Monodispersed LaF_3 nanocrystals: shape-controllable synthesis, excitation-power-dependent multi-color tuning and intense near-infrared upconversion emission

摘要

In this study, monodispersed and highquality hexagonal phase LaF_3 nanocrystals with different shapes and sizes were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The asprepared LaF3 nanocrystals were characterized by transmission electron microscopy (TEM), xray diffraction (XRD), and analysis of the upconversion spectra. The TEM results reveal that the samples present high uniformity and monodispersity and are selfassembled into a twodimensional ordered array. Moreover, the shape, size and structure of the nanocrystals can be readily tuned by adjusting the NaF content.With increasing content of NaF, the shape of the LaF_3 nanocrystals changed from particle to rod and the size gradually increased. More importantly, high NaF content favors the formation of onedimensional nanorods. High Yb~(3+) and Er~(3+) content is beneficial to synthesizing the hexagonal phase of NaLaF_4 nanocrystals. Furthermore, the TEM results show that the shape and size of the LaF_3 nanocrystals can also be tuned by doping lanthanide ions, which provides a new route for size and shape control of nanocrystals. In addition, LaF_3 nanocrystals codoped with Yb~(3+)/Tm~(3+) present efficient nearinfrared (NIR)-NIR upconversion luminescence. More importantly, the upconversion luminescent colors can be readily tuned from bluewhite to blue by adjusting the excitation power. Therefore, it is expected that these LaF_3 nanocrystals with wellcontrolled shape, size and NIR-NIR upconversion emission have potential applications in biomedical imaging fields.