Upconversion luminescence in Yb3+/Tb3+-codoped monodisperse NaYF4 nanocrystals

Upconversion (UC) luminescence in monodisperse NaYF4:Yb3+/Tb3+ nanocrystals was observed under diode laser excitation of 970 nm, which were synthesized by a hydrothermal method. UC emissions at 380, 413, 436 nm and at 488, 542, 584, 620 nm arise from transitions 5D3(5G6) → 7FJ(J = 6, 5, 4) and 5D4 → 7FJ(J = 6, 5, 4, 3) of Tb3+ ions, respectively. UC mechanisms are proposed based on spectral, kinetic, decay time measurements, and pump power dependence analyses. Blue, green and red emissions originate from the same long-lived (milliseconds) upper 5D4 state, which promises the potential applications of these monodisperse Yb3+/Tb3+-codoped NaYF4 nanocrystals in the field of photonics, lasers and biomedicine.