Near infrared fluorescence properties of Tb4+ and Tb3+ inter-conversion states in CaF2:Yb3+nanocrystals

• Published in: Materials research bulletin 2023-10, Vol.166, p.112350, Article 112350
• Main Authors: Coetsee, E., Yagoub, M.Y.A., Swart, H.C.
• Format: Article
• Language: English
• Subjects: NIR emission; Tb4+ charge transfer state; Valance state conversion
• ISSN: 0025-5408; 1873-4227
• DOI: 10.1016/j.materresbull.2023.112350

•Tb4+ ion noticeably enhanced the NIR emission of Yb3+ co-doped CaF2 nanocrystal.•XPS confirmed the presence of Tb4+ ions.•Annealing time affected the NIR emission of the nanocrystal. Near infra-red emission was enhanced in calcium fluoride (CaF2) co-doped with terbium ions and ytterbium (Yb3+) under a 325 nm laser excitation wavelength, during investigations performed for possible application in Si solar cells. Tb3+ ions were oxidized to Tb4+ ions and emission enhancement occurred due to a cooperative energy transfer mechanism between the Tb4+-Yb3+ couple. The oxidation of Tb3+ ions in CaF2 was achieved by annealing the nanocrystals at 450 °C in air. X-ray diffraction analysis confirmed the cubic face of the CaF2 crystal without any impurity peaks. UV–VIS luminescence and X-ray photoelectron spectroscopy were conducted to probe the conversion of Tb3+ to Tb4+ ions as a function of the annealing time. The main conclusion was that the NIR emission from the Tb4+-Yb3+ couple revealed that down-conversion occurred through a cooperative energy transfer mechanism. [Display omitted]