TY - JOUR A1 - Würth, Christian A1 - Grauel, Bettina A1 - Pons, Monika A1 - Frenzel, Florian A1 - Rissiek, P. A1 - Rücker, Kerstin A1 - Haase, Markus A1 - Resch-Genger, Ute T1 - Yb- and Er concentration dependence of the upconversion luminescence of highly doped NaYF4:Yb,Er/NaYF4:Lu core/shell nanocrystals prepared by a water-free synthesis N2 - High sensitizer and activator concentrations have been increasingly examined to improve the performance of multi-color emissive upconversion (UC) nanocrystals (UCNC) like NaYF4:Yb,Er and first strategies were reported to reduce concentration quenching in highly doped UCNC. UC luminescence (UCL) is, however, controlled not only by dopant concentration, yet by an interplay of different parameters including size, crystal and shell quality, and excitation power density (P). Thus, identifying optimum dopant concentrations requires systematic studies of UCNC designed to minimize additional quenching pathways and quantitative spectroscopy. Here, we quantify the dopant concentration dependence of the UCL quantum yield (ΦUC) of solid NaYF4:Yb,Er/NaYF4:Lu upconversion core/shell nanocrystals of varying Yb3+ and Er3+ concentrations (Yb3+ series: 20%‒98% Yb3+; 2% Er3+; Er3+ series: 60% Yb3+; 2%‒40% Er3+). To circumvent other luminescence quenching processes, an elaborate synthesis yielding OH-free UCNC with record ΦUC of ~9% and ~25 nm core particles with a thick surface shell were used. High Yb3+ concentrations barely reduce ΦUC from ~9% (20% Yb3+) to ~7% (98% Yb3+) for an Er3+ concentration of 2%, thereby allowing to strongly increase the particle absorption cross section and UCNC brightness. Although an increased Er3+ concentration reduces ΦUC from ~7% (2% Er3+) to 1% (40%) for 60% Yb3+. Nevertheless, at very high P (> 1 MW/cm2) used for microscopic studies, highly Er3+-doped UCNC display a high brightness because of reduced saturation. These findings underline the importance of synthesis control and will pave the road to many fundamental studies of UC materials. KW - Upconverion KW - Nanoparticle KW - Lanthanides KW - Quantum yield PY - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-551346 SP - 1 EP - 8 PB - Springer AN - OPUS4-55134 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Homann, C. A1 - Krukewitt, Lisa A1 - Frenzel, Florian A1 - Grauel, Bettina A1 - Würth, Christian A1 - Resch-Genger, Ute A1 - Haase, M. T1 - NaYF4:Yb,Er/NaYF4 Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield N2 - 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). KW - Core shell structure KW - Upconversion KW - Non lienear processes KW - Nanoparticle KW - Quantum yield PY - 2018 U6 - https://doi.org/10.1002/anie.201803083 VL - 57 IS - 28 SP - 8765 EP - 8769 PB - Wiley-VCH AN - OPUS4-45574 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Wiesholler, L. M. A1 - Frenzel, Florian A1 - Grauel, Bettina A1 - Würth, Christian A1 - Resch-Genger, Ute A1 - Hirsch, T. ED - Resch-Genger, Ute ED - Hirsch, Thomas T1 - Yb,Nd,Er-doped upconversion nanoparticles: 980 nm versus 808 nm excitation N2 - Yb,Nd,Er-doped upconversion nanoparticles (UCNPs) have attracted considerable interest as luminescent reporters for bioimaging, sensing, energy conversion/shaping, and anticounterfeiting due to their capability to convert multiple near-infrared (NIR) photons into shorter wavelength ultraviolet, visible or NIR luminescence by successive absorption of two or more NIR photons. This enables optical measurements in complex media with very little background and high penetration depths for bioimaging. The use of Nd3+ as substitute for the commonly employed sensitizer Yb3+ or in combination with Yb3+ shifts the excitation wavelength from about 980 nm, where the absorption of water can weaken upconversion luminescence, to about 800 nm, and laser-induced local overheating effects in cells, tissue, and live animal studies can be minimized. To systematically investigate the potential of Nd3+ doping, we assessed the performance of a set of similarly sized Yb3+,Nd3+,Er3+-doped core- and core–shell UCNPs of different particle architecture in water at broadly varied excitation power densities (P) with steady state and time-resolved fluorometry for excitation at 980 nm and 808 nm. As a measure for UCNPs performance, the P-dependent upconversion quantum yield (Φ) and its saturation behavior were used as well as particle brightness (B). Based upon spectroscopic measurements at both excitation wavelengths in water and in a lipid phantom and B-based calculations of signal size at different penetration depths, conditions under which excitation at 808 nm is advantageous are derived and parameters for the further optimization of triple-doped UCNPs are given. KW - Lanthanide KW - Upconversion KW - Nanoparticle KW - Photoluminescence KW - Quantum yield KW - Lifetime KW - Brightness KW - Nd excitation KW - Excitation power density KW - Modelling KW - NIR PY - 2019 U6 - https://doi.org/10.1039/C9NR03127H SN - 2040-3372 SN - 2040-3364 VL - 11 IS - 28 SP - 13440 EP - 13449 PB - Royal Society of Chemistry CY - London AN - OPUS4-48608 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Frenzel, Florian A1 - Würth, Christian A1 - Muhr, V. A1 - Hirsch, T. A1 - Resch-Genger, Ute T1 - Power dependent optical properties of hexagonal β-NaYF4: x % Er3+, 20 % Yb3+ core/ core-shell upconversion nanoparticles in cyclohexane and water N2 - Lanthanide doped photon upconverting nanophosphors (UCNPs) have the unique capability to produce narrow band, multi-color emission in the UV/vis/NIR upon multiphotonic absorption of infrared light, which makes them promising reporters for diagnostic, bioanalytical, and biological applications. This minimizes background signals, which normally occur due to autofluorescence from auxochromes, in biological matrices and enables deep penetration depths in biological applications. Moreover, UCNPs show long luminescence lifetimes in the μs range favorable for time gated emission in conjunction with a high photostability and chemical inertness and they do not blink. One of the most efficient upconversion (UC) phosphors for conversion of 976 nm to 655 nm and 545 nm light presents the hexagonal NaYF4-host crystal doped with 20 % Yb3+ used as sensitizer to absorb infrared light and 2 % Er3+ acting as activator mainly responsible for light emission. The high transparency in the relevant spectral windows of this host together with its low phonon frequencies ensure relatively high luminescence efficiencies. Although UCNPs are ideal candidates for many chemical and biological sensing and imaging applications, compared to other well-known chromophores like organic dyes or QDs, they suffer from a comparatively low brightness due to the low absorption cross sections of the parity forbidden f-f-transitions and low photoluminescence quantum yields (QYUC) particularly in the case of small nanoparticles with sizes of < 50 nm. The rational design of more efficient UCNPs requires an improved understanding of the nonradiative decay pathways in these materials that are influenced by particle architecture including dopant ion concentration and homogeneity of dopant distribution within UCNPs, size/surface-to-volume ratio, surface chemistry, and microenvironment. A promising approach to overcome the low efficiency of UCNPs is to use plasmonic interactions between a noble metal (Ag or Au) structure in the proximity of UCNPs and the incident light. This interaction leads to a modification of the spectroscopic properties due to local field enhancements and can involve an increase of the photoluminescence. In this respect, we study the interactions of UCNPs with metal structures (clusters and shells) by varying shape and size. Here, first results derived from integrating sphere spectroscopy and time-resolved fluorescence measurements are presented. T2 - Summer School "EXCITING NANOSTRUCTURES" CY - Bad Honnef, Germany DA - 17.07.2017 KW - Upconversion KW - Nanoparticle KW - Flourescence KW - Core-shell architecture KW - NIR KW - Absolute flourometry KW - Integrating sphere spectroscopy KW - Er(III) KW - Yb(III) KW - Single particle spectroscopy PY - 2017 AN - OPUS4-41172 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -