C72: A novel low energy and direct band gap carbon phase

Abstract

A novel carbon phase C72 was predicted by using recently developed particle-swarm optimization method on crystal structural search. The structure, mechanical, and electronic properties of the predicted C72 were investigated by performing the first-principles calculations based on density functional theory in this work. The predicted new carbon structure contains 72 carbon atoms per unit cell and adopts an all sp2 hybridized bonding network. The C72 is energetically more favorable (higher than that of diamond only 0.16 eV/atom) than previously theoretical proposed carbon structures (e.g., Hex-C36, hcp-C3, C24-D, BC8-carbon, and C80 etc.) at ambient pressure. The mechanical and dynamic stabilities of C72 are confirmed by the elastic constants and phonon dispersion spectra, respectively. The mass density of C72 is only 1.695 g/cm3, indicating C72 is a relatively lightweight carbon allotrope. The calculations of electronic band structure reveal that C72 exhibits a direct semiconductor character with a band gap of 1.52 eV. Because of its low energy, light weight, and direct band gap, C72 may possess potential applications in energy storage, electronics, optics, and photovoltaics.