Thin film catalyst and substrate interactions on the CCVD (catalytic chemical vapor deposition) growth of carbon nanotubes

A detailed investigation into the growth of single-walled carbon nanotubes (SWNTs) on thin metallic films deposited on various substrates was performed. In this research, Al/Fe/Mo multi-layer thin films were deposited by electron beam deposition on various substrates to determine which substrates promote the growth of SWNTs over multi-walled nanotubes (MWNTs) during the decomposition of methane gas. The chemical vapor deposition (CVD) parameters such as time (10 minutes), temperature (950°C), and gas type and flow (methane at 0.5 slpm) were kept constant. This research varies from previous research in that thin metallic films are utilized instead of prefabricated nanoparticles or liquid phase catalyst. The use of nanoparticles and liquid catalysts is not a viable solution for the application in modern semiconductor integrated circuit (IC) technology. Both, processes contain problems in reproducibility and uniformity over an entire wafer. The advantage of a thin film deposition process is the ease in fabrication and reproducibility and the ability of the thin films to be incorporated into modern IC technology and processes (patterning and etching). The application of thin films also leads to the possibility of controlling the location of SWNT growth and possibly the direction of SWNT growth. Prefabricated nanoparticles and liquid catalyst are not easily selectively deposited on a substrate at specific sites.; The main difficulty with the implementation of thin film catalysts for the production of SWNTs is the ability to produce adequately sized nanoparticles of Fe during the CVD. Particles need to be less than 5 nm in diameter for the production of SWNTs, which contain diameters between 0.8–<5.0 nm. Therefore, the ability to initially form nanosized particles and then disperse the Fe catalyst throughout the substrate is a critical factor in controlling the size of the nanoparticles formed and hence the type of carbon nanotube growth. This ability to form nanoparticles from the evaporated films should be directly related to how the metal layers and the substrate interact.; The previous research has also not tried to understand the direct role of the substrate in influencing SWNT or MWNT growth. Do the substrate characteristics (crystallinity, porosity, stability, chemistry, etc.) influence the growth of SWNTs over MWNTs? This is a major factor in the growth of nanotubes from thin films since the properties of the substrate will directly relate to the formation of nanosized particles. The substrates influence may also lead to the possible selectivity of SWNT growth over MWNT growth.; It was determined in this research that the type of substrate employed as the supporting material for the Fe thin film catalyst has a strong effect on controlling the growth of either SWNTs or MWNTs. There are clear trends noticeable in the growth of either SWNTs or MWNTs on the various substrates. The ideal substrate for SWNT growth is the spun-on alumina film, while, single crystal substrates promote the growth of mixed SWNTs and MWNTs. All remaining substrates grew primarily MWNTs. The actual mechanisms associated with the selectivity of SWNT growth over MWNT growth is difficult to determine due to the many variables and complexities associated with this research.