1D cobalt nanocrystals confined in vertically aligned carbon nanotubes: One-step synthesis and magnetic properties

We report on the one-step synthesis of vertically aligned multiwalled carbon nanotubes (MWCNTs) filled with Co magnetic nanowires via bias-assisted plasma enhanced chemical vapor deposition (PECVD) method. Co/Pd bilayer, both of nanometric thicknesses, deposited onto 50 nm thick TiSiN serving as a diffusion barrier was used as catalysts for nanotube growth. The process gas was a mixture of acetylene (carbon source) and ammonia. A 6 mbar growth pressure combined with 200 W plasma power permitted to achieve simultaneously the growth, alignment and filling of the carbon nanotubes. The plasma was activated by applying a negative bias between - 500 and - 600 V to the substrate. Structural investigations of 1D cobalt rich nanocrystals confined in vertically aligned carbon nanotubes were carried out by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) to elucidate the growth mechanisms. Statistical analysis was performed on the nanotubes peeled from the forest and dispersed. Spatially resolved electron energy loss spectroscopy (EELS) and energy filtered elemental mapping revealed that the nanotubes are mainly filled with cobalt X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co, Pd, C, N and O, with weak degrees of oxidation of the metallic Co and carbide Co formation. Hysteresis cycles, from vibrating sample magnetometer (VSM) measurements at room temperature, exhibited a ferromagnetic behavior with coercive field HC values well above those usually reported for continuous layers. HC is also observed to be higher when the Co nanoparticles embedded in the CNTs exhibit nail shape than spherical or cylindrical one. This can be explained by a lower mean diameter. This shape aspect influences the nature of the magnetization reversal probably operating through curling process. These results open up the potential for one step growth, alignment and filling of carbon nanotubes with potential applications as spintronic devices.& COPY; 2023 Elsevier B.V. All rights reserved.