Optical studies of DNA-wrapped carbon nanotubes

Abstract

This thesis presents a series of detailed optical studies of phonon-assisted relaxation processes in DNA-wrapped single walled carbon nanotubes. Using resonance Raman spectroscopy (RRS) and photoluminescence spectroscopy (PL), a careful characterization has been carried out on different DNA-wrapped SWNT samples to study the effects of DNA wrapping on the electronic and vibrational structure of SWNTs. Then, by using a DNA-wrapped SWNT sample that is highly enriched in (6, 5) SWNTs, I was able to separately identify the individual phonon-assisted relaxation channels that could not be clearly observed in bulk materials system. In light of the recent developments in the excitonic theory of D systems in general and of 1D carbon nanotubes in particular, the observed phonon-assisted processes are interpreted and examined in terms of excitonic states. An intense up-shifted phonon-assisted transition observed in PL spectra of SWNTs suggests the presence of a strongly coupled exciton-phonon bound state due to the strong 1D confinement condition. The presence of such exciton-phonon bound state confirms the excitonic nature of the optical transitions observed in semiconducting SWNTs.

(cont.) To further understand the role of phonon assisted processes in exciton relaxation, the dynamics of such phonon-assisted processes has been studied using time-resolved spectroscopy. By using a carefully chosen Epump that corresponds to E°12 (6, 5) +2hwD and probing at E°A2(6, 5), an intermediate decay time component that is associated with the hot D-band phonon-absorption and relaxation process is identified and studied in detail. The experimental results suggest that in the event of a multi-phonon assisted relaxation process, it is possible for an additional excitonic state to participate in the relaxation process and to give rise to an intermediate relaxation time component. The detailed information obtained from the experimental studies clarifies the role of hot phonon absorption and emission processes, as well as the Auger process, in the filling and depletion of band edge exciton populations for individual SWNTs. The experimental result also gives insights into how dark excitons, which are predicted by theory, might indirectly participate in the exciton relaxation process.

(cont.) Lastly, to further understand the structural-property relation for short nanotubes, a series of RRS studies have been carried out on DNA-wrapped SWNTs sample that are sorted by length using size exclusion chromatography. The consequences of broken translational symmetry in short SWNTs are examined by monitoring the intensities of the Raman features in the extended intermediate frequency mode (IFM) region between 600cm-1 and 1500cm-1. Many of the IFM features show an increased Raman cross section with decreasing average length of the nanotubes. The extent of the Raman intensity increase is found to be dependent on the origin of the IFM features. The changes in the D-band intensity are also examined in the context of the crystalline size effect as the nanotubes become shorter.