A dense, micron-tall layer of carbon nanofibers (CNFs) was grown above a layer of carbon nanotubes (CNTs) during the same synthesis using a thick cobalt catalyst (15 nm). The CNFs had large diameters (100 nm) and were amorphous while the CNTs had small diameter (10-20 nm) and were crystalline. Base growth mechanism was at play for both the nanofibers and the nanotubes. High-resolution transmission electron microscopy characterization suggested that the main mechanisms leading to the growth of the two structures were based on the dewetting of the catalyst layer and its subsequent alloying with the Ta underlayer. We can extend these principles to grow diverse carbon nanostructures during the same synthesis using appropriate multilayer thin films for different applications, especially for electrochemical cells and supercapacitors.
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We thank Professor Carl V. Thompson for useful discussions and for the use of the laboratory equipment at MIT. This research was supported by the MARCO Interconnect Focus Center and the MIT Energy Initiative. G.D.N. was partially supported by an Intel Fellowship. We are grateful to the Center for Materials Science and Engineering for the use of SEM and HRTEM and to Nicola Abate for performing selected experiments.