A proposal has been made to develop bimorph actuators and force sensors based on carbon nanotubes. The proposed devices could make it possible to generate, sense, and control displacements and forces on a molecular scale, and could readily be integrated with conventional electronic circuits. These devices could also enable the development of a variety of novel microelectromechanical systems, including low-power mechanical signal processors, nanoscale actuators and force sensors, and even microscopic robots.

Carbon Nanotubes would grow out from catalytic metal dots on electrodes, eventually becoming attached to each other by van der Waals forces to form a bimorph actuator or sensor.
The proposed devices would exploit the dependence of nanotube length on charge injection that has been observed in mats of disordered carbon single-wall nanotubes (SWNTs) [Baughman, R.H., et al., "Carbon Nanotube Actuators," Science 284, 1340 (1999)]: the nanotubes become elongated or shortened when biased at negative or positive voltage, respectively. This result suggests that one could produce opposing changes in length in pairs of side-by-side, oppositely-biased nanotubes, resulting in a lateral deflection of the unsecured tube ends, as shown in the figure. Fabrication of such a nanotube bimorph device requires the ability to produce and join the tubes in the desired configuration with one end of each tube connected to a suitable electrical contact.

The proposed bimorph device could be fabricated by growing two nanotubes by chemical vapor deposition (CVD) on closely spaced catalyst dots over prepatterned bias electrodes on a substrate. It is likely that during the growth of the nanotubes, the van der Waals attraction would cause the nanotubes to become attached to each other along their sides, as shown in the figure. Because the electrical conductivity of a nanotube perpendicular to its length is much lower than the electrical conductivity along its length, this configuration should make it possible to maintain a significant differential voltage across the two nanotubes, as needed to cause a differential length change in the pair. Conversely, the application of a lateral external force to the tip of the pair should give rise to a voltage between the electrodes so that this device can also function as a sensitive force detector.

To be able to fabricate nanotube bimorph actuators with the configuration shown in the figure, it will be necessary to develop the means to control the positions and orientations of individual nanotubes on such substrates as silicon wafers. This is likely to entail the use of electron-beam lithography, lift-off, and etching for fabricating catalyst dots 5 to 15 nm wide on pre-patterned electrodes. Suitable catalyst materials could include Ni or alloys of Ni, Co, Fe, and/or Mo. In the contemplated CVD process, suitable precursor and carrier gases (e.g., methane, ethylene, or carbon monoxide plus hydrogen plus either argon or nitrogen) would interact with the substrate (which would be heated to a temperature between 600 and 950 °C), yielding selective growth of nanotubes out from the catalyst dots. There are numerous potential variations on this basic fabrication scheme, including orienting the dots so that the nanotubes grow parallel (instead of perpendicular) to the substrate surface and incorporating other materials to modify the electrical and mechanical properties of nanotube pairs.

This work was done by Brian Hunt, Flavio Noca, and Michael Hoenk of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Mechanics category.

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

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Refer to NPO-21153, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
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Carbon Nanotube Bimorph Actuators and Force Sensors

(reference NPO-21153) is currently available for download from the TSP library.

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