Nucleation of carbon-sulfur phases by manipulation of vertically-aligned mm-long films of iron-filled few-wall/multiwall carbon nanotubes

Abstract

We report a novel investigation of the effects of sulfur-precursor concentration on the morphological, structural and magnetic properties of vertically aligned mm-long films of carbon nanotubes (CNTs) filled with Fe3C nanowires. Fabrication was carried out by sublimation and pyrolysis of ferrocene/sulfur mixtures in the presence of 0.65 ml of dichlorobenzene within a custom designed chemical vapour deposition (CVD) reactor, under an Ar flow of 10-100 ml/min. The employed quantity of sulfur was found to have an important impact on the structural properties of the as grown CNTs, yielding either few wall CNTs (FWCNTs) or multiwall CNTs (MWCNTs) in presence of 0.4 mg or 1.2 mg of sulfur (category 1,2). Appearance of RBM (radial breathing modes) bands with variable intensities, was demonstrated in category-1 samples by both point and mapping Raman spectroscopy. Further, an additional broad band indicative of a chlorine/sulfur-induced amorphization of the CNT-walls was detected. Structural manipulation of category-1 and-2 samples was sought through annealing, in an attempt to create carbon-sulfur phases. The latter system has been indeed recently identified as a possible superconductor, under certain conditions of doping. Our findings highlight two different regimes of carbon-sulfur manipulation of the CNT-layers which involve 1) the nucleation of covalent carbon-sulfur phases at T∼150 °C -300 °C, with appearance of Raman bands at 1250 cm−1 and 1450 cm−1 and 2) an enhancement of the CNT-defectiveness (D and D’ bands) at T∼600 °C.

Keywords

• Carbon nanotubes
• Sulfur
• Doping
• Carbon-sulfur
• CVD
• Magnetic-manipulation