Epitaxy-driven vertical growth of single-crystalline cobalt nanowire arrays by chemical vapor deposition
Highly oriented single-crystalline ferromagnetic Co nanowire (NW) arrays were synthesized on sapphire substrates via a single-step chemical vapor deposition (CVD) method. On an m-cut sapphire substrate, Co NWs were vertically grown in epitaxial relationship with the substrate without using any catalysts or templates. On an r-cut sapphire substrate, Co NWs were horizontally grown in two perpendicular directions. Furthermore, we report that the Co NWs were transformed into Co3O4 nanotubes by thermal annealing under dilute O2 conditions. Such formation of hollow structures is ascribed to favored outward diffusion of Co ions. The present vertically aligned arrays of single-crystalline Co NWs could be utilized for advanced magnetic memory applications owing to their uniform orientations.