Dr. Hengzhi Wang and Zhifeng Ren, researchers at Boston College, have found two early stages of carbon nanotube growth when using plasma enhanced chemical vapor deposition method.
During the study, the researchers have discovered that carbon nanotubes that have initially grown as unruly tangles have eventually produced the neatly aligned rows of nanotubes. They have utilized a thin layer catalyst for the method.
During the first stage, the method produced burgeoning nanotubes that were haphazardly entangled, and the second stage yielded partially arranged nanotubes. The third and final phases yielded neatly aligned nanotubes, which are the regular nanotubes utilized by the researchers for applications in several biomedical and materials research activities.
Wang stated that the catalyst’s thickness controls these growth phases. In the plasma enhanced chemical vapor deposition method, the growth of the carbon nanotubes is based on the accretion of the carbon atoms caused by the decay of gases over a catalyst particle, resulting in the fabrication of multilayered carbon material upon a substrate. Scientists want to fabricate millions of carbon nanotubes in a neatly aligned fashion on the substrate.
Wang and Ren stated that for obtaining the third phase of the nanotube growth, the two initial growth phases have been ignored as each phase is etched away by the subsequent plasma application. As per their findings, the absence of these early-stage carbon nanotubes during the application of a thick catalyst is the fact behind their further masking.
The first phase nanotubes, which are smaller in diameter and randomly-tangled carbon nanotubes, are generated in 0-4 minutes. The second phase nanotubes, which are usually smaller diameter partially-aligned carbon nanotubes but taller, are produced in 4-10 minutes. These nanotubes cannot offer higher volumetric density and generate huge surface area, which is critical for heat transfer application in thermal management, Wang said.
The early phase nanotubes have been etched away by plasma etching for 10 minutes, resulting in the emergence of the third phase nanotubes that are tall and in orderly rows over the substrate. At this phase, catalyst particles’ makeshift ‘helmets’ shield the nanotubes for their protection in the final stage of the growth process. Finally, these last traces of catalyst are also washed away.