SWNT and Graphene Colloidal Dispersions: Phase behavior, Material Fabrication and Characterization
Doctor of Philosophy
This dissertation explores the microstructural properties, flow, and phase behavior of aqueous suspensions of single-walled carbon nanotube (SWNT) and graphene. Liquid phase processing with scalable and industrially viable methods is used for fabrication of basic engineering materials like thin films, coatings and 1-D fibrillar structures. The electro-optical transport properties of these novel materials are characterized. A recipe for formulation of an aqueous colloidal suspension with high SWNT concentration is presented. A combination of two surfactants provides optimal rheological behavior for "rod coating" uniform transparent conductive SWNT thin films, with minimal dewetting, rupture and defects. "Doping" with acids and metallic nanoparticles yield SWNT films with electrical sheet resistance of 100 and 300 OJ sq for respective optical transparency of 70% and 90%. SWNT thin films with local nematic ordering and alignment are fabricated using a "slow vacuum filtration" process. The technique is successfully demonstrated on several aqueous SWNT suspensions, employing different ionic and non-ionic surfactants, as well as on dispersions enriched in metallic SWNTs, produced by density gradient ultracentrifugation. Scanning electron microscopy and image analysis revealed a local nematic order parameter of 8 "" 0.7-0.8 for the SWNT films. Aligned SWNT films and fibers are employed as templates, in a simple drop drying process, for large scale ordered (8 "" 0.7-0.9) assembly of plasmonic nanoparticles, like gold nanorods, micro-triangles and platelets. Colloidal self-assembly of surfactant stabilized SWNTs and AC dielectrophoresis (DEP) are combined in a novel two-step technique for fabrication of 1-D SWNT fibrils. The self-assembled SWNT-surfactant colloidal structures are 103 - 104 fold more responsive to external AC electrical fields. The DEP SWNT fibrils show significant Raman alignment ratio ("" 3-5) and good electrical conductivity. iii A detailed study on the phase behavior of giant graphene oxide flakes (aspect ratio > 104) suspended in water is presented. The lyotropic suspensions transition from an isotropic to a biphasic system and to a discotic nematic liquid crystal with increasing flake concentration. Polarizing optical microscopy and colloidal particle inclusions reveal the alignment and orientation flakes in the nematic phase, the nematic order parameter (8 f"V 0.43), low optical birefringence (~n = -0.0018), and an average Frank elastic constant (K f"V 100 pN) which is about 100 fold higher than previously studied discotic liquid crystals.