In-situ characterization of the carbon nanotube growth process by X-ray diffraction

In the last few years carbon nanotubes (CNT) attract more and more attention due to their interesting physical properties especially in the field of micro electronics. Synthesis of CNTs with tailored properties is still a critical point, especially for their applications as interconnects. The growth of CNT by chemical vapor deposition (CVD) is an established synthesis route and it was used in this study.
In-situ X-ray diffraction experiments during growth conditions are performed at the beamline BM20 at the ESRF operated by the Helmholtz-Zentrum Dresden-Rossendorf using a high temperature annealing chamber suitable for reactive gases. Acetylene was used as carbon precursor for the CVD of CNT. Different catalyst systems were studied. For iron nano-particles acting as CNT catalyst, there is still a debate which species can be catalytically active: metallic iron and/or iron carbide. The formation of the metal nano-particles by dewetting and respective crystallisation of the initial thin film was followed by X-Ray reflectivity (XRR) and diffraction (XRD) measurements. We proved that CNT growth can occur without the presence of Fe3C. In general two reaction pathways denoted by a high and low (or zero) iron carbide concentration were observed. Which route the growth follows depends in a statistical way on the ratio of different iron phases (α- and γ-Fe) present in the nano-particles. This can be influenced also by the reaction conditions like temperature and kind of the buffer layer supporting the catalyst film. One challenge is to grow CNT on a conducting support without scarifying the CNT yield and structural quality of the grown CNTs.
Using a Co-buffer layer in-between Si-support and Fe-catalyst leads to the formation of conductive CoSi2 via CoSi by subsequent silicidation during the growth process. Herby a high yield of CNT was obtained.
CoSi2 support is more promising than Ta support, but we used Ta as model system to explain all possible interactions, all occurring phases during processing by side reactions were recorded. Notable CNT yield triggered by high catalytic activity of iron at low temperatures ~550C was observed.
This study shows that in-situ diffraction experiments are a powerful tool to investigate catalytic reactions. The understandings of these processes are the basis of tailoring future materials.