Purity-enhanced bulk synthesis of thin single-wall carbon nanotubes using iron-copper catalysts

Preparation of metallic single-wall carbon nanotubes by selective etching

We report the bulk synthesis of a sample with a high concentration of metallic single-wall carbon nanotubes (m-SWCNTs) using a modified floating catalyst chemical vapor deposition method with methane as the carbon precursor. By tuning experimental parameters, such as species and flux of carrier gases, catalyst concentration, growth temperature, etc., small-diameter semiconducting SWCNTs (s-SWCNTs) and large-diameter m-SWCNTs were obtained. Using identical growth conditions, at a temperature of 1000 °C, it was found that the addition of hydrogen as an etchant gas resulted in the preferential removal of the smaller diameter s-SWCNTs. As a result, a sample enriched with large-diameter metallic SWCNTs was obtained. The self-assembled fishnet-like m-SWCNT network showed excellent optical transparency and electrical conductivity.

Growth of carbon nanotubes via twisted graphene nanoribbons

Carbon nanotubes have long been described as rolled-up graphene sheets. It is only fairly recently observed that longitudinal cleavage of carbon nanotubes, using chemical, catalytical and electrical approaches, unzips them into thin graphene strips of various widths, the so-called graphene nanoribbons. In contrast, rolling up these flimsy ribbons into tubes in a real experiment has not been possible. Theoretical studies conducted by Kit et al. recently demonstrated the tube formation through twisting of graphene nanoribbon, an idea very different from the rolling-up postulation. Here we report the first experimental evidence of a thermally induced self-intertwining of graphene nanoribbons for the preferential synthesis of (7, 2) and (8, 1) tubes within parent-tube templates. Through the tailoring of ribbon’s width and edge, the present finding adds a radically new aspect to the understanding of carbon nanotube formation, shedding much light on not only the future chirality tuning, but also contemporary nanomaterials engineering.

Carbon nanotubes can be considered as rolled-up small sheets of graphene. Here Lim and colleagues demonstrate this process, by fabricating carbon nanotubes through a thermally induced process of self-intertwining of graphene nanoribbons.

Single walled carbon nanotube growth and chirality dependence on catalyst composition

Vertical arrays of single walled carbon nanotubes (VA-SWNTs) were grown using bi-metallic nanoparticle pro-catalysts. Iron oxide particles were doped with varying quantities of first row transition metals (Mn, Co, Ni, and Cu) for a comparative study of the growth of nanotubes. VA-CNT samples were verified using scanning electron microscopy, and characterized using resonance Raman spectroscopy. The length of the VA-CNTs is used as a measure of catalyst activity: the presence of dopants results in a change in the CNT length and length distribution. Cross correlation of the Raman spectra reveal variations in the distribution of radial breathing mode peaks according to the pro-catalyst composition. The formation of various chirality nanotubes is constant between repetitive runs with a particular catalyst, but may be controlled by the identity and concentration of the metal dopants within the iron catalyst. These results demonstrate that the composition of the catalyst is a major driving force toward type selective growth of nanotubes.