An up-conversion luminophore with high quantum yield and brightness based on BaF2_{2}:Yb3+^{3+},Er3+^{3+} single crystals

Up-conversion (UC) of near-infrared radiation to visible light has received much attention because of its use in the conversion of solar radiation, luminescence thermometry, biosensing, and anti-counterfeiting applications. However, the main issue hindering the successful utilization of UC is the relatively low quantum efficiency of the process. In order to design new UC systems with high quantum yield (ϕ$_{UC}$) values, we synthesized two series of co-doped BaF$_{2}$ single crystals with nominal concentrations of Yb$^{3+}$ (2–15 mol%)/Er$^{3+}$ (2 mol%) as well as Yb$^{3+}$ (3 mol%)/Er$^{3+}$ (2–15 mol%). The highest ϕ$_{UC}$ value of 10.0% was demonstrated for the BaF$_{2}$:Er$^{3+}$ (2 mol%) and Yb$^{3+}$ (3 mol%) sample under 490 W cm$^{-2}$ of 976 nm excitation. To study the natural limit of UC efficiency, quantum yield values upon direct excitation (ϕ$_{DS}$) of the $^{4}$S$_{3/2}$ (ϕ$_{DS}$ ≤ 26%) levels were measured. Comparison of experimental values of quantum yields to the ones obtained using Judd–Ofelt theory reveals strong quenching of the $^{4}$S$_{3/2}$ state for all investigated compositions. In addition, we observed an unusually strong contribution of the Er$^{3+}$:4I$_{9/2}$ excited state to both UC and down-shifting luminescent processes. This contribution becomes possible due to the very low maximum phonon energy of BaF$_{2}$ crystals (240 cm$^{-1}$)

Down-conversion luminescence of Ce-Yb ions in YF<inf>3</inf>

© 2019 Elsevier B.V. The single-phase Y1-x-yCexYbyF3 solid solution powders were prepared by melt crystallization technique. The 5d-4f interconfigurational luminescence of Ce3+ ions and 2F5/2-2F7/2 luminescence of Yb3+ ions in YF3 crystalline powder doped with Ce3+/Yb3+ ions pair upon 266 nm excitation were studied. The energy transfer from Ce3+ to Yb3+ after 266 nm excitation has complex character and deals with the UV excitation induced dynamic processes and color centers formation in YF3:Ce3+ powders. It was shown that UV light irradiation leads to in absorbing in 525–650 nm spectral range which is not observed for Ce3+-Yb3+ containing samples. This speaks for additional channels of Ce3+ and Yb3+ excitation as a result of recombination of free charge carriers originated from excited state absorption by Ce3+ ions and trapped by Yb3+ ions. The efficiency of energy transfer from Ce3+ to Yb3+ was 22,6% for YF3:Ce(0.05 mol. %):Yb(10.0 mol.%) solid solution with 0,91% down-conversion luminescence external quantum yield

Luminescence of GdF<inf>3</inf>:Pr:Yb and YF<inf>3</inf>:Pr:Yb Solid Solutions Synthesized by Crystallization from the Melt

© 2019, Springer Science+Business Media, LLC, part of Springer Nature. Spectral and luminescence characteristics of GdF3:Pr:Yb and YF3:Pr:Yb solid solutions synthesized by crystallization from the melt were investigated. Down-conversion luminescence of Yb3+ in the region of 2F5/2 → 2F7/2 transitions with 445-nm excitation was studied. The energy-transfer coefficient from Pr3+ to Yb3+ was <80% for concentration ratios Pr/Yb 0.5/10.0 and 1.0/10.0 for GdF3 and 0.5/10.0 for YF3. The absolute values of Pr3+ and Yb3+ luminescence quantum yields in the region of the greatest crystalline-silicon photosensitivity were low because of highly efficient nonradiative energy transfer. The quantum yield was <1% for Yb3+ intrinsic luminescence and <4% for total luminescence in the range 800–1050 nm (for concentration ratio Pr/Yb = 0.5/1.0 in GdF3 matrix). The low quantum yield was a consequence of the complex energy-exchange pattern between Yb3+ and Pr3+ ions, which was largely influenced by the proximity of 1G4 and 2F5/2 energy states of Pr3+ and Yb3+, respectively, in these matrices. This opened an additional pathway for nonradiative decay of Yb3+ excited states and enabled cross-relaxation between 3P0 → 1G4 transitions of Pr3+ and 2F5/2 → 2F7/2 transitions of Yb3+ that increased the lifetime of the Pr3+ 3P0 state

Synthesis and Reactions of Substituted 3-amino-2-furyl(aryl)-thieno[2,3-b]pyridines

New substituted thieno[2,3-b]pyridines which contain 4-nitropehnyl and 5-nitro-, carboxy-, methoxycarbonyl-2-furyl groups in the 2 position have been obtained