The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase ( β -phase) NaREF 4 (RE = rare earth) is the most commonly used crystal structure for lanthanide-doped upconversion nanoparticles (UCNPs) owing to its high upconversion (UC) efficiency. In this work, we report, for the first time, that more efficient cooperative sensitization upconversion (CSU) can be achieved in cubic-phase ( α -phase) NaREF 4 UCNPs instead of their β -phase counterparts. With the passivation of an inert shell, the UC emission intensity of α -NaYbF 4 :Tb40%@CaF 2 is 10.5 times higher than that of β -NaYbF 4 :Tb40%@NaYF 4 . We propose that the high-symmetry crystal structure of the α phase facilitates the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters, which are particularly beneficial for CSU. Moreover, we prove that such Tb 3+ -based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm −1 ) that existed in Tb 3+ between its lowest emit-ting level ( 5 D 4 ) and next low-lying level ( 7 F 0 ). Finally, their potential application for single-nanoparticle imaging has also been demonstrated. As expected, the α -core-shell UCNPs measured at the single-nanoparticle level are estimated to be 9-fold brighter than their β -core-shell counterparts. Importantly, the α -NaYbF 4 :Tb40%@CaF 2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm 2 ).

中文翻译：

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用于单粒子成像的高效耐水Tb3+基纳米粒子

主晶格的结构对镧系元素掺杂的发光材料的光学性质有重大影响。六方相（β相）NaREF 4（RE = 稀土）是镧系元素掺杂的上转换纳米粒子（UCNP）最常用的晶体结构，因为它具有高上转换（UC）效率。在这项工作中，我们首次报告说，可以在立方相（α 相）NaREF 4 UCNPs 而不是 β 相对应物中实现更有效的协同敏化上转换（CSU）。通过惰性壳的钝化，α-NaYbF 4 :Tb40%@CaF 2 的UC发射强度是β-NaYbF 4 :Tb40%@NaYF 4 的10.5倍。我们提出α相的高对称晶体结构促进了[Yb-Yb]二聚体和[Yb-Yb-Tb]簇的形成，这对 CSU 尤其有益。此外，我们证明这种基于 Tb 3+ 的 UCNPs 几乎不受水淬火的影响，因为 Tb 3+ 中存在的大能隙（~15,000 cm -1 ）在其最低发射水平（ 5 D 4 ）和下一个低层 (7 F 0)。最后，它们在单纳米粒子成像方面的潜在应用也得到了证明。正如预期的那样，在单纳米粒子水平上测量的 α-核-壳 UCNP 估计比其 β-核-壳对应物亮 9 倍。重要的是，α-NaYbF 4 :Tb40%@CaF 2 UCNPs 为在超低辐照度 (30 W/cm 2 ) 下实现单纳米粒子成像提供了令人兴奋的机会。000 cm -1 ) 在 Tb 3+ 中存在于其最低发射水平 ( 5 D 4 ) 和下一个低位水平 ( 7 F 0 ) 之间。最后，它们在单纳米粒子成像方面的潜在应用也得到了证明。正如预期的那样，在单纳米粒子水平上测量的 α-核-壳 UCNP 估计比其 β-核-壳对应物亮 9 倍。重要的是，α-NaYbF 4 :Tb40%@CaF 2 UCNPs 为在超低辐照度 (30 W/cm 2 ) 下实现单纳米粒子成像提供了令人兴奋的机会。000 cm -1 ) 在 Tb 3+ 中存在于其最低发射水平 ( 5 D 4 ) 和下一个低位水平 ( 7 F 0 ) 之间。最后，它们在单纳米粒子成像方面的潜在应用也得到了证明。正如预期的那样，在单纳米粒子水平上测量的 α-核-壳 UCNP 估计比其 β-核-壳对应物亮 9 倍。重要的是，α-NaYbF 4 :Tb40%@CaF 2 UCNPs 为在超低辐照度 (30 W/cm 2 ) 下实现单纳米粒子成像提供了令人兴奋的机会。