This method provides control over the growth density of carbon nanotubes (CNTs) on a relatively coarse scale, with density adjustment over several orders of magnitude, using an applied electrical field or voltage difference that is aligned substantially perpendicular to the substrate surface, which is adjacent to the surface during growth. Control or influence of CNT growth density on a finer scale, estimated at a factor of 2 to 10, is provided using temperature control for the CNT growth process.
For example, an application of a modest electrical field of between 5 and 20 volts over a transverse electrode-to-electrode gap of about 25 μm (electrical field value |E|=(2 to 8) × 103 volts/cm) is estimated to change CNT growth density by 1 to 3 orders of magnitude (coarse scale); and variation of CNT source average temperature between 700 and 850 °C is estimated to change CNT growth density by a multiplicative factor of 2 to 10 (fine scale).
A first region may have a first range of CNT densities, and an adjacent region, spaced apart from the first region, may have a second range of CNT densities that partly overlap, or have no overlap at all, with the density range of the first region. The second region has a higher CNT density, and uses variable heating and/or a reduced electrical field to provide the higher CNT density based on an experimentally determined growth curve and experimental configuration of a device. This approach should be distinguished from masking of regions on a substrate, where the result is binary — where either a CNT array with a fixed density appears, or no CNTs appear in that region at all.
The all-or-nothing approach is fine if the goal is thermal transport because maximum thermal transport benefits if the CNT concentrations are as high as possible. However, if the need is for electron transport (e.g., between adjacent signal processing components on a semiconductor chip), the desired CNT density may lie in an intermediate range, with both a lower bound and an upper bound.
This work was done by Lance D. Delzeit for Ames Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials & Coatings category.