Structure, characterization and exploration of synthesis of conical and polyhedral crystals of graphite

Abstract

The present study describes the structure, Raman spectra and growth model of synthetic and naturally occurring conical and polyhedral nano- and micro- crystals of graphite. While planar graphite and carbon nanotubes have been extensively studied, and their structure and properties are well documented in the literature, the world of nonplanar multishell graphitic materials was somewhat neglected. With a growing tendency to design and utilize materials that will bridge the gap between traditional micro- and “emerging” nano- technologies, the importance of understanding such structures is continuously increasing. In the present work, Raman spectroscopy, scanning and transmission electron microscopy with electron diffraction were used as primary techniques to characterize the morphology and structure of conical and polyhedral nano- and micro- crystals of graphite. Such information was used to understand the environment suitable for their growth.Graphitic carbon cones (GCCs) and graphite polyhedral crystals (GPCs) have an elongated needle-like and, typically, axially symmetric morphology. The length of these low-symmetry crystals ranges from several tens of nanometers up to several microns. Graphite has been known to form seamless graphite nano- and micro-cones that can have up to five different apex angles determined by disclinations in graphite. Scrolled conical structures, which grow with a different mechanism, may have virtually any apex angle between 2-3 degrees up to 150 degrees. However, the “magic” angles of 19.2, 38.9, 60.0, 83.6 and 112.9 degrees are still most frequently observed. These apex angles are found to be energetically preferred as they allow the registry between graphene sheets in the cone. Polygonization of nanotubes accompanied by growth in the radial direction leads to the formation of graphite polyhedral crystals (GPC). They have nanotube cores and graphite crystal faces and remarkably unusual axial symmetries.The structure of carbon cones and polyhedral crystals has been studied, and the mechanism of their formation has been proposed in the present work. The study reports formation of small apex angle cones that have not been predicted by theory, and it also provides detailed information about the occurrence of carbon cones in nature. The morphology, structure and diffraction patterns of graphite polyhedral crystals were explained.%%%%Ph.D., Materials Science and Engineering – Drexel University, 2006