Highlight Publications 2017

The Cassie-Wenzel transition of fluids on nanostructured substrates: Macroscopic force balance versus microscopic density-functional theory
Nikita Tretyakov, Periklis Papadopoulos, Doris Vollmer, Hans-Jürgen Butt, Burkhard Dünweg und Kostas Daoulas
The Cassie-Wenzel transition of fluids on nanostructured substrates: Macroscopic force balance versus microscopic density-functional theory
Classical density functional theory is applied to investigate the validity of a phenomenological force-balance description of the stability of the Cassie state of liquids on substrates with nanoscale corrugation. A bulk free-energy functional of third order in local density is combined with a square-gradient term, describing the liquid-vapor interface. The bulk free energy is parameterized to reproduce the liquid density and the compressibility of water. The square-gradient term is adjusted to model the width of the water-vapor interface. The substrate is modeled by an external potential, based upon the Lennard-Jones interactions. The three-dimensional calculation focuses on substrates patterned with nanostripes and square-shaped nanopillars. Using both the force-balance relation and density-functional theory, we locate the Cassie-to-Wenzel transition as a function of the corrugation parameters. We demonstrate that the force-balance relation gives a qualitatively reasonable description of the transition even on the nanoscale. The force balance utilizes an effective contact angle between the fluid and the vertical wall of the corrugation to parameterize the impalement pressure. This effective angle is found to have values smaller than the Young contact angle. This observation corresponds to an impalement pressure that is smaller than the value predicted by macroscopic theory. Therefore, this effective angle embodies effects specific to nanoscopically corrugated surfaces, including the finite range of the liquid-solid potential (which has both repulsive and attractive parts), line tension, and the finite interface thickness. Consistently with this picture, both patterns (stripes and pillars) yield the same effective contact angles for large periods of corrugation.
© AIP Publishing (2017)
The Cassie-to-Wenzel transition is located for striped (left) and pillared (right) substrates as a function of the corrugation parameters a and cx. Parts of the corrugations re-enter the calculation box through periodic boundary conditions.
Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene
Marco Gobbi, Sara Bonacchi, Jian X. Lian, Yi Liu, Xiao-YeWang, Marc-Antoine Stoeckel, Marco A. Squillaci, Gabriele D’Avino, Akimitsu Narita, Klaus Müllen, Xinliang Feng, Yoann Olivier, David Beljonne, Paolo Samorì & Emanuele Orgiu
Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene
The rise of 2D materials made it possible to form heterostructures held together by weak interplanar van der Waals interactions. Within such van der Waals heterostructures, the occurrence of 2D periodic potentials significantly modifies the electronic structure of single sheets within the stack, therefore modulating the material properties. However, these periodic potentials are determined by the mechanical alignment of adjacent 2D materials, which is cumbersome and time-consuming. Here we show that programmable 1D periodic potentials extending over areas exceeding 104 nm2 and stable at ambient conditions arise when graphene is covered by a self-assembled supramolecular lattice. The amplitude and sign of the potential can be modified without altering its periodicity by employing photoreactive molecules or their reaction products. In this regard, the supramolecular lattice/graphene bilayer represents the hybrid analogue of fully inorganic van der Waals heterostructures, highlighting the rich prospects that molecular design offers to create ad hoc materials.
© Macmillan Publishers Limited (2017)
Calculated differential electrical potential induced by a self-assembled supramolecular lattice on graphene. The supramolecular lattice is superimposed for clarity. The electrical potential is periodically modulated, with negative values in the region below the molecular heads. Carbon atoms are shown in grey, hydrogen in white, nitrogen in red, fluorine in light blue and chlorine in green.
Stable Hydrophobic Metal-Oxide Photocatalysts via Grafting Polydimethylsiloxane Brush
Sanghyuk Wooh, Noemí Encinas, Doris Vollmer, and Hans-Jürgen Butt
Stable Hydrophobic Metal-Oxide Photocatalysts via Grafting Polydimethylsiloxane Brush
We designed a new method for grafting polydimethylsiloxane (PDMS) on surfaces of metal-oxide photocatalysts via simple illumination. By this PDMS grating reaction, stable photocatalytically active liquid-repellent surfaces were realized. The PDMS grafted metal-oxide photocatalysts exhibited improved self-cleaning and anti-biofouling properties by the combination effect of photocatalytic activity and liquid-repellency.
© WILEY-VCH (2017)
The photocatalytically active liquid-repellent surfaces were introduced by developing PDMS grafting reaction on metal-oxide photocatalysts.
Structure–Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells
Bert Conings, Simon A. Bretschneider, Aslihan Babayigit, Nicolas Gauquelin, Ilaria Cardinaletti, Jean Manca, Jo Verbeeck, Henry J Snaith, Hans-Gerd Boyen
Structure–Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells
The power conversion efficiency of halide perovskite solar cells is heavily dependent on the perovskite layer being sufficiently smooth and pinhole-free. It has been shown that these features can be obtained even when starting out from rough and discontinuous perovskite film by briefly exposing the film to methylamine (MA) vapor. The exact underlying physical mechanisms of this phenomenon are, however, still unclear. By investigating smooth, MA treated films based on very rough and discontinuous reference films of methylammonium triiode (MAPbI3) and considering their morphology, crystalline features, local conductive properties, and charge carrier lifetime, we unraveled the relation between their characteristic physical qualities and their performance in corresponding solar cells. We discovered that the extensive improvement in photovoltaic performance upon MA treatment is a consequence of the induced morphological enhancement of the perovskite layer together with improved electron injection into TiO2, which in fact compensates for an otherwise compromised bulk electronic quality simultaneously caused by the MA treatment.
© American Chemical Society (2017)
Methylamine treatment of sensitized perovskite solar cells facilitates the infiltration of the perovskite into the TiO2 and evens out the perovskite capping layer to reduce shunting, resulting in a dramatic improvement of the efficiency.
Shape Controlled Hierarchical Porous Hydrophobic/Oleophilic Metal-Organic Nanofibrous Gel Composites for Oil Adsorption
Kolleboyina Jayaramulu, Florian Geyer, Martin Petr, Radek Zboril, Doris Vollmer, Roland A. Fischer
Shape Controlled Hierarchical Porous Hydrophobic/Oleophilic Metal-Organic Nanofibrous Gel Composites for Oil Adsorption
We designed a simple route to synthesize hydrophobic and superoleophilic, fibrous hierarchically porous hybrid materials composed of metal organic gels (MOGs) with fluorinated graphene oxide (FGO). The integration of FGO layer stacks to light MOGs has proven a great asset in combining functional features and enhancement of materials performance.
© Advanced Materials (2017)
Schematic illustration of the formation of FGO@MOG (top). Coordination perturbation, selective functionalization of MOF-nanoparticles with FGO yields a hybrid composite gel. Water drop (6 µL) on pressed FGO@MOG tablet (bottom left). Oil is adsorbed by the composite material (bottom right).
On-Surface Synthesis and Characterization of 9-Atom Wide Armchair Graphene Nanoribbons
Leopold Talirz, Hajo Söde, Tim Dumslaff, Shiyong Wang, Juan Ramon Sanchez-Valencia, Jia Liu, Prashant Shinde, Carlo A. Pignedoli, Liangbo Liang, Vincent Meunier, Nicholas C. Plumb, Ming Shi, Xinliang Feng, Akimitsu Narita, Klaus Müllen, Roman Fasel, Pascal Ruffieux
On-Surface Synthesis and Characterization of 9-Atom Wide Armchair Graphene Nanoribbons
We present the synthesis of 9-atom-wide armchair-edge GNRs (9-AGNRs) via surface-assisted aryl-aryl coupling and cyclodehydrogenation of a small halogen-substituted precursor molecule on Au(111). In this approach, the molecular precursor precisely defines the width and edge structure of the resulting GNRs down to the single atom, thus providing ultimate control over the band gap by design. We have designed and synthesize suitable monomer with an o-terphenyl-based structure, which selectively provided atomically precise 9-AGNRs as revealed by scanning probe microscopy and Raman spectroscopy. In line with theoretical predictions of their electronic properties, a low band gap of 1.4 eV and small effective masses of 0.1 me for electrons and holes are found for the metal-adsorbed 9-AGNRs, making them an interesting material for room-temperature electronic and optoelectronic switching devices
© American Chemical Society (2017)
High-resolution nc-AFM frequency shift image of 9-AGNR using a CO-functionalized tip with oscillation amplitude of 70 pm (scale bar: 1 nm).
 
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