Role of Adsorbed H, C, O, and CO on the Atomic Structure of Free and MgO(100)-Supported Ir-4 Clusters: An ab Initio Study

We applied density functional theory based on ultrasoft pseudopotentials to study the structural properties of Ir-4 clusters both in the gas phase and adsorbed on a MgO(100) surface. To resolve the discrepancy between experimental data which suggest a tetrahedral structure for Ir-4 and theoretical results which show a strong preference for the square isomer, we investigated the effect of several adsorbates on the equilibrium atomic structure of the clusters. Calculated binding energies of a single H, C, or O atom, as well as one CO or OH molecule, to three stable Ir-4 isomers indicate that C or CO adsorption significantly influences the relative stability of Ir-4 isomers. For MgO(100)-supported Ir-4, atomic carbon is able to change the isomer preference from the square to the tetrahedral geometry.

Properties of Ir4 clusters in the gas phase and on oxide substrates

Vladan Stevanovic

Results of a theoretical study on the properties of Ir4 clusters in the gas–phase and on oxide surfaces are presented. The work is based on density functional theory (DFT) within the generalized gradient approximation (GGA) and ultrasoft pseudopotentials. Properties of a small particle such as Ir4 cluster are entirely determined by its geometry. The already known result that the most stable form of Ir4 in the gas–phase is the square structure which is significantly more stable than the butterfly and tetrahedron is confirmed. This result is in contradiction with experiments which indicate that the oxide supported Ir4 adopts a tetrahedral configuration. It is shown in this thesis that the chemical environment has a strong influence on the relative stability of Ir4 clusters. On MgO(100) surface, the square isomer remains the most stable Ir4 structure, well separated in energy from the other two. Moreover, the tetrahedron is heavily distorted by the interaction with the surface oxygen. Presence of point defects (neutral and charged O vacancies) affects the energy ordering making tetrahedron and square very close in energy, but the structural distortion of the tetrahedron is even bigger and the predicted data do not correspond to experiments. On TiO2(110) the tetrahedron and square structures become degenerate and the butterfly becomes the least stable isomer. Moreover, structural distortions are very small, in agreement with experimental data. It is shown that the TiO2 surface influences the relative stability of the three isomers through a particularly strong electrostatic field. Interactions of Ir4 with H, C and O atoms as well as with CO molecules have been studied. Adsorption of a single C atom strongly influences the relative stability of the three isomers. Upon C adsorption, the butterfly becomes the most stable gas–phase isomer while on both surfaces the tetrahedron is the most probable structure. Adsorption of a single H or O atom does not produce the same effect. The interaction with CO molecules is also important given the experimental procedure used for producing supported Ir4 clusters. It is shown that on MgO(100), CO dissociation is as probable as the competing process CO desorption justifying the presence of carbon adatoms on Ir4 clusters which brings theoretical predictions in better agreement with experimental data.

EPFL2009

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.

2010

First-principles molecular-dynamics study of metal-supported nanosystems

Massimiliano Stengel

This thesis is concerned with the theoretical study of the adsorption of molecules and thin films to single-crystal metal surfaces. First-principles electronic calculations are performed in the framework of density functional theory in the local density approximation (LDA-DFT), by using a plane-wave formalism, norm-conserving pseudopotentials and the supercell method. For the calculation of the electronic ground state and the subsequent structural optimizations we use the Car-Parrinello technique, modified for the optimal treatment of metallic systems. This method has been improved in this work with the introduction of a Lagrangian formalism and a mathematically conserved constant of motion in the presence of variable fractional occupancies. Complex mechanisms involving substantial reconstructions of the substrate are elucidated by the study of the C60/Al(111) system. We find that the interaction of C60 molecules with the Al(111) surface is predominantly covalent, and that the adsorbates bind optimally to the surface if an Al vacancy is created directly underneath. The removed Al atoms form a (6 x 6) array of ad-dimers in the interstices of the C60 overlayer, to which they strongly bind. Large-scale structural relaxations performed directly on a surface unit cell containing three C60 molecules lead to a reconstructed structure with two vacancies and an Al ad-dimer which is significantly more stable than the unreconstructed one. This spontaneous local process, rather than the compression state of the C60 overlayer, explains why one C60 molecule out of three protrudes from the surface upon reconstruction. The delicate interplay of many competing factors occurring at the interface between different media is investigated through the study of ultrathin MgO films deposited on the Ag(100) surface. We consider pseudomorphic MgO films of thickness ranging from one to three atomic layers, and we study the evolution of the surface electronic properties as a function of coverage. We find that already three MgO monolayers are sufficient to fully develop the electronic structure of the single-crystal MgO surface, in agreement with experimental findings. We discuss the reliability of the LDA-DFT results for the band structure by using simple 1D models for the dielectric properties of the system.

EPFL2004