Carbon nanotubes semiconducting tubes

Modifications of the conduction properties of semiconducting carbon nanotubes by B (p-type) and N ( -type) substitutional doping has also been dis-cussed[3l] and, in addition, electronic modifications by filling the capillaries of the tubes have also been proposed[32]. Exohedral doping of the space between nanotubes in a tubule bundle could provide yet an-... 

The electronic properties of single-walled carbon nanotubes have been studied theoretically using different methods[4-12. It is found that if n — wr is a multiple of 3, the nanotube will be metallic otherwise, it wiU exhibit a semiconducting behavior. Calculations on a 2D array of identical armchair nanotubes with parallel tube axes within the local density approximation framework indicate that a crystal with a hexagonal packing of the tubes is most stable, and that intertubule interactions render the system semiconducting with a zero energy gap[35]. 

The ID electronic energy bands for carbon nanotubes [170,171, 172, 173, 174] are related to bands calculated for the 2D graphene honeycomb sheet used to form the nanotube. These calculations show that about 1/3 of the nano tubes are metallic and 2/3 are semiconducting, depending on the nanotube diameter di and chiral angle 6. It can be shown that metallic conduction in a (ra, m) carbon nanotube is achieved when... 

The diameter of the nanotube is an additional important parameter, with smaller tubes presenting enhanced curvature and consequently enhanced reactivity. One last aspect affecting reactivity is the helicity of the carbon nanotubes. In metallic CNTs, the aromaticity is slightly lower than in the semiconducting types, rendering the former more susceptible to functionalization. 

Carbon nanotubes can be single walled (SWNT) or multi-walled (MWNT). SWNTs can be metallic or semiconducting, depending on their chirality. MWNTs contain several walls, so in combination they will tend to be metallic. In addition, as the band gap of semiconducting tubes varies inversely with their diameter, the larger diameter of MWNTs means that effectively most walls are metallic. 

The conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

Attaching chemical functionalities to CNTs can improve their solubility and allow for their manipulation and processability [24]. The chemical functionalization can tailor the interactions of nanotubes with solvents, polymers and biopolymer matrices. Modified tubes may have physical or mechanical properties different from those of the original nanotubes and thus allow tuning of the chemistry and physics of carbon nanotubes. Chemical functionalization can be performed selectively, the metallic SWCNTs reacting faster than semiconducting tubes [25]. 

Figure 24 Structure of a carbon nanotube, (a) the unraveled (4,2) tube, where the indices (m,n) refer to the vector (in terms of a and 32) which spans the circumference of the tube. The vector (4,-5) indicates the size of the unit cell. The angle q is the chiral angle of the tube, (b) Positions of the (m,n) vectors on a graphite sheet. Metallic chiralities are indicted with a full circles and semiconducting tubes are indicated with open circles.

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