Metallic and Semiconducting Carbon Nanotubes
(above) Image of a carbon nanotube taken by a scanning tunnelling microscope (STM), a device that can provide atom-scale images of materials. Considered by many to be a
newly discovered form of pure carbon, nanotubes are rolled-up sheets of carbon atoms. They are just billionths of a meter in diameter but can potentially have lengths of centimeters or
more. This makes them act like one-dimensional wires which have many potential applications in
electronics and other areas of technology. The image shows a nanotube with a diameter of 1.3
nanometers. Individual carbon atoms can be seen as the reddish or yellow blobs in this image,
with dark hexagonal holes between the atoms.
A carbon nanotube is essentially made from graphene (above), a flat sheet of carbon atoms arranged in a hexagonal pattern, wrapped in a certain direction. In this illustration, the carbon atoms occupy the points of the hexagons, and the wrapping direction is indicated by the large arrow. This direction determines the nanotube diameter and its "chirality,"
the amount of "twist" in the tube. The example shown here corresponds to the geometry of the
tube imaged above.
(above) Measurement of how a quantity known as electrical conductance (dI/dV) changes with voltage (V) on 4 different tubes. As the STM tip is brought close to the surface, electrons from the tip can jump to the nanotube, constituting a type of electric current called "tunnelling current." The conductance is simply a measure of how many electronic states are available for electrons to tunnel to at a certain energy. The dI/dV plot is a measure of the electronic states as a function of energy, which is tuned by the voltage. Two classes of tubes--metals and semiconductors--can be distinguished from the plot. Whether a tube is a metal or semiconductor depends on the properties of the electrons in the tubes. Specifically, it depends on how much energy the outermost or
"valence" electrons in the tubes need to need to become "conducting" electrons which flow freely through the material. In metallic conductors, electric current generally flows freely and there is no energy gap between the valence and the conducting states. In a semiconductor, such an energy gap exists and therefore a higher voltage is needed to make electric current flow.
Carbon nanotubes are remarkable in that small differences in their diameter and chirality affect whether it is a
metal or a semiconductor. For most other materials, its metallic or semiconducting nature depends
mainly on its chemical composition and the 2-D arrangement of atoms and molecules.
Thanks to Liesbeth Venema, Jeroen Wildoer, and Cees Dekker at the
DIMES institute at Delft University of Technology for supplying the pictures.
These images are copyright DIMES. Journalists have permission to use these images
if the source is acknowledged.
These measurements are discussed in a paper by J.W.G. Wildoer, L.C.
Venema, A.G. Rinzler, R.E. Smalley & C. Dekker in the 1 January 1998 issue of
Link to Physics News Update Item on this research (Available January 19, 1998)