Evidencing contributions arising from disorder-rich rhombohedral stacking-order regions in S-doped carbon nanotube buckypapers

The recent observations of superconductive-ordering in carbon-based systems consisting of rhombohedral stacking-faults, trilayer graphene (ABC-stacks) or dislocation-rich interfaces (pyrolytic graphite) have attracted a significant attention. Here we report an unusual nucleation of rhombohedral (ABC) stacking-faults in low-dimensional systems comprising of multiwall carbon nanotube (CNT) buckypapers, in experiments involving doping with sulfur. Interestingly, at low sulfur-concentrations, a broad-band at the Raman shift from 1800 to 2000 cm⁻¹ was frequently identified, evidencing contributions arising from disorder-rich regions with rhombohedral stacking-order (ABC-stacks), as confirmed by X-ray diffraction and Rietveld refinements. These observations were further supported by a broadening in the left-shoulder of the 2D band at higher frequencies. T-ESR in the T range from 300 K to 77 K revealed multiple resonant signal splitting, indicative of paramagnetic contributions arising from the conduction π-electrons of the CNT and from amorphous iron-based-sulfides. Presence of C[sbnd]S bonding was revealed by Raman- and X-ray photoelectron-spectroscopy, evidencing a modification of the CNT. A sulfur-induced collapse within the CNTs-structure was found at high concentrations, as a consequence of a substitutional doping process and investigated through XRD, Rietveld refinements, TEM, HRTEM and magnetometry. ZFC/FC and magnetization vs field acquisitions revealed an interplay of ferromagnetic, paramagnetic and diamagnetic components with the diamagnetic-susceptibility χ being −2.85 ∗ 10⁻⁶ (emu/gOe) at T ~ 30 K.