Controlling the growth morphology of carbon nanotubes: from suspended bridges to upright forests.

We have developed two strategies to produce carbon nanotubes (CNTs) from low-density surface growth to high-density forest growth. We have demonstrated that by introducing a C(2)H(2) pulse at the beginning of the growth, where methane is still used as the main carbon feeding gas, the growth tendency of CNTs can be changed and the resulting growth morphology will vary from surface growth to forest growth. Similarly, the growth morphology can be changed when the growth temperature is raised. The further characterization via Raman spectroscopy indicates that an increasing C(2)H(2) pulse time will lead to a rise of the D peak for as-grown CNTs, due to the formation of more multi-walled CNTs and the amorphous carbon contamination introduced by extra C(2)H(2), while a high growth temperature tends to produce high-quality CNTs and to reduce the amorphous carbon contamination. Furthermore, by appropriately adjusting the growth temperature and controlling the C(2)H(2) pulse time, we have managed to produce both suspended CNT bridges and upright forests within a single growth procedure and to form suspended pristine CNT transistors with a relatively high yield. In addition, the electrical properties of these CNT nanostructures have been investigated by electrical transport and scanning photocurrent measurements.