The reason for the low density of horizontally aligned ultralong carbon nanotube arrays

The reason why few carbon nanotubes (CNTs) nucleated in the catalyst region on substrates grow into ultralong ones during gas flow directed chemical vapor deposition (CVD) of horizontally aligned CNT arrays was explored. Small catalyst nanoparticles tend to merge into large ones due to the high processing temperature, which accordingly produces multi-wall CNTs (MWCNTs). These MWCNTs usually follow a base-growth mechanism and cannot be guided by the gas flow during growth. These MWCNTs are often shorter than 20 μm. Only the CNTs that follow the tip-growth mechanism, which are catalyzed by smaller nanoparticles and have fewer walls than most of the CNTs, tend to grow into longer ones. Besides, other factors influencing the areal density of ultralong CNTs, such as the entanglement of CNTs and the falling down of the growing tip of floating CNTs to the substrate, were also discussed.