Raman spectra of hydrocarbons formed in carbon nanotubes – a theoretical study

Recent experimental results based on Raman spectroscopy and mass spectroscopy showed that an appropriate heat treatment of small diameter carbon nanotubes filled with ferrocene molecules results in the formation of hydrocarbon molecules. However, the exact nature of these molecules is still unknown. In order to find possible candidates, we performed density functional theory calculations of the structure, electronic transition energies, and Raman spectrum for a large set of hydrocarbon molecules, including oligoenes and polyaromatic hydrocarbons. To follow the effect of deuteration in the Raman spectra we varied the mass of hydrogen atom from 1 to 2 by 0.1 steps. With increasing mass, the position of the lines red shifted as expected. However, in some rare occasions, the intensity of the red shifted line dramatically decreased and at the same time, an originally weak line gained large intensity at a nearby higher frequency. This can be interpreted in an experiment as an apparent blue shift. From a comparison of the calculations with experiments, based on molecular weight, excitation energy, and Raman spectrum (for both hydrogenated and deuterated forms) either quaterrylene (dimer of perylene) or the dimer of the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is the most likely product of the heat treatment. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim