Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF4:Yb3+, Er3+ Core and Core-Shell Nanocrystals
Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF4:Yb3+, Er3+ Core and Core-Shell Nanocrystals
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DOI：
10.1021/acs.jpcc.7b04567
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发表时间：
2017-08-03
影响因子：
3.7
通讯作者：
Berry, Mary T.
中科院分区：
文献类型：
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作者：
Hossan, Md Yeathad;Hor, Amy;Berry, Mary T.
β-NaYF 4：Yb 3+，Er 3 + Ers的纳米晶体与高质量块体材料相比通常具有较低的NIR至可见光上转换（UC）内部量子效率IQE，并且当用980 nm附近的脉冲源激发时，表现出更快的UC动力学，典型的淬灭。保护壳的添加增加了纳米晶体的IQE，并减缓了整体激发态动力学。在这里，我们表明，最近开发的模型UC的微米级β-NaYF 4：18%Yb 3+，2%Er 3+晶体粉末的扩展正确预测的时间分辨发光曲线的形状，相对强度，和观察到的下降IQE的各种发射线的核心和核壳纳米粒子脉冲激发。该模型清楚地表明，在这些材料中，与典型的高Ybs和低Er 3+掺杂的可见上转换发光的纳米级效应，主要是由于快速的能量迁移之间的Yb 3+（F-2（5/2））和Er 3+（I-4（11/2））离子在1 μ m的能量水平，使内部网站和快速松弛的表面网站之间实现平衡。在可见光发射脉冲NIR激发后观察到的更快的动力学主要是传播的1 μ m水库状态的表面淬火的效果，而不是由于直接淬火的可见光发射状态本身。对于贡献于UC发射的Er 3+离子，蓝色（H-2（9/2））、绿色（H-2（11/2）、S-4（3/2））和红色（F-4（9/2））发射状态的弛豫速率常数与它们的体值基本上不变，表明接近纳米颗粒表面的Er 3+离子对于UC几乎是沉默的。被动β-NaYF 4外壳的加入延缓了1 μ m激发水库的排水，并恢复了UC中外部Er 3+位点的参与。壳厚度的IQE的依赖性很好地解释了福斯特型模型描述的能量供体（Er 3+，Yb 3+）与薄平面层的受体（油酸酯）相互作用。UC的行为的核心和核壳纳米晶体可以建模，几乎定量，仅在1 μ m的水平淬火的基础上，没有单独考虑的近表面Er 3+人口。然而，核心纳米颗粒的双层模型揭示了近表面离子参与UC的适度程度，并更好地表示了NIR发射状态的详细动态。提出了一种方法，允许研究人员估计IQE的任何纳米样品（与18%Yb 3+，2%Er 3+掺杂）作为一个函数的激发功率密度（cw）或脉冲能量密度的基础上的低脉冲能量测量的衰减常数为1 μ m的发射。
Nanocrystals of beta-NaYF4:Yb3+, Er3+ Ers generally have lower NIR-to-visible upconversion (UC) internal quantum efficiency, IQE, compared to high-quality bulk materials, and exhibit more rapid UC dynamics, typical of quenching, when excited with a pulsed source near 980 nm. The addition of a protective shell increases the IQE of the nanocrystals and slows the overall excited-state dynamics. Here, we show that an extension of a recently developed model for UC in powders of micron-sized beta-NaYF4: 18%Yb3+, 2%Er3+ crystals correctly predicts the time-resolved luminescence curve shapes, relative intensities, and observed drop in IQE of the various emission lines for core and core shell nanoparticles following pulsed excitation. The model clearly shows that the nanoscale effect on visible upconversion luminescence in these materials, with typical high-Ybs and low-Er3+ doping, is largely due to rapid energy migration among Yb3+(F-2(5/2)) and Er3+ (I-4(11/2)) ions at the 1 mu m energy level, such that an equilibrium is achieved between interior sites and rapidly relaxing surface sites. The faster kinetics observed in visible emission following pulsed NIR excitation is mainly a propagation of the effect of surface quenching of the 1 mu m reservoir states and is not due to direct quenching of the visible emitting states themselves. For Er3+ ions contributing to UC emission, the relaxation rate constants for the blue (H-2(9/2)), green (H-2(11/2), S-4(3/2)), and red (F-4(9/2)) emitting states are essentially unchanged from their bulk values, indicating that Er3+ ions close to the nanoparticle surface are nearly silent with regard to UC. The addition of a passive beta-NaYF4 shell retards the drain of the 1 mu m excitation reservoir and recovers the participation of outer Er3+ sites in UC. The dependence of IQE on shell thickness is well explained in terms of a Forster-type model describing an energy donor (Er3+, Yb3+) interacting with a thin plane layer of acceptors (oleate). The UC behavior of both the core and the core-shell nanocrystals can be modeled, almost quantitatively, solely on the basis of quenching at the 1 mu m level, without separate consideration of a near-surface Er3+ population. However, a two-layer model for the core nanoparticles is revealing with regard to the modest extent to which near-surface ions do participate in UC and gives a better representation of the detailed dynamics of the NIR emitting states. A method is presented for allowing investigators to estimate the IQE for any nanosample (with 18% Yb3+, 2%Er3+ doping) as a function of excitation power density (cw) or pulse-energy density based on the low pulse energy measurement of the decay constant for the 1 mu m emission.