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Carbon nanotubes have very interesting mechanical and electrical properties.
Mechanically, they are extremely strong and versatile. The tensile strength and
stiffness of carbon nanotubes are extremely high, especially relative to their
weight. Also, two tubes arranged with one inside the other can have a very low
friction interface, similar to the way two flat sheets of graphite can very easily slide
over each other. Therefore, two nanotubes can have very efficient telescoping and
rotation motions, useful for nanomachines [14].
Electrically, a nanotube can potentially behave as a ballistic conductor. This
means that an electron travels through the tube with small, quantized levels of
resistance. The typical levels of resistance are much lower than traditional
conductors. Nanotubes can exhibit properties of a metal or a semiconductor,
depending on how the sheet of graphite is rolled into a tube. It has even been
suggested that nanotubes can behave like a waveguide, guiding the wave-like
properties of an electron similar to the way electromagnetic (optical) waves are
guided through a fiber-optic cable [15]. All of these properties are being
investigated for future switches and wires. In fact, switches, wires, and support
structures have all been demonstrated with carbon nanotubes, but, as with many
nanoscale devices, the ability to fabricate a practical nanoscale device with
nanotubes and nanowires is still an open challenge. Carbon nanotubes are
discussed further in several chapters in this topic. See Chapter 2, Chapter 12,
and Chapter 18.
1.4.3. Quantum Dots and Tunneling Devices
Many quantum phenomena occur when confining electrons to a very small space,
such as the nanoscale range. For example, an electron confined to a small area can
only have a select few discrete levels of energy, similar to the discrete levels of
energy that an electron may have as part of an atom. When a group of electrons is
confined in all axes of movement (i.e., in three dimensions), a quantum dot is
formed. Similarly, a quantum wire is a group of electrons confined along a
1-dimensional line, and a quantum well restricts electrons to a 2-dimensional
plane. These structures can exhibit properties similar to electrons in atoms or
molecules, even if there is no nucleus of protons and neutrons. Their properties
can be fine-tuned with more freedom than atoms or molecules, making them very
interesting structures to use for computing.
Often, the phenomenon of tunneling, described previously, is combined with
quantum dots, wires, and wells to create useful devices. This is in contrast to
traditional transistors, where tunneling is very undesirable. Three such nanoscale
devices are the resonant tunneling diode (RTD), the single electron transistor
(SET), and quantum-dot cellular automata (QCA). An RTD is a device that has a
quantum well where electrons can be confined; therefore, electrons in this region
can assume only a few possible discrete energy states. When the energy of an
incoming electron is close to one of these ''resonant'' discrete energy states, the
electron can tunnel through with high probability. This device can emit
extremely high frequencies, in the hundreds of GHz, making it interesting for
 
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