electric application of Carbon nanotubes


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Introduction
The remarkable properties of carbon nanotubes may allow them to play a crucial role in the
relentless drive towards miniaturization at the nanometre scale.
Nanotechnology is predicted to spark a series of industrial revolutions in the next two decades
that will transform our lives to a far greater extent than silicon microelectronics did in the 20th
century. Carbon nanotubes could play a pivotal role in this upcoming revolution if their
remarkable electrical and mechanical properties can be exploited.
Since the first measurements were made in 1997, these rolled up sheets of graphite have captured
the imagination of researchers around the world. Progress in understanding the basic physics and
chemistry of nanotubes has advanced at a phenomenal rate - and shows no signs of slowing.
Carbon nanotubes can be considered as a single sheet of graphite rolled in the form of a tube,
though it is not actually made by rolling one. They were first observed by a Japanese scientist
Sumio Iijima in the early part of the 1990’s. The tubes that consist of a single layer of graphite is
termed as ‘Single walled nanotubes’ and the ‘Multi-walled tubes are those consisting of more
than a single layer in the form of concentric cylinders.



Electronic Structure of nanotubes.

The remarkable electrical properties of single wall carbon nanotubes stem from the unusual
electronic structure of “graphene”- the 2-D material from which they are made. (Graphene is
simply a single atomic layer of graphite.). The band structure of graphene is not same as that of a
metal or a semiconductor. Instead it is in between these two extremes. In most directions,
electrons moving at the Fermi energy are backscattered by atoms in the lattice whereas in some
others they don’t. Graphene therefore can be considered as a semi-metal, since it is metallic in
these special directions and semiconducting in the others. Thus a nanotube can be either a metal
or a semiconductor, depending on how the tube is rolled up.
Whereas the multiwall nanotubes were tens of nanometres across, the typical diameter of a
single-wall nanotube was just one or two nanometres. The past decade has seen an explosion of
research into both types of nanotube. The multi walled carbon nanotubes should behave slightly
different to their single walled relatives due to the interaction of the adjacent layers. Though
various theories can be incorporated, many of them may not hold true for such microscopic
materials.

Nanotubes as one dimensional metals

Solid –state devices in which electrons are confined to two-dimensional planes have provided
some of the exciting scientific and technological breakthroughs of the past many decades.
However, 1-D systems are also proving to be very exciting. Studies of quasi 1-D systems, such as
conducting polymers , study of ballistic systems, electron waveguides and many other fields that
may transform the face of electronics fall into this category. The 1-d systems on which these
phenomena can be studied have been limited by the fact that they are inherently complex to
make. What has been lacking is the perfect model system for exploring one dimensional transport
– a 1-d conductor that is cheap and easy to make. , can be individually manipulated and
measured, and has little structural disorder. Single walled carbon nanotubes fit this bill
remarkably well.
Nanotubes are ideal systems for studying the transport of electrons in one dimension, and have
commercial potential as nanoscale wires, transistors and sensors. For many years, studies of
quasi-one-dimensional systems, such as conducting polymers, have provided a fascinating insight
into the nature of electronic instabilities in one dimension. In addition, 1-D devices such as
"electron waveguides" - in which electrons propagate through a narrow channel of material - have
been created. Experiments on these devices have shown, for example, that the conductance of
"ballistic" 1-D systems - in which electrons travel the length of the channel without being
scattered - is quantized in units of the charge on the electron squared divided by the Planck
constant.



More on Electronic properties

Carbon nanotubes are giant molecular wires in which electrons can propagate freely, just as they
do in an ordinary metal. This contrasts strongly with conventional "conducting" polymers in
which the electrons are localized. These molecules are actually insulators and only become
conductors if they are heavily doped. Graphite, on the other hand, can conduct electricity because
one of the four valence electrons associated with each carbon atom is delocalized and can
therefore be shared by all the carbon atoms.
However, it turns out that a single sheet of graphite (also known as graphene) is an electronic
hybrid: although not an insulator, it is not a semiconductor or a metal either. Graphene is a
"semimetal" or a "zero-gap" semiconductor.


Field emission
The small diameter of carbon nanotubes is very favourable for field emission - the process by
which a device emits electrons when an electric field or voltage is applied to it. Field emission is
important in several areas of industry, including lighting and displays, and the relatively low
voltages needed for field emission in nanotubes could be an advantage in many applications.
However, as with all new technologies, there are formidable obstacles to be overcome.
To make a field-emission source with just one nanotube, individual multiwall nanotubes were
mounted onto a gold tip. The nanotubes were kept in place by van der Waals forces alone (i.e.
adhesive was not used). The field emissions from multiwall nanotubes with open and closed ends
were compared. Nanotubes grown in arc discharges are normally closed, but they can be opened
by applying a very large electric field, or by treating them with oxygen at high temperature. Field
emission occurred when a potential of a few hundred volts was applied to the gold tip. Both open
and closed nanotubes were capable of emitting currents as high as 0.1 mA, which represents a
tremendous current density for such a small object.