The calculation of the surface energy of high-index surfaces in metals at zero temperature

We used the molecular dynamics simulation with interatomic potentials of the embedded atom method to calculate the high-index surface energies of the surfaces containing the 〈0 0 1〉 axis or 〈−1 1 0〉 axis in f.c.c. metal Al, Cu and Ni at zero temperature. We generalized an empirical formula based on structural unit model for high-index surfaces and present some new formulas that can be used to estimate the surface energy and structural feature of high-index surfaces very well. The results show that the closest surfaces have the lowest surface energy and the surface energies of the closest (1 1 1) surface and the next closest (1 1 0), (1 0 0) surfaces are the extremum on the curve of surface energy versus orientation angle. We also calculated the b.c.c. metal Fe and obtained a similar result. The difference is that in the b.c.c. metal the surface energies of the closest (1 1 0) surface and the next closest (1 0 0), (1 1 2) surfaces are the extremum on the curve of surface energy versus orientation angle. The results of theoretical simulation and the empirical formula consist well with the experiment data.     

Converged properties of clean metal surfaces by all-electron first-principles calculations

All-electron full-potential linearized augmented plane-wave calculations of the surface energy, work function, and interlayer spacings of close-packed metal surfaces are presented, in particular, for the free-electron-like metal surfaces, Mg(0 0 0 1) and Al(1 1 1), and for the transition metal surfaces, Ti(0 0 0 1), Cu(1 1 1), Pd(1 1 1), and Pt(1 1 1). We investigate the convergence of the surface energy as a function of the number of layers in the slab, using the Cu(1 1 1) surface as an example. The results show that the surface energy, as obtained using total energies of the slab and bulk from separate calculations, converges well with respect to the number of layers in the slab. Obviously, it is necessary that bulk and surface calculations are performed with the same high accuracy. Furthermore, we discuss the performance of the local-density and generalized gradient approximations for the exchange-correlation functional in describing the various surface properties.     

Nano-structures in YSZ(1 0 0) surfaces: Implications for metal deposition experiments

The surface morphology of yttria stabilized zirconia (YSZ)(1 0 0) single crystals are examined by AFM and XPS before and after thermal annealing in air to 1000 °C. The surfaces show a large variability in topography which can be categorized in three types: (1) surfaces with well defined terraces, (2) surfaces with etch pits and no visible terraces, (3) surfaces with large square or rectangular holes with flat bottoms. All three types of surfaces show a large number of defects (pits, adatoms, steps) originating from the manufacturing process, and certain samples show large nano-structured arrays of self-organized lines at step edges. The evolution of the surfaces with time at 1000 °C and with higher temperatures was studied. Terraces are ultimately obtained for all sample types at 1300 °C, but the terrace shape is affected by the original defect structure. This history dependence is attributed to defect interactions modifying the annealing process. This is true even for UHV samples prepared using sputter-anneal cycles. The surface type is found to affect the nucleation, growth and sintering behaviour of palladium deposited by electron beam evaporation. For type 3 samples the metal nucleates at step edges outside the holes to particles 6 Å in height, following heating to 135 °C the particles move inside the holes forming agglomerates up to 20 Å.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

Binding energy shift in photoemission spectroscopy study of Ni clusters deposited on rutile TiO2 surfaces

Cluster-size-dependent binding energy (BE) shifts of Ni 2p_3/2 spectra in Ni clusters with respect to bulk Ni metal have been studied as a function of Ni coverage on clean rutile TiO₂(0 0 1) surfaces at room temperature. Auger parameter (AP) analysis of photoelectron spectra has been employed and revealed an obvious initial state contribution at the coverage of 0.5 monolayers (ML). The initial state effect was demonstrated to be strongly affected by the substrate and was assigned to a combination of eigenvalue shift in surface core-level shift (SCLS) and charge transfer between the metal clusters and substrates. The TiO₂(0 0 1) surface stoichiometry was found to introduce different charge transfer behaviors. Our results experimentally present that the Ni clusters are charged positively on stoichiomtric TiO₂ surface and less positively or even negatively on various reduced surfaces.     

STM image visualization of Si(1 1 1) 7 × 7 surface (graphic simulation and implementation)

The possibilities of graphic STM image simulation of a clean Si(1 1 1) 7 × 7 surface at atomic level are indicated. The presented procedure takes into account various types of deformation on the surface near the Fermi level in order to classify them and explain their origin. It also gives a clear hint to insert relevant physical phenomena in a suggested analysis. This goal is achieved exploiting the results of DAS (dimmer adatom stacing fault) model by means of standard mathematical programmes. A clean Si(1 1 1) 7 × 7 surface is considered as the representative example, but similar evaluation is possible for another non-metal and metal surfaces.     

DFT study on surface properties and dissolution trends of Al(1 0 0) surfaces doped with Zn, Ga, In, Sn and Pb

In this paper, the properties and dissolution trends of the surfaces doped with different metal atoms on the Al(1 0 0) surface were investigated by the density functional theory calculations. A surface impurity model was proposed by replacing the topmost surface layer Al atoms by Me (Me = Zn, Ga, In, Sn and Pb) atoms with the coverage of 1/9, 1/4, 1/2, and 3/4 monolayer, respectively. Results show that the surface energy of Me-Al(1 0 0) surfaces depends primarily on the nature of the impurity atom species and the monolayer coverage. The work function of Me-Al surfaces is smaller than that of pure Al(1 0 0) surface, and decreases almost linearly with the amount of Me. It is found that the Me-Al alloys are more easily dissolvable than the pure Al, due to the fact that the electrochemical dissolution potential shifts were negative for all Me-Al(1 0 0) surfaces with respect to the clean pure Al(1 0 0) surface.     

Determination of deuterium adsorption site on palladium(1 0 0) using low energy ion recoil spectroscopy

Ion beam analysis has been recently applied to study the adsorption phenomena of some adsorbates on metal surfaces. In this paper, surface recoils created by low energy Ne⁺ ions are employed to study the adsorption site of deuterium (D) atoms on Pd(1 0 0). This technique is extremely surface sensitive with the capacity for atomic layer depth resolution. From azimuthal angle observations of Pd(1 0 0) specimen, it was found that at room temperature, D was adsorbed in the fourfold hollow site of Pd(1 0 0) at a height of 0.25 ± 0.05 Å above the surface. The adsorbate remains in the hollow site at all temperatures to 383 K though the vertical height above the surface is found to depend on coverage and for the first time evidence is found of a transition to a p(2 × 2) structure for the adsorbate. There is no evidence of D sitting in the Pd(1 0 0) subsurface at room and higher temperatures.     

Adsorption of ethene on Pt(1 1 1) and ordered PtxSn/Pt(1 1 1) surface alloys: A comparative HREELS and DFT investigation

The adsorption of ethene (C₂H₄) on Pt(1 1 1) and the Pt₃Sn/Pt(1 1 1) and Pt₂Sn/Pt(1 1 1) surface alloys has been investigated experimentally by high-resolution electron energy loss spectroscopy and temperature programmed desorption. The experimental results have been compared with density functional theory (DFT) calculations allowing us to perform a complete assignment of all vibration modes and loss features to the species present on the surfaces. On Pt(1 1 1) as well as on the Pt-Sn surface alloys an η² di-σ-bonded conformation of ethene has been found to be the most stable adsorbed form. In addition to this majority species a minor amount of π-bonded ethene has been identified, which is more abundant on the Pt₂Sn surface alloy than on the other surfaces. Additionally the HREELS spectra of ethene on Pt(1 1 1) and the Pt-Sn surface alloys differ only slightly in terms of the energetic positions of the loss peaks.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

Oxygen induced segregation of aluminum to α-Cu-Al(1 0 0) alloy surfaces studied by low energy ion scattering and X-ray photoelectron spectroscopy

The effect of annealing temperature on the surface composition of α-Cu-Al(1 0 0) alloys for aluminum concentrations of 5, 12 and 17 at% was investigated using X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). Two initial states of the sample surfaces were examined: sputter-cleaned and oxidized. The effect of annealing temperature on segregation is different for sputter-cleaned and oxidized samples. Aluminum preferential sputtering and strong oxygen induced aluminum segregation were detected on all examined samples. Whilst for the sputter-cleaned surfaces a small thermal induced segregation was observed, the combination of annealing and oxygen exposure resulted in aluminum enrichment in the 100-300% range relative to the bulk concentration. The segregation rate is proportional to the aluminum concentration for sputter-cleaned surfaces and displays a maximum for the oxidized α-Cu-Al(12 at.%)(1 0 0) surface.     

Predicting the metal growth mode and wetting of noble metals supported on c-ZrO2

Employing periodic density-functional calculations, we predict the metal growth mode for ideal and defective interfaces of noble metals (Pd and Pt) deposited on top of cubic-ZrO₂. We discuss various energetic contributions and surface textures. We propose that isolated noble metal atoms are supported on both steps and terraces on the surfaces. Pt, on the other hand, shows a higher probability to decorate steps and other low oxygen coordinated sites, while Pd forms larger metal aggregates on both high oxygen coordinated sites (terraces) as well as low oxygen coordinated sites (steps). Oxygen defects on the support act as nucleation sites for the metal cluster growth. We also investigate metal wetting of Pd and Pt on the {1 1 1} surface of cubic-zirconia. Our calculations predict that the wetting angles are ca. 110-130° for 1 and 2 mono-layers of Pd and Pt, respectively, suggesting that metal wetting decreases as the metal loading increases.     

Structures and stability of Ag clusters on Ag(1 1 1) and Ni(1 1 1) surfaces

The structures of the lowest total energy for small Ag_N clusters with N = 2-20, which are grown on Ag(1 1 1) and Ni(1 1 1) surfaces, have been determined using a combination of the embedded-atom method and the basin-hopping algorithm. It is found that the particularly stable Ag clusters with N<18 have similar geometries on both surfaces when comparing clusters of the same size. On the other hand, the geometries of the less stable Ag clusters in the same size range differ for the two surfaces. From N?18, the sizes of the particularly stable structures are different for the two different substrates. Due to the large size mismatch of the two types of atoms it is energetically unfavorable for Ag to form a pseudomorphic monolayer structures on Ni(1 1 1) and there is considerable strain produced at the interface. The effect of this strain and the increased adatom-substrate interactions lead to irregular and elongated structures of the adsorbed Ag clusters.     

Preparation and calibration of ultrathin Zn layers on Pd(1 1 1)

The general assumption that metal vapors stick at any surface with probability one at room temperature is a necessary requirement for the correct metal deposition rate determination by quartz microbalance measurements. In a combined thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) study on clean and sulphur covered Pd(1 1 1) surfaces it is shown that Zn exhibits a reduced sticking coefficient on contaminated surfaces which violates the requirements for a proper application of a quartz microbalance. Additional CO titration and AES experiments were applied to calibrate the Zn coverage on Pd(1 1 1).     

Atomistic simulation of point defects at low-index surfaces of noble metals

The formation energies and the migration energies of an isolated vacancy and adatom formed on low-index surfaces are calculated with MAEAM for three noble metals Cu, Ag and Au. The results indicate that the formation energies of an isolated vacancy or adatom increase with increasing atom density in the sequence (1 1 0) → (1 0 0) → (1 1 1), and it is more difficult to form an adatom than to form a vacancy at the same surface. For the mobility of an isolated vacancy, the migration energy grows in the sequence (1 0 0) → (1 1 0) → (1 1 1) for each noble metal. However, a much less migration energy is obtained for the migration of an adatom on (1 1 1) surface.     

First-principles investigation of the electronic properties of niobium and molybdenum mononitride surfaces

Using first-principles calculations we investigated the electronic properties of niobium and molybdenum mononitride (NbN and MoN, respectively) surfaces and their dependence on the surface orientation and termination. Work functions calculated for polar surfaces strongly depend on the surface termination, with nitrogen-terminated surfaces yielding the highest value, up to 6.6 eV for the fully N-covered MoN(0 0 1) surface. The dependence of the work function on coverage for the polar surface is monotonic for nitrogen termination, but does not follow the same trend in the case of metal termination. The work function decreases by ∼2 eV for MoN from a 100% metal-terminated surface to at least 25% metal-terminated surface, and then increases rapidly between 25% and 0% metal-terminated surface to recover its nitrogen termination result. The same trend was obtained for NbN. We observed a significant increase in the charge of the surface metal atom, up to its bulk value, with decreasing metal surface coverage. Electron transfer from the metal surface atoms to the subsurface atoms can explain these submonolayer metal coverage results. Finally we found that for the non-polar surfaces, the mononitrides work functions are generally lower than the work functions of the corresponding simple metal surfaces.     

Computer simulations of the effect of atomic structure and coordination on the stabilities and melting behaviour of copper surfaces and nano-particles

We have studied the structures and stabilities of copper nano-particles and the melting properties of copper surfaces using interatomic potential-based molecular dynamics simulations, where the (1 1 1) surface has been shown to be the most stable in terms of surface energy and melting behaviour. Low energy shapes of nano-particles are influenced by the surfaces present and therefore have a higher proportion of (1 1 1) surface. The effect of surface structure on stability becomes less marked as the size of the nano-particle is increased. Melting is observed to occur below the bulk melting temperature in all the surfaces investigated, at increasingly lower temperatures from the (1 1 1), (1 0 0), (1 1 0) down to the (2 1 0) surface, confirming their order of decreasing stability. The melting processes of defective close-packed copper surfaces were also simulated. Steps, kinks, and facets were all shown to accelerate the melting of the surfaces. The melting is shown to initiate at the site of the defect and the results demonstrate that it is the low-coordinated atoms, at the step edge or kink, that are more mobile at lower temperatures. These features facilitate surface melting even further below the melting temperature than was observed for the perfect surfaces. Furthermore, facets of (1 0 0) surface were shown to be unstable even at moderate temperatures on the close-packed surface.     

Comparative DFT study of the adsorption of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied with density functional theory methods (DFT). The same most stable adsorption modes have been found on both metal surfaces with similar adsorption energies. For 1,3-butadiene the 1,2,3,4-tetra-σ adsorption structure is shown to be the most stable one, in competition with a 1,4-metallacycle-type mode, which is only less stable by 10-12 kJ mol⁻¹. On Pt(1 1 1) these total energy calculations were combined with simulations of the vibrational spectra. This confirms that the 1,2,3,4-tetra-σ adsorption is the most probable adsorption structure, but cannot exclude the 1,4-metallacycle as a minority species. Although similar in type and energy, the adsorption on the Pd(1 1 1) surface shows a markedly different geometry, with a smaller molecular distortion upon adsorption. The most stable adsorption structure for the butene isomers is the di-σ-mode. Similarly to the case of the 1,3-butadiene, the adsorption geometry is closer to the gas phase one on Pd than on Pt, hence explaining the different spectroscopic results, without the previously assumed requirement of a different binding mode. Moreover the present study has shown that the different selectivity observed on Pt(1 1 1) and Pd(1 1 1) for the hydrogenation reaction of butadiene cannot be satisfactory explained by the single comparison of the relative stabilities of 1,3-butadiene and 1-butene on these metals.     

A density functional study of atomic oxygen and water molecule adsorption on Ni(1 1 1) and chromium-substituted Ni(1 1 1) surfaces

Density functional theory (DFT) for generalized gradient approximation calculations has been used to study the adsorption of atomic oxygen and water molecules on Ni(1 1 1) and different kind of Ni-Cr(1 1 1) surfaces. The fcc hollow site is energetically the most favorable for atomic oxygen adsorption and on top site is favorable for water adsorption. The Ni-Cr surface has the highest absorption energy for oxygen at 6.86 eV, followed by the hcp site, whereas the absorption energy is 5.56 eV for the Ni surface. The Ni-O bond distance is 1.85 Å for the Ni surface. On the other hand, the result concerning the Ni-Cr surface implies that the bond distances are 1.93-1.95 Å and 1.75 Å for Ni-O and Cr-O, respectively. The surface adsorption energy for water on top site for two Cr atom substituted Ni-Cr surface is 0.85 eV. Oxygen atoms prefer to bond with Cr rather than Ni atoms. Atomic charge analysis demonstrates that charge transfer increases due to the addition of Cr. Moreover, a local density of states (LDOS) study examines the hybridization occurring between the metal d orbital and the oxygen p orbital; the bonding is mainly ionic, and water bonds weakly in both cases.     

Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.