TY - JOUR A1 - Kraft, Marco A1 - Würth, Christian A1 - Muhr, V. A1 - Hirsch, T. A1 - Resch-Genger, Ute T1 - Explaining the influence of dopant concentration and excitation power density on the luminescence and brightness of beta-NaYF_4:Yb~(3+),Er~(3+) nanoparticles: Measurements and simulations N2 - A systematic study of the luminescence properties of monodisperse β-NaYF4: 20% Yb3+, 2% Er3+ upconversion nanoparticles (UCNPs) with sizes ranging from 12–43 nm is presented utilizing steady-state and time-resolved fluorometry. Special emphasis was dedicated to the absolute quantification of size- and environment-induced quenching of upconversion luminescence (UCL) by highenergy O–H and C–H vibrations from solvent and ligand molecules at different excitation power densities (P). In this context, the still-debated Population pathways of the 4F9/2 energy level of Er3+ were examined. Our results highlight the potential of particle size and P value for color tuning based on the pronounced near-infrared emission of 12 nm UCNPs, which outweighs the red Er3+ emission under “strongly quenched” conditions and accounts for over 50% of total UCL in water. Because current rate equation models do not include such emissions, the suitability of these models for accurately simulating all (de)population pathways of small UCNPs must be critically assessed. Furthermore, we postulate population pathways for the 4F9/2 energy level of Er3+, which correlate with the size-, environment-, and P-dependent quenching states of the higher Er3+ energy levels. KW - Nanoparticle KW - Nanosensor KW - Fluorescence KW - Upconversion KW - Upconversion nanoparticle KW - NIR KW - Photophysics KW - Lanthanide KW - Size KW - Surface chemistry KW - Quantum yield KW - Mechanism KW - Lifetime KW - Decay kinetics KW - Quenching KW - Diameter KW - Cyclohexane KW - Water PY - 2018 DO - https://doi.org/10.1007/s12274-018-2159-9 VL - 11/12 SP - 6360 EP - 6374 PB - Springer Nature AN - OPUS4-50307 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hirsch, A. A1 - Lohmann, S.-H. A1 - Strelow, C. A1 - Kipp, T. A1 - Würth, Christian A1 - Geißler, Daniel A1 - Komoski, A. A1 - Wolter, C. A1 - Weller, H. A1 - Resch-Genger, Ute A1 - Mews, A. T1 - Fluorescence Quantum Yield and Single-Particle Emission of CdSe N2 - The fluorescence quantum yield (QY) of CdSe dot/CdS rod (DR) nanoparticle ensembles is dependent on the Shell growth and excitation wavelength. We analyze the origin of this dependency by comparing the optical properties of DR ensembles to the results obtained in single-particle experiments. On the Ensemble level, we find that the QY of DRs with shell lengths shorter than 40 nm exhibits no dependence on the excitation wavelength, whereas for DRs with shell lengths longer than 50 nm, the QY significantly decreases for excitation above the CdS band gap. Upon excitation in the CdSe core, the ensemble QY, the fluorescence wavelength, and the fluorescence blinking behavior of individual particles are only dependent on the radial CdS shell thickness and not on the CDs shell length. If the photogenerated excitons can reach the CdSe core region, the fluorescence properties will be dependent only on the surface passivation in close vicinity to the CdSe core. The change in QY upon excitation above the band gap of CdS for longer DRs cannot be explained by nonradiative particles because the ratio of emitting DRs is found to be independent of the DR length. We propose a model after which the decrease in QY for longer CdS shells is due to an increasing fraction of nonradiative exciton recombination within the elongated shell. This is supported by an effective-mass-approximation-based calculation, which suggests an optimum length of DRs of about 40 nm, to combine the benefit of high CdS absorption cross section with a high fluorescence QY. KW - Fluorescence KW - Quantum dot KW - Photophysics KW - Single particle spectroscopy KW - Mechanism KW - Theory KW - Ensemble measurements KW - Quantum yield KW - CdSe KW - CdS shell PY - 2019 DO - https://doi.org/10.1021/acs.jpcc.9b07957 VL - 123 IS - 39 SP - 24338 EP - 24346 PB - ACS Publications AN - OPUS4-49556 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -