Structure and dynamics of the missing-row reconstruction on O/Cu(0 0 1) and O/Ag(0 0 1)

The geometry and the vibrational properties of missing row reconstructed O/Cu(0 0 1) and O/Ag(0 0 1) surfaces are investigated by means of density functional theory and density functional perturbation theory, using the local density and the generalized-gradient approximations. Our results predict very similar structural and vibrational properties for the two reconstructed surfaces. In the case of copper our calculations reproduce quite accurately the experimental results, while for the missing row reconstructed O/Ag(0 0 1) surface the agreement between theory and experiment is less satisfactory.     

A structural study of disordered sulfur overlayers on Cu(1 1 1)

The structure of disordered chemisorbed sulfur overlayers on Cu(1 1 1) have been investigated using normal incidence X-ray standing wavefield absorption (NIXSW). The NIXSW data indicates that within the coverage range studied, 0.13 < θ < 0.30 ML, all the disordered layers contain the same structural sub-units. The NIXSW data are consistent with a previously proposed model, which suggests that at room temperature the disordered layers contain both sulfur adatoms and mobile Cu₃S₃ clusters.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

O2 dissociation on Pd(2 1 1) and Cu(2 1 1) surfaces

We have performed ab initio Density Functional Theory (DFT) based calculations to observe the reactivity of the Pd(2 1 1) and Cu(2 1 1) surfaces towards O₂. In order to properly address the adsorption dynamics, the static potential energy surface calculations have been complemented with first principles molecular dynamics calculations, which reveal interesting steering effects that complicate the dissociation dynamics. We have found that on both surfaces the step microfacets are very reactive and the dissociation of the O₂ molecule at room temperature occurs mostly on those sites.     

Structure of enantiopure and racemic alanine adlayers on Cu(1 1 0)

Plane wave density functional theory has been employed to analyze the structure of alanine adlayers on the Cu(1 1 0) surface. Alanine forms (3 × 2) adlayers on Cu(1 1 0) that are closely related to the structures of glycine on the same surface. There is essentially no energy difference between the most stable racemic and enantiopure alanine adlayers. This observation implies that adsorption of racemic alanine on Cu(1 1 0) will result in a pseudoracemate adlayer.     

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(1 1 1)-(7 × 7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(1 1 1) surfaces for which the Fermi level lies near the middle of the band gap.     

Adsorption dynamics of O2 on Cu(1 0 0)

We have studied the adsorption of O₂ on the Cu(1 0 0) surface using both static potential energy surface (PES) calculations and ab initio molecular dynamics. The dynamical calculations complement the PES results, revealing steering effects which could not be predicted based on the static calculations only. We study the effect of oxidation and Ag doping on O₂ adsorption dynamics. The results are discussed in the light of recent molecular beam experiments.     

Coadsorption of methyl radicals and oxygen on Rh(1 1 1)

The chemisorption of CH₃ on Rh(1 1 1) is studied to understand the origin of the weakened symmetric stretch mode. A few different explanations for this weakened mode have been suggested in previous studies. These include C-H bond depletion and donation into C-H anti-bond orbitals either in an upright or tilted geometry. We investigate these possibilities by performing first-principles density functional calculations. Our results show strong adsorption at all high-symmetry sites with methyl in two possible orientations. A thorough analysis of the adsorption geometry shows that C_3v symmetry is preferred over a tilted species, ruling out tilting as a mechanism for C-H mode softening. Evidence of a multi-center bond between methyl and the surface rhodium atoms (similar to the kind shown recently by Michaelides and Hu for methyl on Ni(1 1 1)) is presented, showing that C-H bond depletion is the cause of mode-softening for methyl on Rh(1 1 1). Experimental results have shown that mode-softening diminishes when an electronegative species is coadsorbed, suggesting that donation into C-H anti-bonding orbitals is the mechanism for mode-softening. We therefore examine the coadsorption of oxygen and methyl on Rh(1 1 1). Our results suggest a new model for the effect of O on CH₃. Analysis of charge density differences shows that the dominant initial effects of O coadsorption are the removal of charge from the C-surface bond and the transfer of charge to the C-H bond. Subsequent increase of the H-Rh distance further reduces mode softening.     

Diffusion of oxygen atom in the topmost layer of the Si(1 0 0) surface: Structures and oxidation kinetics

The incorporations and migrations of the atomic oxygen in the topmost layer Si(1 0 0)-p(2 × 2) silicon surface, are investigated theoretically using density functional theory. We show that the diffusion is dependent on the starting and the final surrounding environment and does not simply consist in hops from one silicon-silicon bond to another. The activation energies range from 0.11 eV to 2.59 eV.     

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff-Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

Atomic and molecular adsorption on Pt(1 1 1)

The adsorption of several atomic (H, O, N, S, and C) and molecular (N₂, HCN, CO, NO, and NH₃) species and molecular fragments (CN, CNH₂, NH_2, NH, CH₃, CH₂, CH, HNO, NOH, and OH) on the (1 1 1) facet of platinum, an important industrial and fuel cell catalyst, was studied using self-consistent periodic density functional theory (DFT-GGA) calculations at a coverage of 1/4 ML. The best binding site, energy, and position, as well as an estimated diffusion barrier, of each species were determined. The binding strength for all the species can be ordered as follows: N₂ < NH₃ < HCN < NO < CO < CH₃ < OH < NH₂ < H < CN < NH < O < HNO < CH₂ < NOH < CNH₂ < N < S < CH < C. Although the atomic species generally preferred fcc sites, there was no clear trend in site preference by the molecular species or molecular fragments. The vibrational frequencies of all the stable adsorbates in their best and second best adsorption sites were calculated and found to be in good agreement with experimental values reported in the literature. Finally, the decomposition thermochemistry of NOH, HNO, NO, NH₃, N₂, CO, and CH₃ was analyzed.     

Ab initio density functional study of O on the Ag(0 0 1) surface

The adsorption of oxygen on the Ag(1 0 0) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag₂O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.     

CO adsorption on the Pt(1 1 1) surface: a comparison of a gradient corrected functional and a hybrid functional

The adsorption of CO on the Pt(1 1 1) surface in a pattern has been studied with the gradient corrected functional of Perdew and Wang and the B3LYP hybrid functional. A slab which is periodic in two dimensions is used to model the system. The Perdew-Wang functional incorrectly gives the fcc site as the most favorable adsorption site, in accord with a set of previous studies. The B3LYP functional gives the top site as the preferred site. This confirms results from cluster studies where it was suggested that the different splitting, dependent on the functional, between highest occupied and lowest unoccupied molecular orbital, could be the reason for this change of the adsorption site. This is supported by an analysis based on the projected density of states and the Mulliken population.     

First principles calculations of oxygen adsorption on the UN(0 0 1) surface

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (0 0 1) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(0 0 1) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.     

Density functional theory investigation of the structure of SO2 and SO3 on Cu(1 1 1) and Ni(1 1 1)

Density functional theory (DFT) slab calculations, mainly using the generalised gradient approximation, have been used to investigate the minimum energy structures of molecular SO₂ and SO₃ on Cu(1 1 1) and Ni(1 1 1) surfaces. On Ni(1 1 1) the optimal local adsorption structures are in close agreement with experimental results for both molecular species obtained using the X-ray standing wavefield technique, although for adsorbed SO₂ the energetic difference between two alternative lateral positions of the lying-down molecule on the surface is marginally significant. On Cu(1 1 1) the results for adsorbed SO₂, in particular, were sensitive to the DFT functional used in the calculations, but in all cases failed to reproduce the experimentally-established preference for adsorption with the molecular plane perpendicular to the surface. This result is discussed in the context of previously published DFT results for these species adsorbed on Cu(1 0 0). The optimal geometry found for SO₃ on Cu(1 1 1) is similar to that on Ni(1 1 1), providing agreement with experiment regarding the molecular orientation but not the adsorption site.     

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

First principle calculations of benzotriazole adsorption onto clean Cu(1 1 1)

The adsorption of benzotriazole (BTAH or C₆N₃H₅) on a Cu(1 1 1) surface is investigated by using first principle density functional theory calculations (VASP). It is found that BTAH can be physisorbed (<0.1 eV) or weakly chemisorbed (∼0.43 eV) onto Cu(1 1 1), and the chemical bond is formed through nitrogen sp² lone pairs. The weak chemisorption can be stabilized by reaction with neighboring protonphilic radicals, like OH⁻. Furthermore, the geometries and associated energies of intermolecular hydrogen bonds between adsorbates on Cu(1 1 1) are also calculated. A model of the first layer of BTAH/BTA⁻ on Cu(1 1 1) surface is developed based on a hydrogen bond network structure.