Electronic properties, carbon allotropes

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

Shortly after the discovery of the fullerenes in 1985 (Kroto et al., 1985) and especially after their accessibility in macroscopic quantities (Kraetschmer et al., 1990) these new carbon allotropes raised great interest in the chemical world due to their unique structural and electronic properties. As a direct consequence of the curved conjugated n-systcm fullerenes were predicted to be fairly electronegative with the... 

An attractive feature of fullerenes is the interaction of their n electrons with fluorine, which has given a very broad array of fullerene fluorides as is described in the previous section. Electronic properties of these new fluorides based on the third allotrope of carbon are of great interest and importance from the view points of materials science and potential use. In fact, as will be described in the next section, electrochemical properties such as electromotive force and reduction potential strongly reflect the changes in the C6o electronic structure through fluorination. 

Structural Studies. In a number of communications,22-26 correlations have been sought between both the spectroscopic and chemical properties of the various carbon allotropes and their structures. Thus, an electron-energy-loss spectroscopic study22 of diamond, graphite, and amorphous carbon has shown that the differences in the X-shell ionization loss spectra of the three allotropes (Figure 1) might be the basis of a technique for distinguishing... 

Fullernes form a group of carbon allotropes. There are spherical fullerenes nicknamed buckyballs and cylindrical fullerenes known as buckytubes or nanotubes. Fullerenes have yet to display all of their capabilities to scientists. One of the most promising areas of fullerene research involves the creation of nanotubes. Nanotubes are sheets of carbon that are rolled up into cylinders. These cylinders are strong—due to the hexagonal structure of the carbon atoms—and have unique conducting properties. Fullerene nano-technology on the horizon includes the development of faster computer chips, smaller electronic components, and more advanced space-exploration vehicles. 

The extraordinary electronic properties of graphene have spurred the search for other two-dimensional carbon allotropes. Graphene s electronic properties are related to its exhibiting Dirac cones and points, where the valence and conduction bands meet at the Fermi level at these points it may be considered a semiconductor with a zero band gap. The allotrope 6,6,12-graphyne has been predicted to have two nonequivalent types of Dirac points—in contrast to graphene, in which all Dirac points are equivalent—and may therefore have more versatile applications. ... 

Diamond De (Fig. 11.1), the classical, beautiful and useful diamond has kept its leading interest among the carbon allotropes, even as the newer nano varieties. Along with electronic properties, the mechanical characteristics appear of great... 

This chapter discusses the use of carbon-based electrode materials in the construction of MJs and the use of carbon-based materials in related studies (such as electrochemical experiments and in the construction of other electronic devices). The methods for making MJs are first outlined, followed by the use of the more novel allotropes of carbon. These materials have interesting electronic properties that provide additional opportunities for their application in molecular electronics relative to more conventional carbon materials. Finally, some ofthe considerations that dictate charge transport across molecular layers in MJs are discussed before we leave with some future prospects. 

In addition to traditional carbon materials, novel allotropes of carbon have unique electronic properties that may be exploited to provide some advantages to electronic device fabrication. These may serve as electrode materials in molecular electronics, or may form the basis of extant electronic devices with specifications that exceed those of existing electronics. In any case, carbon electrodes provide a rich area of study for molecular electronic systems. 

This chapter is divided into ten sections (1) introduction (2) allotropic forms of carbon (3) processing routes of carbon (4) structure of some novel phases of carbon (5) electrical and electronic properties of conducting carbon (6) electronic structure to explain electrical and optical properties (here we introduce the mechanism of conduction, interaction between carriers, localization, and the role of hydrogen concentration and dopant in the conductivity of carbon films) (7) optical properties (8) spectroscopic study (IR, Raman) (9) defect study in amorphous carbon and (10) applications and conclusions. We wish to give a view of novel forms of carbon and to analyze their special characteristics rather than review the well-known earlier work. The interrelationship among the different sections gives a complete picture of amorphous carbon and its importance at present from various aspects. 

Since its discovery [199], CNTs have attracted enormous attention as a novel catalyst material due to their high aspect ratio and unique electronic properties [166,167,172,174,175,177,178,199-206]. CNT is an allotrope of carbon with a cylindrical nanostructure made from curved graphite sheets. CNTs can be classified as single-walled nanotubes (SWNTs), double-walled nanotubes (DWNTs),... 

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