Showing papers by "Rodney S. Ruoff published in 2007"

Preparation and Characterization of Graphene Oxide Paper

Abstract: Free-standing paper-like or foil-like materials are an integral part of our technological society. Their uses include protective layers, chemical filters, components of electrical batteries or supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage. Inorganic 'paper-like' materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high-temperature binders, dielectric barriers and gas-impermeable membranes. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite have long been used in packing and gasketing applications because of their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications. Here we report the preparation and characterization of graphene oxide paper, a free-standing carbon-based membrane material made by flow-directed assembly of individual graphene oxide sheets. This new material outperforms many other paper-like materials in stiffness and strength. Its combination of macroscopic flexibility and stiffness is a result of a unique interlocking-tile arrangement of the nanoscale graphene oxide sheets.

Graphene-silica composite thin films as transparent conductors.

Abstract: Transparent and electrically conductive composite silica films were fabricated on glass and hydrophilic SiOx/silicon substrates by incorporation of individual graphene oxide sheets into silica sols followed by spin-coating, chemical reduction, and thermal curing. The resulting films were characterized by SEM, AFM, TEM, low-angle X-ray reflectivity, XPS, UV−vis spectroscopy, and electrical conductivity measurements. The electrical conductivity of the films compared favorably to those of composite thin films of carbon nanotubes in silica.

Simple approach for high-contrast optical imaging and characterization of graphene-based sheets

Abstract: A simple optical method is presented for identifying and measuring the effective optical properties of nanometer-thick, graphene-based materials, based on the use of substrates consisting of a thin dielectric layer on silicon. High contrast between the graphene-based materials and the substrate is obtained by choosing appropriate optical properties and thickness of the dielectric layer. The effective refractive index and optical absorption coefficient of graphene oxide, thermally reduced graphene oxide, and graphene are obtained by comparing the predicted and measured contrasts.

Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets

Abstract: A simple optical method is presented for identifying and measuring the effective optical properties of nanometer-thick, graphene-based materials, based on the use of substrates consisting of a thin dielectric layer on silicon. High contrast between the graphene-based materials and the substrate is obtained by choosing appropriate optical properties and thickness of the dielectric layer. The effective refractive index and optical absorption coefficient of graphene oxide, thermally reduced graphene oxide, and graphene are obtained by comparing the predicted and measured contrasts.

An ultra-thin PDMS membrane as a bio/micro-nano interface: fabrication and characterization

Abstract: We report a method for making ultra-thin PDMS membrane devices. Freely suspended membranes as thin as 70 nm have been fabricated. Bulging tests were performed with a custom built fluidic cell to characterize large circular membranes. The fluidic cell allows the media (such as air or water) to wet one side of the membrane while maintaining the other side dry. Pressure was applied to the membrane via a liquid manometer through the fluidic cell. The resulting load-deflection curves show membranes that are extremely flexible, and they can be reproducibly loaded and unloaded. Such devices may potentially be used as mechanical and chemical sensors, and as a bio-nano/micro interface to study cellular mechanics in both static and dynamic environments.

Modulus, Fracture Strength, and Brittle vs. Plastic Response of the Outer Shell of Arc-grown Multi-walled Carbon Nanotubes

Abstract: The fracture strengths and elastic moduli of arc-grown multi-walled carbon nanotubes (MWCNTs) were measured by tensile loading inside of a scanning electron microscope (SEM). Eighteen tensile tests were performed on 14 MWCNTs with three of them being tested multiple times (3×, 2×, and 2×, respectively). All the MWCNTs fractured in the “sword-in-sheath” mode. The diameters of the MWCNTs were measured in a transmission electron microscope (TEM), and the outer diameter with an assumed 0.34 nm shell thickness was used to convert measured load-displacement data to stress and strain values. An unusual yielding before fracture was observed in two tensile loading experiments. The 18 outer shell fracture strength values ranged from 10 to 66 GPa, and the 18 Young's modulus values, obtained from a linear fit of the stress–strain data, ranged from 620 to 1,200 GPa, with a mean of 940 GPa. The possible influence of stress concentration at the clamps is discussed.

Bond-detach lithography: a method for micro/nanolithography by precision PDMS patterning.

Abstract: We have discovered a micro/nanopatterning technique based on the patterning of a PDMS membrane/film, which involves bonding a PDMS structure/stamp (that has the desired patterns) to a PDMS film. The technique, which we call "bond-detach lithography", was demonstrated (in conjunction with other microfabrication techniques) by transferring several micro- and nanoscale patterns onto a variety of substrates. Bond-detach lithography is a parallel process technique in which a master mold can be used many times, and is particularly simple and inexpensive.

Correction factors for 4-probe electrical measurements with finite size electrodes and material anisotropy: a finite element study

Abstract: In four-probe (4-probe) electrical measurements, especially on highly resistive materials, it is not always possible to configure the electrodes such that the current density is uniform throughout the sample. Under such circumstances, simply considering the material's electrical resistivity to be proportional to the measured resistance with the proportionality constant given by the sample geometry can give an incorrect result. In this paper, a numerical finite element model is presented which can extract a material's true resistivity from co-linear 4-probe electrical measurements on highly resistive samples with large electrodes that extend across the sample width. The finite element model is used to investigate the influence of material anisotropy, the resistance of the sample–electrode interfaces and the relative electrode-to-sample size on the potential and current density distributions in the sample. A correction factor is introduced to account for the impact of these effects on the measured resistivity. In the limit of large interface resistance, excellent agreement is found with an analytical expression derived elsewhere (Esposito et al 2000 J. Appl. Phys. 88 2724–9). The approach presented here can be used to evaluate a variety of effects on co-linear 4-probe electrical measurements, can be extended to complex specimen geometries with arbitrary electrode arrangements and, additionally, could find use in the evaluation of data from 4-probe thermal conductivity measurements.

Electrostatic-Force-Directed Assembly of Ag Nanocrystals onto Vertically Aligned Carbon Nanotubes†

Abstract: A facile dry route electrostatic-force-directed assembly (ESFDA) was used to deposit silver nanocrystals onto the external surface of vertically aligned multiwalled carbon nanotubes (MWCNTs). Charged and nonagglomerated aerosol silver nanocrystals were first produced using a mini-arc plasma source. The nanocrystals were then assembled onto the electrically biased MWCNT array through the enhanced electric field near the CNT surface. The electrostatic field plays a crucial role in the assembly process. Gradients of the nanocrystal size and the nanocrystal areal density along the length of the CNT have been observed, and these are likely due to the variation of the electric field near the CNT surface. The ESFDA technique enables the in situ coating of CNT arrays with nanocrystals to create novel hybrid nanomaterials with tailored properties.

Ripening of silver nanoparticles on carbon nanotubes

Abstract: An electrostatic-force-directed-assembly technique was used to coat multiwalled carbon nano-tubes (MWCNTs) with aerosol Ag nanoparticles produced from a mini-arc plasma source. The deposition of Ag nanoparticles onto CNTs was confirmed by transmission electron microscopy (TEM), high-resolution TEM, scanning electron microscopy, and X-ray photoelectron spectroscopy. Ripening of Ag nanoparticles on CNTs was observed via successive TEM imaging after heating the nanoparticle–nanotube hybrid structures in air to three different temperatures ranging from 100°C to 300°C. With temperatures at and above 200°C, the areal density of Ag nanoparticles decreased and the average particle size increased. In particular, migration and coalescence of Ag nanoparticles have been observed at this relatively low temperature, which suggests a van der Waals nanoparticle–nanotube interaction.

Optimized adhesives for strong, lightweight, damage-resistant, nanocomposite materials: new insights from natural materials

Abstract: From our investigations of natural composite materials such as abalone shell and bone we have learned the following. (1) Nature is frugal with resources: it uses just a few per cent glue, by weight, to glue together composite materials. (2) Nature does not avoid voids. (3) Nature makes optimized glues with sacrificial bonds and hidden length. We discuss how optimized adhesives combined with high specific stiffness/strength structures such as carbon nanotubes or graphene sheets could yield remarkably strong, lightweight, and damage-resistant materials.

Effect of cantilever nonlinearity in nanoscale tensile testing

Abstract: Microcantilevers are widely used in micro-/nanoscale mechanics studies. The nonlinear response of a cantilever at large deflection is sometimes overlooked. A general study of cantilever beam nonlinearity under a variety of loading conditions was performed with analytical and finite element analyses. Analytical equations for the applied load and the cantilever deflection were obtained. The cantilever nonlinearity was found to increase with increasing cantilever deflection and/or angle of loading. Tensile tests were performed on templated carbon nanotubes (TCNTs) with a custom-made nanomanipulator inside a scanning electron microscope. Atomic force microscope (AFM) cantilevers were used to load the TCNTs and sense the force. During the tests the AFM cantilevers were loaded to relatively large deflections with nonvertical loads applied at the AFM tip. Based on the slope and the loading angle measurements, the breaking forces of the TCNTs were obtained through numerical integration of the analytical equations...

Nanoscale Weibull Statistics for Nanofibers and Nanotubes

Abstract: In this paper a modification of the classical Weibull statistics is applied to nanostructures. A comparison is presented of “nanoscale” versus classical Weibull statistics in treating recent experimental results on the fracture strength of C nanofibers and nanotubes, and WS2 nanotubes. “Nanoscale” Weibull moduli of 3.8 for electrospun and then heat-treated carbon nanofibers, 2.7 for arc-discharge synthesized multiwalled carbon nanotubes, 1.8 for chemical vapor deposited multiwalled carbon nanotubes, and 3.0 for multiwalled WS2 nanotubes, are deduced.

Simple and catalyst-free synthesis of silicon oxide nanowires and nanocoils

Abstract: In this paper, a simple method for synthesizing SiOx nanowires and nanocoils is presented. Si substrates with an oxide layer were placed in a tube furnace exposed to temperatures ranging from 900°C to 1200°C for a few hours under a mixture of flowing Ar and H2 gas maintained at ambient pressure. Nanowires were grown from the surface when the furnace temperature was above 1000°C and a high yield could be achieved at 1100°C. SiOx nanocoils have also been observed and the sample treated at 1000°C had the highest concentration of them. TEM images show that the nanowires and the nanocoils have an amorphous structure and analysis of EDX spectra (obtained in the TEM) shows that x varies from 1.2 to 2.0. The mechanism of growth is discussed.