Centrifugation-based Purification of Emerging Low-dimensional Materials and Their Thin-film Applications

Polydispersity in low-dimensional materials offers many interesting challenges and properties. In particular, the one- and two-dimensional carbon allotropes such as carbon nanotubes and graphene have demonstrated exquisite optoelectronic properties that are highly sensitive to their physical structures, where subtle variations in diameter and thickness render them with significantly different electronic band structures. Thus, the carbon nanomaterials have been the subject of extensive studies that address their polydispersity issues. Among these, solution-phase, buoyant density-based methods such as density gradient ultracentrifugation have been widely utilized to enrich subpopulations of carbon nanotubes and graphene with narrow distribution in diameter and thickness, enabling their applications in various next-generation thin-film devices.;In this thesis, I present further advancement of centrifugation-based processing methods for emerging low-dimensional materials through systematic utilization of previously explored surfactant systems, development of novel surfactant types, and study of correlation between the chemical structure of surfactants and the dispersion and optoelectronic properties of the nanomaterials. First, I employ an iterative density gradient ultracentrifugation with a combination of anionic surfactants and addition of excess counter-ions to achieve isolation of novel diameter species of semiconducting single-walled carbon nanotubes. The purification of carbon nanotubes with simultaneous, ultrahigh-purity refinement in electronic type and diameter distribution leads to collaborative studies on heat distribution characteristics and diameter-dependent direct current and radio frequency performances in monodisperse carbon nanotube thin-film transistors. Next, I develop the use of non-ionic polymeric surfactants for centrifugation-based processes. Specifically, I utilize polypropylene and polyethylene oxide-based block copolymers with density gradient ultracentrifugation to enrich high purity semiconducting and metallic carbon nanotubes. Furthermore, by employing the block copolymers as surfactant in a simple centrifugation process, aqueous dispersions of single- to few-layered graphene nanosheets with high concentration are prepared. This study elucidates the correlation between the molecular structure of block copolymers and their dispersion efficiency and degree of defects for graphene nanosheets. Lastly, the application of block copolymers is extended to facilitate solution-phase purification processes for transition metal dichalcogenides, and their photoluminescence in aqueous dispersion is revealed to be dependent on the chemical structure of the surfactant used for exfoliation and stabilization. Overall, these results illustrate the significance of understanding the role of surfactants in solution-phase processing of nanomaterials, and the motivation for further development of surfactant-assisted purification processes for scalable applications of the emerging nanomaterials.