Ultrasensitive Pressure-Induced Optical Materials: Europium-Doped Hafnium Silicates with a Khibinskite Structure for Optical Pressure Sensors and WLEDs

Construction of thermally stable Tb3+-activated green-emitting phosphors: dual driving strategy of doping concentration and excitation wavelength

The realization of thermally stable Tb3+-doped green emission at high temperatures in solid-state lighting is still a crucial challenge. Nevertheless, the study on modulating the thermally stable luminescence at high temperatures is seldom reported. The position of the intervalence charge transfer (IVCT) energy level is used to systematically investigate the thermal quenching performance of Tb3+-activated green-emitting phosphors with varying concentration gradients of Gd1-xTaO4:xTb3+ (x = 0.1%, 0.5%, and 2%) in this study. The IVCT energy levels were determined according to the empirical formula to show a decreasing trend, consistent with the position of the IVCT energy levels measured in the excitation and diffuse reflectance spectra. Moreover, the thermal quenching performance of different wavelength excitation positions (host absorption, 4f-5d of Tb3+, and Tb3+-Ta5+ IVCT band) is quite different. The modulation of thermal quenching performance among distinct phosphors when subjected to host excitation or IVCT excitation can be elucidated through optimal positions within the energy levels associated with IVCT. The diverse concentration gradient samples exhibit varying degrees of thermal quenching performance in the variable-temperature spectra. The fluorescence lifetimes of the samples are generally comparable but slightly lower. The quantum efficiency rapidly improves as the Tb concentration increases. The underlying mechanism governing this phenomenon is elucidated by constructing a model that encapsulates the interplay between the compensating and quenching channels, in addition to the energy conversion of Tb3+ into Gd3+. The abovementioned results indicate that the dual driving scheme of the doping concentration and excitation wavelength is an effective means to regulate the thermal quenching performance of Tb-activated green-emitting tantalate phosphors.

Pressure-Induced Remarkable Spectral Red-Shift in Mn2+ -Activated NaY9 (SiO4 )6 O2 Red-Emitting Phosphors for High-Sensitive Optical Manometry

To settle the low sensitivity of luminescent manometers, the Mn2+ -activated NaY9 (SiO4 )6 O2 red-emitting phosphors with splendid pressure sensing performances are developed. Excited by 408 nm, the resulting products emit bright red emission originating from 4 T1 (4 G) → 6 A1 transition of Mn2+ , in which the optimal concentration of the activator ion is ≈1 mol%. Moreover, the admirable thermal stability of the developed phosphors is studied and confirmed by the temperature-dependent emission spectra, based on which the activation energy is derived to be 0.275 eV. By analyzing the pressure-dependent Raman spectra, the structural stability of the synthesized compounds at extreme conditions is verified. Furthermore, the designed phosphors exhibit remarkable spectral red-shift at elevated pressure. Especially, as pressure increases from 0.75 to 7.16 GPa, the emission band centroid shifts from 617.2 to 663.4 nm, resulting in a high sensitivity (dλ/dP) of 7.00 nm GPa-1 , whereas the full width at half maximum (FWHM) increases from 83.0 to 110.6 nm, leading to the ultra-high sensitivity (dFWHM/dP) of 10.13 nm GPa-1 . These achievements manifest that the designed red-emitting phosphors are appropriate for ultrasensitive optical manometry. More importantly, the developed manometer is a current global leader in sensitivity, when operating in the band-width mode, that is, FWHM.

Highly efficient rare-earth free vanadate phosphors for WLEDs

Yellow-green emitting phosphors of vanadate Ca5Mg4(VO4)6 (CMV) doped with different concentrations of Ta5+ ions were synthesized by a solid-state reaction method. The formation of single-phase compounds with a garnet structure was verified by X-ray diffraction (XRD), Rietveld refinement calculations and energy-dispersive X-ray spectroscopy. Different luminescence properties of CMV phosphors such as spectral shift, luminescence lifetime, quantum efficiency, color coordinates and Stokes shift were measured and have been discussed in detail. PLE and PL spectra showed that CMV : xTa5+ (0 ≤ x ≤ 5%) phosphors could match well to 365 nm LED chips, and showed bright yellow-green emission in the visible range of 400-750 nm, with a peak at 544 nm, which is attributed to the charge transfer (CT) of an electron from the 2p orbital of the oxygen atom to the vacant 3d orbital of V5+ ions in the tetrahedral [VO4]3- group. Compared with the CMV host, the integrated luminescence intensity of CMV : 0.5%Ta5+ increased by 26.31%, and the quantum efficiency increased by 15.98%. The phenomenon can be ascribed to the substitution of V5+ ions by the large Ta5+ ions, which resulted in the squeezed and distorted VO4 tetrahedron. Finally, the white light emitting diode (WLED) devices prepared with UV WLED chips and the CMV : 0.5%Ta5+ phosphor exhibited excellent color temperature (4083 K) and CIE coordinates (0.3677, 0.3409). The CMV : 0.5%Ta5+ phosphor can be considered as a potential yellow-green emitting phosphor in the solid-state lighting field.

Ce3+-Eu2+ Co-doped BaCa13Mg2(SiO4)8 cyan phosphor-ultra-high energy transfer efficiency for white light emitting diodes

The energy transfer of Ce3+-Eu2+ can often greatly increase the luminescence efficiency and expand the scope of application. In this study, blue to cyan color-tunable phosphors BaCa13Mg2(SiO4)8:Ce3+,Eu2+ were prepared. BaCa13Mg2(SiO4)8:Eu2+ cyan phosphors have limited applications in WLEDs because of their disadvantages, including the inadequate luminescence performance and imperfect matching of UV chips. Therefore, Ce3+ ions were used as sensitizers to enhance the optical performance of Eu2+ ions. The energy transfer efficiency between Ce3+ and Eu2+ in the BaCa13Mg2(SiO4)8 host was calculated to be 96.7%, and the incorporation of Ce3+ ions boosted the integrated intensity and quantum efficiency of the emission spectrum by approximately 80% and 20%, respectively. At 140 °C, the integral emission intensities could still keep at 81.5% of the initial integral intensities at 25 °C. The Ce3+, Eu2+ co-doped cyan phosphor-based WLED lamp could produce outstanding warm white light with CIE coordinates of (0.3722, 0.3222), demonstrating the enormous potential for WLED applications.

Phosphors Ba2 LaTaO6 :Mn4+ and its alkali metal charge compensation for plant growth illumination

A series of Mn4+ -doped and Mn4+ ,K+ -co-doped Ba2 LaTaO6 (BLT) double-perovskite phosphors was synthesized using a high-temperature solid-state reaction. The phase purity and luminescence properties were also studied. The optimum doping concentration of Mn4+ and K+ was obtained by investigating the photoluminescence excitation spectra and photoluminescence emission spectra. The comparison of BLT:Mn4+ phosphors with and without K+ ions shows that the photoluminescence intensity of K+ -doped phosphors was greatly enhanced. This is because there was a charge difference when Mn4+ ions were doped with Ta5+ ions in BLT. Mn4+ -K+ ion pairs were formed after doping K+ ions, which hinders the nonradiative energy transfer between Mn4+ ions. Therefore, the luminescence intensity, quantum yield, and thermal stability of phosphors were enhanced. The electroluminescence spectra of BLT:Mn4+ and BLT:Mn4+ ,K+ were measured. The spectra showed that the light emitted from the phosphors corresponded well with chlorophyll a and phytochrome PR . The results show that the BLT:Mn4+ ,K+ phosphors had good luminescence properties and application prospects and are ideal materials for plant-illuminated red phosphors.

A novel efficient red-emitting K7SrY2B15O30: Eu3+ phosphor with non-concentration quenching and robust thermal stability for white LEDs

Inhibiting energy migration between Eu³⁺ ions in a fixed host to get higher doping concentration is a permanent topic. Herein, a novel non-concentration quenching red-emitting K₇SrY_2-2xB₁₅O₃₀: xEu³⁺ (0.1 ≤ x ≤ 1.0) phosphor was synthesized via high-temperature sintering method. XRD measurement, Rietveld refinement results, and radius percentage deviation calculation demonstrated the phase purity and the occupation preference of Eu³⁺ ions. With continuously increasing doping Eu³⁺ ions, the absence of concentration quenching could be explained by long distance between two Eu³⁺ (7.012 Å) and the K₇SrEu₂B₁₅O₃₀ could exhibit striking photoluminescence performance with the highest emission wavelength centered at 617 nm. Meanwhile, under the radiation of 393 nm, the high internal quantum efficiency ( ∼ 78.71 %), excellent color purity ( ∼ 88.32 %) and robust thermal stability whose emission intensity at 140 °C could still reach ∼ 97.31 % could guarantee its potential application. When coating BaMgAl₁₀O₁₇: Eu²⁺, (Ba, Sr)₂SiO₄: Eu²⁺, and K₇SrEu₂B₁₅O₃₀ on a near-ultraviolet chip, the bright white light with a low correlated color temperature of 4211 K and CIE color coordinates of (0.3675, 0.3556) could be obtained. Taking the analytic results above, the non-concentration quenching K₇SrY₂B₁₅O₃₀: Eu³⁺ compound has great potential to act as a candidate for red-emitting phosphors in solid-state lighting field.