On the suitability of carbon nanotube forests as non-stick surfaces for nanomanipulation
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A carbon nanotube forest provides a unique non-stick surface for nanomanipulation, as the nanostructuring of the surface allows micro- and nanoscale objects to be easily removed after first being deposited via a liquid dispersion. A common problem for smooth surfaces is the strong initial stiction caused by adhesion forces after deposition onto the surface. In this work, carbon nanotube forests fabricated by plasma-enhanced chemical vapour deposition are compared to structures with a similar morphology, silicon nanograss, defined by anisotropic reactive ion-etching. While manipulation experiments with latex microbeads on structured as well as smooth surfaces ( gold, silicon, silicon dioxide, Teflon, diamond-like carbon) showed a very low initial stiction for both carbon nanotube forests and silicon nanograss, a homogeneous distribution of particles was significantly easier to achieve on the carbon nanotube forests. Contact-angle measurements during gradual evaporation revealed that the silicon nanograss was superhydrophic with no contact-line pinning, while carbon nanotube forests in contrast showed strong contact-line pinning, as confirmed by environmental scanning electron microscopy of microdroplets. As a consequence, latex microbeads dispersed on the surface from an aqueous solution distributed evenly on carbon nanotube forests, but formed large agglomerates after evaporation on silicon nanograss. Lateral manipulation of latex microbeads with a microcantilever was found to be easier on carbon nanotube forests and silicon nanograss compared to smooth diamond-like carbon, due to a substantially lower initial stiction force on surfaces with nanoscale roughness. Nanomanipulation of bismuth nanowires, carbon nanotubes and organic nanofibres was demonstrated on carbon nanotube forests using a sharp tungsten tip. We find that the reason for the remarkable suitability of carbon nanotube forests as a non-stick surface for nanomanipulation is indeed the strong contact-line pinning in combination with the nanostructured surface, which allows homogeneous dispersion and easy manipulation of individual particles.