Carbon nanotube semiconducting CNTs

Studies on the electronic structure of carbon nanotube (CNT) is of much importance toward its efficient utilisation in electronic devices. It is well known that the early prediction of its peculiar electronic structure [1-3] right after the lijima s observation of multi-walled CNT (MWCNT) [4] seems to have actually triggered the subsequent and explosive series of experimental researches of CNT. In that prediction, alternative appearance of metallic and semiconductive nature in CNT depending on the combination of diameter and pitch or, more specifically, chiral vector of CNT expressed by two kinds of non-negative integers (a, b) as described later (see Fig. 1). 

Carbon nanotube films possess enormous potential for a variety of applications. The major limitations at present are associated with heterogeneity of as-synthesized nanotubes and with difficulties in separating CNTs with semiconducting and metallic characteristics. If this problem will be solved in... 

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. 

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]. 

The G-band is an intrinsic feature of carbon nanotubes that is closely related to vibrations in all sp2 carbon materials, and allows distinguishing whether the nanotube is semiconducting or metallic. The profile shown in Figure 10.3 reveals the semiconductor character of these CNTs. The intensity of the G-band can be related to the SWCNT concentration in the sample. Although is not a quantitative measurement it can be used as a relative indicator of the sample purity. The D-band usually creates ambiguities whether to be assigned to disordered carbon forms (purity) or to SWCNT defects (quality). Nevertheless, the relative strength and width of the D-band has been used as qualitative measurement of the fraction of defects present in the sample. The quality/purity of the SWCNTS can be evaluated by the G/D intensity ratio values obtained for the different SWCNTs are listed in Table 10.1. 

CNTs are commonly classified into single-waUed (SWCNTs) and multi-walled (MWCNTs) nanotubes [28]. SWCNTs consist of a single graphene layer rolled up into a hollow cylinder and are either metallic or semiconducting, whereas MWCNTs are comprised of two, three, or more concentrically arranged cylinders and exhibit only metallic character. Double-wall carbon nanotubes (DWCNTs) are the most basic members of the MWCNT family. The special role of DWCNTs should be emphasized, as they are the link between SWCNTs and the more complex MWCNTs and, therefore, of great interest for a fundamental understanding of these novel nanostructures. 

Electronically, the carbon nanotubes can have either metallic or semiconducting properties depending upon their chirality and diameter. The electronic gap is also controllable within a range of 0-l eV. This gives rise to possible metal-semiconductor or semiconductor-semiconductor junctions for use in nanoelectronic devices. The possibility of integrating carbon nanotubes into logic circuits was demonstrated in 2001. Another property of CNTs, the ability of electron field emission, has reached technological relevance. 

Figure 3.1 DOS for (11,0) semiconducting (left) and (12,0) metallic (right) CNTs showing van Hove singularities based on a tight-binding model. Reprinted (adapted) with permission from Anantram, M. P., and Leonard, F., Physics of carbon nanotube electronic devices. Rep. Prog. Phys., 2006. 69 pp. 507-561. Copyright (2006) Institute of Physics and lOP Publishing Limited.

It is well known that the percolation threshold is strongly dependent on the aspect ratio. One-dimensional (ID) nanostructures, such as nanowires, nanotubes, and nanoribbons, have geometries that are favorable for the maintenance of connectivity at low content of active materials. Carbon nanotubes (CNTs) are a good representative example. Electrical percolation can be achieved in polymer composites with well-dispersed single-walled carbon nanotubes (SWCNTs) at levels as low as 0.03 wt% [69]. Recently, it has been reported that the formation of semiconducting nanofibers facilitates percolation in semiconducting/insulating polymer blends. 

Recently, we have demonstrated the use of reaxFF to the challenging problem of elucidating the growth process of carbon nanotubes (CNTs). Understanding this process is critical for determining the control variables that lead to chiral-specific (with semiconducting or metallic electrical conductivity behavior) mass productimi of CNTs. These results are summarized in the following section. 

For example, Collins et al. (2001) proposed a method based on the selective destruction of metallic nanotubes, which could be realized with bursts of electricity. The ropes containing both semiconducting and metallic tubes are deposited on a flat surface of silicon oxide. Then electrodes on the top of the ropes are fabricated by lithography technique. A voltage is applied to the tubes through the electrodes. The metallic CNTs are destroyed by current-induced oxidation, and only the semiconducting tubes remain (carbon nanotubes transistors, http //www.research.ibm.com). However, this technique is only useful for transistor geometries and cannot be extended to bulk separation. 

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