Atomic structures of iron-based single-crystalline nanowires crystallized inside multi-walled carbon nanotubes as revealed by analytical electron microscopy

Atomic structures of single-crystalline iron-based nanowires crystallized inside multi-walled carbon nanotubes during pyrolysis on silicon substrates with ferrocene as a precursor were analyzed using high-resolution analytical transmission electron microscopy and electron diffraction. Standard crystal lattices, namely body-centered cubic (bcc) α-Fe, face-centered cubic (fcc) γ-Fe and orthorhombic cementite Fe3C, were all found to form inside the nanotubes. A fraction of α-Fe nanowires, thermodynamically favorable at room temperature, was found to be dominant. Both bcc and fcc nanowires display a wide variety of lattice planes being parallel to the nanotube walls, with none of the orientations being preferable. The minor fraction of the nanowires had unidentified long-period crystal lattices with doubled or tripled periodicities as compared to those found in the standard cubic iron phases. The crystal matching of these unusual structures to stable orthorhombic Fe3C or less stable iron carbides, e.g., Fe5C2, Fe7C3, failed. The non-conventional phases were tentatively assigned to rarely seen silicon-doped octahedral iron carbides. Both long-period and standard cementite nanowires exhibited well-defined transient zones in the vicinity of nanowire–tube shell interfaces, where perfectly ordered carbide lattice fringes disappeared. The results suggest the non-existence of metastable equilibrium in the nanoscale Fe–C system between carbide and graphite phases during iron crystallization inside graphitic tubular channels.