Electron carbon onions

Hollow carbon nanostructures are exciting new systems for research and for the design of potential nano-electronic devices. Their atomic structures are closely related to their outer shapes and are described by hex-agonal/pentagonal network configttrations. The surfaces of such structures are atomically smooth and perfect. The most prominent of these objects are ftil-lerenes and nanotubesjl]. Other such novel structures are carbon onions[2] and nanocones[3]. 

S. Tomita, M. Fujii, S. Hayashi, K. Yamamoto, Electron energy-loss spectroscopy of carbon onions. Chem. Phys. Lett., 305(3-4) (1999) 225-229. 

From observations in transmission electron micrography (TEM), carbon soot particles are found to have an idealized onion-like shelled structure, as shown in Fig. 14.1.8. Carbon onions varying from 3 to 1000 nm in diameter have been observed experimentally. In an idealized model of a carbon onion, the first shell is a C6o core of /h symmetry, the second shell is C240 (comprising 22 x 60... 

Figure 9 Transmission electron micrographs of nonplanar, carhon structures in which the inter-layer distances correspond to graphitic spacing ( 3.4A). (a) MultisheU carbon onions. (Reprinted with permission of MacmiUan from D. Ugarte. ) (h) Carhon nanotuhes. (Reprinted with permission of Macmillan from T.W. Ehhesen and P.M. Ajayan. )...

D. Ostling, Electronic Strucuture and Optical Properties of Ceo Nanotubes and Carbon Onions, PhD Thesis, Goteborg University 1996. 

Very little is known about the physical properties of carbon onions. Electron spin resonance measurements on macroscopic quantities of onions, with 3-10 nm sizes, show that these structures have a Pauli-like spin susceptibility close to that of graphite [181]. It demonstrates that carbon onions also belong to the family of conducting carbon structures. 

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). 

The generation of carbon onions in space has not yet been fuUy elucidated. However, it seems reasonable to assume that they originate from nanoscopic diamond particles. These may be converted into carbon onions upon heating, electron bombardment, or intensive irradiation (Section 4.3.5.4). The existence of nanodiamonds in extraterrestrial material could be confirmed by analyses on different meteorites. Especially the AUende meteorite contains significant amounts of tiny diamond particles (Section 5.1.2). 

Actually, the first signs of carbon onions existing in such objects have been found in this very meteorite. Electron microscopic pictures (Eigure 4.8) show... 

Figure 4.17 The conversion of soot (a) into carbon onions (b) may also be affected by the electron beam under an electron microscope (black and white arrows indicate onions with concentric or with spiral core, respectively, ACS 2002).

Figure 4.19 A carbon nanoparticle filled with metal transforms into an onion under electron bombardment while the metal particle emerges from the onion intact. Shown here is the migration of a gold particle from the center of an evolving carbon onion ( Elsevier 1993).

There is a major drawback, however, to the preparation of carbon onions by electron bombardment the amounts obtained are extremely small and thus render the examination of bulk properties virtually impossible. Only high-energy electron sources outside an HRTEM would enable the production of macroscopic amounts. S till a further development of this method is of considerable interest as the carbon onions made from diamond are very uniform in quality. 

Growth Mechanisms of Carbon Onions Obtained by Electron Irradiation... 

Numerous spectroscopic methods have been applied to examine the physical properties and to elucidate the structure of carbon onions. They include IR- and Raman spectroscopy. X-ray diffraction, electron energy loss spectroscopy (EELS), absorption, and photoluminescence spectroscopy and NMR-spectroscopy. Each of these methods gives account of certain aspects of the geometric and electronic structure, so altogether quite a detailed picture is obtained of the situation in carbon onions and related materials. There is, however, a strong dependency on... 

Figure 4.36 Electron energy loss spectra of spherical and faceted carbon onions (a) low-loss region and (b) core-loss region ( Elsevier 1999).

Other authors, on the contrary, postulate the spherical carbon onions to be extremely unstable at least upon observation (not irradiation ) in the HRTEM, so they would retain their ball shape only at sufficient electron intensity. They decompose into unordered graphitic material inward from the outer shell. The instabihty is attributed to the system being far from thermodynamic equihbrium. The electron radiation maintains a steady state of onion formation and decomposition balancing each other. When it stops, the structure will tend toward equilibrium. Thus, carbon onions are considered a high-energy modification of carbon in this case. 

Appreciable amounts of perfectly spherical carbon onions are hard to obtain, and so only few experimental data on their electronic properties are available. For the irregular onion-hke carbon that may for instance be prepared from nanodiamond, on the other hand, the conductivity and other parameters have been studied much more extensively. 

Apart from the irradiation with high-energy electrons, the conversion of carbon onions into diamond also succeeds by bombardment with ions like Ne. The latter are 36000 times heavier than the rather light-weight electrons. Consequently, they require far less velocity and thus smaller accelerator voltages to bear the same effect Diamond-like structures can further be generated by thermal treatment in air at 500 °C or by irradiation with a C02-laser. 

Carbon onions can be produced by converting other forms of carbon by electron bombardment or heating. Furthermore, modified arc discharge methods or the implantation of carbon ions into metal substrates can be employed. 

---