Alan M. Cassell

Synthesis of individual single-walled carbon nanotubes on patterned silicon wafers

Recent progress in the synthesis of high-quality single-walled carbon nanotubes (SWNTs) has enabled the measurement of their physical and materials properties. The idea that nanotubes might be integrated with conventional microstructures to obtain new types of nanoscale devices, however, requires an ability to synthesize, isolate, manipulate and connect individual nanotubes. Here we describe a strategy for making high-quality individual SWNTs on silicon wafers patterned with micrometre-scale islands of catalytic material. We synthesize SWNTs by chemical vapour deposition of methane on the patterned substrates. Many of the synthesized nanotubes are perfect, individual SWNTs with diameters of 1–3 nm and lengths of up to tens of micrometres. The nanotubes are rooted in the islands, and are easily located, characterized and manipulated with the scanning electron microscope and atomic force microscope. Some of the SWNTs bridge two metallic islands, offering the prospect of using this approach to develop ultrafine electrical interconnects and other devices.

CHEMICAL VAPOR DEPOSITION OF METHANE FOR SINGLE-WALLED CARBON NANOTUBES

We report the synthesis of high-quality single-walled carbon nanotubes SWNT by chemical vapor deposition CVD of methane at 10008C on supported Fe O catalysts. The type of catalyst support is found to control the formation of individual 23 or bundled SWNTs. Catalysts supported on crystalline alumina nanoparticles produce abundant individual SWNTs and small bundles. Catalysts supported by amorphous silica particles produce only SWNT bundles. Studies of the ends of SWNTs lead to an understanding of their growth mechanism. Also, we present the results of methane CVD on supported NiO, CoO and NiOrCoO catalysts. q 1998 Published by Elsevier Science B.V. All rights reserved.

Carbon nanotube growth by PECVD: a review

Carbon nanotubes (CNTs), due to their unique electronic and extraordinary mechanical properties, have been receiving much attention for a wide variety of applications. Recently, plasma enhanced chemical vapour deposition (PECVD) has emerged as a key growth technique to produce vertically-aligned nanotubes. This paper reviews various plasma sources currently used in CNT growth, catalyst preparation and growth results. Since the technology is in its early stages, there is a general lack of understanding of growth mechanisms, the role of the plasma itself, and the identity of key species responsible for growth. This review is aimed at the low temperature plasma research community that has successfully addressed such issues, through plasma and surface diagnostics and modelling, in semiconductor processing and diamond thin film growth.

Bottom-up approach for carbon nanotube interconnects

We report a bottom-up approach to integrate multiwalled carbon nanotubes (MWNTs) into multilevel interconnects in silicon integrated-circuit manufacturing. MWNTs are grown vertically from patterned catalyst spots using plasma-enhanced chemical vapor deposition. We demonstrate the capability to grow aligned structures ranging from a single tube to forest-like arrays at desired locations. SiO2 is deposited to encapsulate each nanotube and the substrate, followed by a mechanical polishing process for planarization. MWNTs retain their integrity and demonstrate electrical properties consistent with their original structure.

The fabrication and electrochemical characterization of carbon nanotube nanoelectrode arrays

We report a novel approach for the fabrication of nanoelectrode arrays using vertically aligned multi-walled carbon nanotubes (MWCNTs) embedded within a SiO2 matrix. Cyclic voltammetry and pulse voltammetry are employed to characterize the electrochemical properties of the MWCNT array. The unique graphitic structure of the novel MWCNT nanoelectrodes is compared with model systems such as highly oriented pyrolytic graphite and glassy carbon electrodes. Low-density MWCNT nanoelectrode arrays display independent nanoelectrode behavior showing diffusion-limited steady-state currents in cyclic voltammetry over a wide range of scan rates. Electroactive species can be detected at concentrations as low as a few nM. In addition, ultrasensitive DNA/RNA sensors are demonstrated using the low-density MWCNT arrays with selectively functionalized oligonucleotide probes. This platform can be widely used in analytical applications as well as fundamental electrochemical studies.

Multilayered metal catalysts for controlling the density of single-walled carbon nanotube growth

Abstract Ion beam sputtering has been used for the sequential deposition of metal multilayers on various substrates to control the density of single-walled carbon nanotubes (SWNTs) synthesized by chemical vapor deposition. Underlayers (10–20 nm) of Al and Ir were found to activate the substrates for SWNT growth with Fe as active catalyst. Adding Mo as co-catalyst gives increased production of SWNTs and the density can be controlled by varying the thickness of the different metal layers. High-resolution transmission electron microscopy and Raman scattering are used to characterize the SWNTs.

Pore structure of raw and purified HiPco single-walled carbon nanotubes

Abstract Very high purity single-walled carbon nanotubes (SWNTs) were obtained from HiPco SWNT samples containing Fe particles by a two-step purification process. The raw and purified samples were characterized using high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and thermogravimetric analysis (TGA). The purified sample consists of ∼0.4% Fe and the process does not seem to introduce any additional defects. The N 2 adsorption isotherm studies at 77 K reveal that the total surface area of the purified sample increases to 1587 m 2 /g from 567 m 2 /g for the raw material, which is the highest value reported for SWNTs.

Ultrasensitive label-free DNA analysis using an electronic chip based on carbon nanotube nanoelectrode arrays

We report the detection of DNA PCR amplicons using an ultrasensitive label-free electronic technique based on multiwalled carbon nanotube (MWNT) nanoelectrode arrays embedded in an SiO(2) matrix. Specific PCR amplicons are reliably detected using electrochemical (EC) methods through allele-specific oligonucleotide hybridization. The inherent guanine bases in the DNA amplicon target of [Formula: see text] bases serve as signal moieties with the aid of Ru(bpy)(3)(2+) mediators, providing an amplified anodic current associated with the oxidation of guanine groups at the nanoelectrode surface. The reduced size and density of the nanoelectrode array provided by MWNTs dramatically improves the sensitivity of EC detection. In addition, the abundant guanine bases in target DNA produce a large signal. Less than [Formula: see text] target amplicons can be detected on a microspot, approaching the sensitivity limit of conventional laser-based fluorescence techniques. This method also eliminates the labelling requirement and makes the measurements much simpler. This platform can be employed for developing highly automated electronic chips with multiplex nanoelectrode arrays for quick DNA analysis.

Thermal Contact Resistance and Thermal Conductivity of a Carbon Nanofiber

It has been suggested that CNTs and carbon nanofibers CNFs can be used as thermal interface materials to enhance contact thermal conductance for electronic packaging applications. Several groups have reported mixed experimental results from no improvements to large improvements in the thermal contact conductance due to the CNTs and CNFs 8‐12. These mixed results can be caused by the difference in surface coverage and perpendicular alignment of the CNTs or CNFs. Moreover, the results can be affected by two other factors. First, the CNTs and CNFs grown using different methods possess different defect densities and different intrinsic thermal conductivities. Secondly, the contact thermal resistance of the nanometer scale point and line contacts between a CNT or CNF and a planar surface can be high due to enhanced phonon-boundary scattering at the nanocontacts. We have used a microfabricated device to measure the thermal resistance of an individual CNF from a vertically aligned CNF film for applications as thermal interface materials. The measurement was conducted before and after a platinum Pt layer was deposited on the contacts between the CNF and the microdevice so as to investigate the thermal contact resistance between the CNF and a planar surface. The contact resistance was reduced by the platinum coating for about 9‐13% of the total thermal resistance of the nanofiber sample before the Pt coating. At temperature 300 K, the obtained axial thermal conductivity of the carbon nanofibers was about three times smaller than that of graphite fibers grown by pyrolysis of natural gas prior to high-temperature heat treatment.

Electronic properties of multiwalled carbon nanotubes in an embedded vertical array

We demonstrate integration of carbon nanotubes into large scale vertically aligned electrode arrays, by filling the as-grown samples with conformal SiO2 using chemical vapor deposition. Subsequent mechanical polishing yields a flat surface with only the very ends of the nanotube array exposed. The electronic properties of individual carbon nanotubes in the array are measured using current-sensing atomic force microscopy. These vertical nanotube arrays are suitable for fabricating various electronic devices and sensors.