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The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase (β-phase) NaREF4 (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) NaREF4 UCNPs instead of their β-phase counterparts. With the passivation of an inert shell, the UC emission intensity of α-NaYbF4:Tb40%@CaF2 is 10.5 times higher than that of β-NaYbF4:Tb40%@NaYF4. 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 Tb3+-based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm−1) that existed in Tb3+ between its lowest emit-ting level (5D4) and next low-lying level (7F0). 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-NaYbF4:Tb40%@CaF2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm2).

The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase (β-phase) NaREF4 (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) NaREF4 UCNPs instead of their β-phase counterparts. With the passivation of an inert shell, the UC emission intensity of α-NaYbF4:Tb40%@CaF2 is 10.5 times higher than that of β-NaYbF4:Tb40%@NaYF4. 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 Tb3+-based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm−1) that existed in Tb3+ between its lowest emit-ting level (5D4) and next low-lying level (7F0). 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-NaYbF4:Tb40%@CaF2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm2). [ABSTRACT FROM AUTHOR]