Separation of Single-Walled Carbon Nanotubes By Electronic Type Using Conjugated Polymers

Abstract

Since their discovery over two decades ago, single-walled carbon nanotubes (SWNTs) have become one of the most investigated nanomaterials in materials science. Their exotic optical, electrical, thermal and mechanical properties afford them amazing potential in a variety of different fields. Current SWNT synthetic processes produce heterogeneous mixtures of both semiconducting and metallic SWNTs. The mixed electronic nature of these materials, combined with their limited solubility, has significantly hampered the realization of many applications and necessitates the development of post-synthetic purification techniques. Conjugated polymers offer a significant advantage over other proposed strategies in that not only do they provide a cheaper and scalable route towards the isolation of SWNTs, but they also allow for the preparation of materials with novel properties. Polyfluorenes have been extensively investigated in the literature due to their preference towards dispersing semiconducting SWNTs; however, these dispersions are often quite dilute, and the polyfluorene structure is incompatible with certain device applications for SWNTs. Poly(2,7-carbazole)s offer a viable alternative to polyfluorenes for the purification of bulk SWNT material. At the time of this thesis, there have been relatively few reports investigating the interactions of poly(2,7-carbazole)s with SWNTs, and the majority of examples in the literature have suffered from poor stability and complex dispersal procedures due to the inherent insolubility of the 2,7-carbazole structure. The work presented in this thesis involved the preparation and characterization of a novel poly(2,7-carbazole) structure that displayed excellent solubility in a variety of organic solvents, allowing for the preparation of extremely stable and relatively concentrated dispersions of SWNTs. Thorough characterization of the supramolecular complexes through absorbance, photoluminescence and Raman spectroscopies determined that this polymer preferentially disperses semiconducting SWNTs. A second objective of this work was to investigate how modification of various parameters (including polymer structure, molecular weight and the type of SWNTs) can influence the quality of the resultant composite dispersions. One important study performed was to investigate how the electronic nature of the polymer backbone can affect the separation of SWNTs by electronic type. We demonstrate for the first time that by incorporating an electron-poor functionality into a polyfluorene it is possible to change from dispersing only semiconducting SWNTs to solubilizing both electronic types. This investigation highlights the potential importance of incorporating electron-poor functionalities in the development of polymeric systems that can selectively discriminate metallic SWNTs, which remains a challenging endeavor at the time of this thesis publication.