Metal Surface And The First-principles Calculation Of Atomic / Molecular Interactions

By using first-principles calculations based on the density functional theory(DFT),the dissertation is devoted to the study of the micro-mechanism of the interactions between adatoms/molecules and metal surfaces.The main content includes three parts:(â… ) the quantum size effect(QSE) of metal thin film and its modulation on surface energetics and surface chemical reactivities;(â…¡) the incipient oxidation of metal surfaces;(â…¢) studying the electronic structure and properties of strongly-correlated materials PuO₂ and Pu₂O₃ with DFT+U approach.For partâ… ,studies in chapter 3 and chapter 4 systematically calculate the QSE of Cu(001) and Pb(111) thin films,respectively.In chapter 3,first-principles calculations of Cu(001) free-standing thin films with thickness ranging from 1 monolayer(ML) to 26ML,have been performed to investigate the oscillatory quantum size effects exhibited in surface energy,work function,atomic relaxation.Quantum well states have been identified and clarified at particular k points corresponding to the stationary extrema in the bulk Brillouin zone,and are in good agreement with experimental observations.Furthermore, we have investigated Cs adsorption onto Cu(001) thin films as a function of the film thickness. The calculated surface properties clearly feature quantum oscillations as a function of the film thickness,with oscillation periods characterized by a superposition of long and short length scales, due to the quantum interference between the 'Belly'-and 'Neck' -orbital on the bulk Fermi surface of noble metals.Our systematically calculated results clearly show large-amplitude quantum oscillations in adsorption energetics,which may be used to tailor catalysis,chemical reactions, and other surface processes in nanostructured materials.In chapter 4,we carry out the first-principles calculations Pb(111) films up to 18 ML to investigate the quantum size effects exhibited in the surface energetics and magnetic properties after the monatomic adsorption of 3d-metal,including Mn,Cr,Fe and Co atom.The oscillatory behavior of adsorption energy as a function of the thickness of Pb(111) thin film is same with that of the surface energy of the clean Pb(111) film,which indicate the prototype of QSE in Pb(111) film.The adsorption energy of Cr or Mn adatom is quantitatively much lower than that of Fe or Co adatom,which show the relatively weak interaction between Cr or Mn adatom and outmost Pb atoms.The oscillation in the total magnetic moment of Cr/Pb(111) or Mn/Pb(111) adsorption systems takes on regular oscillation with a period of two Pb monolayers,and is notably different from Fe/Pb(111) or Co/Pb(111) systems,where the oscillation is irregular.The regular oscillation in the magnetic moment can be attributed to the various spin-polarization of Pb(111)-surface conduction electrons modulated by the QSE of Pb(111).According to the adatom-substrate interaction and the screening effect from the surface conduction electrons,we theoretically predict that the QSE of Pb(111) thin film will modulate the Kondo effect of Cr or Mn adatom.For partâ…¡,studies in chapter 5 and chapter 6 systematically explore the incipient oxidation of Pb(111) and Be(0001) surfaces,respectively.In chapter 5,we focus on the interaction between atomic oxygen and Pb(111) surface after the molecular dissociation.The atomic and energetic properties of purely on-surface and subsurface oxygen structures at the Pb(111) surface are systematically investigated for a wide range of coverage and adsorption sites.The fcc and tetra-â…¡sites are found to be energetically preferred for the onsurface and subsurface adsorption,respectively,in the whole range of coverage considered.The on-surface and subsurface oxygen binding energies monotonically increase with the coverage,and the latter is always higher than the former,thus indicating a tendency to the formation of oxygen islands(clusters) and the higher stability of subsurface adsorption.The on-surface and subsurface diffusion path energeties of atomic oxygen,as well as the activation barriers for oxygen penetration from the on-surface to the subsurface sites,are presented at low and high coverage.The activation barrier for the on-surface or the subsurface O diffusion becomes high when increasing the coverage. In particular,we have shown that the activation barrier for the penetration from the on-surface hcp to the subsurface tetra-â… site is as low as 0.065 eV,which indicates that the oxygen atoms can directly incorporate into the lead below the topmost Pb layer right after on-surface O₂ dissociation at low coverage.The other properties of the O/Pb(111) system,including the charge distribution, the lattice relaxation,the work function,and the electronic density of states,are also studied and discussed in detail.It is pointed out that the O-Pb chemical bonding during surface oxidation displays a mixed ionic/covalent character.Here the ionicity is featured by a charge flow from Pb-6p to O-2p states,while the covalency is featured by the Pb-6s² 'lone pair' effect,which results from hybridization of Pb 6s and O 2p states.In chapter 6,the focus of our interest is the adsorption and dissociation of molecular oxygen on the Be(0001) surface.The physisorbed and chemisorbed molecular precursor states are identified to be along the parallel and vertical channels,respectively.It is shown that the vertical channel with O₂ being at the hcp hollow sites of the Be(0001) surface is the most stable channel for the molecular chemisorption.The electronic and magnetic properties of this most stable chemisorbed molecular state are studied,which show that the electrons transfer forth and back between the spinresolved antibondingÏ€^* molecular orbitais and surface Be sp states.A distinct covalent weight in the molecule-metal bond is also shown.The dissociation of O₂ is determined by calculating the adiabatic potential energy surfaces,wherein the T-Y channel is found to be most stable and favorable for dissociative adsorption of O₂.Remarkably,we predict that unlike other simple sp metal surfaces such as Al(111) and Mg(0001),the adiabatic dissociation process of O₂ at Be(0001) is an activated type with a sizeable energy barrier(0.23 eV).In the end,the electronic structure and properties of PuO₂ andβ-Pu₂O₃ of different magnetic structures,including ferromagnetism,anti-ferromagnetism and non-magnetism,have been studied from first principles.The LDA+U and the GGA+U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Pu 5f electrons.We discuss how the properties of PuO₂ and Pu₂O₃ are affected by the choice of U as well as the choice of exchangecorrelation potential.Our results show that by choosing an appropriate U=4 eV,it is promising to correctly and consistently describe atomic structure and anti-ferromagnetic properties of PuO₂ andβ-Pu₂O₃.The inclusion of nonzero U forces the Pu-5f band to split at the Fermi level and,thus, drives the metallic-insulating transition.The insulating band gaps for PuO₂ andβ-Pu₂O₃ have been shown as a function of U.Also,oxidation reaction of Pu₂O₃,leading to formation of PuO₂, and its dependence on U and exchange-correlation potential have been studied.Our results show that the oxidation process of theβ-Pu₂O₃ is an exothermic reaction,which is mostly responsible for the experimentally observed plutonium pyrophoricity at 150-200℃.