Unusual radial breathing mode signals in cm-scale carbon nanotube buckypapers filled with Fe₃C single crystals: Investigating the role of chlorine

Fe₃C filled carbon nanotubes (CNTs) self-organized into flexible-film-morphologies (buckypapers) have been proposed for numerous applications, these include flexible data-storage-devices, exchange-bias, spin-based-systems, flexible ferromagnetic-electrodes and others. However, despite the important progress, the significant concentration of Cl-containing-hydrocarbons which is needed for the buckypaper-growth (i.e. 1 to 10 ml) represents still an important challenge for industrial-production. Here we report the experimental-growth-parameters needed for the nucleation and growth of flexible Fe₃C-filled-CNT buckypapers, under very-low-concentrations of dichlorobenzene, as low as 0.05 ml. By carefully estimating both the heating- and cooling-rate parameters of the chosen chemical-vapor-deposition (CVD) reactor, we demonstrate the identification of two active-growth-regions which allow for the simultaneous nucleation and growth of ferromagnetically-filled buckypapers. The obtained films comprise of selectively grown single-wall (active-region-1) and multiwall (active-region-2) CNTs. The simultaneous growth of the two types of buckypapers within the same reactor is discussed and attributed to the dynamics of dichlorobenzene-pyrolysis, which appears to yield a higher concentration of Cl-radicals in the active-region-1. In the buckypapers grown in the latter (active-region-1) we highlight an unusual separation between the first (inner) and second CNT-cylinders, with the HRTEM analyses revealing an anomalously-large interlayer-distance varying from ∼ 1.6 to ∼ 6.6 nm (an effective void between two CNT-cylinders).