Phase-Transition-Driven Regional Distribution of Rare-Earth Ions for Multiplexed Upconversion Emissions.

Phase transition of the polymorphs is critical for controlled synthesis and property modulation of functional materials. Upconversion emissions from an efficient hexagonal sodium rare-earth (RE) fluoride compound, ß-NaREF4, which is generally obtained from the phase transition of the cubic (α-) phase counterpart, are attractive for photonic applications. However, the investigation of the α → ß phase transition of NaREF4 and its effect on the composition and architecture is still preliminary. Herein, we investigated the phase transition with two kinds of α-NaREF4 particles. Instead of a uniform composition, the ß-NaREF4 microcrystals exhibited regionally distributed RE3+ ions, in which the RE3+ with a smaller ionic radius (smaller RE3+) sandwiched the RE3+ with a larger ionic radius (larger RE3+). We unravel that the α-NaREF4 particles transformed to ß-NaREF4 nuclei with no controversial dissolution, and the α → ß phase transition toward NaREF4 microcrystals included nucleation and growth steps. The component-dependent phase transition is corroborated with RE3+ ions from Ho3+ to Lu3+ and multiple sandwiched microcrystals were obtained, in which up to five kinds of RE components were distributed regionally. Moreover, with rational integration of luminescent RE3+ ions, a single particle with multiplexed upconversion emissions in wavelength and lifetime domains is demonstrated, which provides a unique platform for optical multiplexing applications.

Unravelling the energy transfer of Er3+-self-sensitized upconversion in Er3+-Yb3+-Er3+ clustered core@shell nanoparticles.

Unravelling upconversion (UC) energy transfer mechanisms is significant for designing novel efficient anti-Stokes phosphors. We have studied the correlation of different lanthanide dopants within Er3+-self-sensitized core@shell upconversion nanoparticles (UCNPs). Here, our focus will be on high-concentration dopants that are able to sufficiently produce the clustering effect, especially within the interplay between Er3+ and Yb3+. We demonstrate that whatever the amount of the self-sensitizer (e.g., Er3+), abnormal absorption enhancement will occur as long as Yb3+ clusters are present. This effect originates from the substantial energy transfer between Yb3+-Yb3+ clusters despite the increased energy transfer from Yb3+ to Er3+. Therefore, the energy transfer efficiency is still constrained. However, we conversely used one of the aforementioned quench-paths of UC energy transfer to easily transfer the energy from the in-shell shell layer to the in-core area with the assistance of the energy potential reservoir, which was given by the homogeneous core@shell band offset at the interface region. Indirectly, we actualize the Er3+ UC luminescence with self-sensitization through an extended energy transfer path. This work provides a solid support and analytic theory for unraveling the energy transfer mechanism from recent works on Er3+ self-sensitized UC luminescence.