Enhanced deep-red emission from Mn4+/Mg2+ co-doped CaGdAlO4 phosphors for plant cultivation

Analysis of structural defects and their influence on red-emitting γ-Al2O3:Mn4+,Mg2+ nanowires using positron annihilation spectroscopy

The present paper reported on the analysis of structural defects and their influence on the red-emitting γ-Al2O3:Mn4+,Mg2+ nanowires using positron annihilation spectroscopy (PAS). The nanowires were synthesized by hydrothermal method and low-temperature post-treatment using glucose as a reducing agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), and photoluminescence excitation (PLE) were utilized, respectively, for determining the structural phase, morphology and red-emitting intensity in studied samples. Three PAS experiments, namely, positron annihilation lifetime (PAL), Doppler broadening (DB), and electron momentum distribution (EMD), were simultaneously performed to investigate the formations of structural defects in synthesized materials. Obtained results indicated that the doping concentration of 0.06% was optimal for the substitution of Mn4+ and Mg2+ to two Al3+ sites and the formation of oxygen vacancy (VO)-rich vacancy clusters (2VAl + 3VO) and large voids (~0.7 nm) with less Al atoms. Those characteristics reduced the energy transfer between Mn4+ ions, thus consequently enhanced the PL and PLE intensities. Moreover, this optimal doping concentration also effectively controlled the size of nanopores (~2.18 nm); hence, it is expected to maintain the high thermal conductivity of γ-Al2O3 nanowire-phosphor. The present study, therefore, demonstrated a potential application of γ-Al2O3 nanowire-phosphor in fabricating the high-performance optoelectronic devices.

Defect-induced deep red luminescence of CaGdAlO4-type layered perovskites: multi-cationic sites partial/full substitution and application in pc-LED and plant lighting

A series of CaGdAlO4-type layered perovskite phosphors showing deep red luminescence (λem = 711 nm, λex = 338 nm) were synthesized via a solid-state reaction. A comprehensive analysis performed via photoluminescence, X-ray photoelectron spectroscopy, thermoluminescence, and fluorescence decay revealed that the deep red luminescence is related to oxygen defects and particularly oxygen interstitials. The defect-related luminescence was effectively regulated through partial substitution of multi-cationic sites (the Ca2+ site with Mg2+, Sr2+, and Ba2+; the Gd3+ site with La3+, Y3+, and Lu3+) and full substitution of Gd3+ with Y3+. Remarkably, a 383.3% stronger luminescence was obtained through partial substitution with Lu3+, and the quantum yield of luminescence reached 28.74%, which is higher than those values of most previously reported self-luminescent systems. A pc-LED device was fabricated using this phosphor, and the device was shown to have potential application in indoor 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.

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.

Luminescence studies of a Li2 Ca1-x SiO4 :xSm3+ phosphor for the generation of white light under NUV-excited phosphor converting LEDs

In this paper, we present new aspects of Sm³⁺ -doped pure Li₂ CaSiO₄ as a suitable candidate for white light emitting diode (WLED) applications. The samples were mainly prepared using a conventional modified solid-state synthesis technique. The structural studies were done using X-ray diffraction and Rietveld refinement. Instruments such as a scanning electron microscope (SEM) were used to obtain information about the morphology of the as-prepared samples. Photoluminescence (PL) analysis of phosphor samples for variable concentrations of doping ions with variable excitations were presented. When doped with Sm³⁺ in host Li₂ CaSiO₄ it emitted intense blue, green and red emissions and a more intense red emission peak (605 nm) under 408 nm excitation (near-UV-blue). Our study shows that the as-prepared phosphor may be useful for optical devices and mainly for WLEDs. The corresponding transitions of doping ions and concentration quenching effect were studied in detail. The 1931 Commission Internationale de l'Eclairage (x, y) chromaticity coordinates showed the distribution of spectral regions calculated from PL emission spectra and this was found (0.63, 0.36) in the red region, so the phosphor may be useful for near-UV-blue excited WLED applications.

Achieving high thermal sensitivity from ratiometric CaGdAlO4:Mn4+,Tb3+ thermometers

The pursuit of optical temperature sensing with high thermal sensitivity to discriminate small temperature changes without contact with the subject possesses a crucial technological and scientific significance. Ratiometric temperature detection based on transition metals and lanthanides emerges as a promising strategy to achieve the purpose due to the dopants' distinct thermal quenching rates. In this work, a new CaGdAlO₄:Mn⁴⁺,Tb³⁺ luminescent thermometer was developed. The combination of the highly-thermal-sensitive red emission from Mn⁴⁺ ions with the thermally-robust green emission from Tb³⁺ ions renders the thermometer with a maximum relative thermal sensitivity of 2.3% K^-1 at 398 K. The well-separated red and green channels in digital images enable further evaluation of thermal sensitivity. The estimated thermal sensitivity is 2.23% K^-1 at 398 K from the pixel intensity ratio of red and green channels.

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.

A single-phase white light emitting phosphor Ba3Y(PO4)3:Ce3+/Tb3+/Mn2+: luminescence, energy transfer and thermal stability

A series of Ce³⁺/Tb³⁺, Tb³⁺/Mn²⁺ and Ce³⁺/Tb³⁺/Mn²⁺ doped Ba₃Y(PO₄)₃ were synthesized by the high temperature solid state method. Phase formation, energy transfer, luminescence properties and thermal quenching properties of phosphors were analyzed in detail. For the co-doped samples, the energy transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Mn²⁺ was proved by analyzing the spectra and fluorescence lifetime, and the energy transfer mechanism was calculated to be dipole–dipole interaction. A series of color-tunable phosphors were obtained by the energy transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Mn²⁺. For the tri-doped samples, it was confirmed that the energy transfers from Ce³⁺ to Tb³⁺, Tb³⁺ to Mn²⁺ and Ce³⁺ to Mn²⁺ exist at the same time by analyzing the spectra properties, and it can emit warm-white light with extensive color temperature regulability. In addition, the thermal stability was abnormal and outstanding because the defects exist in the samples. The results show that the phosphors may be novel warm white emitting phosphors for white light emitting diodes.

A series of Ce³⁺/Tb³⁺, Tb³⁺/Mn²⁺ and Ce³⁺/Tb³⁺/Mn²⁺ doped Ba₃Y(PO₄)₃ were synthesized by the high temperature solid state method.