Selective Nitrogen-Doping Structure of Nanosize Graphitic Layers

Nitrogen (N)-doped graphitic layers were uniformly synthesized as shells on pregrown Si nanowires by chemical vapor deposition. The N content (<= 10 atomic %) and thickness (<= 50 nm) of the graphitic layers were finely controlled by adjusting the deposition conditions. X-ray photoelectron spectroscopy revealed that pyridine-like N structures become favorable as the N content increases. For maximum 10% doping, pyridine-like N structures produced selectively at the highly curved graphitic layers of the thinner shell, whereas graphite-like N structures remain dominant at the less curved graphitic layers of the thicker shell. Raman spectroscopy supports the controlled and selective growth of these two N structures upon the change of the shell thickness and N content. The first principles calculation of the carbon nanotube (CNT) and graphene suggests that, for the CNT isomers, the pyridine-like structures, which are characterized by divacancies around the doped N sites, become more favorable than the graphite-like ones at a higher doping level. The preference of the pyridine-like structures reduces at the graphene isomers, which strongly supports the experimental results.