A process has been proposed for growing carbon nanotubes aligned substantially parallel with the nominal planar surfaces of substrates and further aligned with patterns on the substrates. Prior to growth, the patterns would be formed by micro-machining the substrates, which could be silicon or silicon-on-insulator (SOI) wafers. By making it possible to tailor the positions and orientations of individual carbon nanotubes grown on pre-patterned substrates, this process would enable advances in nanotube-based electronic and electromechanical devices.

Figure 1. Carbon Nanotubes Would Grow in Alignment with the tips of micromachined cantilevers on an SOI wafer.
Figure 2. Nanotubes Would Tend To Grow Along Surfaces and Edges because of van der Waals attraction, but could not follow sharp bends. The growing nanotubes tubes would therefore be expected to grow toward the sharp tip of the cantilever. One of the growing nanotubes could be expected to end up protruding from the tip in a predetermined direction.
The process would include chemical vapor deposition (CVD) of the carbon nanotubes on patterned catalysts on the substrates. In each case, the CVD gas would consist of a source of carbon (such as methane, ethylene, or carbon monoxide) either by itself or in a mixture with other gases. Carbon nanotubes grow when a substrate with patterned catalyst is heated and exposed to this CVD gas mixture under appropriate conditions [which can include enhancement by RF (radio frequency) plasmas and/or hot filaments].

The basic process admits of three main variants, each involving a different technique or combination of techniques to position and orient the growing carbon nanotubes. In the first variant (the basic process), the desired alignment would be enforced by use of in-plane pointed silicon cantilevers protruding from an undercut silicon layer on an SOI substrate (see Figure 1). Part of the upper surface of each cantilever would be coated with a thin film of a suitable catalyst (e.g., Ni, Co, or a suitable metal alloy or compound).

On the basis of prior experiments on the growth of nanotubes, it is expected that (1) the nanotubes will tend to nucleate at random times and locations, such that multiple tubes may grow out of each catalyst film, and (2) because of attractive van der Waals forces, the nanotubes will tend to grow along the cantilever surfaces and edges. It is also anticipated that if the tip of a growing nanotube reaches the tip of the cantilever, further growth would likely cause the nanotube to protrude from the tip because strain-energy cost of bending the nanotube to the small tip radius would exceed the energy of van der Waals attraction (see Figure 2).

In the second variant of the process, the micro-machined patterns would comprise narrow, etched trenches in silicon wafers. Enhanced van der Waals forces at the edges of the trenches would preferentially align the growing nanotubes.

In the third variant of the process, electric fields would be used to align the growing nanotubes. In this case, each substrate would be prepared by microfabrication of (1) pointed cantilevers similar to those of the first variant of the process and (2) on-chip electrodes. A bias potential applied during growth would result in a high local electric field between an electrode on the tip of each cantilever and a nearby electrode. The bias circuitry would be designed to prevent the large surges of current that would destroy the growing nanotubes as the interelectrode gaps became bridged by growth of the nanotubes (e.g., by incorporating a large series resistor in the circuit). It may be necessary to adjust the pressure of the CVD gas and/or electrode spacing to prevent electrical discharges between the biased electrodes.

This work was done by Brian Hunt, Daniel Choi, Michael Hoenk, Robert Kowalczyk, and Flavio Noca of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-30205, volume and number of this NASA Tech Briefs issue, and the page number.


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Growing Carbon Nanotubes Aligned with Patterns

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This article first appeared in the October, 2002 issue of NASA Tech Briefs Magazine.

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