Dual Role of Hydrogen in Low Temperature Plasma Enhanced Carbon Nanotube Growth

The gas composition during carbon nanotube (CNT) growth has received much attention because it largely determines the properties of the nanotubes and the yield of the growth. Here, we investigated the influence of hydrogen to hydrocarbon ratio on the CNT growth at low temperatures (<500 °C) using plasma-enhanced chemical vapor deposition (PECVD). Hydrogen was found to have a dual role during the growth process on TiN using catalytic nickel or cobalt nanoparticles. On the one hand, when the ratio of hydrogen to hydrocarbon partial pressure was low, CNT growth terminated well before the total process duration. As a result, no CNT growth was observed without hydrogen added to the gas mixture. The growth process could not be prolonged or reactivated upon performing in situ reducing or oxidizing cycles. On the other hand, when [H2]/[C2Hx] was high, catalyst dependent etching of the CNT forest occurred. For nickel catalyzed CNT growth, the high relative hydrogen content resulted in etching of the CNT forest, wh...