Investigation of hexagonally packed nanostructures of linear carbon chains from first principles

The structural and electronic properties of hexagonally packed nanostructures of linear carbon chains are investigated from first principles calculations based on density functional theory. First of all, polyyne structure of alternating single and triple bonds was shown to be energetically favored structure compared to successive double bonded cumulene structure. However, phonon calculations showed that cumulene is unstable. Band structure calculations showed that polyyne is a semiconductor whereas cumulene is a metallic. Hexagonal formation these chains exhibited three stable structures: tightly bound hexagonal tube, and two loosely bound structures, a hexagonal assembly of polyyne chains and stacks of hexagonal carbon flakes. Charge density profile showed strong chemical bonds both in vertical and horizontal directions for the first structure, whereas second structure of polyyne chains had no strong bonds between chains and third structure of hexagon flakes showed no strong bond between hexagonal flakes. Dynamical stability of these nanostructures were further investigated by phonon calculations. Eventually, electronic band structure calculations showed that all three structures are semiconductors.