First principles studies of Cobalt adatoms on Platinum (111)

Abstract

In the present work the surface diffusion of Co adatoms on Pt(111) is studied. Much attention is paid to this system because of its remarkable magnetic properties. The Vienna Ab-initio Simulation Package (VASP) is used to perform ab initio electronic structure calculations and structural optimisations. The Kohn-Sham equations of density functional theory are iteratively solved within a plane-wave basis set.<br />A generalized gradient approximation (GGA) functional is used to describe the exchange correlation term, and the projector-augmented wave (PAW) method to describe the electron-ion interaction.<br />Before studying the whole system, it is beneficial to discuss its subsystems first. The electronic structure of a single Co atom is briefly described and the Co dimer is discussed. The equilibrium bond length of the dimer is determined as 1.96 Angström. For the clean Pt(111) surface, a surface energy of 0.092 eV/Angström 2 and an outwards relaxation of the surface layer is found. A (5x5) cell for a five layer slab is used for calculations including Co adatoms, and additionally some calculations are performed with seven layers to assess the validity of the results. Non-magnetic as well as magnetic calculations are performed. Two stable adsorption sites, fcc and hcp, for a single Co adatom on Pt(111) are found. These sites are almost energetically degenerate in magnetic calculations. The barrier between the two sites is 0.194 eV for magnetic and 0.241 eV for non-magnetic calculations.<br />Two limiting cases are studied in case of two Co adatoms. First, with maximum distance between the adatoms achieved within the (5x5) cell when both atoms are on either fcc or hcp sites. A difference of the binding energy per adatom is found compared to single Co adatom adsorption, due to a repulsive interaction between the two Co adatoms (mainly mediated by the substrate). And second, the most stable configuration is found with both adatoms close to neighbouring fcc or hcp sites, and relaxed to a bond length of about 2.27 Angström. Nudged elastic band (NEB) calculations with one fixed Co adatom and the other one moving, show that the diffusion barriers depend on the taken path and that the probability to stay separated is higher than to build the most stable system mentioned before. However, if the two adatoms overcome the barriers and come close together, they will stay in this configuration.<br />Results including three adatoms show that the third Co adatom will rather build a triangular configuration with the other two Co adatoms while three Pt adatoms are more stable in a linear arrangement.