Jun 17 2004
Image Credits: Promotive/shutterstock.com
The unique nature of carbon couples with the molecular excellence of buckytubes, also referred to as single-wall carbon nanotubes, to enhance them with remarkably high material properties such as thermal and electrical conductivity, stiffness, strength, and toughness.
No other element in the periodic table can bond itself in an extended network with the same strength as the carbon-carbon bond. The delocalized pi-electron contributed by each atom is free to travel across the entire structure, instead of residing with its donor atom, thus resulting in the formation of the first molecule with metallic-type electrical conductivity. In addition, the high-frequency carbon-carbon bond vibrations offer an intrinsic thermal conductivity greater than even diamond.
However, in a majority of materials, the actual observed material properties—electrical conductivity, strength, etc.—are affected quite markedly by the occurrence of flaws in their structure. For instance, high-strength steel usually fails at about 1 % of its theoretical breaking strength. On the other hand, buckytubes realize values that are near their theoretical limits because of their excellence of structure—their molecular perfection. This feature is part of the exclusive story of buckytubes.
Buckytubes are an example of real nanotechnology: just a nanometer in diameter, but molecules that can be exploited physically and chemically. They open implausible applications in materials, chemical processing, electronics, and energy management.
Buckytubes can be defined as single-wall carbon nanotubes, comprising a single layer of graphite—graphene—that is rolled up into a seamless tube. Graphene has a hexagonal structure that resembles chicken wire. Rolling up graphene or chicken wire into a seamless tube can be realized in many ways.
For instance, carbon-carbon bonds (the wires in chicken wire) can be perpendicular or parallel to the tube axis, leading to a tube where the hexagons circle the tube like a belt, but are oriented differently.
On the other hand, the carbon-carbon bonds need not be either perpendicular or parallel, in which case the hexagons will twist around the tube with a pitch based on how the tube is wrapped.
There is a direct labeling convention to differentiate differently wrapped tubes from one another. The mapping indicates the number of unit vectors needed to link two atoms in the planar hexagonal lattice to form a seamless tube. Such numbers indicate a “vector” for the mapping, usually expressed as (m,n), where m and n are integers. These numbers establish a unique “name” for a tube.
Any kind of tube “named” (n,0) has carbon-carbon bonds that are parallel to the tube axis, and develop a “zig-zag” pattern at an open end; these tubes are termed as “zig-zag” tubes.
Tubes named (n,n), where the two integers are equivalent, contain carbon-carbon bonds that are perpendicular to the tube axis, and are mostly referred to as “armchair” tubes. These two standard types are achiral, which means they do not have a diverse mirror-image, like left and right hands. All the other tubes, called (m,n), where m does not equal n, and neither is 0, are chiral, and have left-and right-handed alternatives.
Properties of Different Tube Types
In many respects, the properties of different types of tubes are fundamentally the same. The exception to this lies in their electrical conductivity, where these minor structural differences can have significant effects. For instance, all armchair tubes—that is, where m=n—have real metallic electrical conductivity.
They move electrons along the tube axis similar to the movement of metals, without a single atom of metal in their structure! This behavior in a molecule is unparalleled. On the other hand, the other tubes are essentially semiconducting, either with moderate band gaps on the order of 1 eV or with a very small band gap of a few meV. The rule, in this case, is that those tubes where (n-m) is a multiple of three are the small-gap type, while the others consist of medium gaps.
Since tubes with varied (n,m) are molecularly distinct, there exists the likelihood of chemically separating different types, and even of growing only particular tube types, although at present, all production processes create an arbitrary mixture.
Another structural feature of tubes is their self-organization into “ropes,” which can be found in many (usually, 10–100) tubes running together along their length in van der Waals contact with one another.
Ropes are much longer than any specific tube in them: while tubes are usually about 100–1000 nm in length (and about 1 nm in diameter) ropes are basically endless, branching off from one another, and subsequently joining others.
These ropes are beneficial in providing extended conductive pathways beyond what would generally be the loading needed to reach a “percolation threshold” for conductivity.