The physical properties and the sensor applications of carbon nanotubes are discussed in the dissertation. Three main topics are (1) The electron spin resonance studies of the magnetic properties embodied in conduction electrons, dangling bonds and exchange interaction of spins of iron based catalyst metals and carbon nanotubes in as-deposited and purified multi-walled carbon nanotubes at various temperatures, (2) the field effects of a single carbon nanotube synthesized by a home-made microwave plasma-enhanced vapor deposition setup, and (3) the fabrication and investigation of gas sensors assembled by straight and circular carbon nanotubes.
The ferromagnetic characters of as-deposited MWCNTs are mainly attributed to the catalyst nanoparticles embedded on the tips. After acid erosion for purification, the trace of magnetic resonance prominently distributed from the remnant of catalyst within the low-sectional parts of the tubes and partially might come from the unpaired spins of CNTs. The free radicals from the unbounded carbons and the iterant conduction electrons of metallic CNTs also contribute to the electron spin resonance. Magnetization measured by a superconducting quantum interference device (SQUID) reveals fairly a small hysteresis loop with a small coercive field for both as-deposited and purified MWCNTs.
Multiwall carbon nanotubes (MWCNTs) were directly grown across the catalyst electrodes fabricated through photolithography by a microwave plasma-enhanced chemical vapor deposition (MPECVD) method. The conductivities were measured repeatedly in various ranges of drain-source voltage VDS at low temperatures showing clearly the quantum dot behavior such as coulomb blockade and coulomb oscillation effects. The electric charging energy increases as the length and diameter of the floated carbon nanowire increases in accord with theoretical estimation for capacitance charging, which is further verified from the experimental I-V characterization. The oscillation of the IV curves is tacitly assumed to be embodied in the irregular variation of conductance. The temperature-resistivity characterisitcs were carried out at various temperatures and explained by the strong localization effect. In addition, the continuous current flow could reduce tunneling barriers existed between the wire and electrodes.
In the case of a gas sensor application, we have fabricated both a straight and a circular-shape carbon nanotube by employing dielectrophoresis and electrostatic attraction, respectively. We firstly implemented a dielectrophoresis method to deposit desired amounts of carbon nanotubes on adjacent electrodes and investigate their conductivity variations at low gas pressures. The sensor exposed to extremely rare traces (in part per million) of CH4, H2, CO2 and acetone is capable of yielding response and showing saturation at a higher concentration. We propose a mechanism to portray that limited reaction surfaces are subjected to be saturated at high gas concentration. It is expected that the resistivity of carbon nanotubes is highly affected by exposing to polar solvent vapor such as acetone. Much sensing of carbon nanotubes based sensors is highly appraised to detect chemical gases and organic vapors. In another topic, a circular shape of carbon nanotubes assembled by ultrasonic agitation and assisted by a surfactant implemented for gas detection. In a well-dispersed CNT solution, we observed that the thinner thickness and larger diameter CNT rings were formed than those of previous studies. This fact can be elucidated more fully that the depletion contact energy is modulated by theoretical estimation with the corresponding tendency in our samples. A CNT ring was exploited as a gas sensor to detect an extremely low concentration of methane gas. The circular CNT sensor has a prominent conduction response embodied in the existence of more defects in the ring CNT sensor than those of the straight CNT sensor. Furthermore, the much fast response time in the circular CNT illustrates the demand of the shorter length of CNTs. The CNT ring features with longer persistent to current conduction after long time measurements appraising to be a robust and durable material in the sensor applications.

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