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In this paper we present the fabrication and characterization of polymer nanomembranes filled with magnetic nanoparticles and attached covalently to a periodic array of free-standing silicon walls, forming an array of micro- channels with the membrane as a cover. The width of a micro-channel of about 1.4 mu m sets a characteristic lateral size and the thickness of the polymer membrane ranges between 100 and 300 nm. The membrane is made of cross-linked hydrophilic polymers possessing a Young's modulus of only a few MPa. The presence of the magnetic particles within the membrane makes the film responsive to external magnetic fields. The mechanical and magnetic properties of the membrane are characterized by bulge tests and with atomic force microscopy.
We show that Scattering Infrared Near-field Microscopy (SNIM) allows chemical mapping of polymer monolayers that can serve as designed nanostructured surfaces with specific surface chemistry properties on a nm scale. Using s- SNIM a minimum volume of 100 nm x 100 nm x 15 nm is sufficient for a recording of a "chemical'' IR signature which corresponds to an enhancement of at least four orders of magnitudes compared to conventional FT-IR microscopy. We could prove that even in cases where it is essentially difficult to distinguish between distinct polymer compositions based solely on topography, nanophase separated polymers can be clearly distinguished according to their characteristic near-field IR response.
We report on the interaction of cationic azobenzene-containing surfactant with DNA investigated by absorption and fluorescence spectroscopy, dynamic light scattering, and atomic force microscopy. The properties of the surfactant can be controlled with light by reversible switching of the azobenzene unit, incorporated into the surfactant tail, between a hydrophobic trans (visible irradiation) and a hydrophilic cis (UV irradiation) configuration. The influence of the trans-cis isomerization of the azobenzene on the compaction process of DNA molecules and the role of both isomers in the formation and colloidal stability of DNA-surfactant complexes is discussed. It is shown that the trans isomer plays a major role in the DNA compaction process. The influence of the cis isomer on the DNA coil configuration is rather small. The construction of a phase diagram of the DNA concentration versus surfactant/DNA charge ratio allows distancing between three major phases: colloidally stable and unstable compacted globules, and extended coil conformation. There is a critical concentration of DNA above which the compacted globules can be hindered from aggregation and precipitation by adding an appropriate amount of the surfactant in the trans configuration. This is because of the compensation of hydrophobicity of the globules with an increasing amount of the surfactant. Below the critical DNA concentration, the compacted globules are colloidally stable and can be reversibly transferred with light to an extended coil state.
Reversible structuring of photosensitive polymer films by surface plasmon near field radiation
(2011)
We report on the fabrication and characterisation of a novel type of hybrid azo-modified photosensitive polymer film with a nanoscale metallic structuring integrated into the substrate. The metal structures permit to generate surface plasmon near fields when irradiated by UV-light from the rear without directly illuminating the polymer. This allows establishment of a localized, complex-shape intensity distribution at sub-wavelength resolution with a corresponding impact on the photosensitive polymer. The possibilities of exploiting this setup are manifold. We find that just by using the change of polarization of the incident light as means of control, the topography can be driven to change between various patterns reversibly. These results are confirmed by numerical simulations and compared with in situ recorded topography changes.
The influence of the subphase temperature on the stripe pattern formation during Langmuir-Blodgett transfer (LB patterning) is investigated in a combined experimental and theoretical study. According to our experiments on the LB transfer of dipalmitoylphosphatidylcholine (DPPC) on planar mica substrates, even small temperature changes between 21.5 and 24.5 degrees C lead to significant changes in the monolayer patterns. For a constant surface pressure and dipper speed, the width of the stripes and the overall spatial period of the patterns increase with increasing subphase temperature. Because the stripe patterns are ascribed to alternating monolayer domains in the liquid-expanded and the liquid-condensed phases, the working regime for the formation of stripes is found to depend strongly on the respective surface pressure-area isotherm. These experimental findings are in accordance with the results of a theoretical investigation based on a model that takes hydrodynamics and the monolayer thermodynamics into account.
We report on the preparation and characterization of photosensitive polymer brushes. The brushes are synthesized through polymer analogous attachment of azobenzene groups to surface-attached poly(methacrylic acid) (PMAA) chains. The topography of the photosensitive brushes shows a strong reaction upon irradiation with UV light. While homogeneous illumination leaves the polymer topography unchanged, irradiation of the samples with interference patterns with periodically varying light intensity leads to the formation of surface relief gratings (SRG). The height of the stripes of the grating can be controlled by adjusting the irradiation time. The SRG pattern can be erased through solvent treatment when the periodicity of the stripe pattern is less than the length of the fully stretched polymer chains. In the opposite case, photomechanical scission of receding polymer chains is observed during SRG formation, and the inscribed patterns are permanent.
We report on a process inducing photomechanical fracture of chemical bonds in photosensitive PMAA polymer brushes. The photosensitive PMAA polymer brushes were prepared by covalent attachment of azobenzene groups to poly(methylacrylic acid) (PMAA) chains generated by surface-initiated polymerization. While homogeneous irradiation leaves the polymer topography unchanged, the azo-PMAA brushes show a strong response upon irradiation with UV interference patterns. The photoisomerization process in the surface-attached polymer films results in the irreversible formation of surface relief gratings (SRG), which are strongly enhanced upon washing with a good solvent for the polymer. The photomechanical forces during mass transport induced by the irradiation lead to the scission of covalent bounds and accordingly to a degrafting of the polymer chains in areas where the polymer is receding from. It is observed that the number of ruptured chains depends strongly on the amount of azo side chains in the polymer.
Using scattering scanning nearfield infrared microscopy (s-SNIM), we have imaged the nanoscale phase separation of mixed polystyrene-poly(methyl methacrylate) (PS-PMMA) brushes and investigated changes in the top layer as a function of solvent exposure. We deduce that the top-layer of the mixed brushes is composed primarily of PMMA after exposure to acetone, while after exposure to toluene this changes to PS. Access to simultaneously measured topographic and chemical information allows direct correlation of the chemical morphology of the sample with topographic information. Our results demonstrate the potential of s-SNIM for chemical mapping based on distinct infrared absorption properties of polymers with a high spatial resolution of 80 nm x 80 nm.
In this paper the study of the tunneling current-voltage (I-V) characteristics of silicon surfaces with n- and p-type conductivity as a function of roughness in the presence of an adsorbed insulating layer of polyethylenimine (PEI) is presented. A new approach is proposed for analysis of the tunnel current-voltage characteristics of a metal-insulator-semiconductor structure based on the combination of two models (Simmons and Schottky). Such joint analysis demonstrates the effect of surface states and evaluates changes in the band bending and electron affinity after the deposition of the polyelectrolyte layer on the semiconductor surface. As a result, we are able to differentiate between the equilibrium tunnel barrier (q phi (0)) and the barrier height (q phi (B)). It is shown that the deposition of the polymer leads to an increase of the equilibrium tunnel barrier by more than 250 meV, irrespective of the roughness and the conductivity type of the silicon substrate. The PEI deposition also leads to changes in the barrier height (less than 25 meV) that are smaller than the equilibrium tunnel barrier changes, indicating pinning of the Fermi level by the electron surface states that are energetically close to it. These surface states can trap charge carriers, a process leading to the formation of a depletion region and band bending on the semiconductor surface. Moreover, the change in the barrier height q Delta phi (B) depends on the conductivity type of the semiconductor, being positive for n-type and negative for p-type, in contrast to q Delta phi (0), which is positive for all substrates. The change is explained by capture of electrons preferably from the semiconductor space-charge region in the presence of a cationic polyelectrolyte, e.g., PEI.
The light-induced reversible switching of the swelling of microgel particles triggered by photo-isomerization and binding/unbinding of a photosensitive azobenzene-containing surfactant is reported. The interactions between the microgel (N-isopropylacrylamide, co-monomer: allyl acetic acid, crosslinker: N,N'-methylenebisacrylamide) and the surfactant are studied by UV-Vis spectroscopy, dynamic and electrophoretic light scattering measurements. Addition of the surfactant above a critical concentration leads to contraction/collapse of the microgel. UV light irradiation results in trans-cis isomerization of the azobenzene unit incorporated into the surfactant tail and causes an unbinding of the more hydrophilic cis isomer from the microgel and its reversible swelling. The reversible contraction can be realized by blue light irradiation that transfers the surfactant back to the more hydrophobic trans conformation, in which it binds to the microgel. The phase diagram of the surfactant-microgel interaction and transitions (aggregation, contraction, and precipitation) is constructed and allows prediction of changes in the system when the concentration of one or both components is varied. Remote and reversible switching between different states can be realized by either UV or visible light irradiation.
We report on sub-wavelength structuring of photosensitive azo-containing polymer films induced by a surface plasmon interference intensity pattern. The two surface plasmon waves generated at neighboring nano-slits in the metal layer during irradiation interfere constructively, resulting in an intensity pattern with a periodicity three times smaller than the wavelength of the incoming light. The near field pattern interacts with the photosensitive polymer film placed above it, leading to a topography change which follows the intensity pattern exactly, resulting in the formation of surface relief gratings of a size below the diffraction limit. We analyze numerically and experimentally how the depth of the nano-slit alters the interference pattern of surface plasmons and find that the sub-wavelength patterning of the polymer surface could be optimized by modifying the geometry and the size of the nano-slit.
We report on reversible structuring of photosensitive azo-containing polymer films induced by near-field intensity patterns emanating from illuminated nano-scale metal structures fabricated by colloidal lithography. Two different sets of these nano-antennas, consisting of either gold or silver, were investigated with respect to their ability to induce topography changes in a photosensitive polymer film placed above. Using in situ recorded atomic force microscopy micrographs of polymer topography changes during UV irradiation, we find that the response of the polymer film differs for the two metals at similar geometries of the metal nanostructures. The maximum topography change is stronger for Ag antennas as compared to the Au pattern, whereas the latter material revealed a pronounced splitting of topography maxima into two, a phenomenon less visible in the case of Ag. Finite difference time domain simulations of the electromagnetic field distribution in the vicinity of the metal structures confirm this remarkable observation. The local intensity is twice as large for the Ag as compared to the Au structures, and in each case, a splitting of the intensity pattern results, with a stronger modulation for Au. For both metals, the topography change was found to be reversible between a patterned and a flat by repeated change of irradiation conditions.
It is well-known that surface plasmon generated near fields of suitably irradiated metal nano-structures can induce a patterning in an azobenzene-modified photosensitive polymer film placed on top. The change in the topography usually follows closely and permanently the underlying near field intensity pattern. With this approach, one can achieve a multitude of morphologies by additionally changing light intensity, polarization and the kind of metal used for nano-structuring. In this paper, we demonstrate that below a critical value of the polymer film thickness, the receding polymer material induces a change in refractive index of the glass-metal-polymer system, modifying the near field intensity distribution and causing a back-reaction on the flow of polymer material. This has a profound influence on the smallest size of topographical features that can be imprinted into the polymer.
Here we demonstrate how a surface plasmon (SP) generated near field pattern in the vicinity of a nano-scale pin hole can be used to generate reversible topography changes in a photosensitive polymer film above the opening. This can be achieved by simply changing the polarization state of the plasmon generating incoming light. In the case of linear polarization, the near field intensity pattern causes the film to laterally expand/contract according to the direction of the polarization. For circular polarization, two pronounced rims corresponding to maxima in the topography are observed. In all cases, the topographical variation is in close agreement with the SP intensity distribution computed from finite difference time domain simulation. Our results demonstrate the versatility of using SP near fields to imprint a variety of structures into photosensitive polymer films using only a single metallic mask.
Nanogradient polymer brushes
(2012)
We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.
In this paper, we report on in-situ atomic force microscopy (AFM) studies of topographical changes in azobenzene-containing photosensitive polymer films that are irradiated with light interference patterns. We have developed an experimental setup consisting of an AFM combined with two-beam interferometry that permits us to switch between different polarization states of the two interfering beams while scanning the illuminated area of the polymer film, acquiring corresponding changes in topography in-situ. This way, we are able to analyze how the change in topography is related to the variation of the electrical field vector within the interference pattern. It is for the first time that with a rather simple experimental approach a rigorous assignment can be achieved. By performing in-situ measurements we found that for a certain polarization combination of two interfering beams [namely for the SP (a dagger center dot, a dagger") polarization pattern] the topography forms surface relief grating with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We believe that this relatively simple method could be extremely valuable to, for instance, produce structural features below the diffraction limit at high-throughput, and this could significantly contribute to the search of new fabrication strategies in electronics and photonics industry.
In this paper we report on an opto-mechanical scission of polymer chains within photosensitive diblock-copolymer brushes grafted to flat solid substrates. We employ surface-initiated polymerization of methylmethacrylate (MMA) and t-butyl methacrylate (tBMA) to grow diblock-copolymer brushes of poly(methylmethacrylate-b-t-butyl methacrylate) following the atom transfer polymerization (ATRP) scheme. After the synthesis, deprotection of the PtBMA block yields poly(methacrylic acid) (PMAA). To render PMMA-b-PMAA copolymers photosensitive, cationic azobenzene containing surfactants are attached to the negatively charged outer PMAA block. During irradiation with an ultraviolet (UV) interference pattern, the extent of photoisomerization of the azobenzene groups varies spatially and results in a topography change of the brush, i.e., formation of surface relief gratings (SRG). The SRG formation is accompanied by local rupturing of the polymer chains in areas from which the polymer material recedes. This opto-mechanically induced scission of the polymer chains takes place at the interfaces of the two blocks and depends strongly on the UV irradiation intensity. Our results indicate that this process may be explained by employing classical continuum fracture mechanics, which might be important for tailoring the phenomenon for applying it to poststructuring of polymer brushes.
We report on conductivity behavior of very thin gold layer deposited on a photosensitive polymer film. Under irradiation with light interference pattern, the azobenzene containing photosensitive polymer film undergoes deformation at which topography follows a distribution of intensity, resulting in the formation of a surface relief grating. This process is accompanied by a change in the shape of the polymer surface from flat to sinusoidal together with a corresponding increase in surface area. The gold layer placed above deforms along with the polymer and ruptures at a strain of 4%. The rupturing is spatially well defined, occurring at the topographic maxima and minima resulting in periodic cracks across the whole irradiated area. We have shown that this periodic micro-rupturing of a thin metal film has no significant impact on the electrical conductivity of the films. We suggest a model to explain this phenomenon and support this by additional experiments where the conductivity is measured in a process when a single nanoscopic scratch is formed with an AFM tip. Our results indicate that in flexible electronic materials consisting of a polymer support and an integrated metal circuit, nano-and micro cracks do not alter significantly the behavior of the conductivity unless the metal is disrupted completely. (C) 2013 AIP Publishing LLC.
We report on a change in the properties of monomolecular films of polyelectrolyte molecules, induced by illuminating the silicon substrate on which they adsorb. It was found that under illumination the thickness of the adsorbed layer decreases by at least 27% and at the same time the roughness is significantly reduced in comparison to a layer adsorbed without irradiation. Furthermore, the homogeneity of the film topography and the surface potential is shown to be improved by illumination. The effect is explained by a change in surface charge density under irradiation of n- and p-type silicon wafers. The altered charge density in turn induces conformational changes of the adsorbing polyelectrolyte molecules. Their photocontrolled adsorption opens new possibilities for selective manipulation of adsorbed films. This possibility is of potential importance for many applications such as the production of well-defined coatings in biosensors or microelectronics.