Enhanced high-order ultraviolet upconversion luminescence in sub-20 nm β-NaYbF4:0.5% Tm nanoparticles via Fe3+ doping

Abstract

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 |²F_7/2, 4T_1g > (Yb³⁺–Fe³⁺ dimer) to ³F_2,3 (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.