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Carbon Nanotubes

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A sheet of graphene rolled to show formation of different types oof single-walled carbon nanotubes. 
Image Credit: NASA Ames Center for Nanotechnology

Carbon nanotubes were discovered in 1991 by Sumiyo Iijima, a Japanese scientist working at the NEC Corporation. A carbon nanotube (CNT) is a tubular form of carbon with a diameter as small as 0.4 nm and length from a few nanometers up to a millimeter.  The length-to-diameter ratio of a carbon nanotube can be as large as 28,000,000:1, which is unequalled by any other material.

Carbon exists in several forms; graphite and diamond are the most familiar. To imagine how a carbon nanotube looks like, think about taking a single layer of a graphite sheet, cutting it into a small piece of any size, and rolling it like you would roll a cigar.  The result is a single-wall carbon nanotube (SWCNT). If you take multiple layers of a graphite sheet and roll them like a cigar, then you get a multiwall carbon nanotube (MWCNT).

Scanning electron microscopy of nanofibers covered with nanotubes,
Image Credit: Kristopher Behler and Mickael Havel, Drexel University

Of course, no one is sitting out there rolling graphite sheets in order to make nanotubes. These are grown in laboratories, often using a process called chemical vapor deposition.

Though many sophisticated commercial growth reactors are available, carbon nanotubes can be made in a standard chemistry laboratory. A quartz tube about 2.5 cm in diameter serves as the growth reactor and is inserted inside a tube furnace (a tube furnace is a standard heating device for conducting syntheses and purifications). The nanotube is grown on a silicon wafer that is placed at a central location inside the quartz tube. A thin layer of iron or nickel or cobalt is applied to the silicon wafer to serve as a catalyst to grow the nanotubes. A hydrocarbon such as methane (high purity form of natural gas) or ethane or acetylene is sent through the reactor tube which is heated to 750-900ºC by the furnace.

Single-walled carbon nanotubes. 
Image Credit: NASA Ames Center for Nanotechnology

In a few minutes, the silicon wafer appears black, indicating that it is covered with nanotubes. Depending on growth conditions (temperature, type of gas, thickness of catalyst, etc.), single-wall or multi-wall carbon nanotubes are found on the silicon wafer.  These can be viewed using powerful microscopes such as a scanning electron microscope or a transmission electron microscope. If iron or nickel catalyst is not used, only amorphous carbon is formed as is known from freshman chemistry. When the growth experiment runs for several minutes, the single wall nanotubes look like spaghetti on a plate since they are very long and have a tiny diameter. A closer look also reveals that these nanotubes tend to cluster together like a rope because of a force called van der Waals attractive force.

A single-walled carbon nanotube is characterized by a set of two integers (n, m) called the chirality vector. When (n-m)/3 is an integer (for example when n is 8 and m is 2), then the nanotube has metallic properties; if (n-m)/3 is not an integer, the corresponding nanotube behaves like it is a semiconductor. The ability to create tubes of either metallic or semiconductor nature is of great practical importance. Today's computer chips use silicon (which is a semiconductor) along with copper (which is a metal) to build circuits. Scientists and engineers envision all carbon-based electronics using semiconducting and metallic carbon nanotubes of different values of n and m.

A tower of multiwalled carbon Nanotubes. 
Image Credit: NASA Ames Center for Nanotechnology

In addition to unique electronic properties, single wall carbon nanotubes exhibit extraordinary mechanical properties. They are a hundred times stronger than steel at one-sixth of its weight. Their ability to carry current and heat along the axial direction is extraordinary, and therefore has the potential to replace copper wires as conductors.

The exciting properties of carbon nanotubes have led to wide ranging studies across the world for their use in high strength but low weight composites, body armor, conducting polymers, electrostatic discharge protection, computer chips, chemical and biosensors and many other applications. One issue that needs to be addressed in all applications is the potential toxicity of carbon nanotubes.