Universität Stuttgart
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Item Open Access Overview: the Janus-nature of molecular CO2 in charge adjustment at wet surfaces(2026) Vogel, Peter; Qaisrani, Muhammad Nawaz; Rasenat, Mattis; Lützenkirchen, Johannes; Sulpizi, Marialore; Beyer, David; Holm, Christian; Palberg, ThomasMolecular CO2 readily dissolves in aqueous electrolyte solutions and partially dissociating to form carbonic acid. The decharging effects of the dissociation products mediated by the ensuing pH-shift and the additional salinity are well established. However, the effects of dissolved molecular CO2 have not been studied systematically. We summarize recent and novel investigations on the role of CO2 regarding charge control at surfaces submersed in aqueous electrolytes. In our electrokinetic and conductometric measurements on representative surfaces, we took special care to control and monitor the electrolyte composition in situ. We discriminate the effects of molecular and dissociated CO2via control experiments using HCl. Depending on the surface under investigation and the charging mechanisms involved, we find that molecular CO2 assists either charging, de-charging and/or recharging. This contrasting charge regulating behaviour reveals the Janus nature of dissolved molecular CO2 with respect to charge control at wet surfaces. In our complementary molecular dynamics simulations, Q4 silica and 9% ionized Q3 silica surfaces are studied as hydrophobic/hydrophilic, respectively charged/uncharged, analogues, as well as uncharged Q3 silica and molecularly rough Isoleucin-coated quartz surfaces. In all cases, we find that the charge-neutral CO2 molecule physisorbs in a thin diffusive layer close to the surface, which leads to pronounced re-structuring of the electric double layer. Based on this result, we suggest to interpret the experimentally observed Janus nature of molecular CO2 in terms of a local decrease of the dielectric permittivity. This in turn leads to a local strengthening of electrostatic interactions dominating the double layer structure next to charged surfaces. Specifically, we propose that CO2 induces a dielectric charge regulation for weakly acidic surface groups, assists the incorporation of OH− into the H-bond network at smooth inert surfaces, and induces significant ion-correlations promoting co-ion binding. Overall, we demonstrate that molecular CO2 allows for a controlled charge-adjustment in opposing directions. We anticipate that our findings on the one hand provide substantial challenges for analytical or numerical modelling as well as for controlled experimental work, but on the other hand bear important practical implications for applications ranging from desalination to bio-membranes.Item Open Access New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase(2024) Nacke, Pierre; Manabe, Atsutaka; Klasen-Memmer, Melanie; Chen, Xi; Martinez, Vikina; Freychet, Guillaume; Zhernenkov, Mikhail; Maclennan, Joseph E.; Clark, Noel A.; Bremer, Matthias; Giesselmann, FrankWe present a new ferroelectric nematic material, 4-((4′-((trans)-5-ethyloxan-2-yl)-2′,3,5,6′-tetrafluoro-[1,1′-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization Ps of up to 6 µC cm-2. The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (NF), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 103, and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase NF are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic ZA (SmZA) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases-N, SmZA and NF-which may shed new light on the understanding of polar and antipolar order in these phases.Item Open Access Adding 161Dy-Mössbauer spectroscopy to a multitechnique investigation of magnetic transitions in a {CoIII3DyIII3} single-molecule toroic(2026) Peng, Yan; Braun, Jonas; Scherthan, Lena; Auerbach, Hendrik; Wolny, Juliusz A.; Schulze, Michael; Alp, E. Ercan; Zhao, Jiyong; Bi, Wenli; Tesi, Lorenzo; Anson, Christopher E.; Moilanen, Jani O.; Brown, Dennis. E.; Chibotaru, Liviu F.; Wernsdorfer, Wolfgang; Perfetti, Mauro; Sessoli, Roberta; Schünemann, Volker; Powell, Annie K.The determination of the orientations of the individual DyIII anisotropy axes in polynuclear complexes is challenging but crucial for the understanding of systems showing Single Molecule Magnet or Single Molecule Toroic behavior. In particular, the experimental proof of a toroidal ground state from magnetization data often remains ambiguous. Here, we report the coordination cluster [CoIII3DyIII3(µ3-OH)4(O2C-C6H4-p-Me)6(pmide)3(H2O)3]Cl2 · 10MeCN (1) (H2pmide = N-2-pyridylmethyldiethanolamine) which crystallizes with threefold symmetry and contains an equilateral DyIII3 triangle surrounded by a triangle of diamagnetic CoIII ions. We also report a multi-technique investigation of its toroidal magnetic spin structure, including 161Dy Synchrotron Mössbauer Spectroscopy which shows an abrupt transition from a non-magnetic to a magnetic state. The experimental orientations of the individual DyIII magnetic axes were assessed using torque magnetometry and micro-SQUID measurements and both experiments converged on a spin structure that is in very good agreement with ab initio calculations. Such a multi-technique approach, including 161Dy Synchrotron Mössbauer Spectroscopy, provides a roadmap for the unambiguous identification of such toroidal states.Item Open Access Plasmonic gratings from highly doped Ge1-ySny films on Si(2021) Berkmann, Fritz; Ayasse, Markus; Schlipf, Jon; Mörz, Florian; Weißhaupt, David; Oehme, Michael; Prucnal, Slawomir; Kawaguchi, Yuma; Schwarz, Daniel; Fischer, Inga Anita; Schulze, JörgPlasmonic modes in metal structures are of great interest for optical applications. While metals such as Au and Ag are highly suitable for such applications at visible wavelengths, their high Drude losses limit their usefulness at mid-infrared wavelengths. Highly n-doped Ge1-ySny alloys are interesting possible alternative materials for plasmonic applications in this wavelength range. Here, we investigate the use of highly n-doped Ge1-ySny films grown directly on Si by molecular beam epitaxy with varying Sn-content from 0% up to 7.6% for plasmonic grating structures. We compare plasma wavelengths and relaxation times obtained from electrical and optical characterization. While theoretical considerations indicate that the decreasing effective mass with increasing Sn content in Ge1-ySny films could improve performance for plasmonic applications, our optical characterization results show that the utilization of Ge1-ySny films grown directly on Si is only beneficial if material quality can be improved.Item Open Access Active colloidal propulsion over a crystalline surface(2017) Choudhury, Udit; Straube, Arthur V.; Fischer, Peer; Gibbs, John G.; Höfling, FelixWe study both experimentally and theoretically the dynamics of chemically self-propelled Janus colloids moving atop a two-dimensional crystalline surface. The surface is a hexagonally close-packed monolayer of colloidal particles of the same size as the mobile one. The dynamics of the self-propelled colloid reflects the competition between hindered diffusion due to the periodic surface and enhanced diffusion due to active motion. Which contribution dominates depends on the propulsion strength, which can be systematically tuned by changing the concentration of a chemical fuel. The mean-square displacements (MSDs) obtained from the experiment exhibit enhanced diffusion at long lag times. Our experimental data are consistent with a Langevin model for the effectively two-dimensional translational motion of an active Brownian particle in a periodic potential, combining the confining effects of gravity and the crystalline surface with the free rotational diffusion of the colloid. Approximate analytical predictions are made for the MSD describing the crossover from free Brownian motion at short times to active diffusion at long times. The results are in semi-quantitative agreement with numerical results of a refined Langevin model that treats translational and rotational degrees of freedom on the same footing.Item Open Access Triggered dissolution of electrostatically crosslinked hydrogels from star‐shaped polyampholytic block copolymers(2026) Grün, Jonas Julius; Beyer, David; Mons, Peter Johannes; Seitel, Sebastian; Fribiczer, Nora; Poudel, Purushottam; Könemann, Nicklas; Zank, Lynn Kendra Renate Jagna; Zylla, Paul Fabio; Košovan, Peter; Seiffert, Sebastian; Holm, Christian; Schacher, Felix HelmutWe explore the possible reversible formation of hydrogels through electrostatic interactions between four‐arm star‐shaped block copolymers, consisting of a polyethylene glycol (PEG) inner block and either an anionic polystyrene sulfonate [PEG27‐b‐PSS108]4 or a zwitterionic polybetaine [PEG27‐b‐PCBMAAm110]4 as outer block. The combination of both can induce attractive or repulsive electrostatic interactions depending on the solution pH value and ionic strength. The polymers were synthesized using controlled atom transfer radical polymerization (ATRP). The charge of [PEG27‐b‐PCBMAAm110]4 was further investigated by potentiometric titration and zeta potential measurements. Using oscillatory shear rheology, we demonstrated the required conditions for hydrogel formation. Stable hydrogel formation is observed within a wide pH range (6.8 -9.5), corresponding to the protonation states of the carboxylic acid groups that facilitate electrostatic interactions. We also showed how the hydrogel stability is influenced by parameters like block copolymer concentration and ionic strength. Coarse‐grained simulations provided molecular‐scale insights, revealing charge regulation effects and the energetic favorability of electrostatic complexation up to high pH values. Overall, our results demonstrated the key design principles, as the polyelectrolyte length, ionic strength, and charge regulation effects, for the formation of partially reversible hydrogels, triggered by changes in the solution pH. Furthermore, we showed that understanding the desired conditions for hydrogel formation requires a combination of experimental characterization with modeling approaches.Item Open Access Heteroepitaxial tuning of resonant forbidden reflections in a spinel(2024) Oka, Ryosuke; Kim, Minu; Wochner, Peter; Francoual, Sonia; Palstra, Thomas T. M.; Takagi, Hidenori; Huang, DennisIn resonant elastic X-ray scattering (REXS), low site symmetries in a crystal may be revealed through resonant Bragg reflections that are normally forbidden in conventional X-ray diffraction due to screw axes and/or glide planes. These resonant forbidden reflections have been observed in spinel compounds, but to better understand and utilize their connection to microscopic material parameters and possible charge and/or orbital ordering, a systematic study of their dependence on growth conditions and applied strain is desired. We performed REXS at the V K edge and examined the resonant forbidden (002) reflection in thin films of the spinel LiV2O4 grown on three substrates: MgAl2O4, SrTiO3, and MgO. The energy dependence of the (002) reflection shows a systematic evolution as epitaxial strain modifies the local anisotropy of the V site. More strikingly, the integrated intensity of the (002) reflection varies by more than an order of magnitude in films on different substrates. We speculate that the large variation in integrated intensity reflects the varying degree of antiphase domains that arise during the epitaxy.Item Open Access Electrically switchable metallic polymer metasurface device with gel polymer electrolyte(2023) Jong, Derek de; Karst, Julian; Ludescher, Dominik; Floess, Moritz; Moell, Sophia; Dirnberger, Klaus; Hentschel, Mario; Ludwigs, Sabine; Braun, Paul V.; Giessen, HaraldWe present an electrically switchable, compact metasurface device based on the metallic polymer PEDOT:PSS in combination with a gel polymer electrolyte. Applying square-wave voltages, we can reversibly switch the PEDOT:PSS from dielectric to metallic. Using this concept, we demonstrate a compact, standalone, and CMOS compatible metadevice. It allows for electrically controlled ON and OFF switching of plasmonic resonances in the 2-3 µm wavelength range, as well as electrically controlled beam switching at angles up to 10°. Furthermore, switching frequencies of up to 10 Hz, with oxidation times as fast as 42 ms and reduction times of 57 ms, are demonstrated. Our work provides the basis towards solid state switchable metasurfaces, ultimately leading to submicrometer-pixel spatial light modulators and hence switchable holographic devices.Item Open Access Multitechnique investigation of Dy3 : implications for coupled lanthanide clusters(2016) Gysler, Maren; El Hallak, Fadi; Ungur, Liviu; Marx, Raphael; Hakl, Michael; Neugebauer, Petr; Rechkemmer, Yvonne; Lan, Yanhua; Sheikin, Ilya; Orlita, Milan; Anson, Christopher E.; Powell, Annie K.; Sessoli, Roberta; Chibotaru, Liviu F.; Slageren, Joris vanIn-depth investigations of the low energy electronic structures of mononuclear lanthanide complexes, including single molecule magnets, are challenging at the best of times. For magnetically coupled polynuclear systems, the task seems well nigh impossible. However, without detailed understanding of the electronic structure, there is no hope of understanding their static and dynamic magnetic properties in detail. We have been interested in assessing which techniques are most appropriate for studying lanthanide single-molecule magnets. Here we present a wide ranging theoretical and experimental study of the archetypal polynuclear lanthanide single-molecule magnet Dy3 and derive the simplest model to describe the results from each experimental method, including high-frequency electron paramagnetic resonance and far-infrared spectroscopies and cantilever torque magnetometry. We conclude that a combination of these methods together with ab initio calculations is required to arrive at a full understanding of the properties of this complex, and potentially of other magnetically coupled lanthanide complexes.Item Open Access CO2-induced drastic decharging of dielectric surfaces in aqueous suspensions(2024) Vogel, Peter; Beyer, David; Holm, Christian; Palberg, ThomasWe study the influence of airborne CO2 on the charge state of carboxylate stabilized polymer latex particles suspended in aqueous electrolytes. We combine conductometric experiments interpreted in terms of Hessinger's conductivity model with Poisson-Boltzmann cell (PBC) model calculations with charge regulation boundary conditions. Without CO2, a minority of the weakly acidic surface groups are dissociated and only a fraction of the total number of counter-ions actually contribute to conductivity. The remaining counter-ions exchange freely with added other ions like Na+, K+ or Cs+. From the PBC-calculations we infer a corresponding pKa of 4.26 as well as a renormalized charge in reasonably good agreement with the number of freely mobile counter-ions. Equilibration of salt- and CO2-free suspensions against ambient air leads to a drastic de-charging, which exceeds by far the expected effects of to dissolved CO2 and its dissociation products. Further, no counter-ion-exchange is observed. To reproduce the experimental findings, we have to assume an effective pKa of 6.48. This direct influence of CO2 on the state of surface group dissociation explains our recent finding of a CO2-induced decrease of the ζ-potential and supports the suggestion of an additional charge regulation caused by molecular CO2. Given the importance of charged surfaces in contact with aqueous electrolytes, we anticipate that our observations bear substantial theoretical challenges and important implications for applications ranging from desalination to bio-membranes.