The catalyst for CNT growth is a metal, typically a transition metal like nickel.
This metal catalyst is deposited on a substrate.
In the referenced study by Hofmann et al. (2003), it was a 6 nm nickel layer on silicon.
The catalyst plays a crucial role in the chemical vapor deposition (CVD) process.
Especially in the case of plasma-enhanced CVD (PECVD), it facilitates the decomposition of the hydrocarbon gas (e.g., acetylene) and the subsequent growth of carbon nanotubes.
The metal catalyst initiates the growth of CNTs by breaking down the hydrocarbon gas into carbon atoms that can then form the nanotubes.
The catalyst particles (aggregation, segregation, or migration) are influenced by various factors including plasma chemistry, electric field effects, and surface chemistry.
The size and density of the catalyst particles can significantly affect the diameter and density of the CNTs.
In the PECVD process, the catalyst not only promotes the decomposition of the hydrocarbon gas but also controls the nucleation and growth of the CNTs.
The catalyst's effectiveness can be influenced by its pretreatment, the nature of the substrate, and the presence of any metal underlayer or diffusion barrier.
For instance, in the study by Hofmann et al., the use of a nickel catalyst allowed for the growth of vertically aligned nanofibers at relatively low temperatures (120°C).
This is significantly lower than the temperatures typically required for CNT growth.
Moreover, the catalyst's role extends to the optimization of the growth process.
Parameters such as the nature of the feedstock, flow rate, pressure, and substrate temperature are critical.
These parameters can be adjusted to control the residence time, which in turn affects the accumulation of the carbon source and the growth rate of the CNTs.
In summary, the catalyst for CNT growth in the context of CVD and PECVD is a metal, typically a transition metal like nickel, which is deposited on a substrate.
This catalyst is essential for the decomposition of the hydrocarbon gas and the subsequent growth of carbon nanotubes.
Its effectiveness is influenced by various process parameters and its interaction with the plasma environment.
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