Self-consistent-charge density-functional tight-binding/MD simulation of transition metal catalyst particle melting and carbide formation

Iron, nickel, and cobalt are commonly employed catalyst transition metals in catalytic chemical vapor deposition (CCVD) growth of SWCNTs. Quantum chemical molecular dynamics simulations (QM/MD) of transition metal particle melting and carbide formation during the early stage of singlewalled carbon nanotube (SWCNT) growth are presented here. The self-consistent-charge densityfunctional tight-binding (SCC-DFTB) method was employed as the potential for MD simulations over timescales of several hundreds of picoseconds at temperatures ranging from 400 K to 2000 K. Model systems consisting of 24C ₂ molecules and a single C ₃₀ 'SWCNT cap-fragment' chemisorbed on the surface of transition metal clusters (Fe ₃₈, Co ₃₈, Ni ₃₈) were employed. The melting behavior is compared with those of corresponding pristine transition metal clusters. Co displayed a larger tendency towards melting and 'bulk' carbide formation over the entire temperature range, whereas Fe and Ni clusters exhibited only surface carbide formation. Carbon surface diffusion was found for all metals, and the growth of carbon clusters by additional ring formation was observed in Fe/Ni trajectories at high temperatures, especially when high electronic temperature was employed. Although no sudden increase in Lindemann indices was observed clearly, we conclude that melting of Fe/Ni clusters starts at lower temperatures when carbon is present on the cluster, whereas the opposite trend is observed for Co clusters.