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- Nanoparticle (2)
- Quantum yield (2)
- Upconversion (2)
- Brightness (1)
- Core shell structure (1)
- Energy transfer (1)
- Excitation (1)
- Lanthanide (1)
- Lifetime (1)
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Organisationseinheit der BAM
- 1 Analytische Chemie; Referenzmaterialien (3) (entfernen)
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 backenergy 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 threephotonic 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 threedimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.
Eine modifizierte Syntheseroute auf der Basis wasserfreier Seltenerdacetate wurde zur Synthese aufwärtskonvertierender Kern/Schale‐Nanokristalle mit mittleren Größen zwischen 15 bis 45 nm eingesetzt. Die nahezu monodispersen Partikel bestehen aus einem NaYF4:Yb,Er‐Kern, dotiert mit 18 % Yb3+ und 2 % Er3+, und einer inerten Schale aus NaYF4, wobei die Dicke der Schale dem jeweiligen Radius des Kernpartikels entspricht. Absolutmessungen der Photolumineszenz‐Quantenausbeuten bei verschiedenen Anregungsleistungsdichten zeigen, dass die Quantenausbeuten von 45 nm Kern/Schale‐Partikeln schon fast an die Quantenausbeute des mikrokristallinen aufwärtskonvertierenden Leuchtstoffs heranreichen. Kleinere Kern/Schale‐Partikel, die nach der gleichen Methode hergestellt wurden, zeigen nur eine moderate Abnahme der Quantenausbeute. Beispielsweise ist die Quantenausbeute von 15 nm großen Kern/Schale‐Partikeln bei hohen Leistungsdichten (100 W cm−2) nur um einen Faktor drei kleiner als die des mikrokristallinen Leuchtstoffpulvers und um ungefähr einen Faktor 10 kleiner bei niedrigen Leistungsdichten (1 W cm−2).
Upconversion core/shell nanocrystals with different mean sizes ranging from 15 to 45 nm were prepared via a modified synthesis procedure based on anhydrous rare‐earth acetates. All particles consist of a core of NaYF4:Yb,Er, doped with 18 % Yb3+ and 2 % Er3+, and an inert shell of NaYF4, with the shell thickness being equal to the radius of the core particle. Absolute measurements of the photoluminescence quantum yield at a series of different excitation power densities show that the quantum yield of 45 nm core/shell particles is already very close to the quantum yield of microcrystalline upconversion phosphor powder. Smaller core/shell particles prepared by the same method show only a moderate decrease in quantum yield. The quantum yield of 15 nm core/shell particles, for instance, is reduced by a factor of three compared to the bulk upconversion phosphor at high power densities (100 W cm−2) and by approximately a factor of 10 at low power densities (1 W cm−2).