Carbon nanotubes (CNTs) are rolled up sheets of graphite and appear in a various forms. One way to produce CNTs is by using chemical vapor deposition (CVD) of carbon containing gases in the presence of catalysts like Fe or Co. For the so called substrate supported catalyst (SSC) method the catalyst is provided in form of particles at the substrate surface and is thus directly accessible for the carbon containing CVD gas flux to induce CNT growth. In this work we studied five different approaches to create and use catalytic nano-particles for CNT growth:
1. Iron based nano-particles out of ferric nitrate solution.
2. Cobalt based nano-particles out of cobalt-acetate solution.
3. Sol-gel supported formation of iron nano-particles.
4. Size and shape controlled synthesis of iron-oxide nano-particles.
5. Dispersion of commercially available iron-oxide nano-particles in polyethylenglycol.
The catalyst containing chemistry was applied on 10x10mm2 quartz-glass substrates by spin-casting or adsorption. Nano-particles for the iron and cobalt based systems were produced in-situ by annealing the samples at temperatures above 650°C. The particle size and lateral distribution of the particles were analyzed using atomic force microscopy (AFM), scanning electron (SEM) and transmission electron microscopy (TEM).
The samples were processed in a hot filament CVD (HF-CVD) reactor using methane (CH4) as carbon source and hydrogen (H2) and argon as supporting gases at pressures between 10 and 800mbar. The filament temperature was in the range of 1900-2600°C resulting in a substrate temperature between 600 and 880°C.
The results of the CVD deposition experiments were analyzed with SEM and TEM. For all five approaches a set of CVD-parameters could be found that led to the formation of dense films of CNTs with different degrees of alignment. HR-TEM analysis showed either fishbone arrangement of the CNTs or multi-walled CNTs depending on the catalyst type. Detailed analysis revealed that the diameters of the CNTs are directly related to the size of the catalyzing particle leading to new methods to produce CNTs with a narrow size distribution.