Carbon Onion Films-Molecular Interactions of Multi-Layer Fullerenes
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Carbon Onion Films-Molecular Interactions of Multi-Layer Fullerenes Raed A. Alduhaileb,1 Virginia M. Ayres,1 Benjamin W. Jacobs,1 Xudong Fan,1 Kaylee McElroy,1 Martin A. Crimp,1 Atsushi Hirata2 and Mutsumi Horikoshi2 1 College of Engineering, Michigan State University, East Lansing, MI 48824, U.S.A. 2 Graduate School of Mechanical Sciences, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
ABSTRACT The evolution of carbon onion structure from spherical to polyhedral is correlated with changes in the sp3/sp2 ratio as a function of increasing synthesis temperature using electron energy loss spectroscopy, high resolution electron microscopy and scanning electron microscopy. Results, which are obtained using asymmetric f-variance and symmetric Gaussian deconvolution of electron energy loss spectrum, are compared. The possibility of a separate peak at 287 eV is also discussed. INTRODUCTION Multi-shell fullerenes, or carbon onions, are under investigation as a nano-property enabled solid lubricant. The potential applications for a carbon onion-based lubricant range from an environmentally benign option for wind power to a vacuum lubricant [1] for solar panel deployment in space. These uses of carbon onions depend on both their individual properties such as mechanical strength, and their interaction properties. When carbon onions are applied as a thin lubricating film, their stiction, rolling, and sliding interactions, with each other and with the wear surfaces govern their ultimate usefulness, in addition to their individual mechanical load-bearing characteristics. Carbon onion physical structures are known to vary with synthesis temperature. Several authors have reported the structural evolution from spherical to polyhedral multi-shells as a function of increasing synthesis temperature [2,3]. It has been generally assumed that the structural evolution is accompanied by a change in the sp3/sp2 ratio, since a reduction in potential sp3 defect sites, which are visible as broken shells in high-resolution electron microscopy (HRTEM) images, is observed. However, broken shells may also terminate in amorphous carbon networks that are more sp2 than sp3, while individual sp3 point defects could be very hard to detect based on HRTEM images alone. Accurate knowledge of a systematic evolution of the sp3/sp2 ratio is important for the synthesis of carbon onions for an optimum lubricating film, especially at the nano-scale, since sp2 carbons interact principally through p-electron overlap, while sp3 carbons exhibit local dangling bond sites. In the present investigations, electron energy loss spectroscopy (EELS) is used as a sensitive measure to quantitatively determine the sp3/sp2 ratio of a series of carbon onion samples grown at increasing synthesis temperatures of 1700oC, 2000oC and 2300oC. This temperature series exhibits the spherical to polyhedral structural transition, as determined using HRTEM. Additionally, scanning electron microscopy (SEM) was used to characterize synthesis un
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