Upconversion luminescence for NaYF:Yb:Er nanoparticles shows a marked decrease in quantum efficiency and much faster rates in the rise and fall of the luminescence decay curves as compared to bulk NaYF:Yb:Er. We propose that in moving from µm-sized to nm-sized β-NaYF:Yb:Er particles, the effects observed in the decay curves can be mostly accounted for by the changes in only three rate constants, namely the non-radiative rate constant of Yb (kYb), the non-radiative rate constant for Er from I to I (k) and the non-radiative rate constant for Er from I to I (k). The increase in the effective values for kYb and k occurs because rapid energy migration among the levels emitting near 1 µm leads to equilibration between interior and surface sites, with a consequent increase over bulk values in the two rate constants relating to the non-radiative decay of these levels. The increase in k also results from this rapid energy migration, since most of the Er: I to I emission comes from surface sites where I is rapidly quenched to I. The changes that occur in the visible upconversion luminescence (red, green, and blue) are simply a reflection of the faster quenching at 1 µm. Computational analysis through a non-linear rate equation model of the upconversion process supports this hypothesis.
South Dakota School of Mines and Technology, University of South Dakota
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