Abstract
Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb3+ and Er3+ ions, we examined energy loss processes in differently sized solid core NaYF4 nanocrystals doped with either Yb3+ (YbNC; 20% Yb3+) or Er3+ (ErNC; 2% Er3+) and co-doped with Yb3+ and Er3+ (YbErNC; 20% Yb3+ and 2% Er3+) without a surface protection shell and coated with a thin and a thick NaYF4 shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the 4G11/2 state of Er3+ to the 2F5/2 state of Yb3+, through which the red Er3+4F9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er3+ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.
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Acknowledgements
The authors gratefully acknowledge financial support from the German Science Foundation (DFG; Nos. RE 1203/18-1 and HA 1649/7-1). We thank Professor Th. Jüstel (FH Münster, Steinfurt) for providing the microcrystalline powder phosphors Yb-Bulk (NaYF4:Yb) and Er-Bulk (NaYF4:Er) and Dr. K. Krämer (University of Bern, Switzerland) for provision of the microcrystalline NaYF4:Yb UC phosphor.
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Grauel, B., Würth, C., Homann, C. et al. Volume and surface effects on two-photonic and three-photonic processes in dry co-doped upconversion nanocrystals. Nano Res. 15, 2362–2373 (2022). https://doi.org/10.1007/s12274-021-3727-y
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DOI: https://doi.org/10.1007/s12274-021-3727-y