CNT can have distinctly different electronic properties depending on the chirality. They can be semiconducting or metallic depending on the type of structure. While armchair tubes are always metallic, others can be semiconducting or metallic. Obviously as the diameter increases, tubes resemble graphite, which can be metallic. Note that all armchair tubes are metallic, as well as tubes with indices (m-n=3). The (n,m) indices determine the nanotube electronic structure. SWNTs for which n-m =3q are metallic, and those for which n-m is not 3q are semiconducting. Here, q is an integer. Armchair CNT is metallic as there is no any Band gap between conduction and valance band but in the case of zig zag a tiny band gap is there that make it semiconducting in nature.
CNT can have distinctly different electronic properties depending on the chirality. They can be semiconducting or metallic depending on the type of structure. While armchair tubes are always metallic, others can be semiconducting or metallic. Obviously as the diameter increases, tubes resemble graphite, which can be metallic. Note that all armchair tubes are metallic, as well as tubes with indices (m-n=3). The (n,m) indices determine the nanotube electronic structure. SWNTs for which n-m =3q are metallic, and those for which n-m is not 3q are semiconducting. Here, q is an integer. Armchair CNT is metallic as there is no any Band gap between conduction and valance band but in the case of zig zag a tiny band gap is there that make it semiconducting in nature.
C.N.R. Rao, FRS A. Govindaraj Nanotubes and Nanowires ISBN 0-85404-832-4
this may help you.
To understand it better, you have to think that CNT are a rolled up sheet of graphite. This means they have the electronic structure of graphene superimposed to a circular simmetry. Graphene is a semimetal, with "few" electrons at the Fermi level. Adding helical simmetry means that from all the continuous energy levels, only the levels which have a helical periodicity (wavefunction = n*2pi with n integer) are allowed. With some basic trigonometry you will find that only cnt with n-m = 3n (with n integer) have an allowed state at the fermi level, so are metallic. About the pathways they are also supposed to have a "ballistic" conduction, but this requires really perfect CNTs and, as far as my knowledge, had been demonstrted only over few hundred nm in CNT length.
continuing on the above correct explanations, if you now lay many nanotubes on a flat surface, they will be mixed. This will create paths of conduction between two electrodes you deposit on the CNT covered surface. The nature of the nanotubes in most powders varies and therefore you will have both semiconducting and metallic CNTs. However, some electrical pathways will have more metallic nanotubes. These are the metallic pathways. If you pass an electric current between the two electrodes and gate the semiconducting nanotubes off, then the current will pass predominantly via the metallic CNTs. That will turn them a little warmer and in the presence of oxygen, the metallic nanotubes are damaged. Then you are left with a mat of CNTs which contain less metallic CNTs than the original powder material. Tah dah
The nature of the electrical pathways in CNT mats depends too much on the nature of the initial CNT powder and the dispersion you have reached in solution. If the nanotubes clamp together it will be difficult to separate metallic from semiconducting pathways. In addition the initial powder contains other species like multiwall CNTs and amorphous carbon. If you create very dense mats, these "contamination" species will dominate your conduction and the mat will appear metallic no mater what you do. What is exactly you are trying to do with CNTs?
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