Luminescence Tuning of Upconversion Nanocrystals

Upconversion luminescence tuning of β‐NaYF 4 nanorods under 980 nm excitation has successfully been achieved by tridoping with Ln 3+ ions with different electronic structures. The effects of Ce 3+ ions on NaYF 4 :Yb 3+ /Ho 3+ as well as Gd 3+ ions on NaYF 4 :Yb 3+ /Tm 3+ (Er 3+ ) have been studied in detail. By tridoping with Ce 3+ ions, not only were unusual 5 G 5 → 5 I 7 and 5 F 2 / 3 K 8 → 5 I 8 transitions from Ho 3+ ions and 5d→4f transitions from Ce 3+ ions observed in NaYF 4 :Yb 3+ /Ho 3+ nanorods, but also an increase in the intensity of 5 F 5 → 5 I 8 relative to 5 S 2 / 5 F 4 → 5 I 8 with increasing Ce 3+ concentration, which can be attributed to efficient energy transfers of 5 I 6 (Ho)+ 2 F 5/2 (Ce)→ 5 I 7 (Ho)+ 2 F 7/2 (Ce) and 5 S 2 / 5 F 4 (Ho)+ 2 F 5/2 (Ce)→ 5 F 5 (Ho)+ 2 F 7/2 (Ce). Interestingly, with increasing pump power density, the luminescence of NaYF 4 :Yb 3+ /Ho 3+ nanorods is always dominated by the 5 S 2 / 5 F 4 → 5 I 8 transition, whereas the luminescence of Ce 3+ ‐tridoped NaYF 4 :Yb 3+ /Ho 3+ nanorods is dominated by the 5 S 2 / 5 F 4 → 5 I 8 and 5 G 5 → 5 I 7 transitions in turn. These observations are discussed on the basis of a rate equation model. Furthermore, Gd 3+ ‐tridoped NaYF 4 :Yb 3+ /Tm 3+ (Er 3+ ) nanorods can emit multicolor upconversion emissions spanning from the UV to the near‐infrared under 980 nm excitation. 6 P 5/2 → 8 S 7/2 (≈306 nm) and 6 P 7/2 → 8 S 7/2 (≈311 nm) transitions from Gd 3+ ions were observed. In addition to the aforementioned luminescence properties, these Gd 3+ ‐tridoped nanorods also exhibit paramagnetic behavior at room temperature and superparamagnetic behavior at 2 or 5 K.