Vasileios Koutsos

Self-affine surface morphology of plastically deformed metals.

We analyze the surface morphology of metals after plastic deformation over a range of scales from 10 nm to 2 mm using atomic force microscopy and scanning white-light interferometry. We demonstrate that an initially smooth surface during deformation develops self-affine roughness over almost 4 orders of magnitude in scale. The Hurst exponent H of one-dimensional surface profiles initially decreases with increasing strain and then stabilizes at H approximately 0.75. We show that the profiles can be mathematically modeled as graphs of a fractional Brownian motion. Our findings can be understood in terms of a fractal distribution of plastic strain within the deformed samples.

Interface magnetism studied by optical second harmonic generation

Optical second harmonic generation studies of Co/Au interfaces show a clear magnetic field induced effect at these interfaces, leading the way to a nonlinear optical probe that is uniquely sensitive to surface and interface magnetism.

Adsorption and self-assembly of linear polymers on surfaces: a computer simulation study

The adsorption and self-assembly of linear polymers on smooth surfaces are studied using coarse-grained, bead-spring molecular models and Langevin dynamics computer simulations. The aim is to gain insight on atomic-force microscopy images of polymer films on mica surfaces, adsorbed from dilute solution following a good-solvent to bad-solvent quenching procedure. Under certain experimental conditions, a bimodal cluster distribution is observed. It is demonstrated that this type of distribution can be reproduced in the simulations, and rationalized on the basis of the polymer structures prior to the quench, i.e., while in good-solvent conditions. Other types of cluster distribution are described and explained. Measurements of the fraction of monomers bound to the surface, the film height, and the radius of gyration of an adsorbed polymer chain are also presented, and the trends in these properties are rationalized. In addition to providing insight into experimental observations, the simulation results support a number of predicted scaling laws such as the decay of the monomer density as a function of distance from the surface, and scaling of the film height with the strength of the polymer-surface interactions.

Electrical and mechanical properties of carbon nanotube-polyimide composites

The use of carbon nanotubes (CNTs) as a conductive and high strength filler into polymer hosts has attracted great attention recently. However, in order to increase the conductivity of the polymer and to reinforce mechanically the polymer structure at low weight fraction of CNTs, the integration of the CNTs into the polymer matrix has to be effective. In this study, CNT-polyimide (CNT-Pi) composite has been fabricated by solution mixing. The electrical and mechanical properties of the composite have been investigated as a function of CNT’s loading fraction as well as temperature. It has been found that the electrical and mechanical properties of the CNT-Pi can be improved by integrating the CNTs into the polymer. In particular, the presence of an ac electric field is believed to induce dielectrophoresis (DEP), hence the possible CNT’s alignment inside the polyimide matrix during curing. The DEP effect can play an important role in enhancing the properties of the polymer matrix by reducing the percolation ...

Atomic Force Microscopy and Real Atomic Resolution. Simple Computer Simulations

Using a simple computer simulation for AFM imaging in the contact mode, pictures with true and false atomic resolution are demonstrated. The surface probed consists of two f.c.c. (111) planes and an atomic vacancy is introduced in the upper layer. Changing the size of the effective tip and its registry with respect to the atoms of the crystal probed, images with completely different qualitative features are obtained. If the effective tip is a single atom the vacancy is clearly imaged. However, if the tip consists of several atoms and is in registry with the sample, a virtual atom appears instead of the vacancy and the crystal lattice is perfectly reproduced. If the tip is out of registry with respect to the sample, artifacts having the size of the effective tip are reported.

Adsorption of star polymers: computer simulations

The behaviour of star polymers adsorbed on smooth surfaces is studied using coarse-grained bead-spring models and Langevin dynamics simulations. The conformational properties of a single adsorbed star polymer in good-solvent conditions are considered as functions of the functionality (number of arms) f, the number of monomers per arm N, and the monomer-surface interaction energy es. Four conformational regimes are identified: a linear-polymer regime; a two-dimensional star polymer regime; a sombrero regime; and a colloidal regime. The latter three correspond to regimes predicted theoretically by Halperin and Joanny [J. Phys. II (France), 1991, 1, 623–636]. Solvent effects are explored by dialing in effective attractions between the monomer beads; with decreasing solvent quality, the star polymers adopt more compact, globular structures. Good-solvent to bad-solvent quenches at finite surface coverages are considered; these correspond to established experimental protocols for adsorbing and then drying polymer sub-monolayers on surfaces. The structure of the polymer film is surveyed as a function of surface coverage, f, N, and es, in good-solvent and bad-solvent conditions. The simulated post-quench structures are in good qualitative agreement with those observed in atomic-force microscopy measurements, while the simulated pre-quench structures shed light on the microscopic mechanisms of film formation. This study draws together much of what is known about surface-adsorbed star polymers from theory, simulation, and experiment.

Elastic properties of suspended multilayer WSe2

We report the experimental determination of the elastic properties of suspended multilayer WSe2, a promising two-dimensional (2D) semiconducting material combined with high optical quality. The suspended WSe2 membranes have been fabricated by mechanical exfoliation of bulk WSe2 and transfer of the exfoliated multilayer WSe2 flakes onto SiO2/Si substrates pre-patterned with hole arrays. Then, indentation experiments have been performed on these membranes with an atomic force microscope. The results show that the 2D elastic modulus of the multilayer WSe2 membranes increases linearly while the prestress decreases linearly as the number of layers increases. The interlayer interaction in WSe2 has been observed to be strong enough to prevent the interlayer sliding during the indentation experiments. The Youngs modulus of multilayer WSe2 (167.3 ± 6.7 GPa) is statistically independent of the thickness of the membranes, whose value is about two thirds of other most investigated 2D semiconducting transition metal dichalcogenides, namely, MoS2 and WS2. Moreover, the multilayer WSe2 can endure ∼12.4 GPa stress and ∼7.3% strain without fracture or mechanical degradation. The 2D WSe2 can be an attractive semiconducting material for application in flexible optoelectronic devices and nano-electromechanical systems.

Imaging of single polymer chains based on their elasticity

In this work we apply a force modulation technique to a standard atomic force microscope (AFM) in order to study the elasticity of individual polystyrene molecules. The sample mounted on a piezoelectric tube was forced to vibrate along the z direction. The corresponding modulation of the cantilever, which reflects the spring constant of the sample, was phase sensitively detected and measured as a function of the surface topography. The image contrast in these images is based on local variations of the surface elasticity. Compared to the conventional AFM topography image, the elasticity image shows an enhanced contrast with pronounced molecular structure.

High-Density Polymer Microarrays: Identifying Synthetic Polymers that Control Human Embryonic Stem Cell Growth

The fabrication of high-density polymer microarray is described, allowing the simultaneous and efficient evaluation of more than 7000 different polymers in a single-cellular-based screen. These high-density polymer arrays are applied in the search for synthetic substrates for hESCs culture. Up-scaling of the identified hit polymers enables long-term cellular cultivation and promoted successful stem-cell maintenance.

Thin Films of Poly(isoprene-b-ethylene Oxide) Diblock Copolymers on Mica: An Atomic Force Microscopy Study

The structural behavior of three amphiphilic semicrystalline poly(isoprene-b-ethylene oxide) block copolymers (PI-b-PEO) with different PEO volume fraction (f(PEO) = 0.32, 0.49, and 0.66), spin-coated on freshly cleaved mica surfaces from aqueous solutions, was investigated by atomic force microscopy. We focus on the dependence of the resulting thin film nanostructures on the molecular characteristics (f(PEO) and molecular weight) and the adsorbed amount. The nanostructures obtained immediately after spin-coating were robust and remained unchanged after annealing and/or aging. The PEO affinity for the highly hydrophilic mica and the tendency of the hydrophobic and low surface energy PI to dewet and be at the free interface caused the soft PI-b-PEO micelles to collapse leading to the formation of 2D dendritic networks over mica. We show that, for all three polymers, the dendritic monolayer thickness can be predicted by a model consisting of a PEO crystallized layer (directly on top of mica) of the same thickness in all cases and a PI brush layer on top. In thicker areas, polymer material self-assembled into conelike multilamellar bilayers on top of the monolayer and oriented parallel to the substrate for both symmetric and asymmetric diblock copolymers with the lowest f(PEO). We compare the lateral morphology of the films and discuss the thickness heterogeneity, which results from the coupling and competition of crystallization kinetics, phase separation, and wetting/dewetting phenomena highlighting the role of the two blocks to inhibit or enhance certain morphologies. We show that the deviation of the f(PEO) = 0.32 thin film from its bulk phase structure (cylinders in hexagonal lattice) continues for several lamellar bilayers away from the substrate. For the asymmetric PI-b-PEO polymer with the higher PEO volume fraction (f(PEO) = 0.66) and higher APT, laterally extensive stacks of flat-on lamellar crystallites formed on the surface demonstrating the crucial role of the PEO crystallization.