A semi-empirical molecular orbital study of freestanding and fullerene-encapsulated Mo nanoclusters

Nanoscopic transition metal clusters are widely used in the catalytic growth of carbon nanotubes (CNTs) synthesised using a chemical vapour deposition process. It is known that addition of oxygen and heavier chalcogens such as sulphur can both increase CNT growth rate and maximum length, and also promote selective growth of single and double wall CNTs. We describe results of semi-empirical molecular orbital calculations, using the AM1^* Hamiltonian as implemented in the VAMP module of Materials Studio^®, of both freestanding and fullerene-encapsulated molybdenum (Mo) clusters, which show that metal clusters above a critical size cause the formation of defects by opening of hexagon–hexagon (6–6) carbon bonds that lead eventually to a splitting open of the fullerene cage. Furthermore, we demonstrate that the addition of oxygen, nitrogen and sulphur to the molybdenum cluster cause changes to its electronic and geometric structure. Our simulations clearly demonstrate the strong interaction of the metal catalyst particle with the fullerene cap of a growing CNT, which may affect one or more stages of carbon incorporation into CNTs according to the commonly accepted vapour–liquid–solid growth model, although the precise mechanism requires further investigation.

Keywords

• semi-empirical molecular orbital theory,
• AM1* ,
• molybdenum nanoclusters,
• carbon nanotube synthesis,
• chemical vapour deposition