Cutting of multiwalled carbon nanotubes by a negative voltage tip of an atomic force microscope: A possible mechanism

Multiwalled carbon nanotubes (MWNT's) on Si(5 5 12) surfaces are demonstrated to be cut only by a negatively biased conducting tip of an atomic force microscope (AFM). By scanning with the AFM tip across a 30-nm-diam MWNT in contact mode, we could cut the MWNT only at a negative tip voltage below a threshold. As the tip-moving speed increased, the magnitude of the threshold voltage was increased. A graphite surface was etched in comparison by the same method. It was also etched only at a negative tip voltage below a threshold. As the magnitude of the bias voltage increased, the etch depth of the graphite surface increased exponentially to reach 7.9 nm, a thickness of 23 atomic layers of graphite, at a bias voltage of -10 V. The etching current from the graphite surface to the negatively biased tip was found to follow the Fowler-Nordheim equation and attributed to field-emission electrons from the negatively biased tip. The etch depth of the graphite surface was also found to follow the bias voltage dependence of the Fowler-Nordheim equation. The graphite etching is thus found to be controlled by the field-emission current so that we may propose a cutting mechanism based on the field-emission current density of the Fowler-Nordheim equation: both the MWNT cutting and graphite etching encounter the same reaction where the activation energy is supplied by electrons that are field emitted from the negatively biased AFM tip.