Greatly enhanced size-tunable ultraviolet upconversion luminescence of monodisperse β-NaYF4:Yb,Tm nanocrystals

Monodisperse beta-NaYF4: Yb, Tm nanocrystals (nanospheres and nanoplates) with a uniform size were successfully synthesized. By tuning the ratios of solvent, reaction temperature, and reaction time, we manipulated their phase, shape, and size in the range of 30-400 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixtures and played a key role in fine-tuning the nanocrystals. Possible mechanisms were proposed for forming the products with various architectures. Spectral analysis showed that the beta-NaYF4: Yb, Tm nanocrystals were excellent materials for intense ultraviolet and blue upconversion luminescence. Here, we demonstrate that, under 980 nm excitation, the intense 5-photon upconversion fluorescence (290 nm and 345 nm) from the I-1(6) level of Tm3+ ions is much stronger than the 4-photon upconversion fluorescence (361 nm and 451 nm) from the D-1(2) level and the 3-photon upconversion fluorescence (474 nm) from the (1)G(4) level. The analysis of the temporal evolution of UC luminescence suggests that long lifetimes benefit the intense ultraviolet upconversion luminescence.