Title

Optoelectronic properties of single-walled carbon nanotubes and their aggregates

Date of Completion

January 2007

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Precise determination of (n,m) abundance and their optical spectroscopy characteristics of single wall carbon nanotubes (SWNTs) is of pivotal importance for their future involvement in high-end electronic and optoelectronic devices as well as in biosensory applications. ^ We first developed a methodology to correlate photoluminescence (PL) (n,m)-intensities with (n,m)-SWNT abundance using two dimensional fluorescence spectroscopy of non-covalently functionalized SWNTs sample. Using the extremely sharp diameter-distributed Co-MCM-41 SWNT sample, in conjunction with spectral reconstruction of its near infrared ES11 absorption spectrum, we have been able to independently assess the accuracy of two current theories. Moreover, the reconstruction of the ES11 absorption spectrum provides additional insights of (n,m)-absorption linewidths and the existence of zigzag (n,0) nanotubes. ^ We also established a methodology to reconstruct the radial breathing modes (RBM) of SWNTs. Using the difference between experimental and E ii(n,m) values of isolated SWNTs. A quantitative assessment was obtained that can be attributed to bundling and possibly excitonic effects. Furthermore, the relative electron-phonon interaction matrix elements (Mph) for 28 (n,m) semiconducting SWNTs species were extracted from the resonance Raman cross sections of individually dispersed HiPco SWNTs. The observed Mph pattern was fitted according to nanotube family (i.e. (2n +m)=constant) and modality ( i.e. mod(n-m,3)=1 or 2) using an empirical equation based on trigonal warping effects. Moreover, the corresponding absolute values of the electron-phonon interaction matrix element were also obtained in both individually dispersed and aggregated states, by utilizing the correlation between RBM overtone and RBM fundamental intensity ratio as a function of laser energy. These values were further used to obtain the Huang-Rhys factor and absolute Mph values for these nanotube species. ^ Recently, we observed a new class of intermediate frequency modes (IFMs) associated with the ES22 and ES11 optical transitions of bundled HiPco SWNTs. "Step-like" dispersive behavior was observed for these IFMs, along with associated clusters of RBM overtones at higher frequencies. The observed IFM maxima were found to obey a resonance behavior based on a combination of the ES22 and E S11 transition energies, scaled by the inverse diameter of the respective nanotube. Moreover, we found the RBM overtone and IFM Raman intensities are very sensitive to the aggregation state. These observations significantly extended the capability of resonant Raman spectroscopy. ^