A Novel Class of Carbon Nanocones
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A Novel Class of Carbon Nanocones Svetlana Dimovski1, Joseph A. Libera2 and Yury Gogotsi1,2 Department of Materials Engineering, Drexel University, Philadelphia, PA 19104, U.S.A. 2 Mechanical Engineering Department, University of Illinois at Chicago, Chicago, IL 60607, U.S.A. 1
ABSTRACT A new class of low-angle conical carbon crystals has been described and their characteristics are reported here. These carbon nanocones were found in pores of a glassy carbon (GC) along with cylindrical multiwall nanotubes and graphite polyhedral crystals. The largest cones reach 2 to 3 microns in length, although most are in the submicron range. Scanning Electron Microscopy (SEM) reveals cones protruding from the inner pore surfaces with the tips oriented toward the inside of the pores. Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) show that cones are made of thick (up to several hundred layers), highlyordered graphitic walls. Their tips are graphitic as well, and exhibit a dome-like morphology. These cones also differ from any carbon cones observed earlier in that they have a much smaller apex angle, sometimes less than 3o. Small angle carbon nanocones can potentially be used for probes and field emission elements. INTRODUCTION It is known that carbon can form nano-size cones [1] or conical filaments, which can further be classified as closed and open cones. The closed cones are formed when one or more pentagonal defects are incorporated into a graphene sheet, thus giving rise to a cone tip and a non-planar geometry of the graphite sheet out of which the cone wall is made. In an ideal case, if the defects are concentrated only at the cone tip, the cone walls remain straight. According to previous observations [2], apex angles of closed cones take discrete values that are in agreement with theoretically calculated values and they depend on number of pentagonal defects incorporated in a graphene sheet. Distribution of the defects determines the cone morphology as well. Open-tip carbon conical filaments were formed during catalytic growth of carbon nanotubes [3,4]. Their shape, apex angle and aspect ratio are determined by the geometry and surface characteristics of the catalytic particle that initiates the growth [4,5], and show a wide range of values. In this paper we describe nano-size graphite conical crystals (GCC) that are different from all carbon cones reported previously. EXPERIMENTAL We observed crystals in GL-200 glassy carbon made by Toyo Tanso Co., Japan. GL-200 glassy carbon was made from phenolic resin by carbonization at 2000°C in an N2 atmosphere at ~10 torr. The density of this GC was 1.48 g/cm3 and the open porosity
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