Michael John Casavant

Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films

Dense, thick films of aligned single wall carbon nanotubes and nanotube ropes have been produced by filtration/deposition from suspension in strong magnetic fields. Electrical resistivity exhibits moderate anisotropy with respect to the alignment axis, while the thermopower is the same when measured parallel or perpendicular to this axis. Both parameters have identical temperature dependencies in the two orientations. Thermal conductivity in the parallel direction exceeds 200 W/mK, within a decade of graphite.

Elastic strain of freely suspended single-wall carbon nanotube ropes

We have induced large elastic strains in ropes of single-wall carbon nanotubes, using an atomic force microscope in lateral force mode. Freely suspended ropes were observed to deform as elastic strings with tension proportional to elongation. Ropes were elastically deformed over >10 cycles without showing signs of plastic deformation. The maximum strain observed, 5.8±0.9%, gives a lower bound of 45±7 GPa for the tensile strength (specifically, yield stress) of single-wall nanotube ropes.

Controlled deposition of individual single-walled carbon nanotubes on chemically functionalized templates

Carbon nanotubes offer great promise as molecular wires because they exhibit high electrical conductivity and chemical stability. However, constructing nanotube-based electronic devices requires a controlled means of assembling the tubes. We report procedures both for producing individual short SWNT segments and for their reliable deposition on chemically functionalized nanolithographic templates. Using this method, we have positioned individual nanotubes at specific locations and orientations in such a way that the nanotubes contact metal electrodes. This discovery is potentially very important for fabrication of simple electrical circuits with nanotubes, and provides a new tool for study of electron transport in nanotubes. q 1999 Elsevier Science B.V. All rights reserved.

Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both ...

Structural anisotropy of magnetically aligned single wall carbon nanotube films

Thick films of aligned single wall carbon nanotubes and ropes have been produced by filtration/deposition from suspension in strong magnetic fields. We measured mosaic distributions of rope orientations in the film plane, for samples of different thicknesses. For an ∼1 μm film the full width at half maximum (FWHM) derived from electron diffraction is 25°–28°. The FWHM of a thicker film (∼7 μm) measured by x-ray diffraction is slightly broader, 35±3°. Aligned films are denser than ordinary filter-deposited ones, and much denser than as-grown material. Optimization of the process is expected to yield smaller FWHMs and higher densities.

In-plane-aligned membranes of carbon nanotubes

We have produced the first macroscopic objects comprised of highly aligned single-wall carbon nanotubes (SWNTs). These objects are thin membranes prepared by producing a suspension of SWNT segments, introducing the suspension to a strong magnetic field to align the segments, and filtering the suspension in the magnetic field to produce an aligned membrane of SWNT. These membranes exhibited natural cleavage planes parallel to the magnetic field. This preparation of macroscopic samples of aligned single-wall nanotubes permits exploitation of their highly anisotropic properties, and will enable measurement of the electronic, thermal, magnetic, mechanical, and optical properties of bulk nanotube materials.

Neat macroscopic membranes of aligned carbon nanotubes

Thick macroscopic membranes of magnetically aligned single-wall carbon nanotubes (SWNT) have been produced via high pressure filtration of aqueous surfactant-suspended SWNT in a magnetic field, resulting in membrane thicknesses of 10 μm and surface areas of 125 cm2. Field strengths of 7 and 25 T were used. Polarized Raman spectroscopy shows an anisotropy of (2.5±0.5), indicating good alignment. A similar degree of alignment was seen at both intensities of magnetic field. Furthermore, the degree of alignment is comparable to that achieved in previous work that produced smaller and thinner membranes at 25 T. The membranes also exhibit uniform anisotropy across their surface and throughout their thickness.

An electron energy-loss study of the structural and electronic properties of magnetically aligned single wall carbon nanotubes

We have performed momentum-dependent electron energy-loss studies of magnetically aligned bundles of single wall carbon nanotubes. Our experiment directly shows the anisotropy in the electron diffraction and Cis absorption spectra. We do not observe any anisotropy for the low energy interband transitions related to the van Hove singularities in the nanotube density of states although we observe a strong anisotropy for the π plasmon.

Roping and wrapping carbon nanotubes

Single-walled carbon nanotubes can be dispersed into solvents by ultrasonication to the point that primarily individual tubes, cut to a few hundred nanometers in length, are present. However, when such dispersions are filtered to a thick mat, or paper, only tangles of uniform, seemingly endless ropes are observed. The factors contributing to this “roping” phenomenon, akin to aggregation or crystallization, will be discussed. We have developed methods for generating “super-ropes” more than twenty times the diameter of those formed by filtration, involving the extraction of nanotube material from an oleum dispersion. Nanotubes have been solubilized in water, largely individually, by non-covalently wrapping them with linear polymers. The general thermodynamic drive for this wrapping involves the polymer disrupting both the hydrophobic interface with water and the smooth tube-tube interaction in aggregates. The nanotubes can be recovered from their polymeric wrapping by changing their solvent system. This sol...