Carbon nanotubes structure features

Several structural characterisations of carbon nanotubes (CNTs) with the cylindrical graphite are reviewed from the viewpoint of transmission electron microscopy (TEM). Especially, electron energy loss spectroscopy (EELS) by using an energy-fdtered TEM is applied to reveal the dependence of fine structure of EELS on the diameter and the anisotropic features of CNTs. 

Raman spectroscopy has been used to determine various features and characterization of carbon nanotubes, and has been instrumental in determining 1D electronic structure and mechanical strength and compliance of these materials (69, 61). 

Single-wall carbon nanotubes are new types of nanomaterial, the study of which generates about five research papers [234] from around the world, each day. An important feature of these structures is that the aromatic rings of the folded graphite sheet that constitutes the tube, are no longer planar. This feature represents a new challenge for accepted theories of 7r-bonding. 

During the last years, a growing interest in the creation of micro- and nanoelectronic devices by use of the swift heavy ion track technology in a combination with carbon nanotubes (CNTs) is observed in several research centers worldwide. The CNTs were grown in etched ion tracks in Si02 layers on Si. For this purpose, Ni-catalyst nanoclusters were electrochemically deposited within the ion tracks. The geometry of the obtained nanostructures has been analyzed. Structure features of CNTs obtained by themial chemical vapor deposition have been investigated. 

Since the discovery of the carbon nanotubes, there has been considerable work on other layered materials such as M0S2, WS2 and BN to explore the formation of nanotubes of these materials. Indeed several of them have been synthesized and characterized [21-23]. Similarly, nanowires of various inorganic materials have also been made [21]. In this chapter, we shall present the various important aspects of carbon nanotubes including their preparation, structure, mechanism of formation, chemical substitution, properties and applications. The methodologies developed for synthesizing nanowires and nanotubes of various inorganic materials as well as their salient features will also be discussed [21-24]. 

The carbon nanotubes also have a lot of structural features in common with the carbon cages described in Chapter 2. Contrasting some fidlerene species, however, they do not occur naturally in any form, neither on earth nor in space, so they are a completely artificial form of carbon indeed. The question of whether nanotubes are a real modification will be discussed further below. In any case they exhibit bent graphene layers, but while the whole three spatial directions are affected in the fuUerenes, the curvature is limited to two dimensions here. Hence, the incorporation of five-membered rings-indispensable to achieve closure of the bowl in fullerenes-is unnecessary for the construction of nanotubes. It suffices to bend the graphene sheet from its plane and make it a cylinder. Compared to fuUerenes... 

A principal distinction can be made between single-walled nanotubes (SWNT) and multiwalled nanotubes (MWNT). Both classes comprise species of most different diameters and lengths. Besides dimensions, it is also the way the graphene layer is rolled up to be a tube that dominantly influences the properties of the resulting materials. Furthermore, there may or may not be caps at the tubes ends, the respective structures then are called closed or open carbon nanotubes. The structural features of single-walled nanotubes will be discussed first in the following before the concept shall be extended to the multiwalled variants then. 

Apart from the hitherto described single-waUed and multiwalled carbon nanotubes, there is a variety of further structures with tubular or related geometries. These include bamboo-hke and so-called cup-stacked nanotubes as well as nano-homs and hehcal nanotubes. like normal SWNT and MWNT they consist of curved graphene layers, but they feature additional structural peculiarities that will be considered in the following sections. 

All known types of carbon nanotubes are potential field emitters (Figure 3.55b). Single and bundled SWNT as well as multiwalled species show this behavior. Individual tubes may be attached to a conductive support, but they might just as well be employed in the shape of ordered or unordered films or as structured arrays (Section 3.3.5). The respective products can be selected depending on the desired application. The choice of single- or multiwalled species influences emission properties, too SWNTs feature a low work function, whereas MWNTs better suit continuous use because they are clearly more stress resistant... 

There is yet another phenomenon that points to a 1,4-addition of fluorine atoms occurring at least partially The products obtained from the reaction of single-walled carbon nanotubes with fluorine are definite nonconductors (resistance >20 MQ), while the nanotubes employed feature a resistance of 10-15 2 only. Considering the structures that result from a consecutive 1,2- or 1,4-addition of fluorine, respectively, reveals that an electric current via conjugated it-bonds would still be possible in the 1,2-adduct (Figure 3.70). In the 1,4-adduct, on the other... 

A wide variety of carbon materials is known for their applicability as a catalyst or support of it. Especially the activated carbons with their large specific surface can be employed. But also carbon nanotubes and, to some degree, the fullerenes may be used to the same end. Besides, carbon onions and onion-like carbons are promising for catalytic applications, too. They feature a considerable specific surface, bear little structural defects (at suitable preparation), and they are stable over a wide range of temperatures. 

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