Highly chiral-selective growth of single-walled carbon nanotubes with a simple monometallic Co catalyst

We report on the growth of single-walled carbon nanotubes from a monometallic Co catalyst on an oxidized Si wafer support by the most simple growth recipe (vacuum annealing, growth by undiluted C${}_2$H${}_2$). Nevertheless, multiwavelength Raman spectroscopy and transmission electron spectroscopy show a remarkable selectivity for chiral indices and thus, e.g., high abundance with a single chirality representing 58% of all semiconducting tubes. In situ x-ray photoelectron spectroscopy monitors the catalyst chemistry during carbon nanotube growth and shows interfacial Co-Si interactions that may help to stabilize the nanoparticle/nanotube diameter. We outline a two-mechanism model explaining the selective growth.

Figure 1

(a) SEM and (b) TEM image for growth with C${}_2$H${}_2$ at 700 ${}^{\circ }$C and (c) SEM and (d) TEM image for 600 ${}^{\circ }$C.

Figure 2

Chiral maps for growth at 700 ${}^{\circ }$C (a) and 600 ${}^{\circ }$C (c). Assigned chiral indices $(n,m)$ are marked in red. Abundance estimations for individual $(n,m)$ are shown by column height [for a numerical table of $(n,m)$ abundances, see Appendix ]. Assigned SWNTs with an abundance of $⩽$1% are kept in faded red. Raman spectra taken on the SWNT grown at 700 ${}^{\circ }$C (b) and 600 ${}^{\circ }$C (d) show the diameter range by the shaded area.

Figure 3

SWNT abundances vs diameter for growth at 700 ${}^{\circ }$C (red) and 600 ${}^{\circ }$C (blue) estimated from the Raman measurements. The lines are a guide for the eye.

Figure 4

AFM images for both temperatures. Height distribution (a) found by AFM for the 0.1 nm Co film annealed in vacuum at 600 ${}^{\circ }$C (blue) and 700 ${}^{\circ }$C (red). The average height of a bare SiO${}_2$ substrate is also indicated. Schematic (b) of a SWNT with diameter $d$ nucleated by a nanoparticle with radius $r$. Oxidation in air increases the particle radius $r$ to $h$.

Figure 5

In situ XPS for as-loaded (bottom), vacuum-annealed (middle), and during C${}_2$H${}_2$ exposure (top) of a 0.6 nm Co film. Note that the top left Si $2p$ spectrum was actually acquired after growth at room temperature. Numbers in brackets are intensity multipliers. See Appendix  for XPS details.

Figure 6

Theoretical Kataura plot (full circles) with experimental data points (colored crosses) for the SWNTs grown at 600 ${}^{\circ }$C. Colored horizontal lines are the excitation energies and the gray area is the SWNT detection range.

Figure 7

Raman overview for increasing Co thickness and both temperatures.