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

Abstract

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.