Growth of mechanically fixed and isolated vertically aligned carbon nanotubes and nanofibers by DC plasma-enhanced hot filament chemical vapor deposition

Abstract

Vertically aligned, mechanically isolated, multiwalled carbon nanotubes (MWCNTs) and nanofibers (MWCNFs) were grown using an array of catalyst nickel nanowires embedded in an anodic aluminum oxide (AAO) nanopore template using DC plasma-enhanced hot filament chemical vapor deposition (HFCVD). The nickel nanowire array, prepared by electrodeposition of nickel into the pores of a commercially available AAO membrane, acts as a template for CNT and CNF growth. It also provides both a mechanical “fixed support” boundary condition and enforces sufficient spatial separation of the CNT/CNFs from each other to enable reliable and well-controlled mechanical testing of individual vertically aligned CNT/CNFs. In contrast with other AAO-templated growth methods, no post-growth etching of the AAO is required, since the CNTs/CNFs grow out of the pores and remain vertically aligned. A mixture of hydrogen and methane was used for the growth, with hydrogen acting as a dilution and source gas for the DC plasma, and methane as the carbon source. A negative bias was applied to the sample mount to generate the DC plasma. The filaments provided the necessary heat for dissociation of molecular species, and also heat the sample itself significantly. Both of these effects assist the CNT/CNF growth. Minimal heating came from the low-power plasma. However, the associated DC field was essential for the vertical alignment of the CNTs and CNFs. Scanning electron, transmission electron, and atomic force microscopy confirm that the CNT/CNFs are composed of graphitic layers, and form a vertically aligned, relatively uniform, and dense array across the AAO template. A significant number of the structures grown are indeed high quality nanotubes, as opposed to more defective nanofibers that are often predominant in other growth methods. This method has the advantage of being scalable and consuming less power than other techniques that grow vertically aligned CNTs/CNFs.