Carbon onions, discovery

As early as 1980, even before the discovery of the fullerenes, that is, S. lijima reported on the preparation of multilayered, spherical particles of graphitic character. He conceived them to be an sp sp -hybrid material, and his results went largely unnoticed. The structures described then were first interpreted as carbon onions only after the determination of the fuUerenes structure and after D. Ugar-te s finding that particles of fullerene soot may be transformed into multilayered fullerenes by electron irradiation (Figure 4.1). 

In the cosmos, however, carbon onions possibly exist. Soon after their discovery they have already been discussed as a potential reason for an up to then not interpretable absorption at 217.5 nm in the spectrum of interstellar space. The absorption spectrum of carbon onions closely resembles that of interstellar dust indeed. A red shift is observed on the occasion, yet this may be explained by the measurements being made in different media (water or vacuum, respectively Section 4.4.1.3). 

Soon after the discovery of carbon onions, their absorption behavior has been studied both in visible and UV hghL One motivation to do so was the assumption that they might be the long-sought source of an absorption band at 217.5 nm (4.60 (tm ) in the spectrum of interstellar dust. 

The question of the thermodynamic stability of carbon onions has been subject to controversial discussion ever since their discovery. Some groups postulate them to be the thermodynamically most stable phase for this size of particles, while... 

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. 

The transformation of carbon nanoparticles to carbon onions can be stimulated by exposure to an intense beam of electrons. Indeed, Ugarte s experiment " demonstrated for the first time that closed, curved carbon nanostructures can be produced not only by the condensation of carbon vapor but also by the transformation of condensed carbon nanoparticles. The other very important aspect of this work was the realization that electron irradiation not only has a destructive effect on carbon materials but can actually be used to synthesize new carbon nanostructures. This discovery opened a completely new field of carbon materials research related to stability and transformation of carbon nanostructures. It was found that not only carbon soot can be converted into carbon onions but also diamond crystals. Qin and lijima demonstrated that carbon onions can form on the surfaces of 1-3 pm diamond crystals under intense electron beam irradiation (150 A/cm ). Growth of carbon onions (which consisted of between 4 and 10 closed graphitic shells) proceeded from the inner shell because of the high mobility of carbon atoms induced by electron irradiation. However, further irradiation resulted in the destruction of the carbon onions. Transformation of nanometer-sized diamond crystals (3-10 nm in diameter) to carbon onions under electron irradiation (20 0 A/cm ) was later observed by Roddatis et al. ° In this case, complete transformation of nanodiamond crystals occurred, starting from their surfaces and proceeding inward. 

Since the discovery of carbon onions, it has been believed that they are a component of the interstellar dust and that they contribute to the strong absorption band centered at a wavelength of 217.5 nm.23 3o. ). 7,88 Optical transmission spectroscopy measurements in the wavelength range of... 

The discovery of perfect geodesic dome closed structures of carbon, such as C o has led to numerous studies of so-called Buckminster fullerene. Dislocations are important features of the structures of nested fullerenes also called onion skin, multilayered or Russian doll fullerenes. A recent theoretical study [118] shows that these defects serve to relieve large inherent strains in thick-walled nested fullerenes such that they can show faceted shapes. 

One of the main scientific issues of the discovery of the bucky-onions is the unresolved question of minimal energy configuration of carbon clusters (onion-... 

The discovery of fullerenes in 1985 led to the era of nanomaterials.1 The three-dimensional geometry of these molecules as well as their unique properties distinguishes them from conventional molecules encountered in organic chemistry. Due to recent discoveries in this field, the horizons of this area have broadened to encompass various new molecules such as endohedral fullerenes, nanotubes, carbon nanohorns, and carbon nano-onions. This chapter discusses the electrochemical behavior of some of these carbon nanoparticles with special emphasis on endohedral fullerenes. Since a large number of fullerene derivatives have been prepared and their various electrochemical studies in different solvents and electrolytes have been reported, the electrochemistry of these derivatives is beyond the scope of this text.2 3 Among the other carbon nanoparticles, the electrochemistry of derivatives of carbon nanotubes has been reported. These studies have been highlighted in the final part of the chapter. 

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