Dr. Hengzhi Wang and Zhifeng Ren, researchers at Boston College, have found two early stages in the growth of carbon nanotube growth while using plasma enhanced chemical vapor deposition method.
During the study, the researchers have discovered that carbon nanotubes that have initially grown in a disoriented manner have eventually resulted in the neatly aligned rows of nanotubes. They have utilized a thin layer catalyst for the method.
During the first stage, the method produced budding nanotubes that are haphazardly entangled, and in the second stage, partially arranged nanotubes were obtained. The third and final phases yield 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 plasma enhanced chemical vapor deposition method, the growth of carbon nanotubes is based on the repeated deposition of the carbon atoms caused by the decay of gases over a catalyst particle, resulting in the formation of multilayered carbon material upon a substrate. Scientists strive to form millions of carbon nanotubes in a neatly aligned fashion on the substrate.
Wang and Ren stated that for obtaining the third phase of nanotube growth, the two initial growth phases have been ignored as each phase is etched away by the subsequent plasma application. Since the early stage carbon nanotubes are subjected to further masking, they are absent while using a thick catalyst.
The first phase nanotubes, which are smaller diameter randomly-tangled carbon nanotubes, are generated in 0-4 minutes. The second phase nanotubes, which are usually of a smaller diameter, partially-aligned carbon nanotubes are taller and 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 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.
Source: http://www.bc.edu