Spectroscopy Improves Nanotube Growth

Hank Hogan

Better displays may be on the way, thanks to work on carbon nanotube growth by researchers at Kyung Hee University in Seoul, South Korea. Using optical emission spectroscopy, the scientists have found the optimum conditions for the growth of vertically aligned nanotubes in a plasma-enhanced chemical vapor deposition system.

Carbon nanotubes could form the basis of long-lasting, high-performance miniature field emission devices that could be used to create the flat equivalent of the standard cathode-ray tube. To do that, the tubes must be vertically aligned, and the best method to do this has been unclear.

The investigators have determined that the abundance of the CH radical is critical in achieving this alignment. The radicals supply carbon to the growing surface and reduce network etching, explained Kyu Chang Park, a professor in the department of information display.

The scientists sputtered nanotubeprecursor nickel catalysts onto a substrate, patterning them via photolithography into 5-µm-diameter disks. They used a mixture of C₂H₂ and NH₃ at a pressure of a few torr, less than 0.01 atmos, for the plasma. They kept the substrate electrode voltage at –600 V while they tried from –300 to +300 V for the mesh electrode, which sat one-third of the way between the substrate and the grounded top electrode.

To characterize what was going on in the plasma, they set up a Horiba Jobin Yvon spectroscope to view the plasma through a shielded viewport. According to Park, optical emission spectroscopy is the easiest means of analyzing the chemical reactions in the system.

Based on the spectral fingerprints of the dominant CN, H, C₂ and CH peaks, the researchers found the +300-V mesh bias to be the best. This setting had the highest emission intensity, possibly because of the dissociation of C₂H₂ and NH₃ with the increased ion flux. Even higher mesh voltages should help nanotube growth, but Park said that this could not be verified in their setup.

Scanning electron microscope images revealed that the straightest and, therefore, the most defect-free carbon nanotubes were grown when the C₂H₂ flow rate was half that of the NH₃. The researchers found that CH radicals were highest at that point, while nanotube-network-destroying H radicals were lowest.

Park said that the team plans to further investigate the mechanisms at work between the density of grown carbon nanotubes and the chemical species.

Applied Physics Letters, Jan. 16, 2006, 033114.