October 3, 2011 — Boston College researchers have discovered two early-stage phases of carbon nanotube (CNT) growth during plasma enhanced chemical vapor deposition (PECVD). A disorderly tangle of tube growth ultimately turns into orderly rows of the nanoscopic tubes.
Using a thin catalyst layer growing CNTs via PECVD, Zhifeng Ren, a Professor of Physics; and researcher Hengzhi Wang discovered two previously overlooked stages of CNT growth. In the first stage, budding tubes appear randomly entangled. Then, the tubes are partially aligned. In the final stage, tubes are in full alignment.
PECVD grows CNTs by accumulating carbon atoms onto a catalyst particle. The catalyst thickness controls these growth phases, explained Wang. "Each stage," Wang said, "has its own merit [and] purpose."
Ren and Wang say that in the process of achieving the third stage of nanotube growth, the two earlier phases of growth have gone overlooked as each stage is etched away by the next application of plasma. Further masking these early-stage carbon nanotubes is the fact that they are not present when a thick catalyst is used, according to their findings.
The first stage tubes, produced in 0-4 minutes, are described as a tangle of random large- and small-diameter carbon nanotubes. The second stage tubes, created in 4-10 minutes, are generally smaller in diameter, but taller and only partially aligned.
Wang says that while these nanotubes are not in neat, orderly rows, they do have the advantage of offer a larger volumetric density and create a larger surface area, which could be an important development in the use of carbon nanotubes in heat transfer in thermal management. A potential application could involve in applying a thin coating of carbon nanotubes to an integrated circuit in order to draw away heat and efficiently cool the device.
After ten minutes of plasma etching, the early stage nanotubes have been washed away and the third stage tubes begin to emerge in tall, ordered rows upon the substrate. At this stage, the tubes themselves are shielded by makeshift "helmets" of catalyst particles, which effectively protect them during the last part of the growth process. Eventually, these last bits of catalyst are etched away as well.
Results are published in the latest edition of the journal Nanotechnology.
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