Length and α-Fe content control of self-organised ferromagnetic nanowires encapsulated by multiwalled carbon nanotubes by low flow-rate CVD

Self-organised ferromagnetic nanowires encapsulated by multiwalled carbon nanotubes produced by CVD methods based on the thermal decomposition of ferrocene commonly contain the elemental phases: α-Fe, γ-Fe, and the carbide Fe3C. A continuous α-Fe nanowire and control of nanowire and nanotube length and diameter are desirable. High α-Fe nanowire content has been achieved through synthesis temperature modification, vapour flow-rate, and post-synthesis heat treatment. Length and diameter are intimately related to the self-organisational growth processes; reported approaches include regulation of the vapour supply to minimise dispersion in the nucleation process and introduction of other growth-modifying precursor elements. High vapour flow-rate produces downstream fluctuation resulting in discontinuous nanowires and diameter dispersion, or external decoration with spherical particles, results from rapid evaporation of the ferrocene precursor. We report a low vapour flow-rate and constant evaporation temperature method which achieves continuous α-Fe nanowires on the same scale as the nanotube for lengths >10 μm without the necessity of post-synthesis heat-treatment or introduction of other precursor elements. The low vapour flow-rate regime has the advantage of sustaining the intrinsic temperature gradient at the tip of the forming structure which drives the vapour feedstock to the growth front to guarantee continuous nanowire formation.