SWNT and Graphene Colloidal Dispersions: Phase behavior, Material Fabrication and Characterization

[摘要] 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.

[发布机构] Rice University