The flow orientation of anisotropic particles through narrow channels is of importance in many fields, ranging from the spinning and molding of fibers to the flow of cells and proteins through thin ...capillaries. It is commonly assumed that anisotropic particles align parallel to the flow direction. When flowing through narrowed channel sections, one expects the increased flow rate to improve the parallel alignment. Here, we show by microfocus synchrotron X-ray scattering and polarized optical microscopy that anisotropic colloidal particles align perpendicular to the flow direction after passing a narrow channel section. We find this to be a general behavior of anisotropic colloids, which is also observed for disk-like particles. This perpendicular particle alignment is stable, extending downstream throughout the remaining part of the channel. We show by microparticle image velocimetry that the particle reorientation in the expansion zone after a narrow channel section occurs in a region with considerable extensional flow. This extensional flow is promoted by shear thinning, a typical property of complex fluids. Our discovery has important consequences when considering the flow orientation of polymers, micelles, fibers, proteins, or cells through narrow channels, pipes, or capillary sections. An immediate consequence for the production of fibers is the necessity for realignment by extension in the flow direction. For fibrous proteins, reorientation and stable plug flow are likely mechanisms for protein coagulation.
Micelles are the simplest example of self-assembly found in nature. As many other colloids, they can self-assemble in aqueous solution to form ordered periodic structures. These structures so far all ...exhibited classical crystallographic symmetries. Here we report that micelles in solution can self-assemble into quasicrystalline phases. We observe phases with 12-fold and 18-fold diffraction symmetry. Colloidal water-based quasicrystals are physically and chemically very simple systems. Macroscopic monodomain samples of centimeter dimension can be easily prepared. Phase transitions between the fee phase and the two quasicrystalline phases can be easily followed in situ by time-resolved diffraction experiments. The discovery of quasicrystalline colloidal solutions advances the theoretical understanding of quasicrystals considerably, as for these systems the stability of quasicrystalline states has been theoretically predicted for the concentration and temperature range, where they are experimentally observed. Also for the use of quasicrystals in advanced materials this discovery is of particular importance, as it opens the route to quasicrystalline photonic band gap materials via established water-based colloidal self-assembly techniques.
Three isomeric π-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed ...organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C70 fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells.
The reorientation of lamellae and the dependence of the lamellar spacing, D lam, on polymer volume fraction, ϕP, D lam ∝ ϕP –β, in diblock copolymer thin films during solvent vapor annealing (SVA) ...are examined by combining white light interferometry (WLI) and grazing-incidence small-angle X-ray scattering (GISAXS). A thin film of lamellae-forming poly(styrene-b-butadiene) prepared by spin-coating features lamellae of different orientations with the lamellar spacing depending on orientation. During annealing with ethyl acetate (EAC) vapor, it is found that perpendicular lamellae behave differently from parallel ones, which is due to the fact that their initial lamellar thicknesses differ strongly. Quantitatively, the swelling process is composed of three regimes and the drying process of two regimes. The first two regimes of swelling are associated with a significant structural rearrangement of the lamellae; i.e., the lamellae first become thicker, and then perpendicular and randomly oriented lamellae vanish, which results in a purely parallel orientation at the end of the swelling process. The rearrangement is attributed to the increase of mobility of the polymer chains imparted by the solvent and to a decrease of total free energy of the thin film. In the third regime of swelling, the scaling exponent is found to be β = −0.32. During drying, the deswelling is nonaffine which may be a consequence of the increase of nonfavorable segmental interactions as the solvent is removed.
The fabrication of bit-patterned media （BPM） is crucial for new types of hard disk drives. The development of methods for the production of BPM is progressing rapidly. Conventional lithography ...reaches the limit regarding lateral resolution, and new routes are needed. In this study, we mainly focus on the dependence of the size and shape of magnetic nanodots on the Ar＋-ion etching duration, using silica dots as masks. Two-dimensional （2D） arrays of magnetic nanostructures are created using silica-filled diblock-copolymer micelles as templates. After the self-assembly of the micelles into 2D hexagonal arrays, the polymer shell is removed, and the SiO2 cores are utilized to transform the morphology into a （Co/Pt）2-multilayer via ion etching under normal incidence. The number of preparation steps is kept as low as possible to simplify the formation of the nanostructure arrays. High-resolution in situ grazing-incidence small-angle X-ray scattering （GISAXS） investigations are performed during the Ar＋-ion etching to monitor and control the fabrication process. The in situ investigation provides information on how the etching conditions can be improved for further ex situ experiments. The GISAXS patterns are compared with simulations. We observe that the dots change in shape from cylindrical to conical during the etching process. The magnetic behavior is studied by utilizing the magneto-optic Kerr effect. The Co/Pt dots exhibit different magnetic behaviors depending on their size, interparticle distance, and etching time. They show ferromagnetism with an easy axis of magnetization perpendicular to the film. A systematic dependence of the coercivitv on the dot size is observed.
Supramolecular assembly allows for enhanced control of bulk material properties through the fine modulation of intermolecular interactions. We present a comprehensive study of a cross-linkable ...amphiphilic wedge molecule based on a sulfonated trialkoxybenzene with a sodium counterion that forms liquid crystalline (LC) phases with ionic nanochannel structures. This compound exhibits drastic structural changes as a function of relative humidity (RH). Our combined structural, dynamical, and transport studies reveal deep and novel information on the coupling of water and wedge molecule transport to structural motifs, including the significant influence of domain boundaries within the material. Over a range of RH values, we employ 23Na solid-state NMR on the counterions to complement detailed structural studies by grazing-incidence small-angle X-ray scattering. RH-dependent pulsed-field-gradient (PFG) NMR diffusion studies on both water and the wedge amphiphiles show multiple components, corresponding to species diffusing within LC domains as well as in the domain boundaries that compose 10% of the material. The rich transport and dynamical behaviors described here represent an important window into the world of supramolecular soft materials, carrying implications for optimization of these materials in many venues. Cubic phases present at high RH show fast transport of water (2 × 10–10 m2/s), competitive with that observed in benchmark polymeric ion conductors. Understanding the self-assembly of these supramolecular building blocks shows promise for generating cross-linked membranes with fast ion conduction for applications such as next-generation batteries.
Sponge-like structures for application in photovoltaics Perlich, Jan; Kaune, Gunar; Memesa, Mine ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
05/2009, Letnik:
367, Številka:
1894
Journal Article
Recenzirano
Large surface areas at an interface between two different materials are desired in many research fields where the interaction between these materials significantly affects the performance of the ...physical system. This behaviour is illustrated on sponge-like structures, which assign for such a high surface area, and demonstrate the development from bulk material to thin films and a variety of applications. The focus is on sponge-like nanostructures consisting of a network of aggregated titania nanoparticles applied in hybrid structures for photovoltaics. Examples based on a sol-gel process for the preparation of titania nanostructures in thin films, mimicking the sponge morphology, are shown. In general, titania films are widely used in photovoltaics, contributing to a large surface area available for interfacial reactions, e.g. charge carrier transfer routes. Interpenetrating networks with dimensions matching exciton diffusion lengths in the polymer component of a hybrid organic-inorganic photovoltaic structure are highly desirable. To characterize the fabricated morphology, atomic force microscopy and field-emission scanning electron microscopy are employed in real space. The advanced scattering technique of grazing-incidence small-angle X-ray scattering complements the characterization in reciprocal space. From the obtained results, the sponge-like morphology is verified, a physical description of the morphology with statistical relevance is constructed and the successful complete filling of the network is shown. According to this description, the presented sponge-like titania nanostructures are well suited for use in hybrid organic-inorganic solar cells.
Herein, we report the synthesis and photocatalytic properties of novel Bi2O3 nanomaterials. Cubic mesoporous films were produced by coassembly of hydrated bismuth nitrate with a ...poly(ethylene-co-butylene)-block-poly(ethylene oxide) diblock copolymer, referred to as KLE, while nanofiber mats were produced via electrospinning. We establish that all materials employed in this work are highly crystalline after thermal treatment and that the nanoscale order of the self-assembled samples is further retained. KLE-templated films can readily produce phase-pure β-Bi2O3 with an optical band gap of about 3.4 eV. Calcination of nanofibers, by contrast, leads to the formation of a composite consisting of β-Bi2O3 and trace amounts of the thermodynamically stable α-Bi2O3 phase. We further show the benefits of a mesoporous morphology in heterogeneous semiconductor photocatalysis. KLE-templated β-Bi2O3 films exhibit much greater photocatalytic activity than nanofiber mats and nontemplated Bi2O3 films (prepared under otherwise identical conditions) as well as than KLE-templated anatase TiO2 films on a mass normalized basis. We associate this exceptional activity with (1) the comparatively high BET surface area, which provides for a large number of adsorption sites, and (2) the high phase purity of the more catalytically active β-Bi2O3.
The possibility of achieving templates of highly ordered nanostructures with crystalline nano‐objects embedded inside an amorphous matrix is addressed. Atomic force microscopy and grazing incidence ...small‐angle X‐ray scattering (GISAXS) are used to probe the morphology at the surface and inside thin films of the diblock copolymer polystyrene‐block‐poly(ethylene oxide) P(S‐b‐EO). Directional benzene vapor flow introduces the orientation of poly(ethylene oxide) (PEO) cylinders inside the PS matrix. Within a limited film thickness regime, the cylinders are oriented parallel to the sample surface in domains of several micrometers due to shearing effects caused by the flow of benzene vapor parallel to the polymer surface. The crystallization of the PEO block by the benzene vapor treatment is also observed and crystals with sizes that approach that of the confined PEO domains are formed. Perpendicular orientation of the PEO crystalline stems with respect to the PEO cylindrical nanodomains is deduced from grazing incidence X‐ray diffraction. X‐ray reflectometry and scanning electron microscopy complement the investigation.
An induced large‐scale alignment of the parallel cylinder morphology of the polystyrene ‐block‐polyethylene oxide thin film via treatment in a saturated benzene vapor that flows parallel to the film surface is realized. The enhanced alignment in the vapor‐flow is attributed to a vapor‐flow‐induced shearing effect of the polymer film. The semicrystalline PEO block forms cylinders that initiate a growth rate with a favorable perpendicular orientation of the PEO crystalline stems with respect to the cylinder axis.
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