Luminescence Tuning of Upconversion Nanocrystals

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.