Concentric sub-micrometer-sized cables composed of Ni nanowires and sub-micrometer-sized fullerene tubes

Abstract

Highly ordered arrays of submicrometer-sized coaxial cables composed of submicrometer-sized C-60 and C-70 tubes filled with Ni nanowires are successfully prepared by combining a sol-gel method with an electrodeposition process. The wall thickness of the submicrometer-sized tubes can be adjusted by the concentration of fullerenes and the immersion time. The thermal stability of the submicrometer-sized C-60 tubes is studied by Raman spectroscopy and it is found that these structures can be easily decomposed to form carbon nanotubes at relatively low temperatures (above 573 K) in an alumina template. These novel coaxial cable structures have been characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning electron microscopy (SEM), field-emission SEM (FESEM), Raman spectroscopy, elemental mapping, energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), vibrating sample magnetometer (VSM) experiments, and superconducting quantum interference device (SQUID) measurements. Magnetic measurements show that these submicrometer-sized cables exhibit enhanced ferromagnetic behavior as compared to bulk nickel. Moreover, submicrometer-sized C-70/Ni cables show uniaxial magnetic anisotropy with the easy magnetic axis being parallel to the long axis of the Ni nanowires. C-70/Ni cables also exhibit a new magnetic transition at ca. 10 K in the magnetization-temperature (M-T) curve, which is not observed for the analogous C-60/Ni structures. The origin of this transition is not yet clear, but might be related to interactions between the Ni nanowires and C-70 molecules. There is no preferred magnetization axis in submicrometer-sized C-60/Ni cables, which implies that the Ni nanocrystals have different packing modes in the two composites. These different crystalline packing modes lead to different magnetic anisotropy in the two composites, although the Ni nanocrystals have the same face-centered cubic (fee) structure in both cases.