Synthesis and luminescence studies of Dy3+ doped Li3Sc(BO3)2 polycrystalline powder for warm white light

Li₃Sc(BO₃)₂, an orthoborate polycrystalline compound, was synthesised using the solid-state synthesis method as a pure and Dy₂O₃-doped material. Dy³⁺ ions were used in various concentrations (from 1 to 6 mol. %). The experimental powder X-ray diffraction data and calculated Rietveld refined data are found to be in good agreement, verifying the effectiveness of the synthesis procedure. The luminescence studies were performed under the excitations of X-ray, proton beam, and UV light. The success of radioluminescence studies verifies the prospects of the Dy³⁺-doped compound for its application as a radiation scintillator in imaging. The experimental study of cross-relaxation processes confirms the lack of potential evidence of the quenching of the emission peak at 765 nm through the absorption via ⁶H_15/2 → ⁶F_3/2 electronic transition cross-relaxation channel ?1. The CIE 1931 colour coordinates and correlated colour temperatures were determined for all doped samples under X-ray, proton beam, and photoluminescence excitations. The obtained combined emission colour appeared in the warm white region, unlike many other Dy³⁺-doped phosphors that suffer suppression of luminescence through the cross-relaxation process.     

Energy transfer and luminescence studies of Pr, Yb co-doped lead borate glass

Lead borate glass samples doped with the tripositive lanthanide ions Pr³⁺ and Yb³⁺ were synthesized by the conventional melting-quenching method. The luminescence properties and energy transfer process from Pr³⁺ to Yb³⁺ were investigated. Upon ultraviolet excitation, the room temperature luminescence decay curve of a sample containing only a low concentration of Pr³⁺ exhibited monoexponential decay from ¹D₂ with the lifetime 37 μs, without emission from ³P₀. The room temperature Pr³⁺ emission intensity decreased with the increase of Yb³⁺ mole ratio in the glass. Under the excitation of 454.5 nm at 10 K, a broad red emission band centered at 605 nm, and an NIR emission band at 995 nm were observed in the co-doped lead borate glass, originating from Pr³⁺ and Yb³⁺ ions, respectively. The decay curves of the ¹D₂ emission from Pr³⁺ with addition of Yb³⁺ in lead borate glass show non-monoexponential character, and are best described by a stretched exponential function. The average ¹D₂ decay time decreases considerably with the addition of Yb³⁺ in the glass. Decay curve fitting using a modified Inokuti-Hirayama expression indicates dipole-dipole energy transfer from Pr³⁺ to Yb³⁺, which is consistent with the expected cross-relaxation scheme. There is a good agreement of the estimated overall energy transfer efficiency obtained from the integrals under the normalized decay curves, or from the lifetimes fitted by the stretched exponential function, or from the average decay times.     

Determination of cross-relaxation efficiency based on spectroscopy in thulium-doped rare-earth sesquioxides

Tm³⁺ doped cubic rare-earth sesquioxides have been recognized as excellent laser materials operating at ～ 2 μm. Upon ³H₄ excitation by commercially available laser diode around 800 nm, a cross-relaxation (CR) process enables efficient Tm³⁺ laser originated from ³F₄→³H₆ transition. The CR process, (Tm1:³H₄, Tm2:³H₆) → (Tm1:³F₄, Tm2: ³F₄), cuts one excitation photon into two photons of 2 μm with quantum efficiency of 200%. As a result, efficient CR is needed. The CR efficiency is usually determined based on the emission lifetime of ³H₄. However, the CR between the nearest Tm³⁺ is too fast to emit, overrating the emission lifetime and underrating the CR efficiency. Here, we propose a spectroscopic method for determination of CR efficiency based on emission intensity ratio. Tm³⁺ doped cubic Y₂O₃, Lu₂O₃, Sc₂O₃, YScO₃, LuScO₃ and YLuO₃ sesquioxides are prepared by solid-state reaction. Near infrared emission spectra and decay curves are studied as a function of Tm³⁺ concentration. The relationship between CR efficiency and emission intensity ratio is established. Large Stark splittings of 1002 cm^?1 and 511 cm^?1 are observed for the ³H₆ ground state and the ³F₄ excited state of Tm³⁺, respectively in Sc₂O₃:Tm³⁺, leading to a long-wave emission at 2.15 μm. The comparison of these six samples reveals that Sc₂O₃: Tm³⁺ has the largest CR parameter and Lu₂O₃:Tm³⁺ has the highest quantum efficiency for populating the ³F₄ from the ³H₄ at the same Tm³⁺ concentration.     

Anomalous upconversion behavior and high-temperature spectral properties of Yb/Ho-SiAlON ceramics

Traditionally, NaYF₄ and glass-based materials are considered the most efficient upconversion materials. However, those materials may show signs of thermal warping or risks of fracture during long-term service life in extreme environments. In contrast, SiAlON ceramics preserve their spectral and physical integrity during continuous use due to their ultra-low thermal expansion coefficient (～3.0 × 10^?6/K) even at extreme temperatures. Here, we investigate the upconversion photoluminescence properties of Ho and Yb co-doped SiAlON (Ho/Yb-SiAlON) ceramics, prepared by hot-press method, at room temperature and at high temperature (298–1023 K). At room temperature, Ho/Yb-SiAlON ceramics show intense red upconversion emission under 980 nm excitation. At the high-temperature range, the spectral shift is absent in Ho/Yb-SiAlON indicating that the SiAlON ceramics have high-temperature spectral stability, which may be attributed to the low thermal expansion coefficient of SiAlON ceramics. Ho and Yb concentration-dependent spectra show an anomalous upconversion emission behavior. It is found that a higher sensitizer and low activator concentration combination is not always an ideal choice for sensitized luminescence in SiAlON ceramics which is in stark contrast to the common understanding of sensitized luminescence. The mechanism involving the dominant cross-relaxation process has been proposed to explain the observed anomalous upconversion behavior.