Enhanced high-order ultraviolet upconversion luminescence in sub-20 nm β-NaYbF4:0.5% Tm nanoparticles via Fe3+ doping†
Despite showing great promise in bioapplications, lanthanide-doped upconversion nanoparticles (UCNPs) are still restricted by low upconversion efficiency. In this work, ultra-small, monodisperse and uniform β-NaYbF4:0.5% Tm NPs were successfully prepared through a solvothermal route, and remarkably enhanced ultraviolet upconversion luminescence (UV UCL) had been achieved via Fe3+ doping. The UV UCL enhancement in NaYbF4:0.5% Tm NPs was investigated in detail. It was found that the UCL intensities of the UV and blue emissions of NPs doped with 5 mol% Fe3+ ions were enhanced by 10 and 6 times, respectively, compared to that of the Fe3+-free sample. The enhancement of UV UCL is mainly ascribed to the high population density of excited Yb3+ and the energy transfer from |2F7/2, 4T1g > (Yb3+–Fe3+ dimer) to 3F2,3 (Tm3+) states. Moreover, through dynamic analysis, we further demonstrate the proposed mechanisms of UC processes. The results suggest that ultra-small UCNPs with enhanced UV UCL may have potential applications in the biomedical field.