Relaxation Of Surface Atoms Research Articles

Surface relaxation and initial surface corrosion of strained Mo(100) surface

Metal surface corrosion is a common phenomenon under atmospheric conditions, however, the design of corrosion resistant materials at the atomistic level is hindered by the limited knowledge about the corrosion mechanism. Here, the initial surface corrosion of metallic molybdenum, an important in industrial materials, is investigated by using the density functional theory calculations. The adsorption energies of several key corrosion intermediates on Mo(100) surfaces are calculated under various strain (-5% to 5%). The concerted effect of strain and adsorbates can cause significant relaxation of surface atoms (the in-plane atom displacement is within 0.1 - 0.4 Å). The small repulsive lateral interactions among O* adsorbates allow high O coverages. The ensuing corrosion follows the uniform corrosion process rather than the pitting corrosion process. Tensile strain can accelerate the initial surface corrosion process. Further analyses indicate that higher coverage of adsorbates correlates to bigger variation of the interlayer distances, which is the consequence of the competition between the adsorption interactions and the interlayer interactions. The findings reveal the oxidation process over Mo(100) and provide a general understanding for metal corrosion.

Vector potential and surface magnetic field in magnetoelectric antiferromagnetic materials

A general formula for the average vector potential of bulk periodic systems is proposed and shown to set the boundary conditions at magnetic interfaces. For antiferromagnetic materials, the study reveals a unique relation between the macroscopic potential and the orientation-dependent magnetic quadrupole, as a result of the different crystalline and magnetic symmetries. In particular, at surfaces and interfaces of a truncated bulk without inversion and time-reversal symmetries, the average vector potential exhibits a discontinuity, which results in an interfacial magnetic field. In general, however, due to the surface and interface electronic and atomic relaxations, additional magnetization may result. For the experimentally-observed magnetoelectric antiferromagnets, in particular, our symmetry analysis suggest that the relaxation effects could well be a system response to the presence of such a potential discontinuity.

Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters.

Ne clusters (∼5000 atoms) were resonantly excited (2p→3s) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Surface atomic relaxation and magnetism on hydrogen-adsorbed Fe(110) surfaces from first principles

We have computed adsorption energies, vibrational frequencies, surface relaxation and buckling for hydrogen adsorbed on a body-centred-cubic Fe(110) surface as a function of the degree of H coverage. This adsorption system is important in a variety of technological processes such as the hydrogen embrittlement in ferritic steels, which motivated this work, and the Haber–Bosch process. We employed spin-polarised density functional theory to optimise geometries of a six-layer Fe slab, followed by frozen mode finite displacement phonon calculations to compute Fe–H vibrational frequencies. We have found that the quasi-threefold (3f) site is the most stable adsorption site, with adsorption energies of ∼3.0eV/H for all coverages studied. The long-bridge (lb) site, which is close in energy to the 3f site, is actually a transition state leading to the stable 3f site. The calculated harmonic vibrational frequencies collectively span from 730 to 1220cm−1, for a range of coverages. The increased first-to-second layer spacing in the presence of adsorbed hydrogen, and the pronounced buckling observed in the Fe surface layer, may facilitate the diffusion of hydrogen atoms into the bulk, and therefore impact the early stages of hydrogen embrittlement in steels.

Surface energy and relaxation in boron carbide (101̄1) from first principles

Abstract The surface energy of the boron carbide polytype B11Cp(CBC) for planar separations along {1011} was determined to be 3.21 J/m2 via first-principles density-functional computations. Surface atomic relaxations are relatively large, thereby lowering the surface energy significantly. The icosahedra are not intact on the surface, i.e., severed polyhedra are the lowest energy surface configuration. Good agreement was found with an experimental average fracture surface energy.

Investigation of pyrite surface state by DFT and AFM

The surface states of pyrite (FeS2) were theoretically investigated using first principle calculation based on the density functional theory (DFT). The results indicate that both the (200) and (311) surfaces of pyrite undergo significant surface atom relaxation after geometry optimization, which results in a considerable distortion of the surface region. In the normal direction, i.e., perpendicular to the surface, S atoms in the first surface layer move outward from the bulk, while Fe atoms move toward the bulk, forming an S-rich surface. The surface relaxation processes are driven by electrostatic interaction, which is evidenced by a relative decrease in the surface energy after surface relaxation. Such a relaxation process is visually interpreted through the qualitative analysis of molecular mechanics. Atomic force microscopy (AFM) analysis reveals that only sulfur atom is visible on the pyrite surface. This result is consistent with the DFT data. Such S-rich surface has important influence on the flotation properties of pyrite.

The structural and electronic properties of spinel MnCo2O4 bulk and low-index surfaces: From first principles studies

The structural and electronic properties of the bulk and low-index (001) and (110) surfaces of the MnCo2O4 spinel have been studied from the spin-polarized GGA+U calculations. Results show that Mn prefers to occupy the octahedral sites and exhibits as high-spin Mn3+ while half Co takes the tetrahedral sites as high-spin Co2+ and the other half takes the octahedral sites as nonmagnetic Co3+. The electronic structure of the MnCo2O4 bulk is insulating with very small gap of about 0.03eV. The surface atomic relaxations and surface energies are calculated with different terminations and stable MnCo2O4 (001) and (110) surface models are obtained. The electronic structures of the Mn and the tetrahedral site Co atoms at the MnCo2O4 (001) and (110) surfaces resemble their bulk characters. However, due to the lowered Co–O coordination, the electronic structure of the octahedral Co atoms changes substantially from the bulk state t32g(↑)t32g(↓) into the surface state t32g(↑)e1g(↓) t22g(↓) at the MnCo2O4 (001) surface (5-coordinated Co) and t32g(↓)e2g(↓)t12g(↓)e1g(↓) at the MnCo2O4 (110) surface (4-coordinated Co).

Numerical study of hetero-adsorption and diffusion on (100) and (110) surfaces of Cu, Ag and Au

Quenched molecular dynamics simulations and density-functional theory (DFT) calculations are used to study the adatom hetero-diffusion on the (100) and (110) surfaces of four systems: Au/Ag, Ag/Au, Au/Cu, Cu/Au. Atomic interactions are described by embedded-atom method potentials, that are validated by a comparison with DFT results on diffusion barriers. The local relaxation of surface atoms around the adatom is analyzed, together with adsorption energies and hopping diffusion barriers. We find that atomic relaxation is qualitatively different between (100) and (110) surfaces, with the exception of the Au/Cu case. Hopping diffusion barriers are larger on the (100) than on the (110) surface. Many-body and relaxation effects cause significant differences between Ag/Au and Au/Ag barriers, and between Cu/Au and Au/Cu barriers.

A comprehensive DFT investigation of bulk and low-index surfaces of ZrO2 polymorphs.

The bulk structure, the relative stability, and the electronic properties of monoclinic, tetragonal, and cubic ZrO(2) have been studied from a theoretical point of view, through periodic ab initio calculations using different Gaussian basis sets together with Hartree-Fock (HF), pure Density Functional Theory (DFT), and mixed HF/DFT schemes as found in hybrid functionals. The role of a posteriori empirical correction for dispersion, according to the Grimme D2 scheme, has also been investigated. The obtained results show that, among the tested functionals, PBE0 not only provides the best structural description of the three polymorphs, but it also represents the best compromise to accurately describe both the geometric and electronic features of the oxide. The relative stability of the three phases can also be qualitatively reproduced, as long as thermal contributions to the energy are taken into account. Four low-index ZrO(2) surfaces [monoclinic (-111), tetragonal (101 and 111), and cubic (111)] have then been studied at this latter level of theory. Surface energies, atomic relaxations, and electronic properties of these surfaces have been computed. The most stable surface is the cubic one, which is associated to small relaxations confined to the outermost layers. It is followed by the monoclinic (-111) and the tetragonal (101), which have very similar surface energies and atomic displacements. The tetragonal (111) was instead found to be, by far, the less stable with large displacements not only for the outermost but also for deeper layers. Through the comparison of different methods and basis sets, this study allowed us to find a reliable and accurate computational protocol for the investigation of zirconia, both in its bulk and surfaces forms, in view of more complex technological applications, such as ZrO(2) doped with aliovalent oxides as found in solid oxide fuel cells.

Termination-dependent electronic and magnetic properties of ultrathinSrRuO3(111) films onSrTiO3

We have investigated electronic and magnetic properties of ultrathin SrRuO$_{3}$ (SRO) film grown on (111) SrTiO$_{3}$ substrate using the {\it ab initio} electronic structure calculations. Ru-terminated SRO (111) film suffers from strong surface atomic relaxations, while SrO$_{3}$-terminated one preserves the surface structure of ideal perovskites. Both Ru- and SrO$_{3}$-terminated SRO (111) film show unexpected interlayer antiferromagnetic (AFM) structure at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO$_{3}$-termination case, the half-metallic nature emerges despite the interlayer AFM structure. Upon reducing the thickness, the collapsing behavior of magnetic anisotropy from out-of-plane to in-plane easy axis is found to occur for the Ru-termination case, which, however, does not pertain to SrO$_{3}$-termination case.

InN growth on BaTiO3 (111) substrates: A first-principles study

The surface stability of the polar (111) BaTiO3 surfaces is systematically studied by first-principle calculations. Surface energy and atomic relaxation have been calculated for BaO3 and Ti polar terminations in perovskite BaTiO3 ceramics. The specific adsorption sites at the initial growth stage and the atomic structure of InN on the BaTiO3 (111) substrate have been systematically investigated. The In adsorption atoms are more favorable than the N atoms for the BaTiO3 (111) surface. The energetically favorable In/BaO3 interface is pointed out among the atomic arrangements of the InN/BaTiO3 (111) interfaces. Oxygen vacancies in the first layer of the BaO3-terminated BaTiO3 substrate induce the occupied defect states of In 5s5p character at the energetically favorable In/BaO3 interface.

Hydrogen induced metallization of ZnO (11̅00) surface: Ab initio study

Results of first principles hybrid calculations are presented for hydrogen atoms adsorbed upon non-polar ZnO (11̅00) surface. The energy of surface atomic relaxation, H adsorption energy, electronic density redistribution and modification of the electronic structure are discussed. It is shown that hydrogen is adsorbed mainly on the surface oxygen ions and forms a strong bonding with them (2.7eV). Adsorption of hydrogen on the surface zinc ions is energetically unfavorable (−4.4eV). It also shown that surface hydrogen atoms are very shallow donors, thus, contributing to the electronic conductivity, and ZnO metallization.

Stability, composition and properties of Li2FeSiO4 surfaces studied by DFT

Surface structures and energies of the Pmn21 polymorph of the electrode material Li2FeSiO4 are studied by first-principles calculations using density functional theory. In total, 29 surface terminations of stoichiometric polar and nonpolar slabs were studied. These surfaces were preselected by energy estimation via a model that accounts for bond cutting and additionally for polarity compensation in the case of polar surfaces. The model provides a way to quantify the most important contributions to the surface energy. Furthermore, we analyze the relaxation of surface atoms statistically. This clearly shows that SiO4 tetrahedra are rather rigid whereas the local environment of Fe and Li can change strongly under relaxation near the surface. We furthermore compare results obtained by generalized gradient approximation (GGA) and GGA+U exchange correlation functionals. Thus, we estimate the thermodynamic equilibrium shape of Li2FeSiO4 by the Wulff construction scheme for stoichiometric surfaces obtaining crystallites that are terminated by {110}, {010}, and {001} surfaces.

Ab initioInvestigation of Hydrogen Atom Adsorption and Absorption on Pd(110) Surface

Ab initio investigation based on density functional theory is performed to determine the behavior of H atom diffusion in Pd(110) surface to the first and second subsurface layers. Potential energy surface is constructed to determine the local minima and activation barriers of H pathways. Contribution of the relaxation of surface atoms in the binding energies of H and activation barriers along the diffusion paths, as well as the zero point energy corrections are also included in this work. The binding energies of H in the second subsurface layer are lower compared to its binding energies in the first subsurface layer and this is attributed to the interaction of H with the surface atoms and the differences in interlayer spacing of the surface layers. Comments on the adsorbate induced Pd(110) (1×2) missing/adding-row reconstruction phenomenon is also given with reference to the observed results in this work as H is absorbed from the surface to the first subsurface layer.

First-principle study on the surface atomic relaxation properties of sphalerite

The surface properties of sphalerite (ZnS) were theoretically investigated using first principle calculations based on the density functional theory (DFT). DFT results indicate that both the (110) and the (220) surfaces of sphalerite undergo surface atom relaxation after geometry optimization, which results in a considerable distortion of the surface region. In the normal direction, i.e., perpendicular to the surface, S atoms in the first surface layer move outward from the bulk (d1), whereas Zn atoms move toward the bulk (d2), forming an S-enriched surface. The values of these displacements are 0.003 nm for d1 and 0.021 nm for d2 on the (110) surface, and 0.002 nm for d1 and 0.011 nm for d2 on the (220) surface. Such a relaxation process is visually interpreted through the qualitative analysis of molecular mechanics. X-ray photoelectron spectroscopic (XPS) analysis provides the evidence for the S-enriched surface. A polysulphide (Sn2−) surface layer with a binding energy of 163.21 eV is formed on the surface of sphalerite after its grinding under ambient atmosphere. This S-enriched surface and the Sn2− surface layer have important influence on the flotation properties of sphalerite.

Properties of rutile TiO 2 surfaces from a Tight-Binding Variable-Charge model. Comparison with ab initio calculations

Up until now, variable charge models appeared to be inadequate for surface simulations of TiO 2. Particularly, the relative stability of the low-index surfaces was not well predicted, and the formation energy of the most stable (110) surface was far from recent ab initio predictions (Surf. Sci. 504, (2002) 115). In the present paper, low index (110), (100) and (001) surfaces of rutile TiO 2 have been successfully described at the atomic scale by means of a new variable-charge model. In this model, the iono-covalent metal–oxygen bond energy is calculated thanks to an analytical expression derived from a tight-binding description of the electronic structure of the oxide. This expression, which is a function of charge, is included in the charge equilibration procedure, which modifies the equilibrium state of the crystal with respect to previous models. Thus, the model becomes very stable with respect to charge transfers and leads to the stabilization of the (110), (100) and (001) surfaces of rutile TiO 2 with the right order in energy. Surface formation energies, atomic relaxations and charge transfers have been calculated and compared with density functional theory (DFT) calculations using the generalized gradient approximation (GGA) and the B3LYP hybrid functional with the CRYSTAL06 code. The surface energies calculated with the atomic model (0.42, 0.49 and 1.26 J m −2 respectively) are in very good agreement with our DFT calculations (0.48, 0.68 and 1.36 J m −2). Moreover, the atomic relaxations and the charge transfers at surfaces obtained with the model compare very reasonably both with the DFT results and the available experimental measurements. Our new model, whose transferability has been previously tested on different polymorphs of TiO 2, appears to be very useful in studying chemical properties of metal oxide surfaces.

Density-functional theory investigation of atomic geometryand oxygen adsorption of Au(110) surface

We have performed density-functional theory calculations of the atomic structure and the oxygen adsorption properties of Au(110) surfaces. The relaxations of missing-row reconstructed Au(110)-(1×2) surface are calculated to be -15.0%(Δd12/d0) and -1.1%(Δd23/d0). The relevant surface energy and workfunction are calculated to be 52.7 meV/2 and 5.00 eV, respectively. In the case of missing-row reconstructed Au(110)-(1×3) surface the surface atomic relaxations are calculated to be -20.5 %(Δd12/d0) and +2.7 %(Δd23/d0) which are quite differente from those of Au(110)-(1×2). However, in the later case, the surface energy and workfunction are found to be very close to those of missing-row reconstructed Au(110)-(1×2) surface, i.e., 53.4 meV/2 and 4.98 eV. We have simulated the scanning tunneling microscope (STM) images of both reconstructed surfaces and found that the missing row exhibits a remarkable hollow in the STM morphology. The further calculation of oxygen adsorption on both surfaces reveals that the adsorption energies in these cases are negative. These results indicate that the Au(110) surface is free from oxygen adsorption and reaction, showing highly chemical inertia.

Origins of visible-light emissions in hydrogen-coated silicon nanocrystals: Role of passivating coating

Abstract We present a theoretical investigation of the electronic and optical properties of hydrogen-coated silicon nanocrystals (Si:H NCs). On one hand, the density-functional theory (DFT) is used to both calculate the total energy and relax the NCs. On a second hand, the tight-binding method, which includes the minimal sp 3 -basis set within the second-nearest-neighbor interaction scheme, is applied to calculate the electronic structures, oscillator strength (OS) and recombination rate (RR) versus the NC size, coating and atomic relaxation. Three main findings are reported: (i) The quantum confinement in these NCs do follow similar rule to the case of a single-particle in a box, where the confinement energy decays in power-law with the increasing NC's size. (ii) The coating is shown to play the essential role in creation of large band-gap energy lying within the visible-light energy spectrum. (iii) The surface atomic relaxation is found to reduce the band-gap energy by about 150 meV and enhance both OS and RR. Our claims are corroborated by the available experimental data.

Surface Oxygen Diffusion into Neutral, Cationic, and Dicationic Oxygen Vacancies on MgO(100) Surfaces

First principles electronic structure investigations of the oxygen diffusion into neutral, cationic, and dicationic oxygen vacancies on MgO(100) surfaces have been carried out within a density functional theory cluster-embedding approach. The transition states and activation barriers for the oxygen diffusion were determined. It was found that the charge of the vacancy has a marked effect on the oxygen diffusion as the activation barrier decreases from 2.84 eV in the neutral, to 2.18 eV in the cationic, and to 0.94 eV in the dicationic oxygen vacancies. An analysis of the relaxation of the surface atoms around the vacancies, and the geometry of the transition states, leads us to conclude that the attractive interaction between the localized positive charge and the electron-rich oxygen atoms, and the relaxation of surface atoms, are responsible for lowering the activation barrier. In addition, our studies on layer relaxations indicate that the oxygen diffusion is strongly confined to the surface atoms. The ...

Quantization of the electron wave vector in nanostructures: Countingk-states

First-principles calculations of Cu(001) free-standing thin films have been performed to investigate the oscillatory quantum size effects exhibited in surface energy, work function, atomic relaxation, and adsorption energy of a cesium adsorbate. 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. The calculated surface energetics and geometry relaxations clearly feature quantum oscillations as a function of the film thickness, with oscillation periods characterized by a superposition of long and short length scales. Furthermore, we have investigated Cs adsorption onto Cu(001) thin films as a function of the film thickness. 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.