Graduate and Postdoctoral Studies
Advanced Quantitative Analysis of Single-Walled Carbon Nanotubes
Monday, April 10, 2017
to 5:00 PM
180 Dell Butcher Hall
Single-walled carbon nanotubes (SWCNTs) have promised wide range of novel applications for containing a mixture of many tubular structures that vary in diameter size, chiral angles, and physical properties. However, their usage has been hindered by structural heterogeneity. The recent application of density gradient ultracentrifugation (DGU) for nanotubes structural sorting has created a need for highly selective removal of closely spaced layers formed in the centrifuged tube.
In the first part of this thesis, a novel custom built automated fractionator is presented to fulfil this purpose. Through the use of fine needles, systematic needle motions, and slow flow rates, multiple sample layers can be aspirated under program control with minimal cross contamination between layers. Surprisingly, with such neat extraction tool, we were able to resolve the homogeneity of enriched fractions with a single enantiomeric type of (6,5) nanotube species. This study highlight how challenging the preparation of homogeneous fraction of nanotubes is even with the most sophisticated separation tool as DGU.
Furthermore, despite of unavoidable heterogeneous properties in CNT samples, we have conducting photoluminescence (PL) and photoluminescence excitation (PLE) studies based on single nanotubes and sorted bulk samples. Both PL and PLE studies have required spectral analysis and modulation of (n,m)-specific electronic transitions and vibronic features. This have allowed us to obtain fundamental physical properties about semiconducting nanotube including their (n,m)-specific E22 absorption cross sections, absolute florescence quantum yields, and absolute fluorimetric efficiencies.
Basic understanding of the photophysical properties of SWCNT gives a new insight toward SWCNT sample characterization using the most routinely characterization tools as absorption and emission spectroscopies. However, the reliable interpretation of SWCNT sample spectra to deduce their compositions is complicated and challenging because of overlapping features from numerous species. Thus, in the rest of this thesis, two practical approaches for spectral analysis of SWCNT samples are presented to provide absolute compositions with a new level of reliability.
The first method covers absorption spectra analysis over the full visible to the short-wave infrared (SWIR) range relying on our new approach to use the (n,m)-specific PLE profiles plus a background component to simplify and improve absorption spectral analysis. The second method covers absorption and emission profiles analyses in the SWIR range. This involves two steps: (1) analysis of emission spectra measured with discrete excitation wavelengths to determine the relative abundance of semiconducting nanotube species, and (2) analysis of E11 absorption spectra to place the findings from (1) on an absolute concentration scale.