Effects of time on the quality of vertically oriented single-walled carbon nanotubes bygravity-assisted chemical vapour deposition

Abstract

The qualities of freestanding single-walled carbon nanotubes (SWCNTs) grown on Co/Si(100)in the gravity-assisted chemical vapour deposition (CVD) process have been investigated.Vertically oriented SWCNTs of high quality appear at a growth time of less than 3 min,verified by the clear radial breathing mode (RBM) in the Raman spectra and by the highresolution images taken by a transmission electron microscope (TEM). At a growth time of3 min, vertical and looped SWCNTs co-appear on the substrate. A SWCNT longer than∼1 µm tends to bend into a semicircular loop. At a longer growth time such as20 min, the coating of amorphous carbon (a-C) on the SWCNT becomesdominant, which is attributed to catalytic poisoning. A relatively long SWCNT(∼10–40 µm)surrounded with a thick a-C layer is the final structure. The bending stiffness of the a-C tube is estimated∼15 times larger than that of the SWCNT, which helps to keep the a-C/SWCNT compositestructure in a nearly vertical shape. A mechanism has been proposed to explain the coatingof a-C on SWCNTs.