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With every atom of a carbon nanotube existing as a surface species, their electronic and optical properties are especially sensitive to their interfacial environment. Control over nanotube surface chemistry is thus vital to modulating their photophysical properties and as a means for incorporating them into novel matrices to enable functional materials. I will present results demonstrating the ability to control non-covalent functionalization chemistry for nanotube separations and development of fluorescent silica-gel based composites and finish with a demonstration of new capability for exploring nanotube interfacial dopant chemistry at the single tube and even single-dopant site level.
Current advances in density-based separations of carbon nanotubes rely on modulation of surfactant interactions at the nanotube surface. We have studied how these interactions can be tuned via addition of electrolyte to surfactant suspensions of nanotubes. Addition of metal chloride salts can act to enhance the density differences between nanotubes of similar diameter and/or metallicity. Use of this phenomenon to separate nanotubes into highly enriched metallic and semiconductor fractions in a diameter-dependent fashion will be discussed. It will also be shown how electrolyte modulation of photoluminescence behavior, paired with the separations results, can act as a probe of the surfactant structure at the nanotube surface. Control over this structure also enables the incorporation of nanotubes into silica gels and aerogels while maintaining the nanotube optical behaviors of interest. I will discuss a new process for creating these novel nanotube/silica composites. Optical properties of the composites will be discussed, with evidence given for a solvent-free optical response. Potential of the aerogel matrices for gas sensing and pursuing previously unaccessed optical behaviors will also be presented.
These results demonstrate the importance of understanding and accessing surface behaviors n detail. I will conclude with a discussion of how photoluminescence imaging at the single-tube level may be used to probe the electronic nature of single dopant species and their dynamic behaviors on the 1-D CNT surface. The contrasting behaviors of mid-level and shallow-level dopants will be presented. While these dopant types behave very differently in their electronic effects, results will be shown demonstrating that both types can be mobile on the nanotube surface.
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