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45078-GB10
The Physical Interaction and Energy Transfer between Single Wall Carbon Nanotubes and a Straight Chain Conjugated Polymer in Solution
The PRF Grant has allowed me to concentrate on completing experiments, in the short term, as opposed to writing proposals. Initial successes in the lab have attracted strong undergraduates to my research and I have allowed my group to blossom, through marketing my research through a newly designed web-page. Now, I recognize the need to follow the success of external funding, and personnel selection/management, with the success of publication (with undergraduate co-authors).
Receipt of this grant allows me to continue to emphasize RIT as a top-tier undergraduate research institution. My personal ambition and career goals have been raised and I currently see greater stability and positional strength within my own institute.
Regarding students who have worked (indirectly and directly) on this research, the three who have graduated from RIT are finding success in PhD programs; Steven Pellizzeri (NC State), Michael Schettini (UC Santa Barbara), April Colleton (UNC Chapel Hill). I currently have seven active students working on projects linked to my proposal. They are learning the importance of critical thinking, reproducibility and accuracy as we look to develop a self-consistent model. I provide them with the tools to be excellent scholars and effective problem solvers. I strongly encourage further education beyond their undergraduate studies and expect strong careers for them in the sciences.
Our work can be broken down into three areas.
1. Consistency/reproducibility of (Single-Wall Carbon Nanotubes) SWNT solutions and purity of SWNT.
2. Physical interactions of SWNTs with substituted phenylene-vinylene conjugated polymers (MEH-PPV).
3. Fluorescence quenching effect of SWNTs on conjugated polymers
We work with as-received CoMoCat nanotubes. The high proportion of (6,5) nanotubes in the CoMoCat samples leads to strong tube-specific spectral features. Changes to optical absorbance features of the tubes suggest interactions of MEH-PPV with nanotubes, despite presence of carbonaceous impurity.
Yet, in fluorescence work, it is particularly important to assign fluorescence quenching to the nanotubes themselves, rather than to a host of possible impurities in a poorly defined as-received sample. We must be able to qualify the purity of our nanotube sample. We are following a procedure similar to that of Landi et al (2005). Based on a gold standard, purified from our as-received sample, we will reconstruct impure samples, adding known amounts of carbonaceous impurity. We will observe the effect of this impurity on the optical absorbance and/or fluorescence quenching of the sample. From this we intend to better qualify our as-received sample. This endeavor has positively attracted the interest of the CoMoCat nanotube supplier.
In parallel we continue to study the effect that debundling of the nanotubes has on our quenching experiments. We must measure the true dispersion limit of the nanotubes in our working solvents since bundling of the tubes leads to experimental artifacts that reduce the ability to make solid interpretations.
The sharpness of NIR absorbance spectroscopy features can indicate the amount of individual nanotubes that are in solution, independent of purity. A systematic study has shown sharpening of the (6,5) S22 nanotube peak as MEH-PPV concentration increases, consistent with a binding of multiple polymer chains to individual carbon nanotubes. Further work is being carried out to solidify our conclusions.
Qualification of sample purity, actual dispersion and debundling is critical when working with nanotubes in solvents. Any spectral data can only be interpreted fully if the actual interactions between nanotube and polymer can be separated from interactions between polymer and impurity.
With results suggesting binding between MEH-PPV and SWNT in solution, we are currently working through a comprehensive set of experiments that address the impact of polymer concentration, polymer solubility and temperature on the fluorescence quenching of the fluorophore by the nanotubes. We include stringent reproducibility tests, as well as comparisons with small molecular fluorophore controls.
To date, our quenching experiments suggest both static quenching and dynamic quenching, which is unusual. Static quenching is complemented by red-shifting in the MEH-PPV spectrum, which is assigned indirectly to polymer complexation with the SWNT. High levels of dynamic quenching can be explained by very high local concentrations of quencher, given this complexation. Our intent is to develop a complete physical model of the microscopic interactions that take into account the statistical binding of multiple polymer chains to single nanotubes.
In summary, our work will provide strong contributions to the further development of purity standards regarding CoMoCat nanotubes. Regarding the fluorescence quenching of conjugated polymers by these nanotubes, we are looking to prove our model of physical interaction and the associated quenching mechanism with a full complement of experimental data. We intend to assess the mechanism's universal application and consider its impact upon photovoltaic efficiency in a polymer bulk heterojunction device. In truth, excited state quenching may lead to loss of performance but, on the other hand, quenching through electron transfer may demonstrate the potential of carbon nanotubes to increase solar cell efficiencies.
