The molecular and magnetic structure of carbon-enclosed and partially covered Fe55 particles.

The structure and magnetic moment distribution are studied for an iron nanoparticle with varying degree of carbon adatom coverage. The limiting models of the study are the clean icosahedral Fe55 particle and the iron particle completely enclosed in carbon cages. Between the two extrema, partially covered particles are considered. The iron cluster with partial coverage of carbon adatoms represents a model of active catalysts in the chemical vapor deposition synthesis of carbon nanotubes. The investigated structures are the bare Fe55 cluster, Fe55N4C(x) (x = 27, 37, 47, 54, 65), and Fe55 encapsulated inside C180 and C240. The two latter are extreme examples of an iron particle completely enclosed in a carbon network. Fe55@C180 and Fe55@C240 present novel structures resembling the endohedral metallofullerenes. Two structural isomers of the Fe55@C180 are considered. Enclosing the Fe55 cluster inside C180 and C240 fullerenes gives rise to changes in the Fe-Fe bond lengths. This alters the magnetic structure of the iron cluster considerably. The interaction between the fullerenes and the enclosed iron cluster is reflected in a charge transfer of 8-13 electrons in the considered endohedral complexes. The localization of atomic charges on the C180 and C240 cages suggests site-selective reactivity of the endohedral complexes. The total magnetic moments of the Fe55N4C(x) nanoparticles vary with the degree of adatom coverage. The magnetic moments of individual Fe atoms depend strongly on the element of the nearest-neighbor atoms and on the coordination number and carry therefore information about the local chemistry.