Lance Delzeit

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.

Dense Vertically Aligned Multiwalled Carbon Nanotube Arrays as Thermal Interface Materials

Carbon nanotube (CNT) arrays are being considered as thermal interface materials (TIMs). Using a phase sensitive transient thermo-reflectance technique, we measure the thermal conductance of the two interfaces on each side of a vertically aligned CNT array as well as the CNT array itself. We show that the physically bonded interface by van der Waals adhesion has a conductance ~105W/m2K and is the dominant resistance. We also demonstrate that by bonding the free-end CNT tips to a target surface with the help of a thin layer of indium weld, the conductance can be increased to ~106W/m2K making it attractive as a TIM

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.

Growth of multiwall carbon nanotubes in an inductively coupled plasma reactor

A high density plasma from a methane–hydrogen mixture is generated in an inductively coupled plasma reactor, and multiwalled carbon nanotubes (MWNTs) are grown on silicon substrates with multilayered Al/Fe catalysts. The nanotubes are vertically aligned, and the alignment is better than the orientation commonly seen in thermally grown samples. A detailed parametric study varying inductive power, pressure, temperature, gas composition, catalyst thickness, and power to the substrate is undertaken. Transmission electron microscopy and Raman spectroscopy are used to characterize the nanotubes. Emission spectroscopy and a global model are used to characterize the plasma. The power in the lower electrode holding the substrate influences the morphology and results in a transition from MWNTs to nanofibers as the power is increased.

Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive

Vertically aligned multiwalled carbon nanotube (MWCNT) arrays can mimic the hairs on a gecko’s foot and act as a dry adhesive. We demonstrate the van der Waals interactions originated dry adhesion between MWCNT array surfaces and various target surfaces over millimeter-sized contact areas. The adhesive strengths were measured over 10N∕cm2 in the normal direction and about 8N∕cm2 in the shear direction with glass surface. The adhesion strength over repeated cycles is limited by the relatively poor adhesion of MWCNTs to their growth substrate, which was improved significantly by adding molybdenum to the catalyst underlayer. We also measured the interfacial work of adhesion as a fundamental adhesion property at the interface. Our measured values of a few tens of mJ∕m2, which falls in the range of typical van der Waals interactions energies, provide a direct proof of the van der Waals dry adhesion mechanism. Furthermore, in contrast to other dry adhesives, we show that MWCNT adhesives are electrically and the...

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.

Carbon nanotube tip probes : stability and lateral resolution in scanning probe microscopy and application to surface science in semiconductors

In this paper we present results on the stability and lateral resolution capability of carbon nanotube?(CNT) scanning probes as applied to atomic force microscopy?(AFM). Surface topography images of ultra-thin films (2-5?nm thickness) obtained with AFM are used to illustrate the lateral resolution capability of single-walled carbon nanotube probes. Images of metal films prepared by ion beam sputtering exhibit grain sizes ranging from greater than 10?nm to as small as ~2?nm for gold and iridium respectively. In addition, the imaging stability and lifetime of multi-walled carbon nanotube scanning probes are studied on a relatively hard surface of silicon nitride (Si3N4). AFM images of the Si3N4 surface collected after more than 15?h of continuous scanning show no detectable degradation in lateral resolution. These results indicate the general feasibility of CNT tips and scanning probe microscopy for examining nanometre-scale surface features of deposited metals as well as non-conductive thin films. AFM coupled with CNT tips offers a simple and nondestructive technique for probing a variety of surfaces, and has immense potential as a surface characterization tool in integrated circuit manufacture.

Improved fabrication approach for carbon nanotube probe devices

An improved process is developed for simple and efficient fabrication of carbon nanotube probe devices. This process requires only two steps to make nanotube probes. First a nanotube cartridge is created using chemical vapor deposition, then the nanotubes are transferred from the cartridge to a device using an electric field. Multiwall nanotube probes are made into different device geometries in this approach. Their applications are illustrated by atomic force microscopy imaging of the surface of a terrestrial rock granule selected to simulate the morphology and consistency of a grain of Mars dust and nanolithography on a silicon substrate.

Growth of carbon nanotubes by thermal and plasma chemical vapour deposition processes and applications in microscopy

Single-walled carbon nanotubes (SWNTs) are grown from a methane feedstock by thermal chemical vapour deposition (CVD). An ethylene-hydrogen plasma generated in an inductively coupled plasma reactor primarily yields multi-walled carbon nanotubes and thicker fibres. In both cases, an iron catalyst layer and an aluminium underlayer are deposited by ion beam sputtering onto silicon wafers for the growth of carbon nanotubes (CNTs). The plasma process provides well-aligned multi-walled nanofibres useful for fabrication of electrodes and sensors and further tip functionalization whereas thermal CVD produces a mat of SWNT ropes. In addition, CNTs grown at the tips of silicon cantilevers are demonstrated to be ideal for high-resolution imaging of biological samples and simulated Mars dust grains using atomic force microscopy.