Semiconducting tubes

It is important to mention DNA strands as one of the moieties that interact most efficiently with CNTs, yielding hybrids that exhibit excellent dispersion in aqueous media. Among the diverse applications, DNA-wrapping was employed to successfully separate metallic and semiconducting tubes [73]. 

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. 

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]. 

The radical breathing, in-plane graphitic G-band and disorder-induced D-band frequencies of SWNTs have been found to be dependent on temperature. A linear decrease in frequency with respect to increasing temperature has been identified. The temperature-dependent >d band for semiconducting tubes at 1565cm is 0.0226cm K. However, experimental and molecular dynamic simulation... 

It is known that the radical breathing mode has an inverse relation with respect to diameter d = A/co bm where A is a constant that can be obtained by theoretical models. A mean value of A from many models appears to be 234nmcm . This correlation is consistent with the experimentally determined semiconducting tube diameter distribution. The van dar Waals intertubular interaction between the individual tubes in a bundle also shifts RBM further to 14cm. Hence, the actual diameter is given by d = (A/curbm) + 14cm . ... 

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.

These properties are mainly originated from the molecular structures of CNTs, which consist of graphene sheets rolled to form hollow cylinders with an extremely high aspect ratio [5]. Moreover, CNTs can be ether metallic or semiconducting tubes, depending on their diameter and/or chirality [5]. There are two types of CNTs SWNTs and MWNTs. Details, such as structure, synthesis methods, application and properties, are well documented [4], [5], [6]. 

Other chemical transformations are suitable as well to discriminate between different types of tubes in a sample. The reaction with diazonium salts, for example, selectively functionalizes metallic nanotubes. Their solubility is enhanced by the surface modification so they may be separated from the unaltered, insoluble semiconducting tubes. Subsequent removal of the functional groups and annealing at elevated temperatures yield nanotubes with most of them being metallic conductors. As metallic nanotubes possess higher electron density close to the... 

Figure 3.51 LCAO-representations of the rolling up of (a) metallic or (b) semiconducting nanotubes, respectively. In the case of metallic tubes, two orbital lobes with identical sign meet, whereas for semiconducting tubes they do not ( Wiley-VCH 2004).

Figure 1. Raman spectra in the radial breathing mode region for a) a Nd YAG-produced sample with a H2 adsorption capacity of 7 wt%, and b) a sample produced with the Alexandrite laser operating with a 200 ns pulse width at a peak power of 10.5 MW/cm. The red curves (highest curve on each figure) are for Raman excitation at 632.8 nm and show the SWNT size distribution for excited metallic and semiconducting tubes. The blue curves (lowest curve on each figure) were obtained at 488 nm and show predominantly semiconducting tubes.

Collins et al. [63] and Sumannasekera et al. [64] later reported that electrical resistance R and thermoelectric power (TEP) of SWCNT bundles and thin films are sensitive to gas adsorption [Figure 14.7(b)]. The film conductance is changed dramatically upon exposure to O2, NO2, and NH3 gases, presumably due to charge transfer from the adsorbates on semiconducting tubes [6,63]. The TEP of SWCNT bundles was also found sensitive to inert gas such as N2 and... 

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