectroscopic analysis, we deciphered that a decreased A-cation is in fact helpful for the enhancements in crystal field strength (D_q/B = 2.12-2.82) and Mn-O covalent bonding, thereby resulting in an improved quantum efficiency, a suppressed nonradiative transition, and a redshift in photoluminescence spectra. Amongst the ALaLiTeO₆:Mn⁴⁺ phosphor series, CaLaLiTeO₆:Mn⁴⁺ exhibits the highest external quantum efficiency of 70.1% and internal quantum efficiency of 96.4% and superior thermal stability (93.3%@423 K), making CaLaLiTeO₆:Mn⁴⁺ very promising as far-red phosphors for pc-LEDs. The findings of this work will serve as a new guide for rational design of high-performance Mn⁴⁺-activated double-double perovskite-type far-red phosphors.

Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn4+ and Mg2

In this report, red-emitting alumina nanophosphors doped with Mn⁴⁺ and Mg²⁺ (Al₂O₃:Mn⁴⁺, Mg²⁺) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al₂O₃:Mn⁴⁺, Mg²⁺ annealed at 500 °C exhibits a cubic γ-Al₂O₃ phase with the space group Fd3m-227. The tetragonal δ-Al₂O₃ and rhombohedral α-Al₂O₃ phase is obtained at 1000 and 1300 °C, respectively. Cube-like nanoparticles in a size of ∼40 nm are observed for the alumina heated at 500-1000 °C. The size and red-emitting intensity of the phosphors remarkably increased with annealed temperature ∼1300 °C. Emission spectra of the phosphors show strong peaks at 678 and 692 nm due to ² E _g → ⁴ A ₂ transitions of the Mn⁴⁺ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al₂O₃:Mn⁴⁺, Mg²⁺ at 298 K, whereas a weak one is observed in those of α- and γ-Al₂O₃:Mn⁴⁺, Mg²⁺. The alumina phosphors exhibited an excellent waterproof ability during 60 days in water and good thermal stability in the range of 77-573 K. A warm-white light-emitting diode (WLED) fabricated using In _x Ga_1-x N nanowire chips with Al₂O₃:Mn⁴⁺, Mg²⁺ red-emitting nanophosphors presents a high color rendering index of ∼95.1 and a low correlated color temperature of ∼4998 K. Moreover, the current-voltage characteristic of the nanowire LEDs could be improved using Al₂O₃:Mn⁴⁺, Mg²⁺ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.

Modulation of Thermally Stable Photoluminescence in Cr3+-Based Near-Infrared Phosphors

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes (pc-LEDs) are desirable for various photonics applications, while developing thermally stable NIR phosphors remains a great challenge. Increasing the temperature accelerates the severe nonradiative relaxation process gorverned by the intrinsic energy gap law, which further suspends the efficient low-energy emission of Cr³⁺ emitters in the inorganic lattice. To address this rule, several state-of-the-art strategies have been put forward in this perspective to modulate the critical law from the viewpoints of (1) crystal structure design, (2) defect engineering, (3) strengthened rigidity, and (4) energy transfer. This perspective suggests avenues for exploring novel broadband NIR phosphors with high thermal stability and will also stimulate further studies on NIR spectroscopy for high-power applications.

Site Preference-Driven Mn4+ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion

The tetravalent-state stability of manganese is of primary importance for Mn⁴⁺ luminescence. Double perovskite-structured A₂B'B″O₆:Mn⁴⁺ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV-VI)O₆ site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A₂B'B″O₆-type materials show no or weak luminescence such as typical Ca₂MgWO₆:Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca²⁺ by Na⁺-La³⁺ to form Ca_2-2xNa_xLa_xMgWO₆:Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO₆] but enlargement of [WO₆] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg²⁺ to W⁶⁺ sites. As a result, the effective Mn⁴⁺/Mn²⁺ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn⁴⁺-related far-red emission. The optimal CaNa_0.5La_0.5MgWO₆:0.9%Mn⁴⁺ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn⁴⁺-doped oxide phosphors. This work may provide a new perspective for the rational design of Mn⁴⁺-activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.

Local Structure Modulation-Induced Highly Efficient Red-Emitting Ba2Gd1-xYxNbO6:Mn4+ Phosphors for Warm WLEDs

Modulating the crystal field environment around the emitting ions is an effective strategy to improve the luminescence performance of the practical effective phosphor materials. Here, smaller Y³⁺ ions are introduced into substituting the Gd³⁺ sites in Ba₂GdNbO₆:Mn⁴⁺ phosphor to modify the optical properties, including the enhanced luminescence intensity, redshift, and longer lifetime of the Mn⁴⁺ ions. The substitution of smaller Y³⁺ ions leads to lattice contraction and then strengthens pressure on the local structure, enhances lattice rigidity, and suppresses nonradiative transition. Moreover, the prototype phosphor-converted light-emitting diode (LED) demonstrates a continuous change photoelectric performance with a correlated color temperature of 4883-7876 K and a color rendering index of 64.1-83.2, suggesting that it can be one of the most prospective fluorescent materials applied as a warm red component for white LEDss. Thus, the smaller ion partial substitution can provide a concise approach to modulate the crystal field environment around the emitting ions for excellent luminescence properties of phosphors toward the modern artificial light.