Luminescence Tuning of Upconversion Nanocrystals

摘要

Upconversion luminescence tuning of β-NaYF₄ nanorods under 980 nm excitation has successfully been achieved by tridoping with Ln3+ ions with different electronic structures. The effects of Ce3+ ions on NaYF₄:Yb3+/Ho3+ as well as Gd3+ ions on NaYF₄:Yb3+/Tm3+(Er3+) have been studied in detail. By tridoping with Ce3+ ions, not only were unusual 5G₅→5I₇ and 5F₂/3K₈→5I₈ transitions from Ho3+ ions and 5d→4f transitions from Ce3+ ions observed in NaYF₄:Yb3+/Ho3+ nanorods, but also an increase in the intensity of 5F₅→5I₈ relative to 5S₂/5F₄→5I₈ with increasing Ce3+ concentration, which can be attributed to efficient energy transfers of 5I₆ (Ho)+2F_5/2 (Ce)→5I₇ (Ho)+2F_7/2 (Ce) and 5S₂/5F₄ (Ho)+2F_5/2 (Ce)→5F₅ (Ho)+2F_7/2 (Ce). Interestingly, with increasing pump power density, the luminescence of NaYF₄:Yb3+/Ho3+ nanorods is always dominated by the 5S₂/5F₄→5I₈ transition, whereas the luminescence of Ce3+-tridoped NaYF₄:Yb3+/Ho3+ nanorods is dominated by the 5S₂/5F₄→5I₈ and 5G₅→5I₇ transitions in turn. These observations are discussed on the basis of a rate equation model. Furthermore, Gd3+-tridoped NaYF₄:Yb3+/Tm3+(Er3+) nanorods can emit multicolor upconversion emissions spanning from the UV to the near-infrared under 980 nm excitation. 6P_5/2→8S_7/2 (≈306 nm) and 6P_7/2→8S_7/2 (≈311 nm) transitions from Gd3+ ions were observed. In addition to the aforementioned luminescence properties, these Gd3+-tridoped nanorods also exhibit paramagnetic behavior at room temperature and superparamagnetic behavior at 2 or 5 K.