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

Enhancing the inherent NIR photoluminescence in SrLaLiTeO6 through Cr3+-Yb3+ co-substitution for high performance pc-LEDs

Until now, double perovskite tellurates have not been reported to exhibit inherent NIR photoluminescence. Therefore, the current study's revelation of inherent NIR luminescence in SrLaLiTeO6 double perovskite centered at 970 nm under 340 nm excitation is particularly intriguing. This phenomenon is attributed to the photoluminescence of Te4+ ions. This study also examines the NIR luminescence of Cr3+-Yb3+ co-doped SrLaLiTeO6. The host and SrLaLiTeO6:3%Cr3+, y%Yb3+ (y = 1, 2, 4, 6, 8, 10 mol%) were synthesized using the solid-state ceramic route. The successful incorporation of Cr3+ and Yb3+ ions into the host lattice was confirmed through XRD, Raman, and diffuse reflectance spectral analyses. Under 270 nm excitation, the photoluminescence (PL) of SrLaLiTeO6:Cr3+ exhibits a blueshift of the PL band to 965 nm due to the 4T2(4F) → 4A2g(4F) emission component of Cr3+ ions. The excited-state lifetime of SrLaLiTeO6:0.5%Cr3+ was measured at 36 μs, but this decreased to 26 μs as the Cr3+ concentration reached 10 mol%, primarily due to the enhancement of non-radiative energy transfer between Cr3+ ions. Incorporating Yb3+ into the system results in additional spectral lines with an enhanced intensity in the range of 970 nm to 1125 nm when excited at 270 nm. These emission lines correspond to the 2F5/2 → 2F7/2 transitions of Yb3+ ions, indicating an efficient energy transfer from Cr3+ to Yb3+. Furthermore, the study also reveals that Yb3+ emission is observed even without Cr3+ ions in SrLaLiTeO6 under 340 nm excitation, suggesting the possibility of energy transfer from the host to Yb3+ ions. The thermal stability and crystal field parameters of the synthesized phosphors are also explained in detail. To explore the potential of these phosphors in practical applications, phosphor-converted NIR LEDs were fabricated using SrLaLiTeO6, SrLaLiTeO6:3% Cr3+, and SrLaLiTeO6:3% Cr3+, 1%Yb3+.

Synthesis and study of luminescence properties of a deep red-emitting phosphor K2 LiAlF6 :Mn4+ for plant cultivation

Light is the most important component in plant growth and development. This study synthesised a novel Mn4+ -doped K2 LiAlF6 red-emitting phosphor using the coprecipitation method. We observed that on addition of dopant Mn4+ ions to the host K2 LiAlF6 , its phase changed from rhombohedral to cubic due to the change in the lattice position of the atoms. When the atoms are excited at 468 nm, the K2 LiAlF6 :Mn4+ phosphor exhibited a red emission band ranging from 630 to 700 nm, centred at 638 nm, which matched well with the absorption spectra of phytochrome PR. The critical quenching content of Mn4+ ions was ~3 mol%. The critical distance between Mn4+ ions was determined to be 19.724 Å, and non-radiative energy transfer among the nearest-neighbour Mn4+ ions was the mechanism used for the concentration quenching effect. The Commission International de l'Eclairage (CIE) chromaticity coordinates of the K2 LiAlF6 :0.03 Mn4+ sample were (x = 0.7162, y = 0.2837). The luminescence mean decay time was calculated to be 8.29 ms. These results demonstrated the promising prospect of K2 LiAlF6 :Mn4+ as a red-emitting phosphor for application in red light-emitting diodes for plant cultivation.

Enriching the Deep-Red Emission in (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) Compositions for Light-Emitting Diodes

Red emission from Mn⁴⁺-containing oxides inspired the development of high color rendering and cost-effective white-light-emitting diodes (WLEDs). Aiming at this fact, a series of new crystallographic site modified (Mg, Ba)₃M₂GeO₈: Mn⁴⁺ (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The Mg₃Al₂GeO₈ host is composed of three phases: orthorhombic-Mg₃Ga₂GeO₈, orthorhombic-Mg₂GeO₄, and cubic-MgAl₂O₄. However, Mg₃Ga₂GeO₈ secured an orthorhombic crystal structure. Interestingly, Mg₃Al₂GeO₈: Mn⁴⁺ showed a 13-fold more intense emission than Mg₃Ga₂GeO₈: Mn⁴⁺ since Mn⁴⁺ occupancy was preferable to [AlO₆] sites compared to [GaO₆]. The coexisting phases of MgAl₂O₄ and Mg₂GeO₄ in Mg₃Al₂GeO₈: Mn⁴⁺ contributed to Mn⁴⁺ luminescence by providing additional [AlO₆] and [MgO₆] octahedrons for Mn⁴⁺ occupancy. Further, these sites reduced the natural reduction probability of Mn⁴⁺ to Mn²⁺ in [AlO₄] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on Mg₃M₂GeO₈: Mn⁴⁺ to improve the luminescence, and Mg_3-xBa_xM₂GeO₈: Mn⁴⁺ (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba²⁺ ions in Mg²⁺ sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned Mg_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ exhibited a 35-fold higher emission than that of Mg₃Ga_1.993GeO₈: 0.005Mn⁴⁺. Additionally, this could retain 70% of its ambient emission intensity at 453 K. A warm WLED with a correlated color temperature (CCT) of 3730 K and a CRI of 89 was fabricated by combining the optimized red component with Y₃Al₅O₁₂: Ce³⁺ and 410 nm blue LED. By tuning the ratio of blue (BaMgAl₁₀O₁₇: Eu²⁺), green (Ce_0.63Tb_0.37MgAl₁₁O₁₉), and red (Mg_2.73Ba_0.27Al₂GeO₈: 0.005Mn⁴⁺) phosphors, another WLED was developed using a 280 nm UV-LED chip. This showed natural white emission with a CRI of 79 and a CCT of 5306 K. Meanwhile, three red LEDs were also fabricated using the Mg_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.

Diffusing Mn4+ into Dy3+ Doped SrAl2O4 for Full-Color Tunable Emissions

Dy³⁺ and Mn⁴⁺ codoped SrAl₂O₄ (SrAl₂O₄:Dy³⁺,Mn⁴⁺) phosphors were obtained by diffusing Mn⁴⁺ ions into Dy³⁺-doped SrAl₂O₄ via the constant-source diffusion technique. The influences of diffusion temperature and diffusion time on the emissions of SrAl₂O₄:Dy³⁺,Mn⁴⁺ were investigated. It was found that: (i) efficient red emission peaking at 651 nm can be readily achieved for SrAl₂O₄:Dy³⁺ by simply diffusing Mn⁴⁺ into SrAl₂O₄:Dy³⁺ at 800 °C and above; (ii) the red emission of Mn⁴⁺ becomes dominant over the characteristic emissions of Dy³⁺ when the diffusion temperature is 900 °C or higher; and (iii) the intensity of the red emission of SrAl₂O₄:Dy³⁺,Mn⁴⁺ is far more sensitive to diffusion temperature than to diffusion time. Our results have demonstrated that full-color tunable emissions can be realized for SrAl₂O₄:Dy³⁺, Mn⁴⁺ by tuning the parameters of diffusion temperature and diffusion time, which opens up a space for realizing easy color control of Dy³⁺-doped inorganic materials.

A review on the structural dependent optical properties and energy transfer of Mn4+ and multiple ion-codoped complex oxide phosphors

The tetravalent manganese Mn4+ ions with a 3d3 electron configuration as luminescence centers in solid-state inorganic compounds have been widely investigated because they emit bright light in the red to far-red region when they are excited by light with a wavelength in the UV to blue light region. Herein, we present an overview of the recent developments of Mn4+ and multiple ion such as Bi3+ and rare earth ion Dy3+, Nd3+, Yb3+, Er3+, Ho3+, and Tm3+ codoped complex oxide phosphors. Most of the specified host lattices of these complex oxide phosphors possess multiple metallic cations, which provide possible substitutions with different codopants and form various luminescence centers with diverse spectra. The luminescence of Mn4+ and multiple ion-codoped materials spans almost the whole visible light to near infrared (NIR) region. The crystal structures of complex oxide phosphors, the spectroscopic properties of Mn4+, and the energy transfer between Mn4+ and multiple ions are introduced and summarized in detail with regard to their practical applications. This review provides an insight into the optical properties of Mn4+ and the energy transfer process in multiple ion-codoped luminescence materials, which will be helpful in the development of novel excellent materials for applications in the lighting industry.

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

In the modern era, growing number of indoor plants for various purposes, such as vegetation, flowering, and decorations, has increased over the traditional follow-up trends for plantation. However, the indoor plantation requires different parameters for their growth; among these, light plays a significant role. In order to control the growth of plants using light-emitting diodes, Mn-doped oxide phosphors have emerged as promising candidates due to their broad and intense emission bands in the red and far-red spectral range. In this review article, recent progress on Mn-doped oxides for indoor plant growth has been reviewed. This review article is mainly divided into three parts. In the first part, different reaction conditions for the synthesis of Mn-doped oxide phosphors are compared. In the second part, the luminescent and other photometric parameters of these are discussed. The influence of different co-dopants on the luminescent characteristics has been elucidated in detail. The third part discusses the properties of light-emitting diodes fabricated using these phosphors for plant growth. The present review article elucidates the synthesis parameters, luminescent properties, and light-emitting diodes fabricated using Mn-doped oxide materials for plant growth applications.