Showing papers by "Pulickel M. Ajayan published in 2001"

Reliability and current carrying capacity of carbon nanotubes

Abstract: The current-carrying capacity and reliability studies of multiwalled carbon nanotubes under high current densities (>109 A/cm2) show that no observable failure in the nanotube structure and no measurable change in the resistance are detected at temperatures up to 250 °C and for time scales up to 2 weeks. Our results suggest that nanotubes are potential candidates as interconnects in future large-scale integrated nanoelectronic devices.

Applications of Carbon Nanotubes

Abstract: Carbon nanotubes have attracted the fancy of many scientists worldwide. The small dimensions, strength and the remarkable physical properties of these structures make them a very unique material with a whole range of promising applications. In this review we describe some of the important materials science applications of carbon nanotubes. Specifically we discuss the electronic and electrochemical applications of nanotubes, nanotubes as mechanical reinforcements in high performance composites, nanotube-based field emitters, and their use as nanoprobes in metrology and biological and chemical investigations, and as templates for the creation of other nanostructures. Electronic properties and device applications of nanotubes are treated elsewhere in the book. The challenges that ensue in realizing some of these applications are also discussed from the point of view of manufacturing, processing, and cost considerations.

Organization of Polymers onto Carbon Nanotubes: A Route to Nanoscale Assembly

Abstract: Atomic scale imaging using scanning tunneling microscopy has been used to investigate the morphology and structure of high molecular weight polymers attached to carbon nanotubes. A surprising degree of regularity is observed in the attached polymers, suggesting a “nanotube-driven” crystallization process is present. The symmetries of this crystalline state of the polymer are directly determined by the underlying nanotube's chirality. Tunneling spectroscopy further demonstrates that the crystallization of the polymer onto the nanotube results in modifications to the nanotube's electronic structure, as would be expected from a strong polymer−tube interaction. The assembly of nanoscale organics into objects with such perfect order clearly foreshadows the construction of large-scale nanoarchitectures with order over many lengths scales.

Ceramic matrix nanocomposites containing carbon nanotubes for enhanced mechanical behavior

Abstract: A ceramic matrix nanocomposite having enhanced mechanical behavior is made up of a nanotube filler composed of at least one nanotube material, and a ceramic matrix composed of a nanocrystalline ceramic oxide. A method for producing ceramic articles having improved fracture toughness includes combining of a nanotube filler made up of a nanotube material and a ceramic matrix made up of a nanocrystalline ceramic oxide, forming an article therefrom, and sintering the article under elevated pressure at elevated temperature.

Electrically conducting nanocomposite materials for biomedical applications

Abstract: Exposing osteoblasts on an electrically conducting nanocomposite, which may be an orthopaedic/dental implant, to electrical stimulation enhances osteoblast proliferation thereon. The electrically conducting nanoscale material includes an electrically conducting nanoscale material and a biocompatible polymer and/or a biocompatible ceramic; carbon nanotubes may be used as the electrically conducting nanoscale material.

Metal atoms in carbon nanotubes and related nanoparticles

Abstract: The paper reviews the present state of research in the field of metal-carbon nanocomposites and the interaction of metal atoms with graphitic structures. Metal crystals can be encapsulated within graphitic shells of cylindrical, spherical, or other geometry. Various chemical and physical production methods to generate metal containing carbon nanotubes and possible microscopic formation mechanisms axe presented. In this context, the role of metals as catalysts in the formation of single-walled carbon nanotubes is discussed. The interaction of metal atoms with the graphitic lattice is of particular interest. In situ electron microscopy is used to study the behaviour of individual metal atoms in a graphitic lattice. Furthermore, novel nanostructures can be generated under electron irradiation. Finally, an overview of theoretical studies using molecular dynamics and tight binding calculations of the carbon-metal interaction is given.

Select Pathways to Carbon Nanotube Film Growth

Abstract: When such superstructures begin to take the role of basic building blocks, the symmetry of these structures assumes importance in deciding the final film structure and morphology These superstructures can be found during the growth of organic and inorganic systems, playing an important role in structural evolution during growth, eg, spherulites during the growth of semi-crystalline polymers [2] When the fundamental building blocks involved in growth are complex macromolecules or rigid nanostructure units, rather than simple atomic species, the structural evolution of the films will depend on how the units are able to self-organize, attach, and space-fill the substrate surface The build up of carbon nanotube films on substrates by chemical vapor deposition (CVD) is a striking example of this phenomenon, and has received much attention due to its practical applications We show in this letter that the film growth of nanotubes occurs via select pathways that involve the nucleation and growth of self-assembled nanotube domains having different shapes and symmetries Moreover, this process can be controlled by changing the rates of nucleation and growth of the nanotubes on the substrates, leading to domains that are distinctly different in morphology, and leading to nanotube films with a characteristic alignment of individual nanotubes Individual carbon nanotubes are macromolecular structures with nanosize diameters and micrometer lengths [3] It has been shown that packed films of nanotubes can be grown on substrates by catalytic CVD of hydrocarbon gases [4‐10] Although

Self-assembled patterns of iron oxide nanoparticles by hydrothermal chemical-vapor deposition

Abstract: Here, we report a hydrothermal chemical-vapor deposition process, which produces self-assembled patterns of iron oxide nanoparticles. By exposing a planar silica substrate to a prevaporized mixture of water, ferrocene [Fe(C5H5)2] and xylene (C8H10), at temperatures of ∼1000 °C, Fe2O3 nanoparticles are deposited on the substrate surface, in regular circular patterns. The particle sizes are less than 100 nm, and are organized into submicron-size patterns. The same process without water produces arrays of carbon nanotubes catalyzed by iron nanoparticles that are formed by the decomposition of ferrocene molecules.

Properties and Applications of Carbon Nanotubes

Abstract: The discovery of fullerenes showed how structures of sp2carbon built based on simple geometrical principles can result in new symmetries that can have fascinating and useful properties. Carbon nanotube is the most striking example. The combination of structure, topology, and dimension create a host of physical properties in nanotubes that are paralleled by few known materials. The combination of high strength, low density, high conductivity, chemical inertness, and low-dimensionality makes nanotubes excellent candidate material for many practical applications. Here we will describe some of these applications of nanotubes, for example, in energy storage, as field emitters, as fillers in novel polymer based composites, and as templates for the creation of unique nanowires. We will discuss recent strategies and challenges in the synthesis of nanotubes tailored for specific applications and their future prospects.

Flash CVD process for synthesis of carbon nanotrees

Abstract: A flash CVD process can be employed to grow micron- and nano-sized tree-like structures, particularly carbon structures on graphite electrodes. This process involves fast cyclic resistive heating of electrodes in an atmosphere of inert gas and hydrocarbons at below atmospheric pressure.

AFM-Based Surface Potential Measurements on Carbon Nanotubes

Abstract: In order to build electrical devices based on carbon nanotubes, it is crucial to understand the effects of the substrate, electrical contacts, defects and junctions on the transport properties of both multi-wall and single-wall carbon nanotubes (MWNT and SWNT). In the present study two different dispositions are analyzed. In one of them, isolated MWNT were deposited onto an insulating oxide layer over a conducting substrate and platinum electrical contacts were prepared via a focused ion beam (FIB) deposition method. An ohmic contact to the substrate allowed us to polarize it at a different potential than the nanotubes themselves. In the second one, a vertical array of MWNT was grown on a template of alumina in which one of the ends was coated with gold, to provide an electric contact. To investigate transport properties, we used an atomic force microscope (AFM) to determine the electric surface potential of MWNT. The technique employed, scanning surface potential microscopy (SSPM), applied a dc-voltage to the tip that equilibrates the local electrostatic potential on the sample so as to eliminate forces on the AFM tip caused by electric repulsion or attraction between tip and sample. There is no alteration of the sample potential caused by the tip. Surface potential measurements on the first system show that the electric transport on the MWNT has a diffusive character. The application of a (4.5V) bias between nanotube and substrate causes the resistance of the nanotube to drop by a factor of four. We can also resolve the voltage drop on the nanotube and at the contacts. The measurements in the second system (vertical arrangement) show that the nanotubes are uncapped and we can observe a contrast that varies with the application of different voltages to the back contact.

Electrical stimulation enhances cellular/molecular functions of osteoblasts relevant to new bone formation in vitro

Abstract: The present study applied electrical stimulation to osteoblasts in vitro and provided cellular/molecular evidence that alternating current electrical stimulation promotes various important, osteoblast functions, such as cell proliferation, expression of genes for collagenous and non-collagenous proteins, and calcium deposition in the extracellular matrix. These functions are responsible for the chemical composition of the organic and inorganic phases of the bone matrix and are required for new bone formation in vivo. Moreover, such functions have major consequences for bone repair, healing, and regeneration and may be responsible for the accelerated bone healing observed in animal models under electrical stimulation. The present in vitro research exemplifies the use of biophysical stimuli and alternative strategies for enhancing osteoblast functions which, although extremely promising, remain as yet unexplored for bone regeneration purposes in either clinical or tissue engineering applications.

Building and testing carbon nanotubes and their architectures

Abstract: Carbon nanotubes have fascinating physical properties. In order to use these novel one-dimensional structures for applications (such as in electronic devices, mechanical reinforcements and nano-electromechanical systems) the structure of nanotubes needs to be tailored and various architectures have to be configured using nanotube building blocks. This paper will focus on the directed and self-assembly of nanotubes on planar substrates into hierarchical structures that include oriented arrays, and ordered bundles. This is achieved by patterning substrates with or without metal catalysts. Growth of nanotubes is typically achieved by chemical vapor deposition (CVD). Various strategies to build two-dimensional and three-dimensional architectures of nanotubes will be described by this method. In addition to creating pristine nanotube arrays on planar substrates, the paper will also cover some of our recent efforts in fabricating nanotube polymer hybrids. Recent efforts and challenges in manipulating nanotube on surfaces and measuring transport properties will be discussed. In conclusion, a perspective will be given on our recent efforts in creating controlled structures with nanotubes and measuring some of their properties.