| Rare earth (RE) ion-doped upconversion materials have presented many advantages, such as narrow emission line-width, abundant emission bands, long fluorescence lifetime, high photo-stability, low background emission, and so forth. Additionally, owing to the specific near-infrared excitation, upconversion nanoparticles have excellent penetration depth in bio-systems, no photo-damage to living organisms, weak auto-fluorescence and light scattering from backgrounds, which improving the signal-to-noise ratio and detection sensitivity. Therefore, RE-based upconversion materials have shown great potential in a wide range of applications, from bioimagine and theranostics to photovoltaic devices and photochemical reactions. And then the design and synthesis of RE-doped upconversion materials have attracted more attention. It is well-known that the upconversion luminescence properties of phosphors are significantly dependent on their crystal phase structure, morphology, size and doping concentrations of rare earth ions. What's more, among the reported upconversion materials, NaYF4, with low phonon energies (?360 cm-1), high thermal/photo-stability and low toxicity, is a most efficient host material. Consequently, we focus our efforts on the controlled synthesis of the Yb3+-Tm3+ co-doped NaYF4 nano/micro-structure upconversion luminescence materials, as well as investigate the influence of crystal phase structure, morphology, size and doping concentrations of rare earth ions on their upconversion luminescence properties.NaYF4:Yb3+, Tm3+ upconversion luminescence materials were synthesized via a facile oleic acid-assisted hydrothermal method, and different crystal phase structures and morphologies were obtained by adjusting the reaction time and the ratio of NH4F to RE3+, respectively. The different NaYF4:Yb3+, Tm3+ crystals were characterized by X-ray diffraction and field emission scanning electron microscopy, the results can be found as follows:firstly, NaYF4 host has two different crystal phase structures, including a cubic phase (a-NaYF4)and a hexagonal phase (?-NaYF4). And the phase transformation can be observed obviously by controlling the reaction time. Second, the ratio of NH4F to RE3+ has a great effect on the morphology of the NaYF4:Yb3+, Tm3+ particles. With increasing the amount of F" ions, the crystals prefer to grow along the c axis, leading to the evolution of morphology from nanodisks to nanoprisms, nanorods and to microrods.Upconversion luminescence of NaYF4:Yb3+, Tm3+ particles has been investigated by using a commercial 980nm diode laser as an excitation source. It can be found that five emission bands located at 450,474,645,695 and 800 nm could be assigned to 1D2?3F4,1G4?3H6,1G4?3F4,3F2,3?3H6 and 3H4?3H6 transitions of Tm3+ ion, respectively.As for NaYF4:Yb3+, Tm3+ upconversion luminescence materials, the upconversion luminescence intensity is sensitive to the crystal phase structure, morphology and the doping concentration of Tm3+ and Yb3+ ions. Firstly, the upconversion luminescence intensity of ?-NaYF4:Yb3+, Tm3+ particles is great stronger than that of a-NaYF4:Yb3+, Tm3+ particles. Secondly, compared to the nanodisks, nanoprisms and nanorods, the ?-NaYF4:Yb3+, Tm3+ microrods with a smaller surface-to-volume ratio present the best upconversion luminescence property. Thirdly, the emission intensities at different wavelengths can be greatly affected by different doping concentrations of Yb3+ ions. The most strongest emission intensity of 800 nm emission band occurs at 60% mol, while others'are at about 80% mol. It's worth noting that the emission intensity of 450 nm emission band can be enhanced suddenly as the doping concentration of Yb3+ increased to 40% mol, which can be assigned to the increasing population of the 1D2 state. Besides, the concentration of Tm3+ ions can also effect the total luminescence intensity, and different excited states have different concentration threshold. As for 1G4?3H6 transition, which can emit blue emission(474 nm), the optimum concentration is 0.05% mol, but for 3H4?3H6 transition, which can emit near-infrared emission(800 nm), the optimum concentration is 1.00% mol. Therefore, a stronger target emission peak can be obtained by adjusting the doping concentrations of Tm3+ and Yb3+ ions in the practice. Finally, it's also striking that the luminescence intensity of 800 nm emission band is always the strongest among the whole emission bands, which can provide a scientific basic for the expansion of NaYF4:Yb+, Tm3+ upconversion luminescence materials in the field of biotechnology. |
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