Showing papers by "Rodney S. Ruoff published in 2002"

Mechanics of carbon nanotubes

Abstract: Soon after the discovery of carbon nanotubes, it was realized that the theoretically predicted mechanical properties of these interesting structures–including high strength, high stiffness, low density and structural perfection–could make them ideal for a wealth of technological applications. The experimental verification, and in some cases refutation, of these predictions, along with a number of computer simulation methods applied to their modeling, has led over the past decade to an improved but by no means complete understanding of the mechanics of carbon nanotubes. We review the theoretical predictions and discuss the experimental techniques that are most often used for the challenging tasks of visualizing and manipulating these tiny structures. We also outline the computational approaches that have been taken, including ab initio quantum mechanical simulations, classical molecular dynamics, and continuum models. The development of multiscale and multiphysics models and simulation tools naturally arises as a result of the link between basic scientific research and engineering application; while this issue is still under intensive study, we present here some of the approaches to this topic. Our concentration throughout is on the exploration of mechanical properties such as Young’s modulus, bending stiffness, buckling criteria, and tensile and compressive strengths. Finally, we discuss several examples of exciting applications that take advantage of these properties, including nanoropes, filled nanotubes, nanoelectromechanical systems, nanosensors, and nanotube-reinforced polymers. This review article cites 349 references. @DOI: 10.1115/1.1490129#

Atomistic simulations of nanotube fracture

Abstract: The fracture of carbon nanotubes is studied by molecular mechanics simulations. The fracture behavior is found to be almost independent of the separation energy and to depend primarily on the inflection point in the interatomic potential. The fracture strain of a zigzag nanotube is predicted to be between 10% and 15%, which compares reasonably well with experimental results. The predicted range of fracture stresses is 65--93 GPa and is markedly higher than observed. The computed fracture strengths of chiral and armchair nanotubes are above these values. Various plausible small-scale defects do not suffice to bring the failure stresses into agreement with available experimental results. As in the experiments, the fracture of carbon nanotubes is predicted to be brittle.

Crystalline Boron Nanowires

Abstract: Ideal nanowire interconnects for nanoelectronics will be refractory, covalently bonded, and highly conductive, irrespective of crystallographic orientation. Theoretical studies suggest that boron nanotubes should be stable and exhibit higher electrical conductivities than those of carbon nanotubes. We describe CVD growth of elemental boron nanowires, which are found to be dense nanowhiskers rather than nanotubes. Conductivity measurements establish that they are semiconducting, with electrical properties consistent with those of elemental boron. High conductivities should be achievable through doping.

Realization of parametric resonances in a nanowire mechanical system with nanomanipulation inside a scanning electron microscope

Abstract: We realize parametric resonances in a nanowire mechanical system using an oscillating electric field. Resonances at drive frequencies near $2{f}_{0}/n,$ where ${f}_{0}$ is the nanowire's fundamental resonance frequency, for n from 1 to 4 were observed inside a scanning electron microscope, and analyzed. Such resonances were found to originate from the amplitude-dependent electric field force acting on the nanowire and can be described by the Mathieu equation, which has known regions of instability in the parameter space.

Cross talk between friction and height signals in atomic force microscopy

Abstract: Atomic force and lateral force microscopes use a four quadrant p-i-n detector to measure the motion of a laser beam reflected from the top of a cantilever. If the detector is rotated slightly in the plane of the p-i-n diode, this will cause frictional forces to be detected as a false height signal. In this article, we will show how this coupling between friction and height signals can adversely affect the measurement of topology at height scales below 10 nm. This will be demonstrated with contact mode images of single-walled carbon nanotubes. We will also show how to detect this effect and possible ways to correct for it.

Effects of Defects on the Strength of Nanotubes: Experimental-Computational Comparisons

Abstract: The failure stresses and strains of nanotubes given by theoretical or numerical predictions are much higher than observed in experiments. We show that defects can explain part of this discrepancy: for an n-atom defect with 2<=n<=8, the range of failure stresses for a molecular mechanics calculation is found to be 36GPa to 64GPa. This compares quite well with upper end of the experimental failure stresses, 11GPa to 63GPa. The computed failure strains are 4% to 8%, whereas the experimental values are 2% to 13%. The underprediction of failure strains can be explained by the slippage that occurred in the experiments. The failure processes of nanotubes are clearly brittle in both the experiments and our calculations.

Crystalline Boron Nanowires.

Abstract: Ideal nanowire interconnects for nanoelectronics will be refractory, covalently bonded, and highly conductive, irrespective of crystallographic orientation. Theoretical studies suggest that boron nanotubes should be stable and exhibit higher electrical conductivities than those of carbon nanotubes. We describe CVD growth of elemental boron nanowires, which are found to be dense nanowhiskers rather than nanotubes. Conductivity measurements establish that they are semiconducting, with electrical properties consistent with those of elemental boron. High conductivities should be achievable through doping.

Structure and photoluminescence of helium-intercalated fullerite C60

Abstract: The intercalation of C60 single crystals with helium is studied by powder x-ray diffractometry. It is established that the intercalation is a two-stage process: octahedral cavities are filled first and then tetrahedral ones, the chemical pressure being negative during both stages. The low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C60 are studied for the first time. The presence of helium in lattice voids is shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C60 molecules, the so-called “deep traps” with the 0–0 transition energy close to 1.69 eV. The mechanism of the effect of intercalation with helium on the pair formation in fullerite C60 is discussed.

Length distribution of single walled carbon nanotubes determined by ac atomic force microscopy

Abstract: A simple method to disperse individual single walled carbon nanotubes (SWCNT ) on an atomically flat substrate is presented. Proper tuning of ac modes of atomic force microscopes(AFM) is discussed. This is needed to discriminate between individual nanotubes and very small bundles. The distribution of lengths of the nanotubes measured by these methods is reported.

Integration of Single Multi-Walled Carbon Nanotube on Micro Systems

Abstract: This paper presents new techniques for depositing single multi-walled carbon nanotube (MWCNT) on metal electrodes based on micro-fabrication and an electric-field-guided assembly technique. It was demonstrated that a single MWCNT can be directionally deposited across a microscale gap guided by a biased AC electric field. The biased AC method could effectively avoid the deposition of undesirable particles, and yet maintained highly ordered deposition results. A specific AC frequency component was applied to attract only desired CNTs into a gap and a DC component was used to diminish electric field significantly when the first CNT is deposited. This method can be easily combined with other micromachining process and possibly applied to batch production.Copyright © 2002 by ASME

Nanomanipulation and Characterization of Individual Carbon Nanotubes

Abstract: We describe the implementation of new tools for the measurement of the mechanics of nanostructures. These tools have been used to measured the stiffness, fracture strength, and various tribological properties, of carbon nanotubes.

What kind of carbon nanofiber is ideal for structural applications

Abstract: We investigate with molecular mechanics the nature of load transfer for a number of candidate CNT structures that shows promise in enhancing the weak load transfer mechanisms. We extend these studies to an amorphous carbon nanotube, where there is a mixture of trivalent and tetravalent carbon in the wall of the tube, to address the question of whether an a-C NT could be a high stiffness and strength fiber. It seems more likely that such "nongraphitized" amorphous carbon nanotubes may be synthesized prior to diamond being mass-produced in nanofiber form.

Mechanics of nanowires

Abstract: A homemade nanomanipulation testing stage (shown in Fig. 1), having X, Y, Z and rotational degrees of freedom, was used in these measurements of the resonance response of C NTs and SiO2 NWs. The measurements were made in an SEM chamber (Hitachi S4500 FEG-SEM). The NTs or NWs were attached on a Pt/Ir wire with conductive carbon tape. A tungsten tip was brought into proximity and an ac electric field was applied between the Pt/Ir wire and the tungsten tip. The NT or NW could be driven into resonance by varying the frequency of the electric field. The bending modulus of multiwall C NTs and SiO2 NWs were obtained by fitting the measured resonate frequencies. [1,2] In addition to this method of electromechanical resonance excitation, the resonance could be mechanically excited by a piezoelectric oscillator (without the presence of an applied electric field). Figure 2 shows a SiO2 NW on resonance.