Pentagons, heptagons and negative curvature in graphite microtubule growth

Abstract

THE standard carbon-arc synthesis for fullerenes also produces graphitic microtubules with helical structures¹. In most cases the cylindrical tubes are closed by polyhedral caps, some being first transformed into a conical shape before closure². Here we present images from transmission electron microscopy of a further kind of growth morphology, in which cone-like growth is transformed into cylindrical growth by the incorporation of a defect that induces negative curvature. We suggest that the defect in the hexagonal network responsible for negative curvature may be a single heptagonal ring. Three-dimensional, open negatively curved graphitic structures have been proposed recently by Terrones and Mackay³. We discuss more generally the effect of pentagons and heptagons on the growth morphologies of these tubules, and the constraints on the number of pentagonal defects imposed by tube closure.