Structural morphology of carbon nanofibers grown on different substrates.

We present a detailed microstructural study comparing conventional carbon nanofibers (CNFs) and novel carbon hybrid CNF materials. The hybrid consists of CNFs grown on top of tetrahedral amorphous carbon (ta-C) thin films on silicon with nickel catalyst and Ti adhesion layers. The conventional CNFs were grown on silicon with nickel catalyst and Cr layers. Even though CNFs can be grown in both systems by tip growth, the micro- and nanoscale features are very different in the two systems. The crystalline structure of the CNF in the hybrid case changes from horizontal alignment to near-vertical alignment from the root to the tip and no bamboo structure is observed. The results show that micro- and nanoscale properties of CNFs grown under the same process conditions can be readily altered by using a sacrificial ta-C layer below the metallic layer to prevent the alloying of Ni with carbide-forming metals used as adhesion promoters and to act as an additional carbon source during the pre-annealing stage. The experimental results are further rationalized with the aid of assessed thermodynamic data and simulations based on density functional theory (DFT) with van der Waals (vdW) corrections. [ABSTRACT FROM AUTHOR]