Photoluminescence studies of single -walled carbon nanotubes: Chirality-resolved length characterization and porphyrin sensitized electronic energy transfer
Photoluminescence spectroscopy has emerged as a powerful technique for characterizing the structure and optical properties of single-walled carbon nanotubes (SWCNT). While SWCNT diameter and chirality information is now routinely available from photoluminescence spectral analysis, the other primary structural parameter, length, has not been measurable except through tedious microscopy. This thesis extends the use of photoluminescence to obtain length information on ensembles of SWCNT in suspension through analysis of their optical anisotropy when aligned by the shear of a flowing fluid. The theoretical background and custom-built instrumentation are described and demonstrated to yield analyses comparable to the standard method of atomic force microscopy. A unique benefit of the new method is the resolution of correlations between length and (n,m) structural indices through spectral analysis of the data. The common sample preparation steps of sonication and centrifugation are found to alter the sample length distribution in a diameter-dependent manner. The shear aligned photoluminescence anisotropy method provides a new means for quickly determining the lengths of SWCNT in bulk suspensions and more thoroughly investigating their structural properties. Photoluminescence characterization techniques such as this depend on radiative decay of SWCNT excited states, which occurs with efficiencies below 10%. Nonradiative relaxation is clearly dominant, yet the detailed decay pathways and their relationship to nanotube structure remain essentially unknown. It is currently suspected that optically forbidden states, such as spin triplet states, are a major factor in the low luminescence efficiency of SWCNT. Experimental studies are described involving energy transfer from optically excited porphyrin sensitizers in an attempt to selectively populate such unexplored SWCNT triplet states. Efficient energy transfer is clearly observed in non-covalent SWCNT-porphyrin complexes. Analysis suggests that singlet rather than triplet interactions are dominant in this system. These studies demonstrate efficient electronic coupling between excited states of the nanotube and porphyrin that make such complexes potentially useful as artificial light-harvesting chromophores.
0494: Physical chemistry