4A2g Transition Research Articles

Enhancing the energy transfer from Mn4+ to Yb3+ via a Nd3+ bridge role in Ca3La2W2O12:Mn4+,Nd3+,Yb3+ phosphors for spectral conversion of c-Si solar cells

In this work, a series of Mn4+, Nd3+ and Yb3+ singly and co-doped Ca3La2W2O12 (CLWO) phosphors were prepared using a high-temperature solid-state reaction approach. In CLWO:0.016Mn4+, the emission spectrum presents a band around 709 nm attributed to Mn4+2Eg→4A2g transition under 365 nm UV excitation, which partly overlaps the excitation spectrum of Nd3+ singly doped CLWO phosphors. This indicates that a non-radiative resonant energy transfer process from Mn4+ to Nd3+ ions would like to take place in Mn4+, Nd3+ co-doped CLWO phosphors. Although scarce spectral overlap between Mn4+ emission and Yb3+ excitation in respective singly CLWO samples is observed, the energy transfer from Mn4+ to Yb3+ ions can also be deduced based on the analyses of excitation spectra, emission spectra and decay times in Mn4+, Yb3+ co-doped CLWO materials, which is proposed to be a phonon-assisted energy transfer process since the energy difference between Mn4+ lowest excited state 2Eg and Yb3+ excited state 2F5/2 is quite large. Moreover, in Nd3+, Yb3+ co-doped CLWO, the energy transfer phenomenon from Nd3+ to Yb3+ ions can be also observed. Therefore, we introduce Nd3+ ions into Mn4+, Yb3+ co-doped CLWO phosphors to realize the Mn4+→Nd3+→Yb3+ successive energy transfer process. Based on this, the Yb3+ emission intensity is highly enhanced in Mn4+, Nd3+ and Yb3+ tri-doped CLWO relative to Mn4+, Yb3+ co-doped CLWO. As a result, with the assistance of two channels Mn4+→Nd3+→Yb3+ and Mn4+→Yb3+ energy transfer processes, the broadband UV/blue light from sunlight spectrum can be effectively converted into Yb3+ near infrared (NIR), matching with the excellent spectral response of solar cells, which is highly expected to improve the c-Si solar cells spectral conversion efficiency.

A far-red-emitting NaMgLaTeO6:Mn4+ phosphor with perovskite structure for indoor plant growth

In this article, we report a far-red-emitting phosphor NaMgLaTeO6:Mn4+ with the perovskite structure synthesized by a high-temperature solid-state reaction method. The XRD patterns for NaMgLaTeO6:Mn4+ and the refinement for the representative NaMgLaTeO6:0.006Mn4+ confirm the purity of the as-prepared samples. The emission spectrum of optimal NaMgLaTeO6:0.02Mn4+ covers a series of narrow lines in the range from 16667 to 12500 cm−1 (600–800 nm), peaking at 14225 cm−1 (703 nm) due to the Mn4+ spin- and parity-forbidden 2Eg→4A2g transition. The excitation spectrum monitored at 703 nm has a broad excitation band with the range from 40000 to 16667 cm−1 (250–600 nm) centered at 28571 (350 nm) and 21505 cm−1 (465 nm), which matches well with UV or blue chips, respectively. The band can be decomposed into four Gaussian bands centered at 31646 cm−1 (316 nm), 28169 cm−1 (355 nm), 24631 cm−1 (406 nm) and 20921 cm−1 (478 nm), corresponding to the O2--Mn4+ charge-transfer and the Mn4+ 4T1g←4A2g, 2T2g←4A2g and 4T2g←4A2g transitions, respectively. The optimal concentration x of Mn4+ in NaMgLaTeO6:xMn4+ was examined to be 0.02 with a corresponding quantum yield of 57.43% under 365 nm excitation, beyond which concentration quenching occurred. The energy transfer mechanism between Mn4+ ions was determined to be a dipole-dipole interaction by analyzing the relationship between concentration and emission intensity. The temperature-dependent luminescence intensity decreased to about 75% at 150 °C of its original value at room temperature, illustrating its thermal luminescence stability of this phosphor. Moreover, a crystal field analysis for the NaMgLaTeO6:0.02Mn4+ representative was conducted. By using the decay lifetimes and time-resolved emission spectra, we discriminated the different occupancies of Mn4+ not only at the Te6+ but also at the Mg2+ sites in the crystal structure. More importantly, the emission band matched well with the absorption band of phytochrome Pfr, which implies the phosphors can be potentially applied in light-emitting diodes (LEDs) for regulating the growth of plants.

A novel rare-earth free red-emitting Li3Mg2SbO6:Mn4+ phosphor-in-glass for warm w-LEDs: Synthesis, structure, and luminescence properties

Developing a novel non-rare-earth activated oxide red phosphor with admirable luminescent performance and low cost is still of great interest in the field of energy-saving warm white light-emitting diodes (w-LEDs) illumination. Here, Li3Mg2SbO6:Mn4+ (LMSO:Mn4+) red-emitting phosphor has been successfully prepared with cheap raw materials via a traditional solid-state method. Crystal structure, morphology, composition as well as optical properties are studied in detail. The prepared LMSO:Mn4+ phosphor exhibits a broad and deep red emission band in the range of 600–750 nm with the maximum centered at 666 nm, which is ascribed to the Mn4+:2Eg→4A2g transition in [SbO6] octahedral environment. Moreover, the nephelauxetic effect of Mn4+ in the LMSO matrix is evaluated by investigating the crystal field strength (Dq) and Racah parameters (B and C). Importantly, LMSO:0.01Mn4+ phosphor demonstrates good thermal stability and high quantum efficiency. By embedding LMSO:Mn4+ and Y3Al5O12:Ce3+ phosphors with various ratios in the TeO2-based glass host, a series of phosphors-in-glass(PiG) composites are achieved and demonstrate to remain the original optical properties of phosphors. Remarkably, by coupling PiG color converters with a blue LED chip, high-power w-LEDs devices with tunable optoelectronic parameters are constructed, with the correlated color temperature evolving from cool white (6783 K) to warm one (4014 K) and the color rendering index increasing from 72.2 to 88.7.

Temperature dependence (13–600 K) of Mn4+ lifetime in commercial Mg28Ge7.55O32F15.04 and K2SiF6 phosphors

The fluorescence lifetime of the Mn4+2Eg→4A2g transition in commercial Mg28Ge7.55O32F15.04 (MFG) and K2SiF6 (KSF) phosphors are measured in the temperature region of 13–600 K. In MFG, the observed decrease in the lifetime with increasing temperature is correlated with increasing probability of phonon induced transitions (vibronic emission) without any contribution of nonradiative transitions. In KSF, the decrease in lifetime in the temperature region of 13–500 K is associated with an increase in the vibronic transition probability. Above 500 K, the rapid decrease in the lifetime is correlated with the onset of non-radiative relaxation. The differences between the thermal quenching behavior of Mn4+ luminescence in the two phosphors in explained with respect to the energy position of the 4T2g state.

The deep red emission of Mn4+ doped SrLaMgNbO6 flower-like microsphere phosphors

We synthesized Mn4+ doped SrLaMgNbO6 phosphors by a solvothermal method. The influence of Mn4+ concentration on emission of SrLaMgNbO6:Mn4+ phosphor was investigated. The synthesized phosphors have a single phase and flower-like microstructure. Under the excitation at 321 nm, SrLaMgNbO6:Mn4+ phosphors show intense red emission with three peaks, which come from the 2Eg, 2T2g → 4A2g transitions of the 3d3 electrons in the octahedral MnO68− complex. SrLaMgNbO6:Mn4+ phosphors have potential applications in WLEDs. The addition of SrLaMgNbO6:Mn4+ phosphor can decrease the CCT value and increase Ra value of white lights emitted by WLEDs.

Preparation, characterization and luminescent properties of red-emitting phosphor: LiLa2NbO6 doped with Mn4+ ions

Series of Mn4+-activated LiLa2NbO6 red emitting phosphors were prepared by the solid state method. The structural and luminescence properties are investigated on the basis of X-ray diffraction (XRD), emission and excitation spectra, and luminescence decay curves. The LiLa2NbO6:Mn4+ phosphors can be efficiently excited by near-UV to blue light and exhibit bright red emission at around 712 nm, which can be assigned to the 2Eg→4A2g transition of the 3 d3 electrons in [MnO6] octahedra. Temperature dependent emission spectra and decay curves from 10 to 480 K are analyzed to understand the luminescence mechanism of Mn4+ in LiLa2NbO6 lattice. Notably, such a novel red emitting phosphor shows special anti-thermal quenching behavior.

Double perovskite Ca2GdNbO6:Mn4+ deep red phosphor: Potential application for warm W-LEDs

A novel Mn4+-doped Ca2GdNbO6 (CGN) phosphor was prepared by high-temperature solid-state reaction. The crystal structure was investigated by X-ray diffraction patterns and unit cell structure. Mn4+ replaced the location of Nb5+ in the CGN lattice, and the value of energy gap (Egap) decreased from 2.16 eV to 1.13 eV, indicating that Mn4+ ions play a great influence on the absorption of CGN hosts. The broad excitation band from 250 nm to 550 nm matches well with commercial near-UV light emitting diodes, and the emission peak centered at 680 nm is due to 2E→4A2g transition in Mn4+ ions. The CIE chromaticity coordinates (0.698, 0.303) of CGN:Mn4+ phosphor was close to standard red color coordinates (0.666, 0.333). These investigations demonstrate CGN:Mn4+ phosphor as an efficient red phosphor for potential applications.

Far-red-emitting double-perovskite CaLaMgSbO6:Mn4+ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs.

A series of Mn4+-activated CaLaMgSbO6 far-red-emitting phosphors were synthesized by a solid-state reaction route and the microstructure and optical characterizations were investigated in detail. Upon excitation at 370 and 469 nm, the samples showed intense far-red emission at about 708 nm originating from the 2Eg → 4A2g transition and the optimal Mn4+ concentration was 0.7 mol%. The as-prepared phosphors also exhibited excellent internal quantum efficiency (88%) and high thermal stability. The emission intensity at room temperature dropped to 54% when the temperature rose to 423 K and the activation energy was 0.34 eV. The outstanding optical properties and the fact that the emission band of the obtained phosphors had a broad overlap with the absorption band of phytochrome PFR demonstrated that the CaLaMgSbO6:Mn4+ phosphors may be promising potential spectral converters for applying to indoor plant cultivation light-emitting diodes.

Synthesis, structure, and luminescence characteristics of far-red emitting Mn4+-activated LaScO3 perovskite phosphors for plant growth.

Far-red emitting phosphors LaScO3:Mn4+ were successfully synthesized via a high-temperature solid-state reaction method. The X-ray powder diffraction confirmed that the pure-phase LaScO3:Mn4+ phosphors had formed. Under 398 nm excitation, the LaScO3:Mn4+ phosphors emitted far red light within the range of 650–800 nm peaking at 703 nm (14 225 cm−1) due to the 2Eg → 4A2g transition, which was close to the spectral absorption center of phytochrome PFR located at around 730 nm. The optimal doping concentration and luminescence concentration quenching mechanism of LaScO3:Mn4+ phosphors was found to be 0.001 and electric dipole–dipole interaction, respectively. And the CIE chromaticity coordinates of the LaScO3:0.001Mn4+ phosphor were (0.7324, 0.2676). The decay lifetimes of the LaScO3:Mn4+ phosphors gradually decreased from 0.149 to 0.126 ms when the Mn4+ doping concentration increased from 0.05 to 0.9 mol%. Crystal field analysis showed that the Mn4+ ions experienced a strong crystal field in the LaScO3 host. The research conducted on the LaScO3:Mn4+ phosphors illustrated their potential application in plant lighting to control or regulate plant growth.

Cymbopogoncitratus assisted green synthesis of doped Yttrium nanopowder: Structural and Photoluminescence properties for wLEDs applications

For the first time green route was used to synthesize pure and Cr3+ (1-11 mol %) doped Yttrium oxide (Y2O3) nanostructures by using Cymbopogon citrates extract as a fuel. The prepared samples were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy, transmission electron microscopy, Diffused reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. The PXRD results shows the formation of single phase, cubic structure of Y2O3 with crystallite sizes ∼15 nm which was further confirmed by TEM results. The SEM results indicated various shaped nanostructures which solely dependent on the fuel concentration. The band gap energy was estimated by DRS and found to be in the range of 5.67 – 5.80 eV. PL results indicate intense sharp peak at ∼ 689 nm along with vibronic side bands 706 and 734 nm. The 689 nm peak was well known R – line and was assigned to the 2Eg → 4A2g transition of Cr3+ions.The CIE (Commission International de I'E' clairage) chromaticity coordinates were located in pure orange- red region. Further, (Commission International de I'E' clairage) CCT values were in the range 3387 – 3486 K. The studies indicate that Y2O3:Cr3+nanophosphors (NPs) were quite useful for display devices

Deep‐red photoluminescence and long persistent luminescence in double perovstkite‐type La2MgGeO6:Mn4+

AbstractA novel non‐rare‐earth doped phosphor La2MgGeO6:Mn4+ (LMG:Mn4+) with near‐infrared (NIR) long persistent luminescence (LPL) was successfully synthesized by solid‐state reaction. The phosphors can be effectively excited using ultraviolet light, followed by a sharp deep‐red emission peaking at 708 nm, which is originated from 2Eg → 4A2g transition of Mn4+ ions. The luminescent performance was analyzed by photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The crystal field parameters were calculated to describe the environment of Mn4+ in LMG host. The LPL behaviors as well as the mechanisms were systematically discussed. This study suggests that the phosphors will broaden new horizons in designing and fabricating novel NIR long phosphorescent materials.

Influence of inversion defects and Cr–Cr pairs on the photoluminescent performance of ZnAl2O4 crystals

ZnAl2O4:Cr3+ crystals were synthesized by using sol–gel method at 1000 °C. The results showed that with the concentration of Cr3+ in spinel lattices increasing, the x-ray diffraction peaks shifted toward lower diffraction angles, and the grain size revealed a rapid increase and became invariable when the Cr3+ dopant concentration was above 0.7 mol%. All the samples showed a series of emission peaks ranging from 660 to 730 nm assigned to spin-forbidden 2Eg→4A2g transition of Cr3+ ions in spinel lattices. The intensity of emission peaks revealed a strong dependence on the Cr3+ concentration. An additional emission peak at 740 nm was present when the Cr3+ ions concentration was above 0.5%. All emission peaks were quenched when the Cr3+ concentration reaches 1.3 mol%. The results of x-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) suggested that the Cr3+ ions occupied the octahedral sites in spinel lattices, and with the Cr3+ concentration increasing, the exchange interaction and coupling interaction between Cr–Cr were enhanced and the antisite defects related to Zn in octahedral sites increased.

Synthesis, deep red emission and warm WLED applications of K2SiF6:Mn4+ phosphors

K2SiF6:Mn4+ phosphors were synthesized by using different materials and methods The influence of synthesis parameters on the luminescent properties was investigated. The phase and composition of three K2SiF6:Mn4+ phosphors were determined by X-ray diffraction, Energy dispersive spectrometer and Fourier transform infrared spectroscopy. The microstructure and morphology of three K2SiF6:Mn4+ phosphors were observed by a field emission scanning electron microscope. The excitation spectrum of K2SiF6:Mn4+ phosphor consists of excitation bands in ultraviolet and blue regions originating from 4A2g → 4T1g and 4A2g → 4T2g transitions of Mn4+. The emission spectrum of K2SiF6:Mn4+ phosphor shows emission bands in red region resulting from the 2Eg → 4A2g transitions of Mn4+ ions. The addition of H2O2 in the synthesis favors the doping of Mn4+ in K2SiF6 host and thus increases the emission intensity of K2SiF6:Mn4+ phosphor. The luminescent properties of K2SiF6:Mn4+ phosphor suggests that it is a suitable candidate for warm white light emitting diodes. By combining blue chip, YAG:Ce yellow and K2SiF6:Mn4+ phosphors, white light emitting diodes were fabricated. They exhibit ‘warm’ white light that suitable use as indoor light.

Preparation, structural and optical characteristics of a deep red-emitting Mg2Al4Si5O18: Mn4+ phosphor for warm w-LEDs

A novel red-emitting phosphor Mg2Al4Si5O18:Mn4+ is successfully synthesized using a solid-state reaction method. Aluminosilicate Mg2Al4Si5O18 of low-symmetry orthorhombic phase contains abundant octahedrons and is suitable for Mn4+ doping. A broad absorption band ranging from 240 nm to 550 nm in the ultraviolet–visible spectra corresponds to the photoluminescence excitation spectrum. The emission spectrum from 650 nm to 750 nm exhibits a strong emission peak at 680 nm because of the 2Eg→4A2g transition of Mn4+ ion. The critical distance Rc for concentration quenching is ∼33.3 Å, and the energy transfer mechanism is determined to be dipole–dipole interaction. Luminescence mechanism is analyzed using the energy level diagram of Mn4+ ion. Racha parameters B and C are evaluated and used to discuss the nephelauxetic effect for the Mn4+ ions in the Mg2Al4Si5O18. Temperature-dependent fluorescence test demonstrates the good thermal optical properties of the phosphor. An assembled light-emitting diode (LED) device with efficient luminescence demonstrates its practical use. Results indicate the potential application of the phosphor for white LEDs.

Deep red emission enhancement in Mg28Ge10O48:Mn4+ phosphor by Zn substitution

Zinc co-doped Mg28Ge10O48:Mn4+ red phosphors are synthesized by a solid-state method. The effects of zinc on the phosphors have been investigated by varying the stoichiometric proportions of raw materials. Zn concentration affects the phosphor crystallinity, morphology, composition and energy structure. We have carried out the X-ray power diffraction (XRD), Field emission scanning electron microscope (FESEM), energy dispersion spectrum (EDS), element mapping, UV–Vis absorption measurement and X-ray photoelectron spectroscopy (XPS). The phosphorescence enhancement centered at around 660nm is ascribed to 2Eg→4A2g transition of Mn4+. The red emission Intensity of optimized sample is 3.08 and 1.64 times of Zn-free samples and conventional magnesium fluorogermanate phosphors, respectively.

Multifunctional applications of self - Assembled 3D CeO2: Cr3+ hierarchical structures synthesized via ultrasound assisted sonochemical route

In the present work, CeO2:Cr3+ (1–11 mol %) nanophosphors (NPs) were prepared via eco-friendly ultrasound assisted sonochemical route using Cicer arietinum (C.A.) seeds extract as a surfactant. The powder X-ray diffraction results confirms the pure cubic structure with average crystallite size of ∼6–10 nm. The optical energy band gap (Eg) were found to be ∼2.95–3.45 eV. Three-dimensional (3D) hierarchical nanostructures are considered as new probe due to their high surface areas and exhibits distinctive properties which furnish excellent performance in luminescent applications. The effect of various experimental parameters on the growth mechanism of 3D morphology of the product was extensively studied. Raman studies confirm the incorporation of Cr3+ ions into the CeO2 matrix, possible electronic states during excitation, oxygen vacancies and defect sites available will change. PL studies have shown an intense sharp peak at ∼689 nm along with vibrancies side bands at ∼706 nm and 734 nm. A ∼689 nm peak was well known as R – line and was assigned to the 2Eg → 4A2g transition of Cr3+ ions. The crystal field splitting and Racah parameters were calculated to know the inter-electrons repulsion in host lattice. The CIE chromaticity co-ordinates were located in pure red region. Further, CCT values were in the range 5342 K–7781 K. The optimal CeO2:Cr3+ (9 mol %) NPs was successfully explored as a fluorescent probe for visualization of latent fingerprints (LFPs) on both porous and non-porous surfaces. Aforementioned studies indicate that the prepared sample were quite useful for multifunctional applications namely display devices and forensic applications.

Deep red-emitting Ca14Al10Zn6O35:Mn4+ phosphors for WLED applications

Novel deep red-emitting Ca14Al10Zn6O35:Mn4+ (CAZ:Mn4+) phosphors were synthesized by a citrate sol-gel method. The X-ray diffraction patterns of CAZ:Mn4+ phosphors confirmed their cubic structure after annealing at 1200 °C. The photoluminescence emission (PL) and excitation (PLE) spectra were measured to examine the emission richness of CAZ:Mn4+ phosphors. The PLE spectra of CAZ:Mn4+ phosphor exhibited a broad band, between UV and blue regions, due to the charge transfer (CT) transition of Mn4+-O2- and 4A2g→4T2g, 4A2g→4T2g, 4A2g→4T2g of Mn4+ ions. The results showed that the deep red emission peak of the CAZ:Mn4+ phosphor was located at 714 nm due to the 2Eg→4A2g transition of Mn4+ ion, which was effectively excited by 450 nm. The optimum concentration of Mn4+ ion in Ca14Al10Zn6O35 host lattice was found to be at 3 mol%. Using CAZ:3Mn4+ phosphors, white light-emitting diode (WLED) was fabricated. The CIE chromaticity coordinates of CAZ:3Mn4+ and fabricated WLED located at (0.709, 0.291) and (0.338, 0.396), respectively, resulting in the generation of warm white light for indoor illumination.

Luminescence of Mn4+ ions in CaTiO3 and MgTiO3 perovskites: Relationship of experimental spectroscopic data and crystal field calculations

Herein, the synthesis, structural and crystal field analysis and optical spectroscopy of Mn4+ doped metal titanates ATiO3 (A = Ca, Mg) are presented. Materials of desired phase were prepared by molten salt assisted sol-gel method in the powder form. Crystallographic data of samples were obtained by refinement of X-ray diffraction measurements. From experimental excitation and emission spectra and structural data, crystal field parameters and energy levels of Mn4+ in CaTiO3 and MgTiO3 were calculated by the exchange charge model of crystal-field theory. It is found that crystalline field strength is lower (Dq = 1831 cm−1) in the rhombohedral Ilmenite MgTiO3 structure due to the relatively longer average Mn4+O2− bond distance (2.059 Å), and higher (Dq = 2017 cm−1) in orthorhombic CaTiO3 which possess shorter average Mn4+O2− bond distance (1.956 Å). Spectral positions of the Mn4+2Eg → 4A2g transition maxima is 709 nm in MgTiO3 and 717 nm in CaTiO3 respectively in good agreement with calculated values.

Luminescence of Mn4+ in the orthorhombic perovskite, LaGaO3

The optical properties of Mn4+ (3d3) in the orthorhombic perovskite, LaGaO3 are investigated. The Mn4+ energy levels are calculated using the exchange charge model of crystal-field theory. The calculated Mn4+ energy levels are in good agreement with the experimental spectroscopic data. The results of our calculations yield the crystal-field splitting and Racah parameters of Dq=1926cm−1, B=780cm−1 and C=2878cm−1, with C/B=3.7. The emission spectrum is assigned on the basis of the zero phonon line corresponding to the 2Eg→4A2g transition and its vibrational sidebands. A comparative study of the variation in the crystal-field splitting and the Mn4+ 2Eg energy level position in materials with the perovskite structure is also presented.

The improvement of structure and photoluminescence properties of ZnAl2O4:Cr3 + ceramics synthesized by using solvothermal method

ZnAl2O4:Cr3+ nanocrystals were fabricated by using solvothermal method at various temperatures ranging from 160°C to 220°C. The influence of sodium acetate (NaAC) on the structural, photoluminescence and magnetic properties of Cr-doped zinc aluminate ceramics was investigated. The results showed that ZnAl2O4:Cr3+ polycrystals with sphere morphology (about 500nm) were formed without NaAC in precursors, while after adding NaAC into the precursors, octahedral and hexagonal single crystals of ZnAl2O4:Cr3+ were present. The PL spectra showed a series of sharp emission peaks ranging from 650nm to 730nm assigned to spin-forbidden 2Eg→4A2g transition of Cr3+ ions in spinel lattices corresponding to the result of EPR. Comparing to the PL spectra of ZnAl2O4:Cr3+ nanocrystals prepared by using sol-gel method, the intensity of R lines (originating from 2Eg→4A2g transition of Cr3+ ions in normal spinel lattices) was much larger than that of N lines (originating from 2Eg→4A2g transition of Cr3+ ions in inverse spinel lattices) for all samples. The EPR and VSM results showed that with grain size increasing, the interaction between Cr-Cr ions became weaker and the ferromagnetism of ZnAl2O4:Cr3+ nanocrystals was enhanced.