Chapter 6 : Carbon Nanotubes
A Carbon Nanotube (CNT) can be modeled as a hexagonal lattice (graphene) “rolled up” into a cylinder.
This is a nanoscopic structure made of carbon atoms in the shape of a hollow cylinder.
The cylinders are typically closed at their ends by semi-fullerene-like structures.
A nanotube may consist of one tube of graphite, a one-atom thick called Single Wall CNT (SWCNT) or a number of concentric tubes called Multi Wall CNT (MWCNT).
When viewed with a transmission electron microscope these tubes appear as planes.
Whereas single walled nanotubes appear as two planes, and in multi walled nanotubes more than two planes are observed, and can be seen as a series of parallel lines.
The SWCNT has only a single wall.
The structure of SWCNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder.
The diameter of a SWCNT varies from 0.3 nm to 3 nm, while its length varies from a several microns to a few millimeters.
Theoretically SWCNT can be infinite in length.
Topics covered in this snack-sized chapter:
MWCNTs are characterized by formation of nanotubes within nanotubes.
MWCNTs are formed with concentric graphitic shells.
The diameter of a MWCNT varies from 5 nm to 80 nm, and they are a few microns long.
In general, the MWCNTs are shorter and wider than the SWCNTs but their structures are more prone to defects.
When the diameter of the MWCNT is in the range of a few microns, the defects are more pronounced and the material is referred to as carbon fibers.
A CNT is characterized by its chiral
Chiral angle with respect to the zig-zag axis,
n, m are integers,
, are unit vectors.
Depending on how the graphene plane is 'cut' before it’s rolled up, three types of carbon nanotubes are obtained:
Rolling up the sheet along one of the symmetry axis gives either a zig-zag (m = 0) tube or an armchair (n = m) tube.
Armchair CNT have (n, n) and a chiral angle of 30.
Zig-zag CNT corresponds to (n, 0) or (0, m) and have a chiral angle of 0°.
Chiral CNT have general (n, m) values and a chiral angle of between 0° and 30°.
It is also possible to roll up the sheet in a direction that differs from a symmetry axis to obtain a chiral nanotube.
The diameter of the nanotubes (n, m) depends on the values of n and m.
There are three methods for CNT fabrication:
- Chemical Vapor Deposition,
The Arc Discharge Method produces a number of carbon nanostructures such as fullerenes, whiskers, soot and highly graphitized carbon nanotubes from high temperature plasma that approaches
In an Arc Discharge Method, a carbon is vaporized between two carbon electrodes, the anode and the cathode in a noble gas (helium or argon) environment.
Schematic representation of a typical arc discharge unit is presented in the figure below:
- Laser Ablation or Pulsed Laser Vaporization.
It has conveniently been used to produce both SWNTs and MWNTs as revealed by Transmission Electron Microscope (TEM) analysis.
An arc discharge with a cathode containing metal catalysts (such as cobalt, iron or nickel) mixed to graphite powder results in a deposit containing SWCNTs.
An arc is struck between two pure graphite electrodes in a gas atmosphere produces MWCNTs.
In this technique, carbon nanotubes grow from the decomposition of hydrocarbons at a temperature range of 500 to 12000
They can grow on substrates such as carbon, quartz, silicon etc. or on floating fine catalyst particles, e.g. Fe, Ni, Co etc. from numerous hydrocarbons e.g. benzene.
Thermochemical decomposition of organic material at elevated temperature deposits carbon (as soot) and carbon nanotubes on reactor wall.
SWCNTs use carbon monoxide (CO) or methane () for a carbon source and a much higher growth temperature (900 - 1200°C).
MWCNTs use acetylene gas for the carbon source and a growth temperature between 600 - 800°C.
Laser Ablation technique involves the use of a laser beam to vaporize a target of a mixture of graphite and metal catalyst such as cobalt or nickel at a temperature approximately 12000
C in a flow of controlled inert gas (argon) and pressure.
The nanotube deposits are recovered at water cooled collector at much lower and convenient temperature.
This method was used in early days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20 nm, and high yield with graphite conversion greater than 70-90%.
The strongest and most flexible molecular material because of C-C
Very high current carrying capacity.
Strain much higher than any material.
Thermal conductivity in the axial direction with small values in the radial direction.
Small size offers faster switching speeds (100GHz) and low power.
Young’s Modulus is over 1 Tera Pascal.