| dc.description.abstract | ABSTRACT
In this study, high-density vertical carbon nanotubes were grown on Si substrate at 400℃ by microwave plasma chemical vapor deposition with Fe, Co, and Ni as catalyst. The characterization of the carbon nanotubes were carried out by SEM, TEM, AFM, Raman, and I-V measurement.
By the observation with high resolution TEM, the appearance of carbon nanotubes were bamboo-shaped and range from 30-100nm in diameter. The bamboo-shaped carbon nanotubes were in a shape of cone and were multi-walled. The catalyst metals were on both top and bottom of the carbon nanotubes. The top metal was with irregular shape and the bottom metal was cone-like similar with the hole-bamboo structure. Base on the above results, we can suppose the grown mechanism of carbon nanotubes achieved by the pill up of carbon atoms with upward movement along the surface of the cone-like catalyst. The catalyst film were separated into nucleation in the pretreatment step, the nanoparticle catalyst were transform into cone-like shape during grown process. In this study, we well focus on the influence of MP-CVD on the grown mechanism of carbon nanotubes.
Single factor experiment was used to study the effects of process parameters on the grown rate of carbon nanotubes. These process parameters includes: source power, pretreatment source power, growth source power, growth temperature, working pressure, flow rate of N2, content of CH4, pretreatment time and growth time. Beside, Taguchi method was used for the optimization of process conditions by the study of the influences of various factors on the growth of carbon nanotubes. The growth rate and the roughness of pretreatment sample were evaluated and served as output response. The larger-the-better analysis which uses growth rate of carbon nanotubes as experiment output response indicates that growth time, pretreatment time, flow rate of N2, working pressure were important half control factors. The confirmation experiment results are as expected. The smaller-the-better analysis which uses the roughness after pretreatment as output response indicates that plasma source power, working pressure, pretreatment time which are in the order of importance. The results of the smaller-the-better analysis are not as expected. This maybe caused by the formation of carbon nanotubes during pretreatment process, which results the error of the measure by AFM.
Carbon nanotubes also can selectively grow in the array of 5μm square. The measured characteristics of field emission of carbon nanotubes shows the threshold voltage is 130V, and a current density of 4.5mA/cm2 can be obtained under a field of 3V/μm. These pole field emission devices formed by carbon nanotubes can deliver different current density by the control of the gate. Besides, a single carbon nanotube was successfully grown on the catalyst metal with dimension of 0.1μm.
The oxidation process at high temperature can exclude amorphous carbon and purify the carbon nanotubes, but some part of the carbon nanotubes were also consumed. The diameter of the carbon nanotubes will become larger after higher temperament annealing. The swelling catalyst metal particles on the top of the grown carbon nanotubes is due to the reorganization of carbon atoms at high temperament. | en_US |