Rational Synthesis of Helically Coiled Carbon Nanowires and Nanotubes through the Use of Tin and Indium Catalysts

Helically coiled carbon nanotubes (HCNTs) and nanowires (HCNWs) represent novel nanostructural morphology and have technological and scientific significance. Their potential applications span high frequency electronics, tactile and magnetic sensors, and structural foams for cushioning and energy dissipation. It is also interesting to make a connection between the CVD synthesized helical carbon nanostructures to organic forms found in nature, such as DNA and proteins, and indeed, these structures can be used for templates in collagen growth. It was also suggested that a coil could correspond to a sequence of alternating metallic/semiconducting junctions, which is very interesting from the point of view of application to nanoelectronic systems. While several groups have previously reported on the synthesis of coiled nanostructures using chemical vapor deposition (CVD), patterned substrates were always used. While mostly HCNWs are seen, there have also been a few reports on the synthesis of HCNTs. Patterned growth of coiled nanotubes based on using previously aligned straight CNTs as templates was also demonstrated. It is typically found that such methods, in addition to limiting the amount of material due to the catalyst distribution, is often accompanied by the formation of linear multiwalled nanotubes. It would be desirable to develop a process that is independent of the underlying substrate, utilizing gas-phase synthesis alone, with controllable coiling characteristics (i.e., length, pitch etc.). In this paper, we report on a liquid-precursor-based synthesis method which has the additional advantage that either coiled nanotubes or nanowires, with differing electrical and mechanical properties, can be fabricated. We provide a rational explanation for the distinct growth modes based on an analysis of the binary equilibrium phase diagrams, where the mutual affinity of secondary catalysts (In/Sn), with the primary catalyst (Fe) in the ferrocene–xylene mixture, promotes nanotube/-wire formation. Coiled carbon nanostructures have been predicted to be energetically stable, and various mechanisms have been posited for their formation. While the curvature, in the case of helically coiled single-walled nanotubes could possibly be due to the regular insertion of pentagon–heptagon pairs at the junctions, it is unclear if a similar mechanism could hold for multiwalled nanotubes and HCNWs. Other formation mechanisms invoke localized stresses and anisotropic rates of carbon deposition on faceted catalyst particles. However, there is no experimental evidence for the above, as it is seen that helical structure is induced even though catalyst particles are not obviously present in the structure. It is also noted that the above mechanisms cannot be invoked for amorphous carbon nanocoils and compound (e.g., boron carbide) nanowires. To provide a comprehensive explanation, we have proposed a thermodynamic model, where helix/coil formation is explained on the basis of the interactions between specific catalyst particles and the growing nanostructure. We use the degree of wettability of the nanostructure surface by the catalysts as a criterion for coiling. It is seen, through the Young equation, that the wetting angle h cos 1 csv csl clv of