Mn4+-activated KLaMgWO6: A new high-efficiency far-red phosphor for indoor plant growth LEDs

Novel Mn⁴⁺-activated KLaMgWO₆ red phosphors with different Mn⁴⁺ concentrations were successfully synthesized via a high-temperature solid-state reaction method. The phase formation, microstructure, photoluminescence properties, decay lifetimes and internal quantum efficiency were discussed to analyze the properties of the as-prepared phosphors. The samples belonged to monoclinic crystal system with enough WO₆ octahedrons that provided suitable sites for Mn⁴⁺ ions. Upon the excitation of 348?nm, KLaMgWO₆:Mn⁴⁺ phosphors gave bright far-red emission around 696?nm due to the ²E_g→⁴A_2g transition of Mn⁴⁺ ions. The critical concentration of Mn⁴⁺ was 0.6?mol% and the concentration quenching mechanism belonged to electric multipolar interaction. Besides, the CIE chromaticity coordinates of the KLaMgWO₆:0.6%Mn⁴⁺ phosphor were (0.7205, 0.2794) which located in deep red range, and its color purity reached up to 96.6%. The KLaMgWO₆:0.6%Mn⁴⁺ sample also exhibited high internal quantum efficiency of 43%. All of the admirable optical properties indicate that the KLaMgWO₆:Mn⁴⁺ phosphors can be applied to indoor plant growth illumination.     

Synthesis and photoluminescence properties of novel Ca2LaSbO6:Mn4+ double perovskite phosphor for plant growth LEDs

A novel Mn⁴⁺ activated Ca₂LaSbO₆ (CLS) far-red phosphor was synthesized by high temperature solid state reaction. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence spectra, ultraviolet-visible spectra, luminescence decay times, emission-temperature relationship and internal quantum efficiency (IQE). It is found that CLS:Mn⁴⁺ phosphor has a strong broad excitation band in the range of 200–550?nm. The samples can be excited by ultraviolet and blue light. There is a wide emission band centered at 685?nm between 600?nm and 760?nm. The optimum doping concentration of Mn⁴⁺ is approximately 0.5?mol%. In addition, all the CIE chromaticity coordinates of CLS:0.005Mn⁴⁺ located at far-red region. The concentration quenching mechanism is the dipole-dipole interaction of Mn⁴⁺ activator. Importantly, the CLS:0.005 Mn⁴⁺ sample has an IQE of up to 52.2%. Finally, a 365?nm ultraviolet light emitting diode (LED) chip combined with 0.5?mol% Mn⁴⁺ far-red phosphor was used to fabricate the LED device. All the results indicated that CLS:Mn⁴⁺ phosphors have potential applications in indoor plant cultivation.     

Color tunable nanocrystalline SrGd2Al2O7:Tb3+ phosphor for solid state lighting

Color tunable Tb³⁺ doped SrGd₂Al₂O₇ nanophosphor is synthesized employing a facile and economic urea assisted solution combustion approach. XRD studies confirm the crystallization of single phased SrGd_2(1-x)Tb_2xAl₂O₇ nanophosphor in tetragonal lattice with I4/mmm (139) space group. Rietveld refinement is performed over SrGd_1.9Tb_0.1Al₂O₇ sample to execute qualitative as well as quantitative phase analysis. TEM analysis confirms the more or less spherical shaped phosphors in nano domain with average particle size ranging 45–80?nm. Photoluminescent investigation reveals that this nanophosphor can be successfully excited by ultraviolet light yielding significant luminescent properties arising due to radiative transitions from ⁵D_3,4 levels to ⁷F_j levels. Dipole-dipole interactions are solely responsible for the energy transfer causing concentration quenching. Concentration controlled luminescent tendency can be employed to induce chromaticity from blue to green region. Findings of the study proclaim the application of this nanophosphor as one of the green component of tricolor based ultraviolet excited white LEDs.     

One-step hydrothermal synthesis and enhanced microwave absorption properties of Ni0.5Co0.5Fe2O4/graphene composites in low frequency band

Ni_0.5Co_0.5Fe₂O₄/graphene composites were synthesized successfully via one-step hydrothermal method. The crystal structure, morphology and corresponding elemental distribution, electromagnetic parameters and microwave absorption performances of the as-prepared composites were measured by XRD, SEM, TEM and VNA, respectively. The results indicated that the microwave absorbing performance can be obviously enhanced through the addition of graphene in a suitable range, the magnetic loss plays a dominant contribution for the microwave absorption of composites. The maximum reflection loss of ?30.92?dB at 0.84?GHz with a ?10?dB bandwidth over the frequency range of 0.58–1.19?GHz is obtained when the composite contains 12?wt% graphene and the thickness of sample is 4?mm. This investigation presents a simple method to prepare Ni_0.5Co_0.5Fe₂O₄/graphene composites with excellent microwave absorption performance in the low frequency band of 0.1–3?GHz.     

Synthesis and characterization of Eu3+-doped C12H18Ca3O18 phosphor

A series of Eu³⁺-doped C₁₂H₁₈Ca₃O₁₈ phosphors were synthesized through a facile hydrothermal method and the properties of as-prepared phosphors were explored by X-ray diffractometer (XRD), scanning electron microscope (SEM), and photoluminescence (PL) spectrometer. The exploration results indicated that the C₁₂H₁₈Ca₃O_18:Eu³⁺ had been successfully synthesized. The morphology of C₁₂H₁₈Ca₃O_18:Eu³⁺ was a strip with the size of 100–4000 nm × 50–400 nm × 50–200 nm and the ratio of length to width of 2–80. The strongest emission peak of C₁₂H₁₈Ca₃O_18:Eu³⁺ around 620 nm was ascribed to ⁵D_o→⁷F₂ transition of Eu³⁺, and the peaks centered at 590, 653 and 694 nm respectively corresponded to ⁵D_o →⁷F₁, ⁷F₃, and ⁷F₄ transitions. C₁₂H₁₈Ca₃O_18: Eu³⁺ gave the red light emission, as indicated by color coordinate analysis. The photoluminescence intensity of the phosphors prepared under the Eu³⁺ concentration of 6% was the highest. The crystal structure of C₁₂H₁₈Ca₃O_18:Eu³⁺ was changed after europium ions occupied the lattice position of calcium ions. Europium ion could displace calcium arbitrarily. As a new kind of matrix, calcium citrate possesses the properties of both organic and inorganic compounds and the luminescent C₁₂H₁₈Ca₃O₁₈: x Eu³⁺ particles may be applied in biological fluorescent tags and luminescent materials.     

Insight into the evolution mechanism of carbon film and Eu valence in carbon coated BaMgAl10O17: Eu2+ phosphor annealed in air

Performing carbon coating on the surface of phosphors has been proven to be an effective strategy to enhance the oxidation resistance, which is an important factor to achieve stable luminescent devices. Therefore, a good understanding of the protection mechanism favors a continuous improvement of oxidation resistance of phosphors. In present paper, the evolution of the carbon layer, Eu valence (Eu²⁺/Eu³⁺), and luminescent properties for the C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature is investigated carefully. Decrease of carbon layer promotes the appearance color transition from black to white as the annealing temperature rises to 1000?°C in air. As expected, the decrease of carbon layer will enhance the luminescence intensity, but risk the possible oxidation of Eu²⁺ to Eu³⁺, which inhibits the blue emission ascribed to Eu²⁺. The results indicate that luminescence intensity of phosphor is dependent on the synergistic effect of carbon thickness and Eu²⁺/Eu³⁺ ratio. Additionally, a reduction reaction of Eu³⁺ to Eu²⁺ is observed in C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature, which also contributes to the higher luminescence intensity.     

Combustion synthesis of Ca1−xCuxAl2O4 (x = 0.0, 0.4 and 0.8) copper doped calcium aluminate

We report the effect of Cu²⁺ ion on CaAl₂O₄ with different molar concentrations of 0.0, 0.4 and 0.8 M prepared by simple combustion method. The materials have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and scanning electron microscopy (SEM). DC electrical conductivity has also been measured to study the electrical behavior of the materials. The XRD patterns confirm the formation of single-phase CaAl₂O₄ along with some impurity phases like CaAl₄O₇, CaAl₁₂O₁₉ and Ca₁₂Al₁₄O₃₃. The FT-IR spectra show the stretching and bending vibrations of the synthesized compounds. DC electrical conductivity of the Ca_1−xCu_xAl₂O₄ is found to vary from 26.46 × 10⁻⁴ to 515.68 × 10⁻⁴ S cm⁻¹ for x = 0.0 to x = 0.8 at the measuring temperature of 1000 °C. SEM images show the morphological features of the compounds.