Hexahedral nanocementites catalyzing the growth of carbon nanohelices

The formation mechanism of carbon nanohelices grown on iron needles by microwave plasma assisted chemical vapor deposition is studied by means of transmission electron microscopy and scanning electron microscopy. The catalyst assisting the helix formation is identified to be the single crystalline cementite (Fe3 C) particle at the tip of each carbon nanohelix. The Fe3 C particles show the general morphology of a hexahedron with six (different) crystallographic planes as the surface planes. The different catalytic effect of different crystallographic surface planes produces an anisotropic growth on the front surface of the carbon nanostructure, which results in a rotation of the cementite particles. The rotating particles catalyze the growth of the carbon nanostructure in a helical way.