We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.
A "stable" electrohydrodynamic jet is used to print arrays of colloidal suspensions on hydrophobic surfaces. Printed lines break up into sessile drops, and capillary forces guide the self-assembly of colloidal particles during the evaporation of the liquid, resulting in arrays of colloidal single particles or particle clusters depending on the concentration of the suspensions. The clusters differ from those formed in the absence of a substrate when the number of particles is larger than three. Multiple structures are found for the same number of particles.
A polymer compn. comprises a polymer matrix comprising an elastomer, and a functional graphene which displays no signature of graphite and/or graphite oxide, as detd. by X-ray diffraction. The functional graphene-contg. polymer compns. have excellent strength, toughness, thermal stability, elec. cond., and can be used for prodn. of gas-barrier materials. Thus, films (200-400 μm) made of a silicone rubber RTV 615 compn. contg. 5% of functional graphene nanoparticles had a Young's modulus 12.18 and a tensile strength 6.52 times higher than the resp. values for neat rubber. Similar films made of a silicone rubber RTV 615 compn. contg. 5% of clay nanoparticles had a Young's modulus 1.28 and a tensile strength 2.38 times higher than the resp. values for neat rubber. [on SciFinder(R)]
Current density inhomogeneities on electrodes (of physical, chemical, or optical origin) induce long-range electrohydrodynamic fluid motion directed toward the regions of higher current density. Here, we analyze the flow and its implications for the orderly arrangement of colloidal particles as effected by this flow on patterned electrodes. A scaling analysis indicates that the flow velocity is proportional to the product of the applied voltage and the difference in current density between adjacent regions on the electrode. Exact analytical solutions for the streamlines are derived for the case of a spatially periodic perturbation in current density along the electrode. Particularly simple asymptotic expressions are obtained in the limits of thin double layers and either large or small perturbation wavelengths. Calculations of the streamlines are in good agreement with particle velocimetry experiments near a mechanically generated inhomogeneity (a "scratch") that generates a current density larger than that of the unmodified electrode. We demonstrate that proper placement of scratches on an electrode yields desired patterns of colloidal particles.
Theoretical predictions of the nonaxisymmetric instability growth rate of an electrohydrodynamic jet based on the measured total current overestimate experimental values. We show that this apparent discrepancy is the result of gas ionization in the surrounding gas and its effect on the surface charge density of the jet. As a result of gas ionization, a sudden drop in the instability growth rate occurs below a critical electrode separation, yielding highly stable jets that can be used for nano- to microscale printing.
A gas diffusion barrier contains a polymer (e.g., natural rubber) matrix and a functional graphene which displays no signature of graphite and/or graphite oxide, as detd. by X-ray diffraction. [on SciFinder(R)]
Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications(1). Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content(2-11). However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for polyacrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet - poly methyl methacrylate) rivaling those for single-walled carbon nanotube-poly methyl methacrylate) composites.
We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study. the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is similar to 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.
We use electrohydrodynamic jets of colloidal suspensions to produce arrays of colloidal crystalline stripes on surfaces. A critical factor in maintaining a stable jet is the distance of separation between the nozzle and the surface. Colloidal crystalline stripes are produced as two wetting lines of the deployed suspension merge during drying. To ensure that the two wetting lines merge, the "deployed-line-width" to "particle size" ratio is kept below a critical value so that the capillary forces overcome the frictional forces between the particles and the substrate. (C) 2008 American Institute of Physics.
Using atomic force microscopy, we show that previous observations on the orientational order of micelles on atomically smooth crystals with directions dictated by the crystal symmetry is only valid for the case of perfectly smooth crystals. On rough surfaces, orientations are independent of the lattice symmetry and the observed directions can be explained by considering the guiding influence of topographic surface features.
We investigate Raman spectra of graphite oxide and functionalized graphene sheets with epoxy and hydroxyl groups and Stone-Wales and 5-8-5 defects by first-principles calculations to interpret our experimental results. Only the alternating pattern of single-double carbon bonds within the sp(2) carbon ribbons provides a satisfactory explanation for the experimentally observed blue shift of the G band of the Raman spectra relative to graphite. To obtain these single-double bonds, it is necessary to have sp(3) carbons on the edges of a zigzag carbon ribbon.
Using liquid-cell atomic force microscopy, we investigate aqueous solutions of alkyltrimethylammonium halide surfactants at the Au(III) surface. The long, micellar surfactant surface aggregates cover the gold surface completely and exhibit two types of orientational order for chloride and bromide counterions, respectively. We observe lateral forces perpendicular to the scanning direction, which we explain by anisotropic friction between the probe and the oriented micelles. Conversely, we show that these friction forces can be employed to modify the spatial conformation of the micellar adlayer. Where previous methods have failed to provide control over the orientation down to the level of individual micelles, we use this technique to achieve a very high degree of order over more than 100 micelle diameters.