Badawi Anis
Stabilization of Carbon Nanotubes: An Infrared and Optical Spectroscopy Study on Peapods and Double-Walled Carbon Nanotubes under Pressure
Supervisor: Prof. Dr. Christine Kuntscher [Experimental physics II]
Date of oral examination: 09/06/2013
167 pages, english
The main goal of the present work is to study the stability of carbon nanotubes via the pressure-induced changes in the optical response using hydrostatic pressure transmitting medium (PTM) up to 20 GPa. For this purpose, fullerene doped SWCNTs, or what is called peapods method, was used to prepare different variety of carbon nanotubes namely: C60, C70 peapods, and double-walled carbon nanotube (DWCNTs) derived from the peapods. Additionally, iodine-filled SWCNTs (I-SWCNTs) were also prepared to extend the comparison to the effect of atoms filling on the stability of the SWCNTs and compare the results with those of the peapods and DWCNTs. The stability of the different carbon nanostructures: peapods, DWCNTs, and I-SWCNTs, against hydrostatic pressure has been mainly addressed by Raman spectroscopy, which monitors the vibrational properties of the nanotubes. Optical spectroscopy coupled with high pressure technique forms a powerful and novel tool to probe the electronic structure of carbon nanotubes. Investigations proposed that at a critical pressure, a modification of the nanotubes cross section from circular to oval, elliptical or collapse at high pressure occurs. Due to the nanotube%92 cross-section modification, the electronic and the optical properties of the deformed tubes are strongly affected. For example, the optical absorption spectra are altered drastically under pressure, where the main absorption bands shift to lower frequencies, broaden, lose spectral weight, and finally vanish. This behaviour was attributed to the symmetry breaking and/or σ%06-π%06 hybridization. The mechanical stability of the SWCNTs by filling the tubes with molecules, atoms, or with inner tube is an important issue. High-pressure Raman measurements showed that the filling with inner tubes or Argon molecules stabilizes the outer tubes and this kind of filling considered as a case of homogenous filling. On the other hand, filling nanotubes with C70 molecules or iodine atoms, a case of inhomogeneous filling, leads to destabilization of the nanotubes. The destabilization of the nanotubes was attributed to the inhomogeneous interaction, non-covalent van der Waals forces, between the nanotubes walls and the inner molecules, which can lead to the tube mechanical instability even at low pressure. In general, optical spectroscopy is a powerful technique to characterize the electronic band structure in terms of the energy position and spectral weight of the excited optical transitions. As demonstrated recently, the optical response is capable of monitoring small pressure-induced deformations of the tubular cross-section, as the characteristic van Hove singularities (vHS) in the density of states in SWCNTs are very sensitive to such deformations. Within the present work, the relevance of PTM regarding the pressure-induced effects in sample under investigation will be clarified. It has been demonstrated perviously that fluids, nitrogen or argon as PTM can intercalate inside the SWCNTs, introducing a steric barrier which is responsible for the SWCNTs stabilization against the applies pressure. Therefore, nitrogen, argon, and alcohol mixture as PTM were used to clarify the effect of the different PTM on the stability of the samples under investigations.