K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+ → Eu3+ Energy Transfer

This paper describes the influence of sintering conditions and Eu³⁺/Tb³⁺ content on the structure and luminescent properties of K₅Eu_1-xTb_x(MoO₄)₄ (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K₅Tb(MoO₄)₄ (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the β ↔ α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-K₅Eu_0.6Tb_0.4(MoO₄)₄ structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group C2/m(0β0)00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to ⁵D₄ → ⁷F_J (J = 3-6) Tb³⁺ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λ_ex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the ⁵D₄ → ⁷F₅ Tb³⁺ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λ_ex = 377 nm exhibited an intense red emission at ∼615 nm due to the ⁵D₀ → ⁷F₂ Eu³⁺ transition, thus indicating an efficient energy transfer from Tb³⁺ to Eu³⁺.

Influence of Dy3+ ion concentration on photoluminescence and energy transfer mechanism of promising KBaScSi3O9 phosphors for warm white LEDs

A novel white light emitting silicate-based phosphor of Dy³⁺ activated KBaScSi₃O₉ (xDyKBS) was prepared by a conventional solid state method. Powder X-ray diffraction patterns represent the pure phase of synthesized materials that was formed with monoclinic structure. The metal-ligand bonding nature and electronic band structure have been examined optical absorption spectra. The luminescence emission curves of the silicate phosphors display an intense yellow emission peak at 579 nm and a blue emission peak at 491 nm. Among the emission bands, the band observed in the yellow region due to ⁴F_9/2→⁶H_13/2 transition was found to be higher intensity. The radiative parameters like transition probabilities (A_R), branching ratios (β_R) and stimulated emission cross-section (σ_P^E) values were calculated using Judd-Ofelt parameters and refractive index values for the observed transitions in emission spectra. The life time measurements were made for ⁴F_9/2 → ⁶H_13/2 transition of all the studied samples by keeping an excitation at 350 nm and emission at 579 nm and decay curves were fitted to bi-exponential fitting method. The CCT values obtained from the color coordinates suggested that present xDyKBS phosphors can emit warm and neutral white light depending upon the dopant concentration under near-UV excitation. Our results demonstrated that the optimum concentration 0.05DyKBS phosphor can be successfully utilized as a promising and potential candidate for various innovative photonic applications like warm white LEDs, solar cells, optical sensors and lasers.

Preparation and investigation of Dy3+/Tm3+-doped NaGd(MoO4)2 with thermal stability and tunable white light emission for LED applications

Combining theoretical calculations and experiments, synthesized NaGd(MoO4)2: 0.06Dy3+zTm3+ phosphors are shown to have good stability. Under 364 nm excitation, the phosphors emit tunable white light and demonstrate good thermal stability.

Crystal growth and properties characterization of Nd3+:Na5Lu(MoO4)4 for continuous multi-wavelength NIR laser emission

The Nd3+:Na5Lu(MoO4)4 crystal was grown in a 4Na2O–5MoO3 flux through the top-seeded solution growth technique, and achieved a continuous multi-wavelength NIR laser emission in the Nd3+:Na5Lu(MoO4)4 crystal for the first time.

The synergistic effect of Er3+ and Ho3+ on temporal color tuning of upconversion emission in a glass host via a facile excitation modulation technique for anti-counterfeiting applications

Lanthanide-doped upconversion luminescent materials are highly promising for diverse applications, e.g., solid-state lighting, volumetric displays, and anti-counterfeiting, owing to their unique optical feature of color-tunable emission under near-infrared excitation. Hence, in this study, emission color tuning of Er3+/Ho3+ ions in a fixed glass host is investigated via a facile excitation modulation technique. The upconversion emission color from green to yellowish is tuned successfully by regulating the frequency of the irradiation source. The population and depopulation rates of related transitions are investigated through time-resolved photoluminescence and Judd-Ofelt analysis in order to elucidate the proposed mechanism of color tuning. Upconversion quantum yield values are measured in the range of 0.12 to 0.17% for a better comparison of the emission properties. Additionally, thermal, and structural properties are investigated to reveal the favorable properties of the selected tellurite glass host. Ultimately, several patterns are designed and constructed by a screen-printing technique using powdered glass to demonstrate its suitability as a multicolor imaging method for anti-counterfeiting applications. The temporal color tuning of upconversion emission via a facile excitation modulation technique in a glass host clearly indicates that the proposed Er3+/Ho3+ co-doped glasses can be potentially applied in the state-of-the-art technologies, especially for anticounterfeiting purposes.

High-efficiency ∼2 µm CW laser operation of LD-pumped Tm3+:CaF2 single-crystal fibers

The Tm:CaF₂ single-crystal fibers were successfully grown by modified temperature gradient technique method. The J-O intensity parameters, spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm³⁺ were calculated with Judd-Ofelt theory. A systematic study of the fluorescence characteristics has been carried out. Simulated emission cross-sections of the ³F₄ → ³H₆ transition were calculated to be 6.68×10^-21 cm^-2 and 4.65×10^-21cm^-2 for crystal doped with 3 at.% and 4 at.% Tm³⁺. The 64.4% slope efficiency with output power of 2.23W was achieved in 3 at.% Tm:CaF₂ single-crystal fiber. The slope efficiency decreased to 44.5% and maximum output power decreased to 1.65W in 4 at.% Tm:CaF₂ single-crystal fiber. Obtained results show that Tm-doped CaF₂ single-crystal fibers are promising materials for IR laser action generation.

Systematic investigation of Eu3+ activated Na2Ln4(MoO4)7 [Ln = La, Gd and Y] narrow band red emitting phosphors for hybrid white LEDs and plant growth

A sequence of Eu³⁺ activated Na₂Ln₄(MoO₄)₇ [Ln = La, Gd and Y] red phosphors has been synthesized using a conventional solid-state method.

Growth, thermal and spectral properties, and laser performance of Tm3+:CNGS crystal

High-quality crystals of Tm³⁺:CNGS were successfully grown by the Czochralski method. The laser performance of the trigonal crystal system at 2 μm is presented for the first time, and the structural, thermal, and spectral properties are also reported.