Mössbauer morphological analysis of Fe-filled multiwalled carbon nanotube samples

We have performed morphological analysis of samples of Fe nanowires encapsulated into aligned multiwalled carbon nanotubes (Fe-MWCNTs) via $Fe57$ Mössbauer spectroscopy. The aligned Fe-MWCNTs were obtained by pyrolysis of ferrocene onto an oxidized Si substrate. Transmission Mössbauer spectroscopy (TMS) and backscattered conversion electron Mössbauer spectroscopy (CEMS) were applied in order to distinguish different Fe phases and their spatial distribution within the whole sample and along the tubes’ height. A characterization (on a large spatial scale) of the aligned CNT samples was performed by obtaining TMS spectra for selected spots positioned at different locations of the sample. While the total Fe content changes considerably from one location to another, the $γ-Fe∕α-Fe$ phase ratio is constant onto a relatively large area. Using TMS and CEMS for all aligned Fe-MWCNT samples it is also shown that along the CNT axes, going to the top of the nanotube the relative content of the $γ-Fe$ phase increases. Going to the opposite direction, i.e., towards the silicon substrate, the relative content of the $Fe3C$ phase increases, which is in agreement with our previous works. The results of an additional Mössbauer spectroscopy experiment in TMS and CEMS modes performed on a nonaligned sample support the conclusion that in our case the iron phases in the channels of carbon nanotubes are spatially separated as individual nanoparticles. The relative intensity ratio of the $α-Fe$ phase Mössbauer sextets shows good magnetic texture along the nanotubes’ axis for one of the aligned samples and the lack of such orientation for the others.