Necessity of base fixation for helical growth of carbon nanocoils

The role played by catalyst aggregates in the growth of carbon nanocoils (CNCs) by a chemical vapor deposition (CVD) method has been studied. The experimental results show that CNCs can be grown from the discrete aggregates on a substrate with a porous surface, while only some irregular carbon nanofibers are grown from those on a flat substrate. It is accepted from the viewpoint of mechanics that the spiral motion of a CNC should generate a torsional moment on its base that attaches to an aggregate. The catalyst particles readily expand on the flat substrate during the CVD process and form a loose aggregate, which cannot provide a strong interaction between the aggregate and the base of a carbon fiber grown from there. On the contrary, the expansion of catalyst particles in a microsized hole is restricted by the surrounding wall of the hole, leading to the formation of a compact aggregate that fixes the base of the grown fiber. A perfect CNC can be grown only under the condition that its base is firmly fixed by an aggregate that can balance the torsional moment of the CNC during its spiral growth.