Nuclear magnetic resonance studies of local magnetic, electronic, and dynamic properties in filled single wall carbon nanotubes
In this dissertation, the local magnetic and electronic properties of SWNTs are investigated. Also, one dimensional (1-D) dynamics of C60 fullerenes encapsulated in SWNTs is investigated using Nuclear Magnetic Resonance (NMR) spectroscopy. In order to remove ferromagnetic catalyst particles present in SWNT samples, which interfere with NMR measurements, we have developed a novel magnetic purification method by which 99% of the ferromagnetic particles are removed. With this new method, we could obtain a well-resolved NMR signal with FWHM of ∼20 ppm from natural carbon based SWNTs. Using 25% 13C enriched C60 encapsulated in the magnetically purified SWNTs as an NMR probe, the local magnetic properties of the 1-D inner space of SWNTs are studied. Surprisingly, SWNTs are found to screen the applied magnetic field by tens of ppm. More interestingly, the diamagnetic shielding is found to be tunable by controlling defects or doping. While defects create paramagnetic currents to destroy the diamagnetic shielding, doping enhances the shielding by increase aromaticity in SWNTs to have stronger diamagnetic ring currents. Encapsulated fullerenes in SWNTs show unique dynamics which is related to 1-D geometry. They are found to undergo dynamics transition from free rotation to hindered rotation at ∼100 K, which is lower than that in 3-D bulk fullerenes by as much as ∼160 K. This huge reduction results from the decrease of Van der Waals interaction and the Coulomb interaction between an electron-rich bond and an electron-poor center of a pentagon or a hexagon. DWNTs were made by high temperature annealing of enriched peapods. The isotropic chemical shift of inner nanotubes was found to have shifted diamagnetically by 26.62 ppm due to the magnetic shielding by outer nanotubes. 75% of the inner nanotubes were proven to be metallic due to a strong interaction between inner and outer nanotubes. ^
Engineering, Materials Science
"Nuclear magnetic resonance studies of local magnetic, electronic, and dynamic properties in filled single wall carbon nanotubes"
(January 1, 2007).
Dissertations available from ProQuest.