Luminescence Tuning of Upconversion Nanocrystals

Upconversion luminescence tuning of beta-NaYF4 nanorods under 980 nm excitation has successfully been achieved by tridoping with Ln(3+) 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 (5)G(5)-> I-5(7) and F-5(2)/K-3(8)-> I-5(8) 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 I-5(5)-> I-5(8) relative to S-5(2)/F-5(4)-> I-5(8) with increasing Ce3+ concentration, which can be attributed to efficient energy transfers of I-5(6) (Ho) F-2(1/2) (Ce)-> I-5(7) (Ho) + F-2(1/2) (Ce) and S-5(2)/F-5(4) (Ho) + F-2(1/2) (Ce)-> F-5(5) (Ho) + F-2(1/2) (Ce). Interestingly, with increasing pump power density, the luminescence of NaYF4:Yb3+/Ho3+ nanorods is always dominated by the S-5(2)/F-5(4)-> I-5(8) transition, whereas the luminescence of Ce3+-tridoped NaYF4:Yb3+/Ho3+ nanorods is dominated by the S-5(2)/F-5(4)-> I-5(8) and (5)G(5)-> I-5(7) 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 multi-color upconversion emissions spanning from the UV to the near-infrared under 980 nm excitation. P-6(5/2)-> S-8(1/2) (approximate to 306 nm) and P-6(7/2)-> S-8(7/2) (approximate to 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.