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

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.