Distribution and stability of Carbon in Fe-C nanoparticles.*

Catalytic Chemical Vapor deposition (CVD) method is widely used to produce carbon nanotubes. To improve our understanding of the CVD growth mechanism, we focus on the thermodynamics and the phase stability of catalyst Fe-C nanoparticles. Using \textit{ab initio} methods and classical molecular dynamics simulations, we investigate 1) the diffusion and solubility of carbon atoms in nanoparticles by calculating the distribution of carbon atoms inside the clusters, 2) the formation and stability of carbides at nanoscale, and 3) the effect of substrates on such structures. We address the implications of these results on NT growth, and give possible strategies to mitigate the problems.