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Tb4+-Yb3+ co-doped NaYF4 nanoparticles (NPs) are prepared by sintering the as-synthesized NaYF4:Tb3+, Yb3+ NPs at 380°C under air atmosphere. The oxidization of Tb3+ ions to Tb4+ ions in NaYF4 NPs after sintering is demonstrated through X-ray photoelectron spectroscopy (XPS). The near-infrared (NIR) downconversion (DC) luminescence of Tb4+-Yb3+ couple is measured and investigated for the first time. The results show that DC luminescence of Tb4+-Yb3+ couple enhance obviously compared with Tb3+-Yb3+ couple in as-synthesized sample. The enhancement factor is about 14 and 19 excited at 379nm and 487nm, respectively. On analyzing the exponential dependence of NIR fluorescence intensity on the pumping power, we reveal that the energy transfer (ET) mechanism from Tb4+ to Yb3+ in NaYF4 NPs occurs by the single-step ET process. Our study may provide a promising DC layer on the top of silicon-based solar cells to improve the photovoltaic conversion efficiency.

Efficient near-infrared downconversion and energy transfer mechanism in Tb^4+-Yb^3+ co-doped NaYF_4 nanoparticles