Other Fullerenes

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

The significance of the magic number 32 found in the experiment may also be stated in a different manner. If a cluster containing Ba and a fuUerene molecule will be stable and, thus, result in a clearly discernible structure in the mass spectra every time there is exactly one Ba-atom situated on each of the rings of the ful-lerene molecule, this property might be used to count the rings of a fullerene. Of course, such a proposal has to be verified using other fullerenes, for example, C70 which is available in sufficient quantity and purity for such an experiment. 

The synthesis of molecular carbon structures in the form of C q and other fullerenes stimulated an intense interest in mesoscopic carbon structures. In this respect, the discovery of carbon nanotubes (CNTs) [1] in the deposit of an arc discharge was a major break through. In the early days, many theoretical efforts have focused on the electronic properties of these novel quasi-one-dimensional structures [2-5]. Like graphite, these mesoscopic systems are essentially sp2 bonded. However, the curvature and the cylindrical symmetry cause important modifications compared with planar graphite. 

Since the discovery of buckminsterfullerene (C o) [85NAT(318)162] and other fullerenes (Cig. C70), these molecules have been intensely studied, both experimentally and theoretically. Likewise, several reports on heterofullerenes containing heteroatoms such as nitrogen and boron have appeared (91JPC4948 91JPC10564). The incorporation of heteroatoms is expected to modify the structural and electronic features of these structures, and have thus attracted some interest. 

Langmuir-Blodgett Films Containing Other Fullerene Materials... 

Let us note in addition that the layered sulfides M0S2 and WS2 have been found to form nanotubes and other fullerene-type structures, on account of their highly folded and distorted nature that favors the formation of rag and tubular structures. Such materials have been synthesized by a variety of methods [78] and exhibit morphologies, which were described as inorganic fiillerenes (IF), single sheets, folded sheets, nanocrystals, and nested IFs (also known as onion crystals or Russian dolls ). 

The hollowness of C60 and other fullerenes has tempted many to encapsulate one or more elements to change the physical and chemical properties of... 

Bang J, Guerrero P, Lopez D et al. (2004) Carbon nanotubes and other fullerene nanocrystals in domestic propane and natural gas combustion streams. J Nanosci Nanotechnol. 4 716-718. 

The first three reduction processes are well formed, whereas there is some doubt concerning the fourth cathodic process, which may be due to overlapping processes or impurities of other fullerenes. 

It is evident that the two fullerenes are strongly electronically communicating in spite of the interposed presence of the Rh6 cluster. A partial contribution to such a strong interaction could, however, arise also from the slight unequivalence of the two C6o environments. In fact, one fullerene is linked to a Rh3 triangle coordinated to a carbon atom (of one isocyanide ligand) and a phosphorus atom (of one diphosphine), respectively, whereas the other fullerene is linked to a Rh3 triangle which coordinates two phosphorus atoms (of the two diphosphines). 

One of the aspects that has been of interest is the incorporation of an external atom in the spheroidal cavity. A variety of metal atoms can, in principle, be trapped in this cavity. Some of the studies have claimed that it is possible to push atoms such as lanthanum, iron and helium inside the spheroidal cavity of CgQ and other fullerenes. Substitution of the carbon in CgQ by boron and nitrogen has been attempted. Interestingly, nitrogen not only substitutes for carbon in the cage but also adds on to Cgo and C-iq. 

Endohedral Fullerene Complexes, Nanotubes and Other Fullerene-based Supramolecular Systems... 

All other fullerene CCbonds are not altered significantly on complexation... 

The C60 molecule is difficult to destroy by ultraviolet radiation or by collisions with other particles. While other molecules have serious difficulties to survive in the interstellar medium, the robustness of C60 and of the other fullerenes allows their long survival. The bonds between carbon atoms make them at least as robust against dissociation in the interstellar medium as polycyclic aromatic hydrocarbons (PAHs) can be. 

Several proposals have been put forward to account for the formation of C 0F and the other fullerenes. Before reviewing these proposals, it seems appropriate to consider the key observations about fullerene synthesis, the kinds of carbon clusters possible, their energetics, and the thermodynamic and kinetics of carbon clustering. 

Our focus is upon the fullerenes. In particular, the stability of C60 and C70 relative to the other fullerenes and to each other is of primary interest because these two species are the fullerenes produced in highest yield in carbon arcs and in combustion. Quite a few calculations mainly using various semi-empirical theories have been carried out to determine the stability of the fullerenes. Figure 2 shows the recent results of Scuseria s group (Strout et al. 1993), who carried out minimal basis set STO-3G SCF calculations on several fullerenes. The smooth curve fitted through... 

As an example of the NEGF-DFT formalism discussed in the last section, we now report an analysis on the transport properties of an Au-Ceo-Au molecular tunnel junction whose device structure is shown in the lower panel of Fig. 1. So far a considerable amount of effort has been devoted to investigate transport properties of Ceo and other fullerene molecules both experimentally [54-59] and theoretically [25,60-62]. However, to obtain a complete picture of the transport properties of such junctions, many details have yet to be clarified, including how conductance and I-V curves depend on the lead material and geometry, and on the position and orientation of the Cgo molecule. Ceo tunnel junctions with Au leads have not been studied before. 

Finally, in a novel application, some uniquely structured hexapyrrolidine derivatives of C60 with Th and D3 molecular symmetries have been synthesized and characterized by analytical methods and x-ray crystallography [260]. This work revealed strong luminescence, indicative of photophysical properties that are unusual in comparison with other fullerene derivatives. Therefore, the hexapyrrolidine adduct was utilized as a chromophore in the fabrication of a white light organic LED [261]. 

These rich bonding and cohesive properties of C60 revealed after its macroscopic production are far beyond the expectation of theorists who dreamed of using clusters as atomlike building blocks of new materials. In the era of nanoscience and nanotechnology, C60 and other fullerenes will continue to play an important role. 

The sample preparation of endohedral metallofullerenes was done by Shino-hara and details are described in the review article [16]. The soot containing M C2 (M=Sc and La) was produced in direct-current (300-400 A) spark mode under He flow at 50 torr and collected under totally anaerobic conditions. The target fullerenes were separated and isolated from the various hollow fullerenes (C60-C110) and other metallofullerenes by the two-stage high-performance liquid chromatography (HPLC) method by using two complementary types of HPLC columns. The purity of the metallofullerenes used for structure analysis relative to other fullerenes was always more than 99.9%. 

In addition to C60, many other higher homologs have been prepared and characterized. Several synthetic routes to fullerenes have yielded gram quantities of pure C60 and C70, whereas C76, C78, Cso, Cs2, Cs4 and other fullerenes have been isolated as minor products. Figure 14.1.5 shows the structures of the following fullerenes C20, C50, C70, and two C78-isomers. 

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