Luminescence tuning of upconversion nanocrystals.

Upconversion luminescence tuning of beta-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) ( approximately 306 nm) and (6)P(7/2)-->(8)S(7/2) ( approximately 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.