Facile synthesis of β-NaLuF4 : Yb/Tm hexagonal nanoplates with intense ultraviolet upconversion luminescence

Monodisperse, hexagonal phase NaLuF4 : Yb,Tm nanoplates (NPs) with uniform size have been successfully synthesized by a novel solution-based method. The nanoplates have a perfect hexagonal shape with a diameter of ∼180 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixture and played a key role in fine-tuning the nanoplates. A possible growth mechanism was proposed for the formation of β-NaLuF4 nanoplates. Spectral analysis showed that the β-NaLuF4 : Yb,Tm nanoplates were excellent materials for intense ultraviolet and blue upconversion luminescence. To our best knowledge, it is the first time such intense 5-photon upconversion fluorescence from the 1I6 level of Tm3+ ions, which is much stronger than the 4-photon upconversion fluorescence from the 1D2 level and the 3-photon upconversion fluorescence from the 1G4 level, has been demonstrated. The analysis on temporal evolutions of UC luminescence suggests that β-NaLuF4 nanocrystals might be a better kind of upconversion material than their β-NaYF4 counterpart. This powerfully demonstrates that β-NaLuF4 is an excellent host lattice for upconversion luminescence materials. Due to the unique luminescence, these β-NaLuF4 nanoplates may be promising for further fundamental research and applications in color displays and solid-state lasers.