Energy transfer and tunable emission in BaSrGd4O8:Bi3+,Eu3+ phosphors for warm WLED

In this work, a series of BaSrGd4O8:xBi3+ blue phosphors was synthesized employing the high-temperature solid-state method. Phase purity of the samples was verified by X-ray diffraction and Rietveld refinement. Time-resolved photoluminescence spectra revealed the existence of two distinct Bi sites. Subsequent optimization of dopant types and doping levels in the batch led to an almost twofold increase in quantum efficiency. The introduction of Eu3+ into the phosphors facilitated the construction of an energy transfer pathway. As the concentration of Eu3+ was increased, the emission color changed from blue to purple and finally to red. In addition, the thermal stability and potential applications of the phosphors were extensively investigated. Finally, two WLED devices were successfully fabricated with color rendering indices of 96.27 and 92.18, and correlated color temperatures of 5198 and 2475 K. This underscores the prospective application of these phosphors in the field of high-quality warm WLEDs.

Utilizing the energy transfer mechanism to realize color tunable luminescence

Bi³⁺/Eu³⁺ co-doped phosphors can realize multi-color luminescence by adjusting the concentration ratio, which makes it possible to manually control the emission color. On this basis, a series of Bi³⁺/Eu³⁺ co-doped phosphors SrLaGa₃O₇:xBi³⁺,yEu³⁺ were prepared. The existence of energy transfer from Bi³⁺ to Eu³⁺ was verified by spectral analysis. The emission spectra of SrLaGa₃O₇:xBi³⁺ show a wide-band peaking at 472 nm. For SrLaGa₃O₇:xBi³⁺,yEu³⁺, the Bi³⁺ → Eu³⁺ energy transfer occurs and a series of sharp emitting peaks of Eu³⁺ can be observed simultaneously. The relative luminescence intensity of Bi³⁺ and Eu³⁺ can be modulated by changing the relative concentrations of Bi³⁺ and Eu³⁺. Using this mechanism, the color tunable luminescence of SrLaGa₃O₇:xBi³⁺,yEu³⁺ from cyan, through white to orange, and finally to red is realized. By using a 320 nm UV chip and SLG:0.06Bi³⁺,0.07Eu³⁺ white phosphor, a white light-emitting diode (WLED) lamp was fabricated with chromaticity coordinates of (0.3199, 0.3083) and a color rendering index R_a of 82. This indicates that the prepared sample is a very promising candidate in the LED field.

Luminescence property improvement and controllable color regulation of a novel Bi3+ doped Ca2Ta2O7 green phosphor through charge compensation engineering and energy transfer

In pursuit of warm WLEDs, exploration of novel phosphors and regulation of the existing phosphors are the two approaches usually used in the luminescent material field. In this work, we prepared green Ca₂Ta₂O₇:Bi³⁺ phosphors firstly and investigated their properties in detail. The as-prepared Ca₂Ta₂O₇:Bi³⁺ exhibits intense green emission in the 450-580 nm range under UV excitation, which matches well with the UV chip and can efficiently avoid the re-absorption problem. The improvement in the emission intensity and thermal stability of the phosphor was achieved using different charge compensation methods including codoping alkali metal ions (Li⁺, Na⁺, and K⁺), creating a cation vacancy, and host co-substitution (Ca²⁺ + Ta⁵⁺ → Bi³⁺ + Si⁴⁺, Ca²⁺ + Ta⁵⁺ → Bi³⁺ + Ge⁴⁺). Through systematic research, the emission intensity at room temperature was improved 2.1 times and the thermal stability was improved 2.9 times at 200 °C. By coating the prepared green sample with other commercial phosphors on the UV chip, warm WLEDs with Ra being 91.1 and CCT being 3990 K were obtained. Moreover, taking the Bi³⁺ → Eu³⁺ energy transfer strategy, the emitting color of the phosphor was tuned and yellow emitting phosphor was obtained. Our study indicates that Bi³⁺ doped Ca₂Ta₂O₇ might be a potential UV excited green phosphor for WLEDs. The charge compensation methods and the Bi³⁺ → Eu³⁺ energy transfer approach are valuable ways to improve and adjust the luminescence properties, which can further derivate a series of novel phosphors for improving the quality of WLED devices.

Photogenerated charge separation and recombination path modification in monocline Lu2WO6via lattice transition and Bi-O antibonding states

Monoclinic Lu2WO6 undergoes diphase-to-perovskite BiLuWO6 transition via selective occupancy of Bi in three Lu sites. The transformation mechanism, process, and structure stabilities are revealed by variable cell nudged elastic band method, video, and phonon spectrum. Lattice transition brings about photogenerated charge separation in BiLuWO6. This is verified by indirect band gap transition, high electron migration rate, weak exciton binding energy, large photocurrent response, and small impedance. The electron-hole life time is elongated to produce abundant superoxide and hydroxyl radicals for the degradation of rhodamine B and phenol molecules. Bi-O antibonding states serve as immediate energy levels to change the recombination path, inducing 340 nm excitation band and 510 nm green light emission of Lu2WO6. Furthermore, multicolor emission of 1 at% Bi3+ + RE3+ (RE = Sm/Eu/Dy)-codoped Lu2WO6 is acquired via synergistic modification of the Bi-O antibonding state and RE3+ 4f states. Thus, the photogenerated charge motion in Lu2WO6 is tuned to expand application fields.