The molecular and magnetic structure of carbon-enclosed and partially covered Fe-55 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 Fe-55 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 Fe-55 cluster, Fe55N4Cx (x = 27, 37, 47, 54, 65), and Fe-55 encapsulated inside C-180 and C-240. The two latter are extreme examples of an iron particle completely enclosed in a carbon network. Fe-55@C-180 and Fe-55@C-240 present novel structures resembling the endohedral metallofullerenes. Two structural isomers of the Fe-55@C-180 are considered. Enclosing the Fe-55 cluster inside C-180 and C-240 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 C-180 and C-240 cages suggests site-selective reactivity of the endohedral complexes. The total magnetic moments of the Fe55N4Cx 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.