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Augmented Cylindrical Wave Green’s Function Technique for Impurities
Published in Pavel N. D’yachkov, Quantum Chemistry of Nanotubes, 2019
Before turning to the calculations of the single-walled nanotubes, let us look at the impurity-related levels in a carbyne, which is the simplest carbon nanowire with cylindrical symmetry. The polyynic carbyne is a linear chain of carbon atoms with alternating single and triple bonds. The LACW band structure shows that the polyynic carbyne is the direct gap semiconductor. Similar to the semiconducting nanotubes, the π bonding and antibonding states of the polyynic carbyne form the top of valence and the bottom of conduction bands.
DFT approach on stability and conductance of nine different polyyne and cumulene molecules
Published in Molecular Physics, 2020
AbhayRam Balakrishnan, R. Shankar, S. Vijayakumar
Carbynes are infinite linear carbon chains (LCC). The finite counterparts of carbynes are polyynes (alternate single and triple bonds) and cumulenes (consecutive double bonds). The nature of terminating group substitution determines the cumulene or polyyne structure of the finite LCC. If the terminating group induces a double bond between the two terminal carbon atoms in the LCC, the chain tends to have cumulenic structure, if the substitution induces a single bond or triple bond between the two terminal carbon atoms it leads to a polyynic structure. LCC with polyynic and cumulenic structures exhibit different physical and chemical properties. Bond Length Alternation (BLA) is the parameter which predicts the metallic or semiconducting behaviour of LCC. Even though cumulenes are having consecutive double bonds, it is seen from theoretical as well as experimental studies that they also possess a small BLA instead of zero [1,2]. But still the BLA values of cumulenes are much lesser than that of polyynes. LCC having low values for BLA are expected to show metallic behaviour and those with higher values for BLA show semiconducting behaviour. Hence cumulenes are expected to be metallic and polyynes are expected to be semiconductors. Both polyynes and cumulenes with different chain lengths (containing different number of carbon atoms) and different terminating groups have been synthesised experimentally and their crystal structures have been analysed in various studies [1,3]. Cumulenes with its predicted metallic nature is more desirable candidate for use as molecular wires. But due to their instability, lesser number of studies was made on it and comparatively more studies are done on polyynes.