Showing papers by "Rodney S. Ruoff published in 2006"

Graphene-based composite materials

Abstract: The remarkable mechanical properties of carbon nanotubes arise from the exceptional strength and stiffness of the atomically thin carbon sheets (graphene) from which they are formed. In contrast, bulk graphite, a polycrystalline material, has low fracture strength and tends to suffer failure either by delamination of graphene sheets or at grain boundaries between the crystals. Now Stankovich et al. have produced an inexpensive polymer-matrix composite by separating graphene sheets from graphite and chemically tuning them. The material contains dispersed graphene sheets and offers access to a broad range of useful thermal, electrical and mechanical properties. Individual sheets of graphene can be readily incorporated into a polymer matrix, giving rise to composite materials having potentially useful electronic properties. Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects1,2,3; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties4,5,6,7,8. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite9 and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold10 of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes11; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications12. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate)

Abstract: For the first time, stable aqueous dispersions of polymer-coated graphitic nanoplatelets can be prepared via an exfoliation/in-situ reduction of graphite oxide in the presence of poly(sodium 4-styrenesulfonate).

Dielectrophoretic assembly of nanowires

Abstract: Nanowire (NW) assembly is currently of great interest, partly because NWs are considered as a fundamental component in the fabrication of a variety of devices. A powerful method has been developed to model the assembly of NWs. The three-dimensional dielectrophoretic (DEP) assembly of NWs across opposing electrodes is, for the first time, comprehensively studied using this new method. It is found that the DEP force reaches a maximum when the ratio of gap size to NW length is in the range 0.85-1.0. Both the magnitude and sign of the DEP torque on each NW varies with this ratio, and also with the orientation angle and the geometry and configuration of the electrode. The simulation of the dynamic assembly of individual and bundled NWs agrees with experiment. This method is of sufficient power that it will be of direct use in modeling DEP-based assembly and thus the manufacturing of nanoelectronic devices.

Stable dispersions of polymer-coated graphitic nanoplatelets

Abstract: A method of making a dispersion of reduced graphite oxide nanoplatelets involves providing a dispersion of graphite oxide nanoplatelets and reducing the graphite oxide nanoplatelets in the dispersion in the presence of a reducing agent and a polymer. The reduced graphite oxide nanoplatelets are reduced to an extent to provide a higher C/O ratio than graphite oxide. A stable dispersion having polymer-treated reduced graphite oxide nanoplatelets dispersed in a dispersing medium, such as water or organic liquid is provided. The polymer-treated, reduced graphite oxide nanoplatelets can be distributed in a polymer matrix to provide a composite material.

Nanoscale Weibull statistics

Abstract: In this paper a modification of the classical Weibull statistics is developed for nanoscale applications. It is called nanoscale Weibull statistics. A comparison between nanoscale and classical Weibull statistics applied to experimental results on fracture strength of carbon nanotubes clearly shows the effectiveness of the proposed modification. A Weibull’s modulus of ∼3 is deduced for nanotubes. The approach can treat (also) a small number of structural defects, as required for nearly defect-free structures (e.g., nanotubes) as well as a quantized crack propagation (e.g., as a consequence of the discrete nature of matter), allowing to remove the paradoxes caused by the presence of stress intensifications.

Mechanical properties of nanoparticle chain aggregates by combined AFM and SEM: isolated aggregates and networks.

Abstract: Mechanical properties of nanoparticle chain aggregates (NCA) including tensile strength and Young's modulus were measured using an instrument incorporating an AFM tip under SEM imaging. The NCA were studied individually and as network films. Carbon NCA were made by laser ablation of graphite, and SnO2 NCA were made by oxidation of a tin compound. The films were deformable and showed elastic behavior. NCA serve as reinforcing fillers in rubber and films of SnO2 NCA for trace gas detection.

In situ mechanical testing of templated carbon nanotubes

Abstract: A new microelectromechanical system (MEMS)-based tensile testing stage (with integrated actuator, direct load sensing beam, and electrodes for controlled assembly of an individual nanostructure) was developed and used for in situ tensile loading of a templated carbon nanotube (T-CNT) inside a scanning electron microscope (SEM). Specifically, an increasing tensile load was applied to the T-CNT by actuating the device and high-resolution scanning electron microscopy images were acquired at different loads. The load (from the bending of the direct force-sensing beam), the elongation of the specimen during loading, and the specimen geometry were all obtained from analysis of SEM images. The stress versus strain curve and Young’s modulus were thus obtained. A model is presented for the tensile loading experiment, and the fit value of Young’s modulus from this model is compared to values obtained by an independent method. The results of this experiment on a T-CNT suggest the use of this device for loading other...

Carbide-Derived Nanoporous Carbon and Novel Core−Shell Nanowires

Abstract: Carbide-derived carbon (CDC) nanowires (NWs) have been synthesized by the high-temperature treatment of small-diameter β-SiC whiskers with Cl2/H2 A variety of physical measurements indicate that Si was extracted by exposure to Cl2 and that the C in the carbon nanowires is primarily sp2-bonded From BET measurements, the specific surface area of these carbon nanowires is 13 × 103 m2/g and they contain a network of nanopores Nanoindentation measurements indicate that the SiC-derived C is not a stiff material, the elastic modulus being 50 ± 12 GPa High-temperature treatment of the CDC nanowires under an inert gas significantly increases the degree of graphitization In addition, partial extraction was used to obtain core−shell structures having a thin and also very high surface area CDC shell; further treatment at high temperature was used to produce graphitized carbon shell−crystalline SiC core NWs

Analysis of a microelectromechanical system testing stage for tensile loading of nanostructures

Abstract: A new analytical model is developed for interpreting tensile loading data on “templated carbon nanotubes” (T-CNTs, amorphous carbon nanotubes made by pyrolysis with the channels of nanopores in anodized alumina nanopore arrays) obtained with a microelectromechanical-system (MEMS)-based mechanical testing stage. It is found that the force output from the actuation unit of the testing stage depends on the stiffness of the force sensing beam and the nanostructure being loaded, as well as the power input. A superposition method is used to treat the mechanics of the device structure in the linear elasticity response regime. To our knowledge this is a new approach for solving the mechanical response of MEMS structures with variable force output and of the configuration described herein. An in situ mechanical testing of individual T-CNTs was undertaken in a scanning electron microscope (LEO1525) using a new device fabricated with integrated electrodes for controlled deposition of T-CNTs by electric-field guided ...

Time, temperature, and load: the flaws of carbon nanotubes.

Abstract: The “mechanics of nanostructures” is of intrinsic and practical interest. An acorn turning into an oak tree can lead one to consider the (often unknown) mechanical forces exerted by, and acting on, nanostructures present in the tree. A mantra of nanotechnology [which may ultimately outpace (1) “natural” evolution] is having “a place for every atom and every atom in its place” (www.foresight.org/nano/whatismm.html). What level of perfection might be achieved considering the known laws of physics and the constraints of chemistry? In principle, there is no limitation to achieving essentially perfect covalent bonding in material structures. With increasing atom number, a size is eventually reached where the defect-free structure is not the most stable (consider the role of entropy) (2), but it may be kinetically stable if there are high barriers to the nucleation of defects. In a recent issue of PNAS, Dumitrica et al. (3) consider carbon nanotubes (CNTs) and, building on prior theoretical work by themselves and others, present the pathways to failure caused by tensile load as a function of time and temperature. Because CNTs can have different chiralities, the issue of the orientation of the C—C bonds in the different CNTs is treated and shown to critically influence the ultimate strength, the type of defects that nucleate and how they grow or propagate, and the modeled time to failure (3). The possibility of having structures entirely free of defects would seem more likely for small structures than large structures, and living organisms routinely achieve such perfection. The remarkable mechanics of biological motors (4, 5) and viral DNA packaging and ejection … *E-mail: r-ruoff{at}northwestern.edu

Boron nanowires and novel tube-catalytic particle-wire hybrid boron nanostructures

Abstract: Catalyst-assisted growth of boron nanowires and novel tube–catalytic particle–wire hybrid boron nanostructures were achieved by pyrolysis of diborane at 820–890°C and ~ 200 mTorr in a quartz tube furnace. Electron microscopy imaging and diffraction analysis reveal that most of the nano-structures are amorphous. Elemental analysis by EELS and EDX shows that the nanostructures consist of boron with a small amount of oxygen and carbon. Possible growth mechanisms for the tube–catalytic particle–wire hybrid boron nanostructures are discussed.

Resonance of curved nanowires

Abstract: The effects of non-ideal experimental configuration on the mechanical resonance of boron (B) nanowires (NWs) were studied to obtain the corrected value for the Young's modulus. The following effects have been theoretically considered: (i) the presence of intrinsic curvature, (ii) non-ideal clamps, (iii) spurious masses, (iv) coating layer, and (v) large displacements. An energy-based analytical analysis was developed to treat such effects and their interactions. Here, we focus on treating the effect of the intrinsic curvature on the mechanical resonance. The analytical approach has been confirmed by numerical FEM analysis. A parallax method was used to obtain the three-dimensional geometry of the NW.

Fabrication of nanopores in a 100-nm thick Si3N4 membrane.

Abstract: Textured alumina films have been used to fabricate nanoscale pores in Si3N4 membranes. A few nanometer-thick alumina layer was used as a masking material for nanopore fabrication, and the pattern was transferred into a 100-nm thick, 200 microm x 200 microm Si3N4 membrane by reactive ion etching (RIE). The nanopores were found to be concentrated in a approximately 150-microm diameter region at the center of the membrane.

Mechanics of Nanostructures

Abstract: Quantized fracture mechanics is applied for studying the mechanics of nanostructures. An application for predicting the strength of defective nanotubes, compared with atomistic simulations and experiments, clearly shows that materials are sensitive to flaws (also) at nanoscale.

Scanning Electron Microscopy Methods for Analysis of Polymer Nanocomposites

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Modeling the electrode-electrolyte interface for recording and stimulating electrodes.

Abstract: The design of metal microelectrodes that produce minimal damage to tissue and can successfully record from and stimulate targeted neural structures necessitates a thorough understanding of the electrical phenomena generated in the tissue surrounding the electrodes. Computational modeling has been a primary strategy used to study these phenomena, and the Finite Element Method has proven to be a powerful approach. Much research has been directed toward the development of models for electrode recording and stimulation, but very few models reported in the literature thus far incorporate the effects of the electrode-electrolyte interface, which can be a source of very high impedance, and thus likely a key component of the system. To explore the effects that the electrode-electrolyte interface has upon the electric potential and current density surrounding metal microelectrodes, simulations of electrode- saline systems in which the electrodes were driven at AC potentials ranging from 10 mV to 500 mV and frequencies of 100 Hz to 10 kHz have been performed using the Finite Element Method. Solutions obtained using the thin layer approximation for the electrode-electrolyte interface were compared with those generated using a thin uniform layer, a representation that has previously appeared in the literature. Solutions using these two methods were similar in the linear regime of the interface, however, the thin layer approximation has important advantages over its competitor including ease of application and low computational cost. 1

Transmission Electron Microscopy of Polymer-Graphene Nanocomposites

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Nanocrack detection may be possible in vibrating nanowires

Abstract: Resonance and tensile tests of boron nanowires are reported and discussed assuming the presence of nanocracks. From the measured resonant frequency shift a procedure to localize the nanocrack and quantify its depth is proposed. The nanocrack depth is expected to strongly influence the failure stress and its position can be assumed to be at the fractured cross-section: thus nanocrack depth and position might be independently deduced by resonance and tensile tests. Introduction In recent years nanostructures such as nanotubes and nanowires have attracted great attention due to the promise of applications in sensing, materials reinforcement and micro/nano-electromechanical systems. Both the mechanical resonance method and the tensile testing method are two methods used to study the mechanical properties of nanostructures. The mechanical resonance method has been used to study the mechanical properties of onedimensional nanostructures such as carbon nanotubes, nanowires and nanobelts [1-4]. The uniaxial tensile test is the most popular method for bulk material mechanical property characterization, and it has been adapted for use in mechanics measurements of nanostructures such as carbon nanotubes [5]. Crystalline boron (B) nanowires (NWs) have been synthesized recently with the chemical vapor deposition (CVD) method [6]. We have experimentally investigated their dynamical resonance (i) and mechanical strength (ii) [7]. Both of these independent methods suggest the possible presence of nanocracks in the tested B NWs. Nanocrack detection can in principle be achieved by analytical calculations quantifying crack position and depth. The mechanical resonances of cantilevered B NWs were excited and the resonance peak frequencies measured. Shifts in the natural frequencies were observed, suggesting the possibility of the presence of nanocracks; other possible shift causes, such as intrinsic NW curvature, non-ideal clamps, presence of spurious masses, coating layer, large displacements, etc., have been discussed elsewhere [8]. Assuming the existence of a nanocrack, analytical calculations were obtained to quantify its depth and position based on the measured frequency shifts. A newly developed rapid electron beam induced deposition (EBID) method [9] was used to clamp the B NWs and test them in tension inside an SEM with a home-built nanomanipulator. High-resolution SEM images were acquired at each loading step, and two independent methods of analysis of each image were used to obtain the corresponding tensile load. The B NW geometries were measured by TEM after the tests. The stress vs strain, Young’s modulus, and tensile strength of the B NWs were obtained through data analysis. The strength measurements strongly suggest the presence of nanocracks in the B NWs. Assuming the existence of a nanocrack, placed at the fracture section, its depth can be quantified by applying Quantized Fracture Mechanics [10] starting from the measured fracture strengths. The possibility of detecting nanocracks through mechanical resonance and tensile testing of nanostructures is thus exemplified, even though it is not possible to know whether nanocracks alone were responsible for the frequency shifts [8]. The challenge on the experimental side in the future will be ensuring that other contributors to shifts in mechanical resonance are not present or have sufficiently different functional dependence, such that the nanocrack contribution, if present, can perhaps be “uncovered”. Frequency shift and strength reduction due to the presence of a nanocrack Mechanical resonance can be induced in nanowires when the frequency of the applied force approaches their resonant frequency. According to simple beam theory, the n mode mechanical resonance frequency fn of a clampedfree uniform beam is given by: A I E L f b n n ρ π β 2 2

Fracture mechanics of one-dimensional nanostructures

Abstract: One-dimensional (1D) nanostructures such as nanotubes and nanowires have attracted considerable attention in recent years due to their promise of applications in sensing and materials reinforcement. Over the past decade various 1D nanostructures has been synthesized. To develop applications with these nanostructures, it is important to first understand their fundamental properties. Our work focused on characterizing the mechanical properties of these novel 1D nanostructures.

Nanocrack Detection in Vibrating Nanowires

Abstract: Crystalline boron (B) nanowires (NWs) have been synthesized by the CVD method with preformed metal catalyst particles. We have experimentally investigated their dynamical resonance (i) and mechanical strength (ii). Both the two independent methods suggest the possible presence of nanocracks in the tested B NWs. Nanocrack detection can in principle be achieved by analytical calculations quantifying crack position and depth.