Structure and properties of graphullerene: a semiconducting two-dimensional C60 crystal

Abstract

Graphullerene is a recently discovered, two-dimensional allotrope of carbon formed from C60 molecules. It has been synthesized in the form (C60Mg4)n and subsequently transformed into (C60)n by removal of the Mg atoms. Ab initio calculations are employed to examine the structure and properties of this material. Structurally, graphullerene is composed of strained C60 molecules. Each of these molecules is connected to six neighbors in a hexagonal network with a total of eight chemical bonds. We find this structure to be meta-stable, owing to the strain produced by the covalent bonding of the molecules. However, the inclusion of Mg atoms transforms the cohesion energy from negative to positive values by forming additional C-Mg bonds, creating an energetically stable material. In the absence of Mg, this allotrope is a pure carbon semiconductor with an indirect band gap. Phonon spectrum calculations confirm the dynamical stability of the material and yield its in- and out-of-plane sound velocities. In addition, symmetry breaking of the C60 molecules results in a distribution of bond lengths and creates vibrational modes that serve as a signature of graphullerene.