Influence of Fe/Cr on nitrogen doped carbon nanotube growth

Using electron energy loss spectroscopy in a 100 kV VG scanning transmission electron microscope we study nitrogen doped carbon nanotubes grown via electron cyclotron resonance (ECR) microwave plasma techniques. The process is controlled by direct current (dc) biasing the grid separating the ECR source and the substrate. We show that plasma induced sputtering of the ECR source wall (stainless steel) can lead to significant iron and chromium contamination of growth samples. We identify various Fe, Cr, Ni nitride phases, and propose a growth model based on nitridation-induced metal segregation of steel based FeCrN alloys. Trace Cr doping of nanotube catalysts appears a promising route for introducing large nitrogen concentrations into both single and multi-walled nanotubes and may accelerate nanotube growth rates.