Synthesis and upconversion luminescence of uniform beta-NaYF4:Yb3+/Tm3+ hexagonal nanoplates.

: Yb3+ and Tm3+-codoped hexagonal-phase NaYF4 powders were prepared by a facile hydrothermal method. The results of X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) showed that the as-prepared powders were well crystallized nanoplates with high size-uniformity. Under the excitation from a 980 nm laser diode, upconversion (UC) emissions centered at approximately 291 nm (1I6 --> 3H6), approximately 346 nm (1I6 --> 3F4), approximately 361 nm (1D2 --> 3H6), approximately 451 nm (1D2 --> 3F4), approximately 474 nm (1G4 --> 3H6), approximately 644 nm (1G4 --> 3F4), and approximately 799 nm (3H4 --> 3H6) were observed in the sample. Furthermore, the intensity dependence of UC emissions on excitation power was measured. The results indicated that populating the 1I6, 1D2, 1G4, and 3H4 states were five-photon, four-photon, three-photon, and two-photon UC processes, respectively.