NANOPHENSIM · COMPUTER SIMULATIONS OF OPTICAL AND TRANSPORT PHENOMENA IN CARBON NANOTUBES

Computer simulations of optical and transport phenomena in carbon nanotubes

The project covers one of most exciting topics in present research in physics - the carbon nanotubes and graphene. Significant progress has been achieved both in theory and experiment for almost twenty years of work in the area. Yet quite a few properties and phenomena in nanotubes have still to be studied in depth in view of the industrial applications of nanotubes. Here, we focused on one of the fundamental optical properties of nanotubes - the optical absorption and its major characteristics - the optical transitions. We calculated the excitonic effects on the electronic structure of carbon nanotubes and confirmed previous predictions by other groups for weak diameter dependence and almost chirality independent optical transitions. The estimation of the excitonic corrections were successfully applied in the assignment of experimental Raman data on isolated individual single-walled carbon nanotubes on substrate [1]. An important part of the project is the study of the effect of defects on the electronic structure, phonon dispersion, and Raman spectra of carbon nanotubes and graphene. The calculations revealed that the Raman spectra were strongly influenced by the type of the defects even for small defect concentrations. The obtained results allow to determine the predominant defect type, present in nanotube samples, and possibly, the defect concentration, based on Raman data [2-4]. One of the manifestations of the peculiar properties of the nanotubes as one-dimensional systems is the large modification of the phonon dispersion of graphene and carbon nanotubes due to strong electron-phonon interactions and known as the Kohn anomaly. We performed calculations for almost a hundred observable metallic nanotube types and obtained, in particular, the modified G bands frequencies and linewidths. Our results agree well with previous partial estimations and available experimental data [5]. The nanotubes are usually doped by charges from the substrate and the surrounding gases, which may strongly influence the Raman signatures of the nanotubes. We studied the effect of doping on the phonon dispersion of graphene and metallic nanotubes, and concluded that this effect is largest for the phonons close to the centre and the K point the Brillouin zone. The obtained results for almost a hundred observable metallic nanotubes compare well to available data. They can be used for estimation of the doping level of graphene and nanotubes by Raman spectroscopy [6-8]. Finally, the calculation of defect-induced and second-order Raman bands of carbon nanotubes, which uses the studied earlier in this project effects of defects, dynamic and doping effects, is in progress and publishable results are expected soon. Similarly, transport phenomena in carbon nanotubes are currently being simulated but this part of the project may be finalized in the continuation of the project. [1] A. Débarre, M. Kobylko, A. M. Bonnot, A. Richard, V. N. Popov, L. Henrard, and M. Kociak, Electronic and Mechanical Coupling of Carbon Nanotubes: A Tunable Resonant Raman Study of Systems with Known Structures, Phys. Rev. Lett. 101 (2008) 197403. [2] V. N. Popov, L. Henrard, and Ph. Lambin, Resonant Raman spectra of graphene with point defects, Carbon 47 (2009) 2448-2455. [3] V. N. Popov and Ph. Lambin, Theoretical Raman intensity of carbon nanotube (7,0) with point defects, phys. stat. sol. (b) 246 (2009) 2602-2605. [4] V. N. Popov and Ph. Lambin, Theoretical phonon dispersion of armchair and metallic zigzag carbon nanotubes beyond the adiabatic approximation, phys. stat. sol. (b) 1-5 (2010) / DOI 10.1002/pssb.201000112. [5] V. N. Popov and Ph. Lambin, Intermediate frequency Raman spectra of defective single-walled carbon nanotubes, phys. stat. sol. (b) 247 (2010) 892-895. [6] V. N. Popov and Ph. Lambin, Dynamic and charge doping effects on the phonon dispersion of graphene, Phys. Rev. B 82 (2010) 045406/1-9. [7] V. N. Popov and Ph. Lambin, Non-Adiabatic Phonon Dispersion of Metallic Single-Walled Carbon Nanotubes, Nano Research (2010) / DOI 10.1007/s12274-010-0052-2. [8] V. N. Popov, Theoretical study of the doping effects on the phonon dispersion of metallic carbon nanotubes, Physica E (2010) doi:10.1016/j.physe.2010.10.007.

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