Colloidal synthesis and blue based multicolor upconversion emissions of size and composition controlled monodisperse hexagonal NaYF4 : Yb,Tm nanocrystals

Monodisperse beta-NaYF4 : Yb, Tm nanocrystals with controlled size (25-150 nm), shape (sphere, hexagonal prism, and hexagonal plate), and composition (Yb: 20-40%, Tm: 0.2-5%) were synthesized from the thermolysis of metal trifluoroacetates in hot surfactant solutions. The upconversion (UC) of near-infrared light (980 nm) to ultra-violet (360 nm), blue (450 and 475 nm), red (650 and 695 nm) and infrared (800 nm) light in the beta-NaYF4 : Yb, Tm nanocrystals has been studied by UC spectroscopy. Both the total intensity of UC emissions and the relative intensities of emissions at different wavelengths have shown a strong dependence on different particle sizes and different Tm3+ and Yb3+ concentrations. As a result, different overall output colors of UC emissions can be achieved by altering sizes and Yb3+/Tm3+ doping concentrations of the beta-NaYF4 : Yb, Tm nanocrystals. The intensity-power curves of a series of samples have proved that emissions at 360 and 450 nm can be ascribed to four-photon process (D-1(2) to H-3(6) and D-1(2) to H-3(4), respectively), while emissions at 475 and 650 nm are three-photon processes ((1)G(4) to H-3(6) and (1)G(4) to H-3(4), respectively) and emissions at 695 and 800 nm are two-photon ones (F-3(2) to H-3(6) and F-3(4) to H-3(6), respectively). A UC saturation effect would occur under a certain excitation intensity of the 980 nm CW diode laser for the as-obtained beta-NaYF4 : Yb, Tm nanocrystals, leading to the decrease of the slopes of the I-P curves. The results of our study also revealed that the successive transfer model instead of the cooperative sensitization model can be applied to explain the UC behaviors of the beta-NaYF4 : Yb, Tm nanocrystals. Further, an unexpected stronger emissions of four-photon process at 360 and 450 nm for similar to 50 nm beta-NaYF4 : Yb, Tm nanocrystals than those for the bigger (similar to 50 nm) nanocrystals was observed and explained in terms of the effects of crystallite size, surface-to-volume ratio and homogeneity of the doping cations.