Highlight Publications 2018

Interfacial Conformation of Hydrophilic Polyphosphoesters Affects Blood Protein Adsorption
C. Bernhard , K. N. Bauer, M. Bonn , F. R. Wurm , and G. Gonella
Interfacial Conformation of Hydrophilic Polyphosphoesters Affects Blood Protein Adsorption
Synthetic polymers are commonly used as protein repelling materials for a variety of biomedical applications. Despite their widespread use, the fundamental mechanism underlying protein repellence is often elusive. Such insights are essential for improving existing and developing new materials. Here, we investigate how subtle differences in the chemistry of hydrophilic polyphosphoesters influence the adsorption of the human blood proteins serum albumin and fibrinogen. Using thermodynamic measurements, surface-specific vibrational spectroscopy, and Brewster angle microscopy, we investigate protein adsorption, hydration, and steric repulsion properties of the polyphosphoester polymers. Whereas both surface hydration and polymer conformation of the polymers vary substantially as a consequence of the chemical differences in the polymer structure, the protein repellency ability of these hydrophilic materials appears to be dominated by steric repulsion.
© Christoph Bernhard (2018)
Hydrophilic polyphosphoesters prevent human blood proteins adsorption though steric repulsion.
Functional protein nanostructures: a chemical toolbox
Seah Ling Kuan, Fernando R. G. Bergaminic and Tanja Weil
Functional protein nanostructures: a chemical toolbox
Protein nanostructures are ubiquitous in biological systems and serve many functions. Inspired by Nature, scientists have devised synthetic approaches to derive functional and well-defined protein nanostructures for a broad range of applications including imaging, catalysis and in medicine. In the review article by Kuan, Bergamini and Weil, a step-by-step guide on the design principles of preparing synthetic protein nanostructures and their resultant impact in biomedical applications are highlighted.
© Royal Society of Chemistry (2018)
Chemical toolbox for creating versatile protein nanostructures
Improved Hole Injection into Perovskite Light‐Emitting Diodes Using A Black Phosphorus Interlayer
Antonio Gaetano Ricciardulli, Sheng Yang, Naresh B. Kotadiya, Gert-Jan A. H. Wetzelaer, Xinliang Feng, Paul W. M. Blom
Improved Hole Injection into Perovskite Light‐Emitting Diodes Using A Black Phosphorus Interlayer
The light‐output and efficiency of perovskite based light‐emitting diodes (PeLEDs) is limited by hole injection and high leakage current, generated by a high hole injection barrier and poor perovskite morphology, respectively. Here, a feasible strategy is reported to overcome both constraints by introducing 2D black phosphorus (BP) as hole injection layer in the PeLED stack. A continuous film composed of high‐quality, ultrathin, and large BP sheets on top of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate simultaneously improves the hole injection and morphology of the green‐emitting inorganic CsPbBr3 perovskite. Inclusion of the BP enhances the external quantum efficiency of CsPbBr3 based PeLEDs from 0.7% to 2.8%.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (2018)
Hole injection improvement in perovskite light-emitting diodes upon the application of 2D black phosphorus. EQE and luminance of the device are greatly enhanced.
Robustness of elastic properties in polymer nanocomposite films examined over the full volume fraction range
E. Alonso-Redondo, L. Belliard, K. Rolle, B. Graczykowski, W. Tremel, B. Djafari-Rouhani & G. Fytas
Robustness of elastic properties in polymer nanocomposite films examined over the full volume fraction range
Polymers with nanoparticle inclusions are attractive materials because physical properties can be tuned by varying size and volume fraction range. However, elastic behavior can degrade at higher inclusion fractions when particle-particle contacts become important, and sophisticated measurement techniques are required to study this crossover. Here, we report on the mechanical properties of materials with BaTiO3 nanoparticles (diameters < 10 nm) in a polymer (poly(methyl methacrylate)) matrix, deposited as films in different thickness ranges. Two well-known techniques, time and frequency domain Brillouin light scattering, were employed to probe the composition dependence of their elastic modulus. The time domain experiment revealed the biphasic state of the system at the highest particle volume fraction, whereas frequency domain Brillouin scattering provided comprehensive information on ancillary variables such as refractive index and directionality. Both techniques prove complementary, and can in particular be used to probe the susceptibility of elastic properties in polymer nanocomposites to aging.
© MPI-P (2018)
MPIP researchers in collaboration with Prof. Laurent Belliard (UPMC Paris), Prof. Wolfgang Tremel (JGU Mainz) and Prof. Djafari-Rouhani (University of Lille) used two complementary Brillouin scattering techniques to evidence crossover in elastic property behavior with volume fraction variation of polymer nanocomposites.
Specific Ion Effects on an Oligopeptide: Bidentate binding matters for the Guanidinium Cation
Vasileios Balos, Bogdan Marekha, Christian Malm, Manfred Wagner, Yuki Nagata, Mischa Bonn, Johannes Hunger
Specific Ion Effects on an Oligopeptide: Bidentate binding matters for the Guanidinium Cation
Ion‐protein interactions are important for protein function, yet challenging to rationalize due to the multitude of ion‐protein interaction possibilities. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy experiments show that salts affect the peptide’s reorientational dynamics, with a more pronounced effect of denaturing cations (Li+, guanidinium Gdm+) and anions (I‐, SCN‐) than weakly denaturing ones (K+, Cl‐). Notably, we find the effect of Gdm+ and Li+ to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm+ and Li+ to triglycine, yet with a different binding geometry: While Li+ predominantly binds to the C‐terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm+. This bidentate binding markedly affects peptide conformation. As such, this bidentate binding geometry may help explain the high denaturation activity of Gdm+ salts.
© Vasileios Balos (2018)
How the guanidinium cation unfolds a protein
Light guided motility of a minimal synthetic cell
Solveig M. Bartelt, Jan Steinkühler, Rumiana Dimova and Seraphine V. Wegner
Light guided motility of a minimal synthetic cell
Cell motility is an important but complex process; as cells move new adhesions form at the front and adhesions disassemble at the back. To replicate this dynamic and spatiotemporally controlled asymmetry of adhesions and achieve motility in a minimal synthetic cell, we controlled the adhesion of a model giant unilamellar vesicle (GUV) to the substrate with light. For this purpose, we immobilized the proteins iLID and Micro, which interact under blue light and dissociate from each other in the dark, on a substrate and a GUV, respectively. Under blue light the protein interaction leads to adhesion of the vesicle to the substrate, which is reversible in the dark. The high spatiotemporal control provided by light, allowed partly illuminating the GUV and generating an asymmetry in adhesions. Consequently, the GUV moves into the illuminated area, a process that can be repeated over multiple cycles. Thus, our system reproduces the dynamic spatiotemporal distribution of adhesions and establishes mimetic motility of a synthetic cell.
© MPI-P (2018)
Light guided motility of a minimal synthetic cell.
Ultrathin Shell Layers Dramatically Influence Polymer Nanoparticle Surface Mobility
Eunsoo Kang, Hojin Kim, Laura A. G. Gray, Dane Christie, Ulrich Jonas, Bartlomiej Graczykowski, Eric M. Furst, Rodney D. Priestley , and George Fytas
Ultrathin Shell Layers Dramatically Influence Polymer Nanoparticle Surface Mobility
The vibration spectrum of individual polymer colloids (red) is a sensitive probe of their adhesion and in a colloidal cluster (blue) as schematically shown. Nanometer thick shells can drastically affect the particle surface mobility (softening temperature) of the glassy core and the elastic shear modulus (G). The particles can be either more flexible or rigid at the surface than the core. We added a shell atop the nanoparticle either by seeded surfactant-free emulsion polymerization or layer-by-layer method. Our group has recently introduced this direct probe of particle surface mobility via the polymer colloid vibrations recorded by BLS spectroscopy (Kim et al., Nat. Commun. 9, 2918,2018).
© Bartlomiej Graczykowski, Eunsoo Kang (2018)
MPIP researchers in collaboration with Prof. Priestley’s and Prof. Furst’s group, and Prof. Jonas investigated ultrathin shell layer effect on polymer nanoparticle surface mobility and elastic modulus by Brillouin Light Spectroscopy.
Fast Access to Amphiphilic Multiblock Architectures by the Anionic Copolymerization of Aziridines and Ethylene Oxide
Tassilo Gleede, Elisabeth Rieger, Jan Blankenburg, Katja Klein, and Frederik R. Wurm
Fast Access to Amphiphilic Multiblock Architectures by the Anionic Copolymerization of Aziridines and Ethylene Oxide
An ideal system for stimuli-responsive and amphiphilic (block) polymers would be the copolymerization of aziridines with epoxides. However, to date, no copolymerization of these two highly strained three-membered heterocycles had been achieved. We report the combination of living oxy- and azaanionic ring-opening polymerization of ethylene oxide (EO) and sulfonamide-activated aziridines. In a single step, well-defined amphiphilic block copolymers are obtained by a one-pot copolymerization. The highest difference of reactivity ratios ever reported for an anionic copolymerization (with r1=265 and r2=0.004 for 2-methyl-N-tosylaziridine/ EO) led to the formation of block copolymers in a closed system. The amphiphilic diblock copolymers were used a novel class of nonionic and responsive surfactants. In addition, this unique comonomer reactivity allowed fast access to multiblock copolymers: we prepared the first amphiphilic penta- or tetrablock copolymers containing aziridines in only one or two steps, respectively. These examples render the combination of epoxide and aziridine copolymerization to a powerful strategy to sophisticated macromolecular architectures and nanostructures.
© MPI-P (2018)
Block copolymers of aziridines and ethylene oxide are prepared in a closed-reactor without sequential monomer addition.
Plastics of the future? The impact of biodegradable polymers on the environment and on society
Tobias Haider, Carolin Völker, Johanna Kramm, Katharina Landfester, Frederik R. Wurm
Plastics of the future? The impact of biodegradable polymers on the environment and on society
We are living in a plastic age. For most of us, life without polymers and plastics is unthinkable. However, in recent years the littering of plastics and the problems related to their persistence in the environment have become a major focus in research and the news. Biodegradable polymers might be a suitable alternative to commodity plastics to minimize the impact of plastics on the environment. However, their degradation rates crucially depend on the environments they end up in, such as soil or marine water, or when used in biomedical devices. We show that biodegradation tests carried out in artificial environments lack transferability to real conditions and, therefore, highlight the necessity of environmentally authentic and relevant field‐testing conditions. In addition, we focus on ecotoxicological implications of biodegradable polymers. We also consider the social aspects and ask how biodegradable polymers influence consumer behavior and municipal waste management. Taken together, this study is intended as a contribution towards evaluating the potential of biodegradable polymers as alternative materials to commodity plastics.
© Wiley VCH (2018)
Biodegradable polymers. Currents materials and future perspectives are discussed.
Evidence for auto-catalytic mineral dissolution from surface-specific vibrational spectroscopy
Jan Schaefer, Ellen H. G. Backus & Mischa Bonn
Evidence for auto-catalytic mineral dissolution from surface-specific vibrational spectroscopy
In nature, many geologically relevant processes are driven by non-equilibrium interfacial effects: Water typically flows and doesn’t stand still. A key example is the dissolution of minerals, like silica, in water. By using time-dependent surface specific spectroscopy in the presence and absence of flow of water, we determine the dissolution kinetics of silica in a direct way. The interfacial insights of this approach reveal that the macroscopic dissolution process of silica is limited by diffusion. The molecular dissolution mechanism appears autocatalytic: the presence of dissolved silicate close to the interface speeds up the dissolution process.
© Nature (2018)
Interfacial spectroscopy reveals: Silica dissolution is self-accelerating and limited by diffusion.
Vibrational coupling between organic and inorganic sub‐lattices of hybrid perovskites
Maksim Grechko Simon A. Bretschneider Laura Vietze Heejae Kim Mischa Bonn
Vibrational coupling between organic and inorganic sub‐lattices of hybrid perovskites
The excellent photovoltaic performance of perovskite solar cells has inspired significant interest in the fundamental photophysical properties of this promising material. One of the most challenging fundamental issues is coupling between different motions of electrons and nuclei in the lattice of perovskite semiconductors. In the present work, we reveal direct coupling between phonon vibrations of inorganic sub-lattice and high-frequency molecular vibrations of organic sub-lattice of hybrid organic-inorganic perovskites. Mixing of the phonon and molecular vibrations can provide additional energy relaxation pathways for photoexcited electrons in the conduction band and, thus, elucidate the faster hot-carrier cooling rates in the MA/FAPbI3 as compared to the CsPbI3 perovskites.
© Wiley VCH (2018)
A newly developed spectroscopy reveals coupling between low-frequency phonon modes and high-frequency molecular vibrations in a hybrid organic – inorganic perovskite.
Red-Light-Controlled Release of Drug–Ru Complex Conjugates from Metallopolymer Micelles for Phototherapy in Hypoxic Tumor Environments
Wen Sun, Yan Wen, Raweewan Thiramanas, Mingjia Chen, Jianxiong Han, Ningqiang Gong, Manfred Wagner, Shuai Jiang, Michael S. Meijer, Sylvestre Bonnet, Hans-Jürgen Butt, Volker Mailänder, Xing-Jie Liang, and Si Wu
Red-Light-Controlled Release of Drug–Ru Complex Conjugates from Metallopolymer Micelles for Phototherapy in Hypoxic Tumor Environments
Amphiphilic metallopolymers, which contain photocleavable drug-Ru complex conjugates, self-assemble into micelles. The micelles are biocompatible and carry the conjugates into tumor cells. Subsequent red light irradiation induces intracellular release of the drug-Ru complex conjugates. Because the photoinduced release is oxygen-independent, the novel metallopolymer provides a new platform for phototherapy against hypoxic tumors in vivo.
© Wiley (2018)
Layered Thiadiazoloquinoxaline Containing Long Pyrene‐fused N‐Heteroacenes
Ben-Lin Hu, Ke Zhang, Cunbin An, Dieter Schollmeyer, Wojciech Pisula, and Martin Baumgarten
Layered Thiadiazoloquinoxaline Containing Long Pyrene‐fused N‐Heteroacenes
Three thiadiazoloquinoxaline containing long pyrene-fused N-heteroacenes with 8, 13 and 18 rings were designed and synthesized, which show high electron affinities (EAs) of ~4.1 eV derived from the onset of reductive peaks in cyclic voltammetry. Crystal structure analysis demonstrated in-plane extension through close contacts of thiadiazoles and layered packing enabling in-plane and interlayer electron transport. Organic field-effect transistor devices provided electron mobilities, which supplies a potential way to enhance the charge transport in long N-heteroacenes.
© Wiley VCH (2018)
Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature
Hai I. Wang, Ivan Infante, Stephanie ten Brinck, Enrique Cánovas, and Mischa Bonn
Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature
The efficiency of a single bandgap solar cell, such as silicon, is limited to ~31%. This limit is primarily the result of very rapid relaxation of electrons, once they are generated by a photon. For photons that have energies exceeding the bandgap energy, this excess energy is lost in this relaxation process. In order to circumvent such losses, and increase solar cell efficiency, the extraction of hot carriers towards selective contacts is required to be faster than thermalization in the absorber. Quantum dot (QD)-sensitized oxides have long been proposed as an appealing system to harvest hot carriers for solar energy conversion. Previous work has demonstrated the possibility of hot electron extraction in quantum dot (QD)-sensitized systems, but only at low temperatures (e.g. 77 Kelvin). Here, we demonstrate a room-temperature hot electron transfer with unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous oxide. Such achievement is realized by enhancing the electronic coupling between QDs and oxides, which ensures an ultrafast hot electron transfer process (sub-100 fs) that can effectively compete with the hot carrier thermalization processes. These results provide new insights into circumventing thermal losses in sensitized systems, with potential relevance for low-cost solar energy conversion schemes.
© ACS (2018)
Room-temperature hot electron transfer with unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous oxide
Detaching microparticles from a liquid surface
Frank Schellenberger, Periklis Papadopoulos, Michael Kappl, Stefan A.L. Weber, Doris Vollmer, Hans-Jürgen Butt
Detaching microparticles from a liquid surface
The work required to detach microparticles from fluid interfaces depends on the shape of the liquid meniscus. However, measuring the capillary force on a single microparticle and simultaneously imaging the shape of the liquid meniscus has not yet been accomplished. To correlate force and shape, we combined a laser scanning confocal microscope with a colloidal probe setup. While moving a hydrophobic microsphere (radius 5-10 µm) in and out of a 2-5 µm thick glycerol film, we simultaneously measured the force and imaged the shape of the liquid meniscus. In this way we verified the fundamental equations (D.F. James, J. Fluid Mech. 63, 657 (1974); A.D. Scheludko, A.D. Nikolov, Colloid Polymer Sci. 253, 396 (1975)) which describe the adhesion of particles in flotation, deinking of paper, the stability of Pickering emulsions and particle-stabilized-foams. Comparing experimental results with theory showed, however, that the receding contact angle has to be applied, which can be much lower than the static contact angle obtained right after jump-in of the particle.
© MPI-P (2018)
Macroscopic and microscopic particle withdrawn from a liquid. The microsphere was imaged with a confocal microscope while simultaneously the force was measured.
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Wei Huang, Jeehye Byun, Irina Rörich, Charusheela Ramanan, Paul W. M. Blom, Hao Lu, Di Wang, Lucas Caire da Silva, Run Li, Lei Wang, Katharina Landfester, Kai A. I. Zhang
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Complex multiple‐component semiconductor photocatalysts can be constructed that display enhanced catalytic efficiency via multiple charge and energy transfer, mimicking photosystems in nature. In contrast, the efficiency of single‐component semiconductor photocatalysts is usually limited due to the fast recombination of the photogenerated excitons. Here, we report the design of an asymmetric covalent triazine framework as an efficient organic single‐component semiconductor photocatalyst. Four different molecular donor–acceptor domains are obtained within the network, leading to enhanced photogenerated charge separation via an intramolecular energy transfer cascade. The photocatalytic efficiency of the asymmetric covalent triazine framework is superior to that of its symmetric counterparts; this was demonstrated by the visible‐light‐driven formation of benzophosphole oxides from diphenylphosphine oxide and diphenylacetylene.
© Wiley VCH (2018)
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
Wang, Xiao-Ye; Urgel, José I.; Barin, Gabriela Borin; Eimre, Kristjan; Di Giovannantonio, Marco; Milani, Alberto; Tommasini, Matteo; Pignedoli, Carlo A.; Ruffieux, Pascal; Feng, Xinliang; Fasel, Roman; Müllen, Klaus; Narita, Akimitsu
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
Bottom-up synthesis of graphene nanoribbons (GNRs) has significantly advanced during the past decade, providing various GNR structures with tunable properties. The synthesis of chiral GNRs, however, has been underexplored and only limited to (3,1)-GNRs. We report herein the surface-assisted synthesis of the first heteroatom-doped chiral (4,1)-GNRs from the rationally designed precursor 6,16-dibromo-9,10,19,20-tetraoxa-9a,19a-diboratetrabenzo[a,f,j,o]perylene. The structure of the chiral GNRs has been verified by scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy in combination with theoretical modeling. Due to the presence of oxygen–boron–oxygen (OBO) segments on the edges, lateral self-assembly of the GNRs has been observed, realizing well-aligned GNR arrays with different modes of homochiral and heterochiral inter-ribbon assemblies.
© JACS (2018)
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
Direct observation of polymer surface mobility via nanoparticle vibrations
Hojin Kim, Yu Cang, Eunsoo Kang, Bartlomiej Graczykowski, Maria Secchi, Maurizio Montagna, Rodney D. Priestley, Eric M. Furst, George Fytas
Direct observation of polymer surface mobility via nanoparticle vibrations
MPIP researchers in collaboration with Prof. Priestley (Princeton Univeristy), Hojin Kim, Prof. Furst (University of Delaware), Dr. Maria Secchi and Prof. Montagna (University of Trento) introduced a new technique (Brillouin Light Spectroscopy) to directly observe surface mobility and shear modulus of polymer colloids. When laser impinges an assembly of polymer nanoparticles (NP)s, the inelastically scattered light is frequency resolved by a high resolution Tandem Fabry-Perot Interferometer to record the particle vibration spectrum in the GHz frequency range. For individual non-interactive nanoparticles, the single lower frequency mode (1,2) of the spectrum (red in the scheme) splits into a blue shifted doublet and a new interaction induced (1,1) mode (blue in the scheme) emerges in the presence of attractive interparticle interactions. The proposed assignment and analysis of the particle vibration spectrum allows the measurement of the particle shear modulus and leads to the first direct observation of NP surface mobility. NP’s possess lower shear modulus than the contiguous films and this softening effect is not size but surface chemistry dependent. The results provide new insight in the glass transition phenomenon and NP elasticity. Understanding thermomechanical properties is crucial for applicatios such as pressure-sensitive adhesives, drug carriers, surface coating, and nanoparticle reinforced plastics.
About the Project
Dr B.Graczykowski is supported by the Alexander von Humboldt Foundation
This project is funded by ERC SmartPhon (No. 694977)
© Nature Communications (2018)
Schematic picture of low frequency vibration spectrum of individual (red line) and interacted (violet lines) nanoparticles in a colloidal cluster.
Synthesis of Triply Fused Porphyrin-Nanographene Conjugates
Qiang Chen, Luigi Brambilla, Lakshya Daukiya, Kunal S. Mali, Steven De Feyter, Matteo Tommasini, Klaus Müllen, Akimitsu Narita
Synthesis of Triply Fused Porphyrin-Nanographene Conjugates
Syntheses of Pi-extended porphyrins have attracted immense interests for their unique optical and electronic properties, which render them highly valuable for a variety of applications, e.g., as near-infrared (NIR) dyes, organic semiconductors, and nonlinear optical materials. Various aromatic hydrocarbons, including benzene, naphthalene, pyrene, azulene, corannulene, anthracene and coronene, have thus been fused to the meso- and β-positions of porphyrin. Large PAHs, which can be regarded as nanographenes, are known to exhibit attractive (opto)electronic properties and self-assembly behavior, which can be further fine- tuned by precise control over their size, shape, and edge structure. Fusion of porphyrin core to large PAHs has not been achieved because of lacking appropriate methods. In this paper, two unprecedented porphyrin fused nanographene molecules 1 and 2 have been synthesized by Scholl reaction of tailor-made precursors based on benzo[m]tetraphene-substituted porphyrins. The chemical structures were validated by a combination of high-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (HR MALDI- TOF MS), IR and Raman spectroscopy, and scanning tunnelling microscopy (STM). The UV- vis-near infrared absorption spectroscopy of 1 and 2 demonstrated broad and largely red- shifted absorption spectra extending up to 1000 and 1400 nm, respectively, marking the significant extension of the Pi-conjugated systems.
© John Wiley and Sons (2018)
Molecular structures and UV-vis-NIR absorption spectra of triply fused porphyrin nanographene conjugates 1 and 2, demonstrating the absorption extending to 1000 and 1400 nm, respectively (inset shows pictures of THF solutions of compound 1 and 2).
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
A. Tomadin, S.M. Hornett, H.I. Wang, E.M. Alexeev, A. Candini, C. Coletti, D. Turchinovich, M. Kläui, M. Bonn, F.H.L. Koppens, E. Hendry, M. Polini and K.J. Tielrooij
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
For many of the envisioned optoelectronic applications of graphene, it is crucial to understand the subpicosecond carrier dynamics immediately following photoexcitation and the effect of photoexcitation on the electrical conductivity—the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on Fermi energy have been put forward. We present a unified and intuitive physical picture of the ultrafast carrier dynamics and the photoconductivity, combining optical pump–terahertz probe measurements on a gate-tunable graphene device, with numerical calculations using the Boltzmann equation. We distinguish two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy (EF ≲ 0.1 eV for our experiments), broadening of the carrier distribution involves interband transitions (interband heating). At higher Fermi energy (EF ≳ 0.15 eV), broadening of the carrier distribution involves intraband transitions (intraband heating). Under certain conditions, additional electron-hole pairs can be created [carrier multiplication (CM)] for low EF, and hot carriers (hot-CM) for higher EF. The resultant photoconductivity is positive (negative) for low (high) EF, which in our physical picture, is explained using solely electronic effects: It follows from the effect of the heated carrier distributions on the screening of impurities, consistent with the DC conductivity being mostly due to impurity scattering. The importance of these insights is highlighted by a discussion of the implications for graphene photodetector applications. The Mainz-based researchers Dr. H. Wang, Prof. D. Turchinovich, Prof. M. Kläui, and Prof. M. Bonn, in collaboration with scientists from various European labs, have now succeeded in understanding these processes. The project was led by Dr. K.-J. Tielrooij from ICFO in Spain, who was recently elected visiting professor at the MAINZ Graduate School.
© Fabien Vialla (2018)
Schematic representation of the ultrafast optical pump – terahertz probe experiment.
How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells
Stefan A.L. Weber, Ilka M. Hermes, Silver-Hamill Turren-Cruz, Christopher Gort, Victor W. Bergmann, Laurent Gilson, Anders Hagfeldt, Michael Graetzel, Wolfgang Tress, Rüdiger Berger
How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells
Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. Very efficient (>20 %) devices with perovskite layers of low defect density can be prepared by cheap and simple solution based processes at moderate temperatures (<150°C). For commercializing this technology, a stable and reliable operation is required. In perovskite solar cells, however, the output power strongly depends on the history of the device in terms of bias voltage (causing hysteresis) or illumination (known as light soaking effect). The underlying process is connected to the migration of ionic charges within the perovskite layer. In our study, we were able to map and follow the vertical charge distribution in the perovskite layer of an operating device. In particular, we found that thin layers of localized charge were forming at the electrode interfaces when we changed the external voltage or illuminated the device. Our results show that the formation and release of these ionic interface charges determine the time scales for current-voltage hysteresis in perovskite solar cells. Our study demonstrates that a precise control over the interfaces in perovskite solar cells is the key for controlling and suppressing hysteresis in perovskite solar cells.
© RCS (2018)
In this study, we use time-resolved Kelvin probe force micrscopy to investigate the mechanisms of current–voltage hysteresis in a hybrid lead-halide perovskite solar cell.
Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion
Yunbin Hu, Peng Xie, Marzio De Corato, Alice Ruini, Shen Zhao, Felix Meggendorfer, Lasse Arnt Straasø, Loic Rondin, Patrick Simon, Juan Li, Jonathan J Finley, Michael Ryan Hansen, Jean-Sébastien Lauret, Elisa Molinari, Xinliang Feng, Johannes V. Barth, Carlos-Andres Palma, Deborah Prezzi, Klaus Müllen, and Akimitsu Narita
Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion
Amongst organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, non-planarity can be an additional parameter to modulate their property without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with “cove”-type edges, having an identical aromatic core, but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the “cove”-type edges can “bend” the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and non-planar GNRs through UV-Vis-near infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.
© ACS (2018)
Molecular models and band structures of planar and non-planar graphene nanoribbons with the identical aromatic structure, demonstrating that the structural distortion induced by the proper positioning of alkyl chains can lower the bandgap.
Magnetic edge states and coherent manipulation of graphene nanoribbons
Michael Slota, Ashok Keerthi, William K. Myers, Evgeny Tretyakov, Martin Baumgarten, Arzhang Ardavan, Hatef Sadeghi, Colin J. Lambert, Akimitsu Narita, Klaus Müllen & Lapo Bogani
Magnetic edge states and coherent manipulation of graphene nanoribbons
Here we use molecular graphene nanoribbons functionalized with stable spin-bearing radical groups to demonstrate delocalized magnetic edge states and test theoretical models of the spin dynamics and spin–environment interactions. Comparison with a non-graphitized reference material enables us to clearly identify the characteristic behaviour of the radical-functionalized graphene nanoribbons. We quantify the parameters of spin–orbit coupling, define the interaction patterns and determine the spin decoherence channels. Even without any optimization, the spin coherence time is in the range of microseconds at room temperature, and we perform quantum inversion operations between edge and radical spins. Our approach provides a way of testing the theory of magnetism in graphene nanoribbons experimentally. The coherence times that we observe open up encouraging prospects for the use of magnetic nanoribbons in quantum spintronic devices.
© Lapo Bogani (2018)
Molecular graphene nanoribbons functionalized with stable spin-bearing nitronyl nitroxide radical groups, demonstrating delocalized magnetic edge state.
Quantifying Polaron Formation and Charge Carrier Cooling in Lead-Iodide Perovskites
Simon A. Bretschneider, Ivan Ivanov, Hai I. Wang, Kiyoshi Miyata, Xiaoyang Zhu, and Mischa Bonn
Quantifying Polaron Formation and Charge Carrier Cooling in Lead-Iodide Perovskites
Perovskite-based solar cells have been reaching record efficiencies. In these solar cells, light is converted into current (i.e., conducting electrons) by liberating otherwise bound electrons in the material. Despite their promise for solar cells, the fundamentals of this excitation process have yet to be fully understood in hybrid organic-inorganic perovskites. When light of sufficient energy hits a perovskite, electrons are generated that typically have excess energy: they are ‘hot.’ Electrons cool rapidly by giving off energy to the material. In perovskites, the electrons further distort the ionic lattice – attracting positive and repulsing negative ions in the lattice: a so-called polaron is formed, an electron ‘dressed’ with a lattice deformation. In our experiments, we follow the generation of (hot) electrons, their cooling and polaron formation in real-time. While the polaron formation time was found to be independent of the perovskite structure, marked variations in hot electron cooling were observed, in dependence on the structure. Our results show what material parameters can be used to tune – and to some extent control – the hot electron cooling process.
© MPI-P (2018)
The generation of conductive charge carriers in perovskites (lattice shown) occurs very quickly.
Beyond the Protein Corona - Lipids Matter for Biological Response of Nanocarrier
J. Müller, D. Prozeller, A. Ghazaryan, M. Kokkinopoulou, V. Mailänder, S. Morsbach, K. Landfester
Beyond the Protein Corona - Lipids Matter for Biological Response of Nanocarrier
It is well accepted that nanomaterials developed as carrier systems need to be well characterized in terms of biological responses. It was shown that proteins adsorb to the surface of a nanomaterial and define its biological identity. However, the presence of other surface-active components of blood plasma and how they interact with nanomaterials has been much less investigated. Thus, this study aims at providing a significant contribution to understanding the interaction mechanism between lipoproteins and nanomaterials. Since lipoproteins transport a high amount of lipids, which are surface-active molecules, the demonstrated interactions can go as far as complete lipoprotein disintegration.
© Elsevier (2018)
Lipoproteins may adsorb onto nanomaterial surfaces involving disintegration and thus present an explanation for an enrichment of apolipoproteins in the protein corona.
Ultra-low voltage high-sensitivity ion detection with current-driven organic electrochemical transistors
Matteo Ghittorelli, Leona Lingstedt, Irina Crăciun, Zsolt Miklós Kovács-Vajna, Paul W. M. Blom & Fabrizio Torricelli
Ultra-low voltage high-sensitivity ion detection with current-driven organic electrochemical transistors
Ions dissolved in aqueous media play a fundamental role in plants, animals, and humans. Therefore, the in-situ quantification of the ion concentration in aqueous media is gathering relevant interest. The fundamental limitation of approaches based on electrochemical transistors is the trade-off between sensitivity, ion concentration range and operating voltage. Here we show a new current-driven configuration based on organic electrochemical transistors that overcomes this fundamental limit. The measured ion-sensitivity exceeds by one order of magnitude the Nernst limit at an operating voltage of only few hundreds millivolts, opening new opportunities for high-performance bioelectronics.
© Nature Communications (2018)
Device structure of an organic electrochemical transistor using as ion-permeable conducting polymer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and an aqueous solution of NaCl as electrolyte.
Molecular Firefighting – How Modern Phosphorus Chemistry Can Help Solve the Flame Retardancy Task
Maria M. Velencoso, Alexander Battig, Jens C. Markwart, Bernhard Schartel, and Frederik R. Wurm
Molecular Firefighting – How Modern Phosphorus Chemistry Can Help Solve the Flame Retardancy Task
As polymers carry an inherent risk of fire, the use of flame-retardants is unavoidable. Current legislation, that is the ban of several halogenated flame-retardants, and novel scientific developments have further made the search for new flame-retardants an important topic. This review article describes the current state-of-the-art of phosphorus-based flame-retardants and highlights examples of modern strategies to a sustainable use of phosphorus-containing materials (low molecular weight and polymeric) as flame-retardants. State-of-the-art phosphorus-based flame-retardants and their mode of action are discussed, showing how they affect the properties of the polymer matrix. Future trends for sustainable phosphorus sources are discussed as well.
© Wiley VCH (2018)
Halogenated and phosphorus-based flame-retardants: How modern phosphorus chemistry can help to establish a sustainable future for flame-retardants.
Saturation of Charge-Induced Water Alignment at Model Membrane Surfaces
Lisa B. Dreier, Yuki Nagata, Helmut Lutz, Grazia Gonella, Johannes Hunger, Ellen H.G. Backus, Mischa Bonn
Saturation of Charge-Induced Water Alignment at Model Membrane Surfaces
The primary building blocks of biological membranes are lipids. The water molecules present at these membrane interfaces are oriented by the charged headgroups of the lipid molecules. A combined theoretical and experimental approach shows that there is a critical charge density above which the water molecules stop responding to the increase in charge. The insensitivity of the water molecules to the increase in charge can be traced to molecular rearrangement occurring at the lipid interface, in combination with the adsorption of counterions. These findings have important implications for reactions occurring at biological membranes.
© MPIP (2018)
Water molecules at negatively charged and net neutral lipid monolayers
On-Surface Synthesis of Indenofluorene Polymers by Oxidative Five-Membered Ring Formation
Marco Di Giovannantonio, José I. Urgel, Uliana Beser, Aliaksandr V. Yakutovich, Jan Wilhelm, Carlo A. Pignedoli, Pascal Ruffieux, Akimitsu Narita, Klaus Müllen, and Roman Fasel
On-Surface Synthesis of Indenofluorene Polymers by Oxidative Five-Membered Ring Formation
On-surface synthesis is a successful approach to the creation of carbon-based nanostructures that cannot be obtained via standard solution chemistry. In this framework, we have established a novel synthetic pathway to one-dimensional conjugated polymers composed of indenofluorene units. Our concept is based on the use of ortho-methyl groups on a poly(para-phenylene) backbone. In this situation, surface-assisted oxidative ring closure between a methyl and the neighboring aryl moiety gives rise to a five-membered ring. The atomically precise structures and electronic properties of the obtained indenofluorene polymers have been unambiguously characterized by STM, nc-AFM, and STS, supported by theoretical calculations. This unprecedented synthetic protocol can potentially be extended to other polyphenylenes and eventually graphene nanoribbons, to incorporate five-membered rings at desired positions for the fine-tuning of electronic properties.
© ACS (2018)
Synthesis of indenofluorene polymer from 4,4"-dibromo-2,2"-dimethyl-1,1':4',1"-terphenyl on Au(111) surface and visualization by noncontact atomic force microscopy (nc-AFM).
Origin of Negative Capacitance in Bipolar Organic Diodes
Quan Niu, N. Irina Crăciun, Gert-Jan A. H. Wetzelaer, and Paul W. M. Blom
Origin of Negative Capacitance in Bipolar Organic Diodes
An important parameter for devices as organic light-emitting diodes (OLEDs) is the capacitance, which provides information on the build-up of charges. However, in OLEDs the capacitance frequently becomes negative, a phenomenon that is poorly understood. We have revealed the origin of the negative capacitance by varying the number of electron traps in OLEDs. When adding electron traps, the negative capacitance became more pronounced, whereas the negative capacitance could be completely eliminated by reducing the number of traps by means of trap dilution. Now that the phenomenon is understood, the negative capacitance can be used to determine the number of traps in organic semiconductors.
© MPI-P (2018)
Reducing the negative capacitance in an OLED by blending with an insulating polymer
Calcium-Induced Molecular Rearrangement of Peptide Folds Enables Biomineralization of Vaterite Calcium Carbonate
Hao Lu, Helmut Lutz, Steven J. Roeters, Matthew A. Hood, Arne Schäfer, Rafael Muñoz-Espí, Rüdiger Berger, Mischa Bonn, Tobias Weidner
Calcium-Induced Molecular Rearrangement of Peptide Folds Enables Biomineralization of Vaterite Calcium Carbonate
Calcium carbonate (CaCO3) is the most abundant mineral on earth and plays an important role for life. CaCO3 is a key component of marine animals such as mollusk, mussels and sponges. The biogenesis of CaCO3-based hard tissue is tightly controlled by proteins, which exert control over the nucleation and growth of the specific phase of CaCO3, calcite, aragonite and vaterite. The latter phase occurs almost exclusively as a biomineral and has applications in drug delivery, implant design and surface coating. In this article we used surface spectroscopy and molecular dynamics simulations to determine, at the molecular level, how proteins control the formation of vaterite phase CaCO3.
© ACS (2018)
Calcium carbonate biogenesis is tightly controlled by proteins. Vaterite phase CaCO3 occurs almost exclusively as a biomineral and has applications in drug delivery and implant design. This article describes how proteins control the formation of vaterite phase CaCO3.
Precision synthesis versus bulk-scale fabrication of graphenes
Xiao-Ye Wang, Akimitsu Narita, Klaus Müllen
Precision synthesis versus bulk-scale fabrication of graphenes
Graphene is a fascinating material with unique properties, such as extreme mechanical strength, ultrahigh electrical and thermal conductivities and remarkable transparency. Further reduction in the dimensionality of graphene in the form of graphene quantum dots and graphene nanoribbons has compensated for the lack of a bandgap in the extended 2D material. These nanoscale graphenes exhibit finite bandgaps because of quantum confinement, making them attractive as next-generation semiconductors. Numerous fabrication methods for various types of graphenes have been developed, which can generally be categorized into ‘top-down’ and ‘bottom-up’ procedures. These methods afford, on different production scales, a wide range of graphene structures of different sizes, shapes and quality (defect density, edge roughness and so on). Atomically precise syntheses are indispensable for fundamental research and future technological development, but the projection of the existing methods to cost-effective bulk-scale fabrication techniques is required for upcoming industrial applications of graphenes.
© Nature Publishing Group (2018)
Precision synthesis versus bulk-scale fabrication of graphenes
Hydrophilicity regulates the stealth properties of polyphosphoester-coated nanocarriers
Johanna Simon, Thomas Wolf, Katja Klein, Katharina Landfester, Volker Mailänder and Frederik Roman Wurm
Hydrophilicity regulates the stealth properties of polyphosphoester-coated nanocarriers
Increasing the plasma half-life is an important goal in the development drug carriers. Attachment of polymers, especially poly(ethylene glycol) (PEG), is an effective method to increase the plasma half-life of drugs. Even though it was assigned to be a result of a decreased overall protein adsorption on the hydrophilic surface in combination with the adsorption of specific proteins, the molecular reasons for the success of PEG and other hydrophilic polymers are still widely unknown. We prepare poly(phosphoester)-coated nanocarriers with adjusted hydrophilicity to control the stealth properties of the polymer shell. We find that the logP-values of the copolymers control the pattern of the protein corona and the cell interaction. In spite of a significant change in hydrophilicity, the overall protein amount adsorbed from blood on the nanocarrier is unchanged, while the protein pattern is altered. This underlines the importance of the protein type in the protein corona and cell uptake.
© Wiley-VCH (2018)
A Rational Delamination Strategy towards Defect-Free, High-Mobility, Few-Layered Black Phosphorus Flakes
Sheng Yang, Ke Zhang, Antonio Gaetano Ricciardulli, Panpan Zhang, Zhongquan Liao, Martin R. Lohe, Ehrenfried Zschech, Paul W. M. Blom, Wojciech Pisula, Klaus Müllen and Xinliang Feng
A Rational Delamination Strategy towards Defect-Free, High-Mobility, Few-Layered Black Phosphorus Flakes
Based on electrochemical engineering, a simple and scalable exfoliation method was developed to bring truly defect-free black phosphorus (BP) flakes to a macroscopic scale. The delaminated flakes exhibited excellent electronic properties, well comparable with mechanically exfoliated BP flakes. This strategy paves great opportunities for the future development of BP-based devices and technologies.
© WILEY-VCH (2018)
Cathodic exfoliation of bulk black phosphorus in organic electrolytes.
CO2-Triggered Switchable Hydrophilicity of Heterogeneous Conjugated Polymer Photocatalyst for Enhanced Catalytic Activity in Water
Jeehye Byun, Wei Huang, Di Wang, Run Li, Kai A. I. Zhang
CO2-Triggered Switchable Hydrophilicity of Heterogeneous Conjugated Polymer Photocatalyst for Enhanced Catalytic Activity in Water
Water-compatibility of heterogeneous photocatalysts has been pursued for energy and environmental applications. However, there exists a trade-off between hydrophilicity and recyclability of the photocatalyst. MPIP researchers have designed a conjugated polymer photocatalyst with tertiary amine terminals that reversibly binds CO2 in water, generating a switchable hydrophilicity. The CO2-assisted hydrophilicity boosted up the photocatalytic efficiency in aqueous medium with minimum dosage. When CO2 was desorbed, the photocatalyst could be simply regenerated from reaction media, facilitating the repeated use of photocatalyst. The hydrophilicity/hydrophobicity control of the polymer photocatalyst has successfully showcased through a variety of organic photo-redox reactions under visible light irradiation in water.
© Wiley VCH (2018)
CO2-triggered switchable hydrophilicity of a conjugated polymer photocatalyst for photocatalysis in water
Hybrid silver nanowire and graphene based solution-processed transparent electrode for organic optoelectronics
Antonio Gaetano Ricciardulli, Sheng Yang, Gert-Jan A. H. Wetzelaer, Xinliang Feng, Paul W. M. Blom
Hybrid silver nanowire and graphene based solution-processed transparent electrode for organic optoelectronics
The research on transparent conductive electrodes (TCEs) is in rapid ascent in order to respond to the requests of novel optoelectronic devices. The synergic coupling of silver nanowires (AgNWs) and high-quality solution-processable exfoliated graphene (EG) enables an efficient transparent conductor with low surface roughness of 4.6 nm, low sheet resistance of 13.7 Ω sq-1 at high transmittance and superior mechanical and chemical stabilities. The developed AgNWs-EG films are versatile for a wide variety of optoelectronics. As an example, when used as bottom electrode in organic solar cell (OSC) and polymer light-emitting diode (PLED), the devices exhibit a power conversion efficiency of 6.6% and an external quantum efficiency of 4.4% respectively, comparable to their commercial ITO-counterparts.
© Wiley VCH (2018)
Flexible OSC and PLED based on spray-coated hybrid AgNWs-EG transparent electrodes.
 
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