Force output, control of film structure, and microscale shape transfer by carbon nanotube growth under mechanical pressure

We demonstrate that a film of vertically aligned multiwall carbon nanotubes (CNTs) can exert mechanical energy as it grows, and in our experiments the average force output is approximately 0.16 nN per CNT, for CNTs having an outer diameter of 9 nm and five walls. The film thickness after a fixed growth time and the alignment of CNTs within the film decrease concomitantly with increasing pressure which is applied by placing a weight on the catalyst substrate prior to growth, and CNTs grown under applied pressure exhibit significant structural faults. The measured mechanical energy density of CNT growth is significantly less than the energies of primary steps in the CNT formation process yet, based on the film volume, is comparable to the energy density of muscle and based on the volume of CNTs is comparable to hydraulic actuators. We utilize this principle to fabricate three-dimensional structures of CNTs which conform to the shape of a microfabricated template. This technique is a catalytic analogue to micromolding of polymer and metal microstructures; it enables growth of nanostructures in arbitrarily shaped forms having sloped surfaces and nonorthogonal corners and does not require patterning of the catalyst before growth.