Epitaxy-driven vertical growth of single-crystalline cobalt nanowire arrays by chemical vapor deposition

Abstract

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.