Beijing National Laboratory for Molecular Sciences, Peking University, Beijing, 100871, China.
Nanoscale. 2010 Jun;2(6):953-9. doi: 10.1039/b9nr00397e. Epub 2010 Mar 29.
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 (1D2 to 3H6 and 1D2 to 3H4, respectively), while emissions at 475 and 650 nm are three-photon processes (1G4 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 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 approximately 50 nm beta-NaYF4:Yb,Tm nanocrystals than those for the bigger (approximately 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.
单分散β-NaYF4:Yb，Tm 纳米晶具有可控的尺寸（25-150nm）、形状（球体、六方棱柱和六方板）和组成（Yb：20-40%，Tm：0.2-5%），是通过热解金属三氟乙酸盐在热表面活性剂溶液中合成的。通过上转换光谱研究了β-NaYF4:Yb，Tm 纳米晶中近红外光（980nm）到紫外光（360nm）、蓝光（450nm 和 475nm）、红光（650nm 和 695nm）和红外光（800nm）的上转换。上转换发射的总强度和不同波长处发射的相对强度均强烈依赖于不同的粒径和不同的 Tm3+和 Yb3+浓度。因此，通过改变β-NaYF4:Yb，Tm 纳米晶的尺寸和 Yb3+/Tm3+掺杂浓度，可以获得不同的上转换发射整体输出颜色。一系列样品的强度-功率曲线证明，360nm 和 450nm 处的发射可归因于四光子过程（1D2 到 3H6 和 1D2 到 3H4），而 475nm 和 650nm 处的发射是三光子过程（1G4 到 3H6 和 1G4 到 3H4），695nm 和 800nm 处的发射是双光子过程（3F2 到 3H6 和 3F4 到 3H6）。对于获得的β-NaYF4:Yb，Tm 纳米晶，在一定强度的 980nmCW 二极管激光激发下，会发生上转换饱和效应，导致 I-P 曲线的斜率减小。我们的研究结果还表明，连续转移模型而不是合作敏化模型可用于解释β-NaYF4:Yb，Tm 纳米晶的上转换行为。此外，对于大约 50nm 的β-NaYF4:Yb，Tm 纳米晶，观察到并且解释了四光子过程在 360nm 和 450nm 处的出乎意料的更强发射，而对于更大的（大约 150nm）纳米晶，这种发射则较弱，这是由于纳米晶的晶粒尺寸、表面-体积比和掺杂阳离子的均匀性的影响。
