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The growth control of α-Fe inside multiwall carbon nanotubes has challenged researchers for more than a decade owing to the coexistence of this phase with Fe3C and γ-Fe. Previously, long heating treatments of 20 h have been used to decompose the encapsulated Fe-phases in C and Fe; however, these methods were limited by an unusual oxidation process leading to nanotube decomposition. In this letter, we report an alternative chemical vapour deposition approach that through an accurate control of the ferrocene-vapour flow-rate allows to achieve the direct encapsulation of 95% of α-Fe without additional heating treatments.

The electrical conductance of single- and double-wall carbon nanotube systems was measured by a mechanically controllable break-junction technique using freestanding nanotubes not subject to any chemical modification. For metallic single-wall carbon nanotubes, two channels with transmission coefficient of 0.88 contribute to ballistic electronic transport at room temperature. In double-wall carbon nanotubes, one metallic channel contributes to ballistic electronic transport and additional field- and temperature-dependent two-channel contributions were observed.