Showing papers by "Pulickel M. Ajayan published in 2002"
TL;DR: Observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes.
Abstract: Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions Stable junctions of various geometries are created in situ in a transmission electron microscope Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes
647 citations
TL;DR: A chemical-vapour deposition method with gas-phase catalyst delivery is used to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements.
Abstract: Cunning refinements help to customize the architecture of nanotube structures. Nanoscale structures need to be arranged into well-defined configurations in order to build integrated systems. Here we use a chemical-vapour deposition method with gas-phase catalyst delivery to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements. The preference of nanotubes to grow selectively on and normal to silica surfaces forces them to inherit the lithographically machined template topography of their substrates, allowing the sites of nucleation and the direction of growth to be controlled.
471 citations
TL;DR: Evidence is provided that electrical stimulation delivered through novel, current-conducting polymer/nanophase composites promotes osteoblast functions that are responsible for the chemical composition of the organic and inorganic phases of bone.
Abstract: The present study demonstrates that novel nanocomposites consisting of blends of polylactic acid and carbon nanotubes effectively can be used to expose cells to electrical stimulation. When osteoblasts cultured on the surfaces of these nanocomposites were exposed to electric stimulation (10 μA at 10 Hz) for 6 h/day for various periods of time, there was a 46% increase in cell proliferation after 2 days, a 307% increase in the concentration of extracellular calcium after 21 consecutive days, and upregulation of mRNA expression for collagen type-I after both 1 and 21 consecutive days. These results provide evidence that electrical stimulation delivered through novel, current-conducting polymer/nanophase composites promotes osteoblast functions that are responsible for the chemical composition of the organic and inorganic phases of bone. Furthermore, this evidence elucidates aspects of the cellular/molecular-level mechanisms involved in new bone formation under electrical stimulation. © 2001 Wiley Periodicals, Inc. J Biomed Mater Res 59: 499–506, 2002
360 citations
TL;DR: It is accidentally discovered that SWNTs ignite when exposed to a conventional photographic flash, another unusual property of single-walled carbon nanotubes.
Abstract: Single-walled carbon nanotubes (SWNTs) exhibit a range of unusual mechanical and electronic properties because of their unique structure and dimensions. Here we report another unusual property. We accidentally discovered that SWNTs ignite when exposed to a conventional photographic flash. This
304 citations
TL;DR: In this paper, the authors studied the adsorption of H2O molecules on a single-wall carbon nanotube (NT) using first-principles gradient-corrected density-functional theory.
Abstract: We have studied the adsorption of H2O molecules on a single-wall carbon nanotube (NT) using first-principles gradient-corrected density-functional theory Subsequently, Green’s function-based Landauer–Buttiker multichannel formalism within a tight-binding model is used to calculate the electron transport, and our results suggest that H2O molecules adsorbed on the NT surface reduce the electronic conduction in the tube in agreement with recent experimental measurements The decrease of conductance with water adsorption is explained on the basis of charge transfer between the adsorbate and the NT
149 citations
143 citations
TL;DR: In this article, the local interfacial electronics between multiwalled carbon nanotubes and several supporting substrates has been investigated by utilizing the current transients in scanning tunneling spectroscopy, and a systematic variation in this local potential, as a function of tube diameter, is observed for both metallic and semimetallic substrates.
Abstract: By utilizing the current transients in scanning tunneling spectroscopy, the local interfacial electronics between multiwalled carbon nanotubes and several supporting substrates has been investigated. Voltage offsets in the tunneling spectra are directly correlated with the formation of a dipole layer at the nanotube-substrate interface, strongly suggesting the formation of interface states. Further, a systematic variation in this local potential, as a function of tube diameter, is observed for both metallic substrates (Au) and semimetallic substrates (graphite). In both cases, for tubes with diameters between \ensuremath{\sim}5 nm and 30 nm, the interfacial potential is nearly constant as a function of tube diameter. However, for tube diameters 5 nm, a dramatic change in the local potential is observed. Using ab initio techniques, this diameter-dependent electronic interaction is shown to derive from changes in the tube-substrate hybridization that results from the curvature of the nanotubes.
135 citations
TL;DR: In this article, the Stokes and anti-Stokes Raman spectra of a multi-walled carbon nanotube (MWNT) sample are studied by four excitation energies and the observed Raman modes are assigned based on the double resonance Raman effect and the previous results in graphite whiskers.
Abstract: The Stokes and anti-Stokes Raman spectra of a multiwalled carbon nanotube (MWNT) sample are studied here by four excitation energies and the observed Raman modes are assigned based on the double resonance Raman effect andthe previous results in graphite whiskers. There exists frequency discrepancy between Stokes and anti-Stokes lines (FDSA) of many Raman modes in MWNT's and the discrepancy values are strongly dependent on the excitation energy, in which the FDSA value of the D' mode even changes from a positive value (9 cm - 1 , 1.58 eV) to a negative value (-11 cm - 1 , 2.54 eV). The laser-energy dependence of the FDSA values of some modes in MWNT's is attributed to the nonlinear frequency dependence of Stokes and anti-Stokes Raman lines of these modes on the excitation energy. Raman results and the theoretical analysis of the intravalley and intervalley double resonance processes of Stokes and anti-Stokes Raman scatterings both show that the frequency of an anti-Stokes peak excited by e L is equal to that of the corresponding Stokes peak excited by a laser excitation of e L +∞ω S where ∞ω S is the phonon energy of the Raman mode. Stokes and anti-Stokes double-resonance Raman scatterings have been used to probe the phonon dispersion relations of graphite. The Raman data of the well-known disorder-induced D mode are in good agreement with the theoretical results.
111 citations
TL;DR: The diameter of the single-walled carbon nanotubes varies from 1.1 to 1.7 nm, with domination of the 1.5 nm tubes and corresponding lattice constant of 1.42 nm as mentioned in this paper.
Abstract: Centimeter long single-walled carbon nanotube strands synthesized using chemical vapor deposition method were characterized using microscopy, micro-Raman spectroscopy, and small-angle X-ray diffraction techniques. The strands are large collections of well-aligned nanotube bundles, which consist of well-arranged single-walled nanotubes in a two-dimensional triangular lattice. The diameter of the single-walled nanotubes varies from 1.1 to 1.7 nm, with domination of the 1.1 nm tubes and the corresponding lattice constant of 1.42 nm.
92 citations
TL;DR: Density-functional calculations of charge distribution on negatively and positively charged nanotubes result in charge density profiles characterized by a significant increase of charge density at the tube ends, in quantitative agreement with classical electrostatic analysis.
Abstract: Density-functional calculations of charge distribution on negatively and positively charged nanotubes result in charge density profiles characterized by a significant increase of charge density at the tube ends. These results are in quantitative agreement with classical electrostatic analysis, which assumes constant electrostatic potential on the conductive tube surface. At high charging levels, the tube ends are observed to be unstable due to Coulomb repulsion. By combining ab initio calculations with classical electrostatics, we determine, as a function of tube length and geometry of the tube end, the critical voltage beyond which nanotubes are unstable.
77 citations
Patent•
12 Feb 2002
TL;DR: An ultrafast all-optical nonlinear switch has been proposed in this article with a switching speed of less than 1 picosecond for light with a wavelength of about 1.55 micrometers.
Abstract: An ultrafast all-optical nonlinear switch. The switch has as components a substrate and a material disposed on the substrate. In one embodiment, the material includes a plurality of single-walled carbon nanotubes and a polymer forming a composite. Preferably, the polymer is polyimide. In another embodiment, the material includes a plurality of single-walled carbon nanotubes incorporated into a silica. The nanotube loading in the material is less than about 0.1 wt %. The material is a substantially transparent, third-order nonlinear optical material. The switch has a switching speed of less than 1 picosecond for light with a wavelength of about 1.55 micrometers. Also disclosed is a process for preparing the ultrafast all-optical nonlinear switch.
TL;DR: In this paper, the icosahedral multiply twinned particles of boron carbide were discovered among soot produced during the arc evaporation of carbon and Boron in the presence of magnesium.
Abstract: We have discovered massive icosahedral multiply twinned particles of boron carbide among soot produced during the arc evaporation of carbon and boron in the presence of magnesium. The observation is striking because 5-fold symmetry in large micron size crystals is extremely rare. It is suggested that the icosahedral structural unit that builds up the rhombohedral crystal structure of bulk B4C provides a template for the formation of these large icosahedral crystals. The physical properties of boron carbide, known for its hardness, could be further improved in these crystals because of their unusually high symmetry.
TL;DR: In this article, a new method using palladium (Pd) seeds to ensure the needed selectivity is presented for the tailored growth of individual nanotubes and nanotube networks.
TL;DR: In this article, a coupling reaction between nitrosobenzene and aniline to produce azobenzene was shown to be catalyzed by functionalized multiwalled carbon nanotubes (CNTs).
Abstract: Aniline is oxidized to nitrosobenzene as the initial product, which undergoes further oxidation to nitrobenzene. The nitrosobenzene formation is catalyzed by functionalized multiwalled carbon nanotubes (CNT) followed by a coupling reaction between nitrosobenzene and aniline to produce azobenzene. This coupling requires close proximity of the reactants. It proceeds rapidly resulting in the UV-VIS absorption spectrum showing maxima at 327 nm and 425 nm. The nitrosobenzene yield in the presence of CNTs is controlled by the amount present in the medium. As the reaction is not catalyzed by unfunctionalized CNTs or graphitic particles, the uniqueness of the functionalized multiwalled CNTs in this catalysis suggests a nanodimensional reaction pathway.
TL;DR: In this article, the reduction temperature of Fe 2 O 3 is determined by the desorption temperatures of SiO or GeO molecules, and the resulting nanoparticles are magnetic and of uniform size and shape.
TL;DR: Carbon nanotube self-assembly into honeycomb-networks via controlling the ratio of the catalyst over hydrocarbon in the vapor phase using a tunable chemical vapor deposition process.
TL;DR: In this article, a simple procedure to synthesize carbon-reinforced silica fibres on planar silicon substrates covered with a thin silica cap layer was reported.
Abstract: In this letter, we report a simple procedure to synthesize carbon-reinforced silica fibres on planar silicon substrates covered with a thin silica cap layer The procedure contains two steps, which are first activating the substrate with ferrocene and then exposing it to methane at ~ 1100 °C, which transforms the silica cap layer into nanowires of ~ 200 nm in diameter The silica nanowires are reinforced by glassy carbon in the core, and have an optical band gap of ~ 31 eV During growth the nanowires are grown randomly inside circular areas surrounded by circular walls of large fibres, thus forming wreath-like patterns randomly distributed on the substrate surface The growth of the nanowires and the formation mechanism of the wreath-like patterns are also discussed
01 Jan 2002
TL;DR: In this article, a photothermoelectric technique is employed to measure the anisotropic thermal diffusivity of an aligned multi-walled carbon nanotube array, where a modulated laser beam incident to the front surface of the sample creates a thermal wave which is detected by a fast responding thermocouple formed between the back surface of a sample and the tip of a sharp metallic probe.
Abstract: This work employs a photothermoelectric technique to measure the anisotropic thermal diffusivity of an aligned multiwalled carbon nanotube array. A modulated laser beam incident to the front surface of the sample creates a thermal wave which is detected by a fast responding thermocouple formed between the back surface of the sample and the tip of a sharp metallic probe. The anisotropic thermal diffusivity values are obtained by fitting the radial and frequency dependent thermal signals with an anisotropic heat conduction model. The room temperature thermal diffusivity measured perpendicular to the alignment direction is 0.246×10−5 m2 /s, an order of magnitude smaller than thermal diffusivity along the CNTs alignment direction 4.4×10−5 m2 /s. However, the thermal diffusivity of the aligned multiwalled CNT is two orders of magnitude smaller than expected for an individual multiwalled CNT.Copyright © 2002 by ASME
TL;DR: In this paper, the authors investigated the merge at the atomic level using tight-binding molecular dynamics (TBMD) and confirmed that the presence of N is responsible for introducing donor states near the Fermi Level.
Abstract: Self-assembly pyrolytic routes to arrays of aligned CNx nanotubes are described. The electronic properties and the density of states (DOS) of these N doped tubes characterized by scanning tunneling spectroscopy (STS) are also presented. Using tight-binding calculations, we confirm that the presence of N is responsible for introducing donor states near the Fermi Level. Finally, it will be shown that high electron irradiation during annealing at 700-800degreesC, is capable of coalescing single-walled nanotubes (SWNTs). We investigate the merge at the atomic level using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, responsible of a continuous reorganization of atoms on individual tube lattices within the adjacent tubes. The latter results pave the way to the fabrication of nanotube contacts, nanocircuits and strong 3D composites using irradiation doses under annealing conditions.
22 Apr 2002
TL;DR: In this paper, multiwalled carbon nanotube films can be used to engineer mechanically robust, light-weight, minimally-intrusive (ultra-thin) damping films, which could be applied to coat the surfaces of metallic structures or embedded within composite/heterogeneous systems.
Abstract: High structural damping is an important design parameter for a wide variety of structural components and machineelements, including those used in aircraft systems. We show here that multiwalled carbon nanotube films can be used to engineer mechanically robust, light-weight, minimally-intrusive (ultra-thin) damping films, that could be used to coat the surfaces of metallic structures or embedded within composite/heterogeneous systems. For high temperature applications, the damping performance of nanotube films is expected to be superior to conventional damping treatments that use viscoelastic material.
Journal Article•
TL;DR: In this paper, the authors demonstrate that high-energy electron irradiation at elevated temperatures (700-800degreesC) results in the molecular merging of adjacent single-walled carbon nanotubes (SWNTs) via a zipper-like mechanism.
Abstract: Using state-of-the-art electron microscopy, we demonstrate that high-energy electron irradiation at elevated temperatures (700-800degreesC) results in the molecular merging of adjacent single-walled carbon nanotubes (SWNTs) via a zipper-like mechanism. In order to elucidate this coalescence process, we perform tight-binding molecular dynamics (TBMD) calculations at 1000degreesC. These simulations indicate that only a few vacancies (generated experimentally by knock-on effects on the tube surfaces) between two adjacent tubes of the same chirality trigger tube coalescence via a zipper-like mechanism. We further demonstrate theoretically that two crossing tubes containing a limited number of vacancies (dangling bonds) connect molecularly at 1000degreesC, resulting in the creation of an "X" molecular nanotube junction. Along this line, we propose a method for creating novel nanotube "X" and "Y" junctions, which could be developed in the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using SWNTs.
Journal Article•
TL;DR: In this paper, nanoscale alumina powder and carbon nanotubes were mixed and hot-pressed to form dense ceramic-matrix composites, and the results indicated that lightweight composites of high strength and fracture-toughness can be made from composite of nanophase alumina, or other ceramics, and carbon-nanotubes.
Abstract: Nanoscale alumina powder and carbon nanotubes were mixed and hot-pressed to form dense ceramic-matrix composites. The strength and fracture toughness of hot-pressed alpha-alumina was much greater than that of conventional grain size polycrystalline alumina. The addition of carbon nanotubes to the alumina resulted in composites with even greater strength and fracture toughness. Hot pressing in a vacuum improved both of these properties over hot pressing in argon. These results suggest that lightweight composites of high strength and fracture toughness can be made from composites of nanophase alumina, or other ceramics, and carbon nanotubes.
TL;DR: In this paper, the authors used scanning surface potential microscopy (SSPM) and conductive tip atomic force microscopy(CT-AFM) to study the electrical properties of nanotube circuits.
Abstract: Carbon nanotubes have the potential of being used as interconnects and active semiconducting material in future electronic circuits. It is necessary to study such nano-scale circuits with probes that can make measurements with molecular precision. We describe results using two nanoprobe techniques, namely scanning surface potential microscopy (SSPM), and conductive tip atomic force microscopy (CT-AFM), in the investigation of electrical properties of nanotube circuits. Vertical arrays of multi-walled nanotubes, grown in a porous alumina template with a metal back contact were analyzed. Current mapping confirmed that the nanotubes were electrically connected to the back contact. Isolated single-walled nanotube bundles deposited on an oxidized silicon wafer, and contacted electrically through chromium electrodes were also studied. Contact potential differences between the metal and nanotubes, and the current in some connected nanotubes were measured. Measurements of contact potential with different metals, and the nature of microscopic transport is crucial. Contact potential measurements can also provide fast and reliable characterization of junctions between metallic and semiconducting nanotubes and metals electrodes.
TL;DR: In this article, a strong attenuation of surface acoustic waves in the range of 30 to 100 MHz by singlewalled carbon nanotube layers deposited on the surface of piezoelectric lithium niobate single crystal has been observed.
Abstract: A strong attenuation of surface acoustic waves in the range of 30 to 100 MHz by singlewalled carbon nanotube layers deposited on the surface of piezoelectric lithium niobate single crystal has been observed. The attenuation exhibits non-monotonous dependence on nanotube density. This attenuation is attributed to the acoustoelectronic interaction between electric fields of the SAW and charge carriers in the nanotubes. The experimental results are in the qualitative agreement with the theory of acoustoelectronic interaction in inhomogeneous structures.