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DDC: 540 × Subject: transport ×

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    Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals
    (Washington, DC : ACS Publications, 2022) Graf, Lukas; Liu, Fupin; Naumann, Marco; Roth, Friedrich; Debnath, Bipasha; Büchner, Bernd; Krupskaya, Yulia; Popov, Alexey A.; Knupfer, Martin
    High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure─unknown before─was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.
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    Nanometer-thick lateral polyelectrolyte micropatterns induce macrosopic electro-osmotic chaotic fluid instabilities
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2014) Wessling, M.; Morcillo, L. Garrigós; Abdu, S.
    Electro-convective vortices in ion concentration polarization under shear flow have been of practical relevance for desalination processes using electrodialysis. The phenomenon has been scientifically disregarded for decades, but is recently embraced by a growing fluid dynamics community due its complex superposition of multi-scale gradients in electrochemical potential and space charge interacting with emerging complex fluid momentum gradients. While the visualization, quantification and fundamental understanding of the often-chaotic fluid dynamics is evolving rapidly due to sophisticated simulations and experimentation, little is known whether these instabilities can be induced and affected by chemical topological heterogeneity in surface properties. In this letter, we report that polyelectrolyte layers applied as micropatterns on ion exchange membranes induce and facilitate the electro-osmotic fluid instabilities. The findings stimulate a variety of fundamental questions comparable to the complexity of today's turbulence research.