Keeping an Eye on Carbon Nanotube Growth

Hank Hogan

Thanks to their electrical characteristics, high thermal conductivity and great tensile strength, carbon nanotubes could be the material of the future. At present, however, they are too expensive for widespread use. Now a group from Toyota Central R&D Laboratories Inc. in Nagakute, Japan, has shown that an optical microscope and an image processor can be used to monitor the growth rate of carbon nanotubes as the particles are synthesized. Having this information enabled the scientists to achieve a higher growth rate through the adjustment of various process parameters.

Using the experimental setup illustrated here, researchers measured a nanotube forest as the carbon particles were deposited onto a substrate. The arrangement enabled them to measure the deposition rate and to adjust process parameters such as heat and gas flow to produce a constant growth rate. Images reused with permission of the American Institute of Physics.

Itaru Gunjishima, a member of the team along with Takashi Inoue and Atsuto Okamoto, said that their technique could yield big dividends someday. “I hope this method will be one of the ways to reduce the production cost of carbon nanotubes,” he said.

The technique was based on a long-working-distance zoom microscope from Union Optical Co. of Tokyo, which the researchers set up so that it allowed them to observe the nanotubes through a quartz window as they were synthesized via chemical vapor deposition. The distance between the silicon substrate where the growth occurred and the microscope was ∼100 mm.

An optical microscopy image shows the result of carbon nanotube growth, with the measurement region indicated in the space between the arrows.

The investigators captured the magnified image of a cross section of the growing nanotube “forest.” They measured its thickness 50 times per second using a CCD camera and an image processing package from Keyence Corp. of Osaka, Japan. They fed these measurements into a programmable logic controller, a specialized computer typically used for automation tasks. The controller calculated the growth rate at each sampling point, smoothing the data in various ways. The researchers logged the growth rate and manually adjusted the temperature, the pressure and the gas-flow rate.

They observed that the growth rate would start out as high as 8 μm/min for certain process conditions. It would then fall off, declining by as much as 50 percent over the course of a 25-min session. They found that they could abate this trend by raising the growth temperature from, for example, an initial 700 °C to 760 °C.

Similarly, changing the flow rates of the process gases helped stabilize the growth rate. The scientists adjusted the hydrogen flow downward and the acetylene flow upward over time. In addition, increasing the pressure from ∼250 Pa to ∼350 Pa also helped keep the growth rate constant. With these changes, they achieved a nearly constant growth rate, although eventually the growth stopped for reasons that are not yet understood.

Aside from the adjustment being a manual affair, this scheme generated other concerns. One resulted from fluctuations in the measured carbon nanotube thickness, a consequence of failures in edge detection and of tiny oscillations in the system. Gunjishima noted that they are working to diminish these fluctuations.

“Reduction of minute vibrations in the equipment will be one of the ways we do this,” he said.

Applied Physics Letters, Nov. 5, 2007, Vol. 91, 193102.