Density functional theory investigation of linear carbon chains
In this thesis the structural and electronic properties of linear carbon chains are investigated using density functional theory. Polyyne structure of alternating single and triple bonds was shown to be energetically favored structure compared to successive double-double bonded cumulene structure. Band calculations showed that polyyne is a semiconductor whereas cumulene is a metal. Phonon calculations showed that cumulene is unstable. When put in a hexagonal formation these chains are found to form three possibly stable structures, one tightly bound hexagonal tube, and two loosely bound structures one which can be described as a hexagonal assembly of polyyne chains and one which can be considered stacks of hexagonal carbon flakes. Electronic band structure calculations showed that all three structures are semiconductors. 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 hexagon flakes. It is also found that as hexagon size shrinks the favored structure of chains changes from polyyne to cumulene and a band structure calculation showed that a semiconductor to metal transition happens.