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Stefan Thiel «  Komalavalli Thirunavukkuarasu »  Michael Thorwart
Komalavalli Thirunavukkuarasu
Carbon nanostructures under high pressure studied by infrared spectroscopy
Supervisor: Prof. Dr. Christine Kuntscher [Experimental physics II]
Date of oral examination: 05/25/2009
162 pages, english
This dissertation presents the various investigations on the carbon nanostructures, namely, fullerene-based compounds and single-walled carbon nanotubes (SWCNTs), under high pressure using infrared spectroscopy. The reflection/transmission measurements on these compounds have been performed for the first time over a broad frequency range (120-20000 cm-1) for pressures up to 10 GPa. The first part consists of the pressure-dependent infrared transmission measurements on the fullerene-based compounds: pristine C70, C60C8H8 and C70C8H8. Both vibrational as well as electronic properties of these compounds were investigated under pressure. These studies on the fullerene-based compounds identify the pressureinduced orientational ordering transition which occurs below 1 GPa for all three studied compounds. At higher pressures, the pressure-induced changes in the vibrational and electronic properties could be attributed to an increased intermolecular interaction in these compounds. The second final part of this work deals with the pressure-dependent infrared spectroscopic measurements on the SWCNT films. The measurements were performed on unoriented SWCNTs films and first for the time also on oriented SWCNT films, namely, the oriented nanotubes in polyethylene matrix and the magnetically-aligned SWCNT film, for the first time over the frequency range from 120-20000 cm-1 for pressures up to 8 GPa. With the application of pressure, the optical absorption bands of the SWCNTs (in both oriented and unoriented SWCNT films) exhibit a redshift and an anomaly exists in the pressure dependence of these transitions at a critical pressure Pc=2-3 GPa. This anomaly is related to a structural phase transition, where the cross-section of the SWCNTs changes from circular to oval/racetrack-like. The farinfrared absorbance of the unoriented SWCNT films decreases monotonically with increasing pressure, indicating an increase of the localization effects upon pressure application. The polarization-dependent infrared spectroscopic measurements on oriented SWCNT films show a strong anisotropy in the optical conductivity for both polarizations parallel and perpendicular to the alignment direction.