TY - CONF A1 - Saloga, Patrick E. J. A1 - Thünemann, Andreas T1 - Size-selective microwave-assisted synthesis of pure zinc oxide nanoparticles for precise band gap engineering N2 - Zinc oxide (ZnO) nanoparticles find manifold applications, most prominently in photovoltaics, where their unique optical properties are exploited. Particularly important are their band gap energy of 3.37 eV, which can be widely tuned through doping, and a large exciton binding energy of 60 mV. As a wide-bandgap II-VI semiconductor, the optical band gap energy and fluorescence energy become size-dependent when moving to particle radii of a few nanometers. To gain a deeper insight into this issue, we report on a microwave-assisted, size-selective synthesis of pure ZnO nanoparticles. By hydrolysis of the metal precursor in presence of a strong base at temperatures exceeding the solvent’s boiling point, the reaction is dramatically accelerated, and narrowly dispersed, spherical particles are yielded within seconds – instead of hours at lower temperatures. The determination of their size distributions in high resolution using small-angle x-ray scattering (SAXS) allows for a precise mapping of the optical properties (UV/Vis absorption and fluorescence) to particle size. We observed that the mean particle radii increase from 2.6 ± 0.1 nm with increasing synthesis temperature from 125 °C to 200 °C. This is accompanied by a red shift of the optical band gap and the fluorescence energies, the latter of which can be seen in Figure 1. Thus, undoped ZnO nanoparticles with narrow size distributions and pre-defined size as well as optical properties can be obtained through a microwave-assisted synthesis. T2 - 6th Nano Today Conference CY - Lisbon, Portugal DA - 16.06.2019 KW - SAXS KW - Zinc oxide nanoparticles KW - Microwave synthesis PY - 2019 AN - OPUS4-48340 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Saloga, Patrick E. J. T1 - SAXS in Analysis of Ultra-small Size-adjustable Zinc Oxide Nanoparticles for Controlled Band Gap Engineering N2 - Zinc oxide (ZnO) as a wide-bandgap II-VI semiconductor finds application in areas like optoelectronics, photocatalysis as well as in detection systems. While band-gap engineering in macroscopic ZnO can be performed by alloying, the band-gap of ZnO nanoparticles is also dependent on their size. Since small-angle X-ray scattering (SAXS) provides a much higher resolution in terms of ultra-small nanoparticle size analysis compared to other techniques, it allows for a careful examination of the correlation between particle size and band-gap. We report on the microwave-assisted synthesis of oleate-capped, photoluminescent zinc oxide nanoparticles with adjustable size as dispersions in organic solvents. The spherical particles were obtained by hydrolysis of the metal precursor in presence of a strong base at temperatures above the solvent’s boiling point. Hence, the reaction is dramatically accelerated and within seconds – instead of hours at lower temperatures –, narrowly dispersed particle systems are yielded. The particles’ sizes as derived from SAXS strongly depend on the reaction temperature and time. Choosing the right reaction conditions, the particle size and thus their band gap can be finely tuned. A size increase can be achieved both by increasing the reaction temperature and the reaction time. See Figure 1 for an exemplary comparison of five-minute syntheses at different temperatures. Here, the yielded particles display diameters between 5.0 and 7.6 nm and corresponding band-gaps of 3.32 up to 3.41 eV. The size increase is accompanied by a red-shift of the UV/Vis absorption edges and fluorescence emission. Furthermore, these particles can be transferred into water by coating with polysorbates. T2 - SAS2018 XVII International Small Angle Scattering Conference CY - Traverse City, Michigan, USA DA - 07.10.2018 KW - SAXS KW - Zinc oxide nanoparticles KW - Microwave synthesis PY - 2018 AN - OPUS4-46348 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -