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Carbon Nanotubes -- tiny tubes about 10,000 times thinner than a human hair -- consist of rolled up sheets of carbon hexagons. Multiwalled carbon nanotubes were discovered in 1991 by Sumio Iijima of NEC. In early 1993, Iijima and Almaden's Don Bethune discovered that transition metals can catalyze the growth of single-wall carbon nanotubes. The precisely-defined structures of the single-walled nanotubes led to an explosion of experimental and theoretical research into their properties and applications. They show great potential for use as minuscule wires or in ultrasmall electronic devices. To build those devices, scientists must be able to manipulate the Nanotubes in a controlled way. IBM researchers using an atomic force microscope (AFM), an instrument whose tip can apply accurately measured forces to atoms and molecules, have recently devised a means of changing a nanotube's position, shape and orientation, as well as cutting it.

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How it works
Tobias Hertel, Robert Walkup, Richard Martel and Phaedon Avouris at IBM's Thomas J. Watson Research Center found that van der Waals forces -- attractive forces among atoms and molecules -- hold Nanotubes firmly against the surfaces they are placed on. Thus the researchers were able to change the Nanotubes' positions and orientations, and to alter their shape, by bending them. They distorted the Nanotubes in various ways using calibrated AFM forces; the strong interaction with the surface then stabilized the distorted Nanotubes. By applying particularly large forces, the researchers were able to cut the Nanotubes. For that to happen, however, the Nanotubes had to be anchored to the surface more firmly than normal, by means of chemical bonds rather than the physical van der Waals forces.

These studies led to the important conclusion that the van der Waals interaction between the Nanotubes and the surfaces on which they rest is itself strong enough to change the shape of Nanotubes. In general, they tend to adapt to the shape of the surface on which they sit by bending and becoming slightly squashed. Those changes can cause the properties of Nanotubes on surfaces to differ from those of perfect Nanotubes, which are straight and have circular cross-sections. This raises the possibility of tailoring Nanotubes' properties by intentionally changing their shapes.
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Structure of a single-walled carbon nanotube

Electron microscope image of a multi-walled carbon nanotube across metal electrodes

Figure 1. Schematic of a carbon nanotube field-effect transistor. The nanotube acts as the channel that bridges the source and drain electrodes. The current flow in the nanotube is controlled by the voltage applied to the gate electrode.

Figure 2. Graph showing the amount of current flowing through a nanotube field-effect transistor as a function of the voltage applied to the gate electrode. When the gate voltage is positive the current involves negative electrons, while for negative voltage the current involves positive holes.