UC San Diego

Low dimensional nanostructures have drawn tremendous interest owing to their remarkable electrical, optical, and mechanical properties, which can be used for novel electronics, energy approaches, and size/power efficient applications. Examples include quantized electrical conduction in single molecules (zero-dimension)/ semiconducting nanotubes (one-dimension). Additionally, nanotubes have been shown to have high mechanical stiffness/resilience, anisotropic optical absorption due to their one-dimensional morphology. Such aspects were investigated in detail in this thesis. In Chapter 1, a brief introduction to molecular electronics and the properties of single molecules such as Mn₁₂ Acetate, and linear structures such as carbon nanotubes and their applications are addressed. The quantized electrical transport behavior and its optical dependence, of Mn₁₂ Acetate have been studied through novel electro-migration assisted nano-junctions in Chapter 2. In Chapter 3, a comprehensive study of the synthesis of carbon nanotubes (CNTs) using thermal chemical vapor deposition (TCVD) is presented. The characterization in terms of nanotube length and quality were compared, as a function of synthesis temperature, gas mixture, types of substrate, etc., and a growth mechanism was then determined. An improved synthesis procedure was successfully proposed based on the study. In Chapter 4, the electrical transport behavior of SWCNTs, functionalized with different molecules (e.g., dodecyl, phenyl groups), were compared and investigated. The doping effects from those covalently bonded functional groups were manifested in the change in transport properties (e.g., p-, n-type, or metallic). The flexural rigidities (EIs) of MWCNT bundles were investigated through optical methods including laser transmission measurement and imaging techniques, in Chapter 5. The deflections of CNT bundles under fluid flow were monitored in situ and the flexural rigidities derived from the measured deflection through modified Stokes-Oseen formalisms. The potential of a CNT based gas flow sensor was demonstrated. In Chapter 6, the anisotropic optical absorption properties of MWCNTs were investigated and the absorption cross-sections, considering the misalignment of nanotube mats, were calculated and compared to those from SWCNTs. The nanotube diameter dependence of the optical absorption anisotropy was also studied. Chapter 7 summarizes the thesis