Regular arrays of vertically-aligned carbon nanofibers are actively sought: Due to their anisotropy and high surface area-to-volume ratio, they could be useful in many areas in nanotechnology. They can be grown from catalyst nanoparticles deposited on a substrate. Fibers with monodisperse diameters above 100 nm can be grown from dewetted catalyst thin films. For diameters below 100 nm, electron beam lithography is very successful, but the cost of this technique is an obstacle for commercialization. Alternatively, for these sizes, chemically synthesized nanoparticles can be used as seeds for nanofiber growth. M.F. Sarac, R.M. Wilson, A.V. Melechko, J.B. Tracy from North Carolina State University, K.L. Klein from the National Institute of Standards and Technology, and their colleagues show the influence of the ligands capping Ni catalyst nanoparticles for obtaining monodisperse, regular arrays of vertically-aligned nanofibers with diameters up to 100 nm.

As reported in the March 16th online edition of ACS Applied Materials & Interfaces (DOI: 10.1021/am101290v), the researchers compared nanofibers grown by plasma-enhanced chemical vapor deposition. The seeds were Ni catalyst nanoparticles, either capped by organic ligands (trioctylphosphine and oleylamine), or after ligand removal with UV-ozone treatment. The fibers obtained from ligand-capped nanoparticles are monodisperse of diameter and more regularly spaced, whereas the ones obtained from nanoparticles without ligands have a larger mean diameter and a broad dispersion of diameters. The UV-ozone treatment itself leaves the Ni nanoparticle core sizes unchanged, but during heating to 700°C, particles without ligands agglomerate and coalesce, which results in polydisperse seeds for nanofiber growth. Coalescence occurs only prior to nanofiber growth: Once growth is initiated, no further coalescence can occur, and the fibers grow separately.

The researchers have also demonstrated the formation of graphitic shells around the ligand-capped particles during the pre-growth heating. The shells are made of the carbon atoms from the ligand molecules. They protect the particles from agglomeration and also serve as a carbon source for the initial stage of carbon nanofiber growth.

Scanning electron micrographs of vertically aligned carbon nanofibers grown by plasma-enhanced chemical vapor deposition, with and without ligands on the catalyst Ni nanoparticles. Reproduced with permission from ACS Appl. Mater. Interfaces (2011) DOI: 10.1021/am101290v. © 2011 American Chemical Society.