O. Zhou

Large current density from carbon nanotube field emitters

Field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm/sup 2/. At electric fields as low as 4-7 V//spl mu/m, they emit technologically useful current densities of 10 mA/cm/sup 2/. The emission originates from nanotube ends with a characteristic structured ring pattern.

Deformation of carbon nanotubes in nanotube–polymer composites

Composites of uniaxially oriented multiwalled carbon nanotubes embedded in polymer matrices were fabricated and investigated by transmission electron microscopy. In strained composite films, buckling was ubiquitously observed in bent nanotubes with large curvatures. By analyses of a large number of bent nanotubes, the onset buckling strain and fracture strain were estimated to be ≈5% and ⩾18%, respectively. The buckling wavelengths are proportional to the dimensions of the nanotubes. Examination of the fracture surface showed adherence of the polymer to the nanotubes.

Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition

The nucleation and growth of aligned multiwall carbon nanotubes by microwave plasma-enhanced chemical vapor deposition have been studied. The nanotubes nucleate and grow from catalytic cobalt islands on a silicon substrate surface, with both their diameter and length dependent on the size of the cobalt islands. Electron microscopy reveals that the nanotubes grow via a ‘‘base growth’’ mechanism. The nanotubes grow initially at a very rapid and constant rate ~;100 nm/s! that decreases sharply after the catalyst Co particles become fully encapsulated by the nanotubes. We propose a detailed model to explain these experimental observations on nucleation and growth of nanotubes.

Deposition of aligned bamboo-like carbon nanotubes via microwave plasma enhanced chemical vapor deposition

Aligned multiwall carbon nanotubes have been grown on silicon substrates by microwave plasma enhanced chemical vapor deposition using methane/ammonia mixtures. Scanning electron microscopy shows that the nanotubes are well aligned with high aspect ratio and growth direction normal to the substrate. Transmission electron microscopy reveals that the majority phase has a bamboo-like structure. Data are also presented showing process variable effects on the size and microstructure of the aligned nanotubes, giving insight into possible nucleation and growth mechanisms for the process.

Application of carbon nanotubes as electrodes in gas discharge tubes

turn-on fields. Recent experiments have reported turn-on electric fields in the range of 1.5‐3 V/mm. 3‐5 The nanotubes emitters, especially the SWNTs, are capable of producing stable electron emission with a current density exceeding 4 A/cm 2 ~Ref. 5! which make them attractive cold-cathode materials for microvacuum electronic applications. Assynthesized SWNTs are in the form of either free-standing mat or powder, unsuitable for device applications. We have processed the raw materials to uniform films by a spray method. 6 Adhesion between the substrates and the films is enhanced by introducing either a carbon-dissolving or a carbide-forming metal interlayer. In this letter, we report the effects of carbon nanotube coating on the performance of the gas discharge tubes. The direct current ~dc! breakdown voltages of GDTs fabricated using SWNT-coated electrodes were measured and compared with commercial GDTs. A significant reduction in the breakdown voltage and voltage fluctuation ~over 1000 surges! was observed for the nanotubebased GDTs as compared to typical commercial devices.

Photoemission spectroscopy of single-walled carbon nanotube bundles

Abstract The electronic structures and the work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated by C 1 s and valence band photoemission spectroscopy. The C 1 s spectrum of the pristine material showed a Doniach–Sunjic type asymmetry, indicating the existence of metallic tubes. The work function of the pristine bundles was found to be 4.8 eV, which is about 0.2 eV larger than that of graphite. A drastic decrease of the work function to about 2.0 eV was observed in the Cs-intercalated sample. The Cs intercalation also caused a nearly two-order increase in the spectral intensity at the Fermi level.

Fabrication and Field Emission Properties of Carbon Nanotube Cathodes

A variety of carbon nanotube films have been fabricated and tested as cold cathodes. A spray deposition technique was developed for processing as-grown bulk nanotubes, both single-walled and multi-walled, into films of randomly oriented nanotubes. Films of randomly oriented multi-walled nanotubes were grown using thermal chemical vapor deposition, and arrays of well-aligned multi-walled nanotubes have been fabricated using a microwave plasma enhanced chemical vapor deposition technique. The emission current-voltage (I-V) characteristics of these nanotube cathodes have been measured. Both multi-walled (random and aligned) and single-walled carbon nanotubes exhibit low turn-on fields (∼ 2 V/μm to generate 1 nA) and threshold fields ( 2 ). Significantly, these cathodes were capable of operation at very large current densities (> 1A/cm 2 ), making them candidates for application in a variety of vacuum microelectronic devices.

Energy distribution of field emitted electrons from carbon nanotubes

Abstract Field emission properties of single-walled carbon nanotubes (SWNTs), deposited on n-type Si substrates, were investigated by a combination of classical I–V characterization and field emission energy distribution (FEED) measurements. I–V characterization showed that current densities on the order of 1xa0mA/cm2 could be obtained at nominal electric fields as low as 3xa0V/μm. A current density of up to 25xa0mA/cm2 was observed, and long-term current stability and reproducibility were excellent. FEED measurements revealed that at low current densities, the field emitted electrons originated from energy levels close to the Fermi level of the Si substrate. At larger applied voltages and larger current densities, I–V characterization of the field emitters showed primarily an ohmic behavior; FEED data taken under the same experimental conditions showed a shift of the main spectral peak toward lower kinetic energies with increasing applied voltages. This energy shift was found to be linear with the applied voltage and emission current over a wide range. Both FEED and I–V data thus indicated that the field emission current was primarily limited by the ohmic resistance of the nanotubes and/or the contact resistance between the nanotubes and the Si substrate; typical measured values for the cathode surface resistances were 100xa0kΩxa0cm2.

Structure and Electrochemical Properties of Carbon Nanotube Intercalation Compounds

Abstract This article summarizes the current status of carbon nanotube intercalation compounds. It focuses on the structure and electrochemical properties of intercalated single-walled carbon nanotubes (SWNTs). Materials synthesis, purification and characterization methods are also discussed. This article draws mostly from works performed at UNC.

Lithium storage in single wall carbon nanotubes

The effects of structure and morphology on lithium storage in single wall carbon nanotubes (SWNTs) were studied by electrochemistry, x-ray diffraction and nuclear magnetic resonance (NMR) techniques. Purified SWNT bundles were chemically etched to variable lengths and were reacted with Li via the electrochemical and solid state routes. The reversible Li storage capacity increased from LiC6 in close-end SWNTs to LiC3 after etching. The increase is attributed to diffusion of the Li ions into the interior space of the individual SWNTs through the open ends and defects on the sidewalls.