Analysis of the catalytic growth of carbon nanotubes
This thesis covers the analysis of the catalytic growth of carbon nanotubes under well de fined conditions, the optimization of the field emission properties of those structures and introduces a model for the growth mechanism based on the experimental results. Experimental investigations are presented which allow to get a comprehensive picture of the catalytic growth of carbon nanotube films. These films were generated by patterning silicon surfaces with transition metal catalysts usingmicro contact printingand subsequent generation of nanotubes by means of chemical vapor deposition (CVD). The development of the used catalyst was characterized as function of growth time and applied temperature. The subsequent studies of the morphology of the carbon structures grown by CVD revealed a significant influence of the deposition temperature and the catalyst material on the quality of the carbon structures. Electron microscopy and Raman spectroscopy were used to investigate the character of these structures in more detail. It was found that the increase in temperature above 800°C resulted in the formation of a polycrystalline outer shell over a nanotube core. Based on those experimental results a mechanism for the growth of carbon nanotubes is suggested. Acetylene is dissociated catalytically on the catalyst nanoparticles spread on the substrate surface. In a first stage the acetylene reduces the metal oxide grains to pure metal. The further catalytic dissociation of acetylene takes presumably place at facets of well-defined crystallographic orientation and the carbon diffuses into the particle. The resultingdensit y gradient of carbon dissolved in the particle drives the diffusion of carbon through the particle. In order to avoid dangling bonds, the carbon atoms assemble at a less reactive facet of the particle, which leads to the formation of a nanotube. Thicker nanotubes at higher temperatures are generated due to the dissociation of acetylene in the gas phase, which leads to the formation of carbon flakes that condense on the catalytically grown structures. In order to support the growth model, simple classic calculations and simulations were performed, which yielded formulas that allow to estimate nanotube growth properties, like the growth velocity. The theoretical results correspond well with the experimental. Furthermore a mechanism for the cessation of the nanotube growth was proposed. Complementary, the field emission properties of different carbon structures were determined. It turned out that the thinnest nanotubes emit at lowest fields. Furthermore, in regard to applications nanotubes have been grown on glass substrates and the field emission properties of such samples have been characterized. Finally, a scanningprob e microscope is presented which exploits the field emission of carbon nanotubes. It would offer new possibilities to characterize samples on a nanometer scale.
Section de physique
Faculté des sciences de base
Institut de physique des nanostructures
Jury: Jean-Marc Bonard, Joël Chevrier, Laszlo Forro, Jean-Jacques Meister
Public defense: 2003-6-5
Record created on 2005-03-16, modified on 2016-08-08