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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 …