Fullerene concentric shells

Another important group of fullerene-related compounds are the carbon nanotubes, the reference compounds of the carbon nano-science. They may be obtained by convenient variations of the preparation procedure mentioned. These are long tubular fullerenes with a concentric shell structure a few nanometres wide and often capped with C60-like hemisphere or faceted tips. They are mechanically very strong and either metallic conducting or semiconducting types have been obtained. 

Indeed, the smallest tube that we measured had a diameter of 10 A, which is of the size of C o- predicted to be the limiting case of vapor-grown graphitic tubes with monolayer thickness (14). Our observation of hemispherically capped 10 A tubes suggests that an incomplete cluster is the nucleation seed for these tubes. The 60 derived tube could be the core of possible multilayer concentric graphitic tubes. After the fullerene-based tube has been formed, further concentric shells can be added by graphitic cylindrical layer growth. 

In contrast to the spherical carbon onions observed in the first experiments by Ugarte, OLC particles were subsequently produced with polyhedral facets, more closely matching the polyhedral structures predicted from the consideration of nested fullerene structures described above. These polyhedral onion-like particles were synthesized by vacuum heat treatment of carbon sooF and diamond nanoparticles." Figure 10.5 presents HRTEM images of the polyhedral OLC particles produced in the experiments of Kuznetsov et al. The range of synthesis methods available has led to the production of different types of OLC. In addition to their shape, such carbon onions can be characterized by other parameters, such as the number of concentric shells, the spacing between adjacent shells, the size of the innermost shell, and the presence of different types of defects. 

Henrard, L., Malengreau, F., Rudolf, P. et al., 1999, Electron-energy-loss spectroscopy of plasmon excitations in concentric-shell fullerenes, Phys. Rev. B 59, 5832-5836. 

Fig. 4.25. Diagram respresenting a hypothetical carbon vapor nucleation scheme leading to concentric shells as those observed in graphite microparticles. Fullerene formation may occur by statistical closure at stage (c). Reproduced with permision from Kroto HW, McKay KG (1988) Nature 331 328...

Carbon onions are a member of the family of nanometer-scale graphite-like aU-carbon allotropes, the emergence of which was catalyzed by the Nobel Prize-winning discovery of the first member, the fullerene, by Kroto et al. in 1985. Initially, carbon onions were observed by lijima in 1980, and were brought to popular attention by the experiments of Ugarte in 1992. Structurally, they consist of concentric spherically closed carbon shells and receive their name from the close resemblance between their nanoscale structure and the more familiar concentric layered structure of an onion. Closely related to carbon onions is a class of material known as onion-like carbons (OLCs), which include polyhedral nanostructures such as ideal nested fullerenes. This material, rather than ideal spherical carbon onions, can be currently produced in macroscopic quantities, and, hence, be used for future applications. 

There have been numerous IR, Raman and resonance Raman studies on single-walled carbon nanotubes. " IR and Raman spectroscopy were used to characterise multilayer carbon nanotubes at different stages of their purification. The radial vibrational mode of the nanotube walls was found at 120 cm". The Raman spectra of carbon spherical shells, onions , contained a band at 1572 cm" - the shift of 10 cm" to lower wavenumber compared to graphite was ascribed to the effects of shell curvature. FUR spectra were reported for carbon onions produced by carbon-ion implantation at 500 °C. Two families of bands were identified one due to Ceo and one to C240 species. Therefore these onions comprise concentric spheres of Ceo and C240 fullerenes. Other vibrational studies on carbon nanosystems have also been reported. " ... 

Xu et al. [78] have synthesized gold (Au)-silica nanorods with shell thickness of 0-10 nm incorporated into the bulk heterojunction of organic solar cell and observed that at optimal (1 wt. %) concentration, Au-silica nanorods with 5 nm shell thickness resulted in the highest power conversion efficiency of 8.29%. A 26% relative enhancement in PCE was observed for a low bandgap polymer fullerene bulk heterojunction (BHJ) device after incorporation of 1% weight ratio of Au-silica nanorods by Xu et al. [79]. A similar enhancement was found when Ag-silica NPs were introduced at the interface of the anode buffer layer and active layer of a polymer fullerene BHJ device by Choi et al. [80]. Yang et al. [81] have developed a novel hole transport layer (HTL) composed of ultrathin two-dimensional, molybdenum disulfide (M0S2) sheets decorated with 20 nm... 

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