Atmospheric pressure plasma jet process for carbon nanotube (CNT) growth: Characterization and composite fabrication

Understanding the influence of process conditions on film characteristics is an important study in process engineering; more so in relatively newer technologies like deposition of Carbon nanotubes (CNTs), due to the lack of a library of experimental evidence. This dissertation examines the effect of plasma power and substrate temperature on the gas-phase chemistry, during Chemical Vapor Deposition (CVD) of CNTs using an atmospheric pressure plasma jet (APPJ). Increasing the plasma power to 45 W from 0 W results in the dissociation of 1.5% more C-H bonds in the acetylene precursor, due to collisions with the helium metastables generated by the plasma. The collisions also increase the vibrational temperature of the acetylene molecules by 50 K, in part due to the torque imparted by the dissociating hydrogen atom. Consequently, the resultant CNT films demonstrate a linear enhancement in cross sectional height and film purity. C-H bond cleavage gave rise to unsaturated and isolated carbon atoms; species that are energetically better suited for CNT growth. A similar improvement in film characteristics at higher substrate temperatures (600°C to 680°C) is attributed not to additional acetylene dissociation in the gas-phase but rather more efficient catalysis on the substrate. Finally, in situ introduction of catalyst in the form of ferrocene was shown to overcome CNT growth saturation---a phenomenon where the nanotubes form a canopy that hinders the acetylene flux from reaching and reacting with the catalyst on the substrate; thus terminating CNT growth.; Filling the as-grown CNT forests with various materials is envisioned as a composite fabrication method bereft of the hassles involved with CNT dispersion---a very necessary condition for manufacturing capable composites. Low pressure chemical vapor deposition (LPCVD), a process characteristic of low sticking coefficients and conformality was ideal to fill the CNT forests with inorganic materials like polysilicon or silicon nitride. Preliminary studies suggested that filling began and proceeded by coating the nanotubes with the inorganic material. Large coating thicknesses lead to bridging the gap between adjacent nanotubes and resulted in a continuous composite layer. In the lower reaches of a tall CNT forest however, coating rates were significantly less than on the forest surface. Such non-conformality caused the formation of a capping layer near the surface, and consequent voids below; symptomatic of poor step coverage. Reducing the temperature from 630°C to 550°C or decreasing the silane flow rate from 75 sccm to 10 sccm improved the step coverage by a factor of 6. (Abstract shortened by UMI.)...