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

Monodisperse β-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 β-NaYF ₄: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 β-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 (¹D₂ to ³H₆ and ¹D₂ to ³H₄, respectively), while emissions at 475 and 650 nm are three-photon processes (¹G ₄ to ³H₆ and ¹G₄ to ³H₄, respectively) and emissions at 695 and 800 nm are two-photon ones (³F₂ to ³H₆ and ³F₄ to ³H₆, respectively). A UC saturation effect would occur under a certain excitation intensity of the 980 nm CW diode laser for the as-obtained β-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 β-NaYF₄:Yb,Tm nanocrystals. Further, an unexpected stronger emissions of four-photon process at 360 and 450 nm for ∼50 nm β-NaYF₄:Yb,Tm nanocrystals than those for the bigger (∼150 nm) nanocrystals was observed and explained in terms of the effects of crystallite size, surface-to-volume ratio and homogeneity of the doping cations.