Marangoni effect in $\mathrm{Si}{\mathrm{O}}_2$ during field-directed chemical vapor deposition growth of carbon nanotubes

Intense local heating of $\mathrm{Si}{\mathrm{O}}_2$ is shown to occur during chemical vapor deposition growth of single-walled carbon nanotubes in the presence of an electric field. This gives rise to the Marangoni effect where strong convection currents are induced in the molten $\mathrm{Si}{\mathrm{O}}_2$ layer. Nanoscale trenches and bumps are formed in the insulating layer directly below the growing nanotube.

Figure 1

Left: SEM image of a gap where the upper electrode was negatively biased and the lower electrode positively biased during growth. The dark areas surrounding the CNTs grown from the upper electrode are structural modifications of the substrate surface. Inset: higher resolution SEM picture of the nanotube shown in the AFM scan of (b). (a) AFM cross section image of a part of a nanotube grown from the upper electrode showing that the tube is embedded in a trench. (b) AFM cross section image of a nanotube grown from the lower electrode showing the nanotube lying on top of the substrate with no surface modification. Note that the lateral resolution of the AFM measurements is reduced due to tip convolution effects.

Figure 2

AFM height profiles and cross sections of substrate modifications caused by (a) nanotubes and (b) trench ridges observed from nanotubes growing from the same electrode. The AFM lateral resolution is poor due to tip convolution effects; the nanotube shows up as a central “bump” on the AFM scan. (c) Another sample showing a much higher ridge with the nanotube clearly visible as a small bump on top of the cross section image.

Figure 3

Right: AFM height profile showing a long nanotube-induced trench. The nanotube can still be seen at the bottom of the trench which increases in width and depth as the nanotube extends away from the electrode. Left: Trench depth versus distance from the electrode.

Figure 4

Schematic diagram of the Marangoni effect.