Local Structure Modulation Induced Highly Efficient Far-Red Luminescence of La1–xLuxAlO3:Mn4+ for Plant Cultivation

Co-Doping Effect of Mn2+ and Eu3+ on Luminescence in Strontiowhitlockite Phosphors

A new series of Sr-based phosphates, Sr9-xMnxEu(PO4)7, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as strontiowhitlockite, which is a β-Ca3(PO4)2 (or β-TCP) structure. The concentration of Mn2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu3+ to Eu2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d-4f Eu2+ and 4f-4f Eu3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn2+ concentration due to quenching effect of Eu3+ by the Mn2+ levels. The temperature dependence of Eu2+ photoluminescence in a Sr9-xMnxEu(PO4)7 host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.

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.

Broadband UV-Excitation and Red/Far-Red Emission Materials for Plant Growth: Tunable Spectrum Conversion in Eu3+,Mn4+ Co-doped LaAl0.7Ga0.3O3 Phosphors

Broadband ultraviolet (UV) excitation and red/far-red emission phosphors can effectively convert solar spectrum to enhance photosynthesis and promote morphogenesis in plants. Based on the above application requirements, Eu³⁺ single-doped LaAl_1-yGa_yO₃ solid solutions and Eu³⁺,Mn⁴⁺ codoped LaAl_0.7Ga_0.3O₃ phosphors were designed and synthesized in this work. The LaAl_0.7Ga_0.3O₃:0.05Eu³⁺ (LAG:Eu³⁺) phosphor exhibits a strong charge transfer band (CTB) excitation and characteristic ⁵D₀ → ⁷F₂ transition red emission (619 nm), which is very similar to the luminescence properties of Eu³⁺-organic ligand compound (EuL₃). Rietveld refinement studies further revealed that the cation substitution disturbs the site symmetry. The optimal Eu³⁺, Mn⁴⁺ co-doped LaAl_0.7Ga_0.3O₃ (LAG:Eu,Mn) phosphor possesses a dual-band excitation spectrum in broadband ultraviolet (UVA, UVB) area and a dual-band emission spectrum within red/far-red area. Under the sunlight radiation, the real-time spectrum of luminous laminated glasses fabricated by coating the LAG:Eu,Mn phosphor shows the percentage of radiant intensity in the red/far-red region significantly increases, suggesting that the phosphor can be a promising candidate for solar spectral conversion in plant cultivation. We believe this work provides a new idea for developing novel broadband ultraviolet excitation and red/far-red emission phosphors.

Investigation of multicolor emitting Cs3GdGe3O9:Bi3+,Eu3+ phosphors via energy transfer for WLEDs

Bi³⁺/Eu³⁺ doped Cs₃GdGe₃O₉ luminescent materials were prepared by a solid-state reaction. The energy band and density of states of Cs₃GdGe₃O₉ were calculated by density functional theory. The Cs₃GdGe₃O₉ host presents a broadband emission peaking at 520 nm. Systemic measurement and analysis of luminescence properties were performed to confirm the energy transfer in Cs₃GdGe₃O₉:Bi³⁺,Eu³⁺. The multicolor modulated emission from blue (0.1678, 0.1568) to red (0.5931, 0.3251) can be achieved by varying the doping ratio of bismuth to europium. A white light-emitting diode (WLED) was produced by combining the Cs₃GdGe₃O₉:0.05Bi³⁺,0.1Eu³⁺ phosphor, a commercial green phosphor, and a 310 nm ultraviolet chip. The color rendering index of the WLED driven by 20 mA bias current is 89.6 with the CIE coordinates of (0.3520, 0.3626). The results reveal that the Cs₃GdGe₃O₉:Bi³⁺,Eu³⁺ phosphor is a potential material that can be used in multicolor tunable luminescence and WLEDs.

Sr/Ba substitution induced higher thermal stability far red-emitting Ba1-ySryLaLiWO6:Mn4+ phosphors for plant growth applications

A series of red-emitting BaLaLiWO₆:Mn⁴⁺ (BLLW:Mn⁴⁺) phosphors were successfully synthesized by a high-temperature solid-state reaction method. The crystal structure and luminescence properties of the obtained samples were systematically investigated. The emission spectra exhibited a deep red emission band peaking at 716 nm with a full width at half-maximum (FWHM) of 44 nm under 340 nm excitation. The optimal Mn⁴⁺ molar concentration was about 1.2%. In addition, the luminescence mechanism was analyzed using a Tanabe Sugano energy level diagram. With the substitution of Sr for Ba, there was a red shift in the emission spectrum and a blue shift in the excitation spectrum. The emission intensity of BLLW:1.2%Mn⁴⁺ at 150 °C was about 22% of the initial value at room temperature. In contrast, the emission intensity of SrLaLiWO₆:1.2%Mn⁴⁺ still maintained 79% of the initial emission intensity at room temperature at 150 °C. This was due to the fact that with the substitution of Sr for Ba, the W-O bond length gradually decreases, which gradually enhanced the crystal field strength of Mn⁴⁺.

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.

Enhancing the luminescence performance of an LED-pumped Mn4+-activated highly efficient double perovskite phosphor with A-site defects via local lattice tuning

Enhancing luminescence performance of Mn4+-activated highly efficient double perovskite phosphor with A-site vacancies via local lattice tuning (left), and warm white and far-red light-emitting diodes prepared with the phosphors (right).

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.

Research on the quantum confinement effect and enhanced luminescence of red-emitting P5+-doped CaAl12O19:Mn4+,Mg2+ phosphors

Mn⁴⁺-activated oxide red phosphors are always a hot topic in the luminescent material field to solve the lack of red light components in white-light-emitting diodes (WLEDs). Herein, a series of novel deep red-emitting CaAl_12-mP_mO_19+m:0.01Mn⁴⁺,0.2Mg²⁺ (m = 0-0.15) phosphors were synthesized and their crystal structure, luminescence properties and thermal stability were investigated in detail. The high-valence P⁵⁺ is used to replace low-valence Al³⁺ in the luminescent host CaAl₁₂O₁₉ to improve the photoluminescence quantum yield (PLQY) of phosphors. The doping of P⁵⁺ does not change the crystal phase structure of phosphors, and the luminescence intensity and PLQY are significantly enhanced. The analysis of the photocurrent and fluorescence lifetime shows that an electron trap with a quantum-confinement structure is formed in the phosphor host, which plays a key role in buffering photogenerated electrons. Therefore, the PLQY of the P⁵⁺-doped CaAl_11.90P_0.1O_19.10:0.01Mn⁴⁺,0.2Mg²⁺ phosphor increased from 9.8% (P⁵⁺-undoped) to 70.2%, and the mechanism of PLQY enhancement is proposed based on the analysis of the crystal structure. Furthermore, the phosphor has superior thermal stability and color purity (96.8%). Overall, this work provides new insights and ideas on quantum confinement effects for improving the quantum yield of Mn⁴⁺-activated luminescent materials.

Enhancing Luminescence and Controlling the Mn Valence State of Gd3Ga5-x-δAlx-y+δO12:yMn Phosphors by the Design of the Garnet Structure

Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn solid solutions with improving luminescence properties were prepared via cation substitution and a controllable Mn valence state. The abnormal autoreduction from Mn⁴⁺ to Mn²⁺ ions was observed during the formation of Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn. The doped manganese ions occupy octahedral Ga³⁺(1) and Al³⁺(1) sites to form the Mn²⁺ luminescent center with red emission at 630 nm and Mn⁴⁺ luminescent centers with deep red light emission at 698 nm, respectively, matching well with the red light absorption of phytochrome (P_R) and the far-red light absorption of phytochrome (P_FR). With the design of the concentration of Al³⁺ and doped manganese ions, the photoluminescence (PL) of Mn⁴⁺/Mn²⁺ (corresponding to P_FR/P_R) can be tuned, which is very useful for controlling the plant growth. Moreover, the PL intensity of Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn can be increased by 6.8 times by substituting Al³⁺ for Ga³⁺. The thermal stability is also enhanced significantly. Finally, a series of warm white-light-emitting diodes (WLEDs) with good performance were fabricated using the as-prepared Gd₃Ga_5-x-δAl_x-0.012+δO₁₂:0.012Mn phosphor. The results show that the designed Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn phosphors have potential practical values in plant-growth light-emitting diodes (LEDs) and high-performance WLEDs. Moreover, our strategy not only provides a unique inspiration for tuning the valence states of Mn but also designs new advanced luminescent materials.

Enhanced red luminescence in CaAl12O19:Mn4+via doping Ga3+ for plant growth lighting

A series of solid solution CaAl12-xGaxO19:Mn4+ phosphors were prepared via a high-temperature solid-state reaction. Their structural properties were characterized by X-ray diffraction (XRD) and the luminescence was investigated via photoluminescence spectra. The obtained CaAl12-xGaxO19:Mn4+ phosphor has a strong broad excitation band in the range of 250-550 nm, which can be easily excited by the UV, NUV and blue light, and a broad emission band centered at 655 nm between 600 nm and 800 nm due to the 2Eg → 4A2g transition of the Mn4+ ion. The PL spectra indicate that the intensity of CaAl12O19:Mn4+ can be enhanced when the Ga3+ concentration equals 1. Furthermore, the element mapping, optical properties, thermal stability, fluorescence lifetime and CIE chromaticity reveal that the CaAl12-xGaxO19:Mn4+ phosphors can be considered as potential candidates in indoor plant cultivation.

Synthesis, Crystal Structure, and Photoluminescence Characteristics of High-Efficiency Deep-Red Emitting Ba2GdTaO6:Mn4+ Phosphors

In this article, a series of novel Mn4+-doped Ba2GdTaO6 (BGT) red-emitting phosphors were successfully synthesized via a high-temperature solid-state method. The crystal structure, morphology, and luminescent performance of the samples were investigated in detail with X-ray diffraction, field emission scanning electron microscopy, photoluminescence (PL) spectra, decay curves, and internal quantum efficiency (IQE). Excited at 358 nm, these samples showed an intense deep-red emission band peaking at 688 nm in the wavelength region of 620-800 nm. The excitation spectra of these samples monitored at 688 nm exhibited two broad excitation bands from 250 to 600 nm with peaks at 358 and 469 nm. The systematic investigation of the concentration-dependent PL properties of BGT:Mn4+ phosphors revealed that the deep-red emission intensity reached the maximum when the Mn4+ doping concentration was 0.6 mol %. The critical distance (R c) between Mn4+ ions for concentration quenching was 36.57 Å, and the major mechanism of energy transfer among Mn4+ activators in BGT:Mn4+ was dipole-dipole interaction. The decay lifetimes decreased from 0.285 to 0.248 ms with the increasing Mn4+ doping concentration from 0.2 to 1.2 mol %. The Commission Internationale de l'Éclairage coordinates of the optimal BGT:0.6%Mn4+ sample were (0.7294, 0.2706). The values of the IQE for all BGT:Mn4+ samples were measured, and the highest value could reach up to 62%. The above results revealed that these high-efficiency BGT:Mn4+ deep-red-emitting phosphors had promising potential for application in indoor plant growth lighting.