Determination of a (4 × 4) structure formed on a Cu(0 0 1) surface by adsorption of calcium

The adsorption of calcium (Ca) atoms on a Cu(0 0 1) surface has been studied by low-energy electron diffraction (LEED) at 130, 300 and 400 K. It is found that a (4 × 4) was the only LEED pattern appeared at 400 K while a quasi-hexagonal structure was formed in a wide range of submonolayer coverage at 130 K. At 300 K, the (4 × 4) LEED spots were broad and weak. The (4 × 4) structure formed at 400 K was determined by a tensor LEED I-V analysis. It is a new-type of surface alloys consisting of five substitutional Ca atoms, nine surface Cu atoms, and two atomic vacancies in the unit cell. In spite of a quite large size-difference between Ca (3.94 Å) and Cu (2.55 Å) atoms, all Ca atoms are located at the substitutional sites. Among surface alloys so far reported, the atomic size ratio between Cu and Ca in the (4 × 4), 1.54, is the largest. Optimized structural parameters reveal that large lateral displacements of surface Cu atoms, being enabled by the appearance of the vacancies, allow the formation of the (4 × 4) structure.     

Substitutional adsorption geometry for Pb(1 1 1)-(√3 × √3)R30°-K

Intermixed structures for alkalis (larger than Li) on close-packed substrates have previously been observed only on Al(1 1 1). This study shows that K forms an ordered intermixed structure on Pb(1 1 1). The structures of clean Pb(1 1 1) and Pb(1 1 1)-(√3 × √3)R30°-K were studied using dynamical low-energy electron diffraction (LEED). The clean Pb(1 1 1) surface at 47 K was found to be a relaxed version of the bulk structure, in agreement with an earlier study of the same surface [Y.S. Li, F. Jona, P.M. Marcus, Phys. Rev. B 43 (1991) 6337]. At room temperature, adsorption of K on this surface results in a (√3 × √3)R30° structure, which was shown using dynamical LEED to consist of K atoms substituted in surface vacancies. The K-Pb bond length was found to be 3.62 ± 0.3 Å, with no significant change to the Pb interlayer spacings.     

An ordered surface ternary alloy of a c(6 × 4) structure formed on Cu(0 0 1) by substitutional coadsorption of Mg and Bi

A c(6 × 4) structure formed on Cu(0 0 1) by the coadsorption of Mg and Bi atoms at room temperature has been determined by a tensor low energy electron diffraction analysis. It is an ordered surface ternary alloy with a thickness of single layer, in which Mg, Bi and Cu atoms are mixed in the top layer. In the primitive unit cell, there are one Mg, four Bi, six Cu atoms and one vacancy in the top layer, and substituted Mg and Bi atoms form MgBi₄ plane clusters being arranged in the c(6 × 4) order. Structural parameters show that Mg-Bi bond distances in the MgBi₄ cluster are 3.01 and 3.07 Å, which are shorter than the summation of metallic radii of Mg and Bi. It is concluded that a direct, attractive interaction between Mg and Bi atoms plays critical role in the formation of the c(6 × 4) structure.     

Comparative investigation of the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloys using DFT

The electronic structure of the c(2 × 2)-Si/Cu(0 1 1) surface alloy has been investigated and compared to the structures seen in the three phases of the (√3 × √3)R30°Cu₂Si/Cu(1 1 1) system, using LCAO-DFT. The weighted surface energy increase between the alloyed Cu(0 1 1) and Cu(1 1 1) surfaces is 126.7 meV/Si atom. This increase in energy for the (0 1 1) system when compared to the (1 1 1) system is assigned to the transition from a hexagonal to a rectangular local bonding environment for the Si ion cores, with the hexagonal environment being energetically more favorable. The Si 3s state is shown to interact covalently with the Cu 4s and 4p states whereas the Si 3p state, and to a lesser extent the Si 3d state, forms a mixture of covalent and metallic bonds with the Cu states. The Cu 4s and 4p states are shown to be altered by approximately the same amount by both the removal of Cu ion cores and the inclusion of Si ion cores during the alloying of the Cu(0 1 1) surface. However, the Cu 3d states in the surface and second layers of the alloy are shown to be more significantly altered during the alloying process by the removal of Cu ion cores from the surface layer rather than by the addition of Si ion cores. This is compared to the behavior of the Cu 3d states in the surface and second layers of the each phase of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloy and consequently the loss of Cu-Cu periodicity during alloying of the Cu(0 1 1) surface is conjectured as the driving force for changes to the Cu 3d states. The accompanying changes to the Cu 4s and 4p states in both the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloys are quantified and compared. The study concludes with a brief quantitative study of changes in the bond order of the Cu-Cu bonds during alloying of both Cu(0 1 1) and Cu(1 1 1) surfaces.     

Improved growth and the spin reorientation transition of Ni on (√2×2√2)R45° reconstructed O/Cu(0 0 1)

Ni films between 1 and 20 monolayers (ML) thick are deposited at room temperature on clean and (√2×2√2)R45° reconstructed--via oxygen adsorption--Cu(0 0 1). A significant expansion of the out-of-plane Ni phase by about 5 ML is revealed by ferromagnetic resonance experiments. This shift of the spin reorientation transition is attributed to a huge change of about 90 μeV/atom in the surface anisotropy due to the presence of half a monolayer of oxygen atoms on the top of Ni. Furthermore, the growth of Ni on the preoxidized Cu surface is found to be closer to the layer-by-layer one as compared to the growth on the clean Cu(0 0 1) due to the presence of oxygen which acts as a surfactant.     

Investigation of the (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloy using DFT

The electronic structure of the FCC, HCP and 2-fold bridge phases of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) surface alloy have been investigated using LCAO-DFT. Analysis of the total electron density, partial density-of-states (PDOS) and crystal orbital overlap population (COOP) curves for the system have shown a surprising similarity between the intra- and inter-layer Si-Cu bond for each phase. Low hybridization between the Si 3s and 3p orbitals results in a low directionality of the Si-Cu bond within each of phase. The Si 3s orbitals are shown to form covalent bonds with their surrounding Cu atoms whereas the Si 3p and 3d orbitals are shown to form combinations of covalent and metallic bonds. The Si-Cu interaction is shown clearly to extend to the second layer of the alloy in deference to previous studies of Si/Cu alloys.     

Surface phase transition and electronic structure of c(5√2 × √2)R45°-Pb/Cu(1 0 0)

The c(5√2 × √2)R45°-Pb/Cu(1 0 0) surface phase is investigated by means of angle resolved ultraviolet photoemission and low energy electron diffraction in the temperature range between 300 and 550 K. We identify and characterize a temperature-induced surface phase transition at 440 K from the room temperature c(5√2 × √2) R45° phase to a (√2 × √2)R45° structure with split superstructure spots. The phase transition is fully reversible and takes place before the two-dimensional melting of the structure at 520 K. The electronic structure of the split (√2 × √2)R45° phase is characterized by a metallic free-electron like surface band. This surface band is backfolded with c(5√2 × √2)R45° periodicity phase at room temperature, giving rise to a surface band gap at the Fermi energy. We propose that a gain in electronic energy explains in part the stability of the c(5√2 × √2)R45° phase.     

Deduction of a three-phase model for the (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloy

The (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) surface alloy that forms during high temperature dosing of silane (SiH₄) on Cu(1 1 1) has been investigated using LCAO-DFT. Simulated STM images have shown that experimental images may be interpreted as a mixed phase system consisting of Si ion cores bound in HCP, FCC and twofold bridge sites with a ratio of 25:25:50 rather than previously proposed models where the Si ion cores were bound in only FCC and HCP sites. The new model is shown to be consistent with previously published NIXSW studies.     

Structure and stability of Sb/Au(1 1 0)-c(2 × 2) surface phase

Adsorption of 0.5 monolayers (ML) of Sb on the Au(1 1 0) surface resulted in the formation of a c(2 × 2) surface reconstruction. Analysis of surface X-ray diffraction data by a direct method revealed the existence of an ordered substitutional surface alloy, with every other hollow site occupied by Au and Sb atoms. Quantitative conventional χ² refinement showed a contraction of 0.12 ± 0.03 Å in the spacing of the first Au layer to the second, an expansion of 0.13 ± 0.03 Å in the second-to-third layer distance, and an inward Sb displacement (rumpling) of 0.21 ± 0.04 Å. This surface phase proved to be extremely robust, with the long-range order of this arrangement remaining up to substrate temperatures of 900 K.     

LCAO study of the Cu(1 1 0)-p(2 × 1)O surface

We have performed semi-empirical LCAO calculations of the electronic structure of the Cu(1 1 0)-p(2 × 1)O surface. This has been done accounting for the Cu-Cu interactions by means of a recently proposed set of parameters, which give very good results for the bulk as well as for the surfaces of lowest Miller indices. Furthermore, the O-O interactions, which have been neglected in the preceding similar studies, have been taken into account. The resulting surface bands are in very good agreement with the overall set of the available experimental data. Several issues concerning the physical properties of this surface are addressed in the present paper: the changes induced on the clean surface bands by the adsorption and the reconstruction; the arrangement of the Cu and O atoms in the added rows; the position of the p_y antibonding band of the oxygen. In particular, we have found that the latter has an energy of −0.2 eV at the point. This result confirms an experimental indication in the same direction previously reported by Courths et al. [R. Courths, S. Hüfner, P. Kemkes, G. Wiesen, Surf. Sci. 376 (1997) 43].     

Formation and structure of a (√19 × √19)R23.4°-Ge/Pt(1 1 1) surface alloy

An ordered (√19 × √19)R23.4°-Ge/Pt(1 1 1) surface alloy can be formed by vapor depositing one-monolayer Ge on a Pt(1 1 1) substrate at room temperature and subsequently annealing at 900-1200 K. The long-range order of this structure was observed by low energy electron diffraction (LEED) and confirmed by scanning tunneling microscopy (STM). The local structure and alloying of vapor-deposited Ge on Pt(1 1 1) at 300 K was investigated by using X-ray Photoelectron Diffraction (XPD) and low energy alkali ion scattering spectroscopy (ALISS). XPS indicates that Ge adatoms are incorporated to form an alloy surface layer at ∼900 K. Results from XPD and ALISS establish that Ge atoms are substitutionally incorporated into the Pt surface layer and reside exclusively in the topmost layer, with excess Ge diffusing deep into the bulk of the crystal. The incorporated Ge atoms at the surface are located very close to substitutional Pt atomic positions, without any corrugation or “buckling”. Temperature Programmed Desorption (TPD) shows that both CO and NO adsorb more weakly on the Ge/Pt(1 1 1) surface alloy compared to that on the clean Pt(1 1 1) surface.     

Structural analysis of Pt(1 1 1)c(√3 × 5)rect.-CO using photoelectron diffraction

Core level shift scanned-energy mode photoelectron diffraction using the two distinct components of the C 1s emission has been used to determine the structure of the Pt(1 1 1)c(√3 × 5)rect.-CO phase formed by 0.6 ML of adsorbed CO. The results confirm earlier assignments of these components to CO in atop and bridging sites, further confirm that the best structural model involves a 2:1 occupation ratio of these two sites, and provides quantitative structural parameter values. In particular the Pt-C chemisorption bondlengths for the atop and bridging sites are, respectively, 1.86 ± 0.02 Å and 2.02 ± 0.04 Å. These values are closely similar to those found in the 0.5 ML coverage c(4 × 2) phase, involving an atop:bridge occupation ratio of 1:1, obtained in earlier quantitative low energy electron diffraction studies. The results also indicate a clear tilt of the molecular axis of atop CO species in this compression phase, consistent with the finding of an earlier electron-stimulated desorption ion angular distribution investigation.     

Observation of a (√3 × √3)-R30° reconstruction on GaN(0 0 0 1) by RHEED and LEED

A new reconstruction of √3 × √3-R30° has been observed on a GaN film grown on a 6H-SiC (0 0 0 1)-√3 × √3 surface using RHEED and LEED experimental techniques. The experimental LEED PF shows that the GaN film is Ga-terminated hexagonal. The surface is a mixture of two structures with a single bilayer height difference between them. One is a √3 × √3-R30° reconstruction with Ga-adatoms occupying the T4 sites. Another is a Ga-terminated 1 × 1 with no extra Ga on top. The area ratio of the √3 × √3 part to the 1 × 1 part is slightly larger than 1. The first principle total energy calculations and Tensor-LEED I-V curves simulations further confirm this structure model.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

STM study of acetylene reaction with Si(1 1 1): observation of a carbon-induced Si(1 1 1)√3×√3R30° reconstruction

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C₂H₂ pressure of 2 × 10⁻⁴ Pa (2 × 10⁻² mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si-C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T₄ and/or S₅ sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer.     

Formate-induced destabilization of the Cu(1 1 0) surface

Scanning tunneling microscopy (STM) images show that adsorbed formate has a profound affect on the step edges of Cu(1 1 0) surfaces at room temperature. For low exposures, the presence of formate enhances step fluctuations as confirmed by a correlation function analysis. For formate coverages approaching 0.5 monolayers, drastic restructuring of step edges is observed. Quantum chemical calculations help to explain this behavior.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.     

The CuGaSe2(0 0 1) surface: A (4 × 1) reconstruction

We report on the formation of a stable (4 × 1) reconstruction of the chalcopyrite CuGaSe₂(0 0 1) surface. Using Ar⁺ ion-bombardment and annealing of epitaxial CuGaSe₂ films grown on GaAs(0 0 1) substrates it was possible to obtain flat, well-ordered surfaces showing a clear (4 × 1) reconstruction. The cleanliness and structure were analyzed in situ by AES and LEED. AES data suggest a slight Se-enrichment and Cu-depletion upon surface preparation. Our results demonstrate that (0 0 1) surfaces of the Cu-III-VI₂(0 0 1) material can show stable, unfacetted surfaces.     

Reconstructions 3 × 3 and √3 × √3 on SiC(0 0 0 1) studied using RHEED

The 3 × 3 and √3 × √3 reconstructions on 6H-SiC(0 0 0 1) surface were obtained via depositing thin silicon layer and annealing it in ultrahigh vacuum (without Si flux). Rocking curves of reflection high energy electron diffraction (RHEED) were measured for integer and fractional order beams. They were fitted with results of many-beam calculation on the basis of dynamical theory of RHEED to determine structural parameters. For √3 × √3 superstructure, it was found that the occupancy of adatom states is 0.45 (incomplete coverage). In the sequence of Si-C double layers ABCACB, the lattice is terminated with the layer A. For 3 × 3 superstructure, the rocking curves support the model with twisted tetra-cluster. The best-fit twist is as half of that predicted in ab initio calculations; it is due to limited source of Si atoms to build up the superstructure. Larger twist correlates with higher occupancy of corner sites and with slower cooling rate of the sample after annealing.     

Surface alloy formation after deposition of Ce on Rh(1 1 0)

Vapour deposition of Ce onto a Rh(1 1 0) single crystal at room temperature is studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). The thicknesses of the deposited Ce layers are estimated to be between 2 and 9 Å. To study the changes in the Ce-Rh surface layer, the samples are annealed at temperatures between 500 and 1000 °C after Ce deposition.After heating, a c(2 × 2) LEED pattern appears for the sample with the thinnest deposited Ce layer (2.4 Å). For samples with thicker Ce-films, the LEED pattern co-exists of a c(2 × 2) structure and a more diffuse 6% contracted (2 × 1) structure. This appears at the same temperature as the Ce 3d and Rh 3d core levels exhibit sharp intensity changes and binding energy shifts.The intensity of the f⁰, f¹ and f² multiplets in the Ce 3d core level spectra change when the annealing temperature is increased. The relative intensity of the Ce 3d f⁰ and f² features compared to the Ce 3d f¹ features is largest after annealing to 500 °C. This is below the temperature at which the ordered surface alloy is formed. When the sample is heated above the formation temperature of the surface alloy, the relative intensity of the Ce 3d f⁰ and f² features decrease.