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.     

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.     

Electronic structure study of reconstructed Au-SiC(0 0 0 1) surfaces

A study of surface and interface properties of reconstructed Au-SiC(0 0 0 1) surfaces is reported. Two reconstructions were prepared on SiC(0 0 0 1), a √3 × √3R30° and a Si-rich 3 × 3, before Au deposition and subsequent annealing at different temperatures. For the Si-rich 3 × 3 surface the existence of three stable reconstructions 2√3 × 2√3R30°, 3 × 3 and 5 × 5 are revealed after deposition of Au layers, 4-8 Å thick, and annealing at progressively higher temperatures between 500 and 950 °C. For the 2√3 surface two surface shifted Si 2p components are revealed and the Au 4f spectra clearly indicate silicide formation. The variation in relative intensity for the different core level components with photon energy suggests formation of an ordered silicide layer with some excess Si on top. Similar core level spectra and variations in relative intensity with photon energy are obtained for the 3 × 3 and 5 × 5 phases but the amount of excess Si on top is observed to be smaller and an additional weak Si 2p component becomes discernable.For the √3 surface the evolution of the core level spectra after Au deposition and annealing is shown to be distinctly different than for the Si-rich 3 × 3 surface and only one stable reconstruction, a 3 × 3 phase, is observed at similar annealing temperatures.     

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(1 0 0) and Pt(1 0 0) electrodes in 0.1 M HClO₄, revealing the formation of well-ordered adlattices of Au(1 0 0)-(√2 × √5) between 0.2 and 0.9 V and Pt(1 0 0)-(√2 × √2)R45° between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(1 0 0)-(√2 × √5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(1 1 1). In comparison, the coverage of Pt(1 0 0)-(√2 × √2)R45° - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that benzenethiol admolecules on Pt(1 0 0) could stand even more upright than those on Au(1 0 0). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(1 0 0) and Pt(1 0 0) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(1 0 0) electrode resulted in irreversible desorption of admolecules at E ? 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E ? 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(1 0 0) at potentials as negative as the onset of hydrogen evolution. Raising the potential rendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Structural investigation of the c(√2 × 5√2)R45° surface alloy formed by Ti deposition on Cu(0 0 1)

The alloying process of Ti deposited on Cu(0 0 1) was studied by means of XPS, LEIS, XPD and LEED intensity analysis. With the sample held at 570 K, a linear decrease of the Cu LEIS signal as a function of the amount of Ti deposited is observed in the early stages of deposition until a constant value is reached. At the onset of the plateau a c(√2 × 5√2)R45° LEED pattern starts to be visible. XPD and LEED intensity measurements were performed for the c(√2 × 5√2)R45° phase prepared depositing ca. 1.5 monolayer of Ti. The angle-scanned XPD curves measured for the phase c(√2 × 5√2)R45° reveal that Ti atoms substitute Cu atoms in the fcc lattice of the substrate. The polar XPD curves show that at least the first four layers of the substrate are involved in the alloying process. We found that the (3 1 0) plane of the Cu₄Ti alloy (D1_a type-structure) fits, without significant contraction or expansion of the lattice parameters, the c(√2 × 5√2)R45° structure. The intensity versus energy curves of the diffracted beams were calculated on the basis of this structural model using the tensor LEED method. The results of the LEED intensity analysis provide a further evidence of the formation of a slab of Cu₄Ti(3 1 0) layers.     

Interdiffusion of adsorbed Pb and Bi on Cu(1 0 0)

The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(1 0 0) has been studied, at T = 513 K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consist of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features lead to the conclusion that Pb atoms are more strongly bound at surface alloyed sites in Cu(1 0 0) than Bi atoms. Once the total coverage (Pb + Bi) on the substrate reaches about one monolayer, Pb and Bi are dealloyed from the substrate, and the interdiffusion profiles become essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of ∼10⁻¹³ m²/s. This is considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion is different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction, leading to the conclusion that binding of Bi atoms to the Cu(1 0 0) substrate is stronger than that of Pb atoms in the highest-coverage surface dealloyed layers.     

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.     

Surface structures formed by individual adsorption and coadsorption of Mn and Bi on Cu(0 0 1), studied by LEED

We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi₄ clusters are arranged in the top Cu layer instead of MgBi₄. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment.     

Bi induced step-flow growth in the homoepitaxial growth of Au(1 1 1)

Homoepitaxial growth of Au on Bi-covered Au(1 1 1) was studied at room temperature using reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). From observations of RHEED it is found that the Au(1 1 1) (23 × 1) reconstruction structure changes to a (1 × 1) by about 0.16-0.5 ML deposition of Bi and to a (2√3 × 2√3)R30° by about 1.0 ML deposition of Bi, respectively. The surface morphology evolution by Bi deposition leads to a change of Au homoepitaxial growth behavior from layer-by-layer to step flow. This indicates that the surface diffusion distance of Au atoms on the Bi-precovered (1 × 1) and (2√3 × 2√3)R30° surfaces is longer than that on the Au(1 1 1) (23 × 1) clean surfaces. A strong surface segregation of Bi was found at top of surface. It is concluded that Bi atoms acted as an effective surfactant in the Au homoepitaxial growth by promoting Au intralayer mass transport.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Asymmetric adsorption-site of potassium atoms in the (3 × 2)-p2mg structure formed on Cu(0 0 1)

The adsorption of K atoms on Cu(0 0 1) has been studied by low-energy electron diffraction (LEED) at room temperature (RT) and 130 K. At RT, a (3 × 2)-p2mg LEED pattern with single-domain was observed at coverage of 0.33, whereas the orthogonal two-domain was found at 130 K. At 130 K, a c(4 × 2) pattern with orthogonal two-domain was observed at coverage 0.25. Both the (3 × 2)-p2mg and c(4 × 2) structures have been determined by a tensor LEED analysis. It is demonstrated that K atoms are adsorbed on surface fourfold hollow sites in the c(4 × 2), while in the (3 × 2) structure two K atoms in the unit cell are located at an asymmetric site with a glide-reflection-symmetry. The asymmetric site is at near the midpoint between the exact hollow site and bridge-site but slightly close to the hollow site. A rumpling of 0.07 Å in the first Cu layer was confirmed, which might stabilize K atoms at the asymmetric site. Surface structures appearing in a coverage range 0.25-0.33 are discussed in terms of the occupation of the asymmetric site with increase of coverage.     

Fabrication of nanostructures by selective growth of C60 and Si on Si(1 1 1) substrate

Using two types of selective growth, selective C₆₀ growth and selective Si growth, on a common Si(1 1 1) substrate, an array of C₆₀ nanoribbons with controlled values of width and thickness is fabricated. On a surface that has Si(1 1 1)√3 × √3R30°-Ag (referred to as Si(1 1 1)√3-Ag hereafter) and bare Si(1 1 1) regions at the same time, the preferential growth of C₆₀ multilayered film is recognized on the Si(1 1 1)√3-Ag region. The growth of Si selectively occurs on a bare Si(1 1 1) region if the substrate surface has C₆₀-adsorbed and bare Si(1 1 1) regions at the same time. As a demonstration of the use of these selective growths, we fabricate an array of well-isolated C₆₀ nanoribbons, which show a well-ordered molecular arrangement and have sizes of about 40 nm in widths and 3-4 nm in thicknesses.     

Structural study of Si(1 1 1)-6 × 1-Ag surface using surface X-ray diffraction

The surface structure of Si(1 1 1)-6 × 1-Ag was investigated using surface X-ray diffraction techniques. By analyzing the CTR scattering intensities along 00 rod, the positions of the Ag and reconstructed Si atoms perpendicular to the surface were determined. The results agreed well with the HCC model proposed for a 3 × 1 structure induced by alkali-metals on a Si(1 1 1) substrate. The heights of the surface Ag and Si atoms did not move when the surface structure changed from Si(1 1 1)-√3 × √3-Ag to Si(1 1 1)-6 × 1-Ag by the desorption of the Ag atoms. From the GIXD measurement, the in-plane arrangement of the surface Ag atoms was determined. The results indicate that the Ag atoms move large distances at the phase transition between the 6 × 1 and 3 × 1 structures.     

Infrared reflection absorption spectroscopic study of the adsorption structures of dimethyl ether and methyl ethyl ether on Cu(1 1 1) and Ag(1 1 1)

Infrared reflection absorption (IRA) spectra measured for dimethyl ether (DME) adsorbed at 80 K on Cu(1 1 1) and Ag(1 1 1) give IR bands belonging only to the A₁ and B₂ species, indicating that the adsorbate takes on an orientation in which the C₂ axis bisecting the COC bond angle tilts away from the surface normal within the plane perpendicular to the substrates. The DFT method was applied to simulate the IRA spectra, indicating that the tilt angles of DME on Cu(1 1 1) and Ag(1 1 1) are about 50° and 55°, respectively, at submonolayer coverages. The results are in contrast to the case of DME on Cu(1 1 0) and Ag(1 1 0), where the C₂ axis is perpendicular to the substrates [T. Kiyohara et al., J. Phys. Chem. A 106 (2002) 3469]. Methyl ethyl ether (MEE) adsorbed at 80 K on Cu(1 1 1) gives IRA bands mainly ascribable to the gauche (G) form, whereas the IRA spectra measured for MEE on Ag(1 1 1) are characterized by the trans (T) form. The rotational isomers are identical with those on Cu(1 1 0) and Ag(1 1 0); i.e., MEE on Cu(1 1 0) takes the G form and the adsorbate on Ag(1 1 0) the T form [T. Kiyohara et al., J. Phys. Chem. B 107 (2003) 5008]. The simulation of the IRA spectra indicated that (i) the G form adsorbate on Cu(1 1 1) takes an orientation, in which the axis bisecting the COC bond angle tilts away from the surface normal by ca. 30° within the plane perpendicular to the surface to make the CH₃-CH₂ bond almost parallel to the surface, and (ii) the T form adsorbate on Ag(1 1 1) takes an orientation, in which the bisecting axis tilts away by ca. 60° from the surface normal within the perpendicular plane. Comparison of these adsorption structures of MEE on the (1 1 1) substrates with those of MEE on Cu(1 1 0) and Ag(1 1 0) indicates that the structures are mainly determined by a coordination interaction of the oxygen atom to the surface metals and an attractive van der Waals interaction between the ethyl group of MEE and the substrate surfaces. The coordination interaction plays an important role on Cu(1 1 1) and Cu(1 1 0), which makes the adsorbate on the Cu substrates to take the orientations with the bisecting axis near parallel to the surface normal and to assume the G form in order to make the ethyl group parallel to the surface, which is favorable for the van der Waals interaction. In the case of MEE on the Ag substrates the attractive van der Waals interaction plays a dominant role, resulting in the T form which is more favorable for the interaction than the G form.     

Sulfidation of a Cu submonolayer at the Au(1 1 1)/electrolyte interface - An in situ STM study

We describe the electrochemical preparation of an ultrathin copper sulfide film on Au(1 1 1) and its structural characterization by in situ STM. The first step, underpotential deposition of a Cu submonolayer from CuSO₄/H₂SO₄ solution, is followed by two electrolyte exchanges for (i) Cu-free (blank) H₂SO₄ solution and (ii) NaOH/Na₂S solution. The well-known (√3 × √3)R30° structure of the upd Cu layer is stable in the blank electrolyte for at least 2 h. After exposure to bisulfide, the Cu layer contracts and forms two-dimensional islands of two distinct ordered surface phases, i.e. a rectangular and, at higher potentials, a hexagonal phase, with Cu-free Au(1 1 1) regions between them, the latter exhibiting the characteristic (√3 × √3)R30°-S adlayer structure. Potential changes lead to a complex phase behaviour including HS⁻ ? S_x oxidation/reduction and, at strongly anodic potentials, dissolution of the Cu adlayer.     

An IRAS study of CO bonding on Sn/Pt(1 1 1) surface alloys at maximal pressures of 10 Torr

The adsorption of CO on Pt(1 1 1), (2 × 2) and (√3 × √3)R30° Sn/Pt(1 1 1) surface alloys has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED) and infrared reflection adsorption spectroscopy (IRAS). The presence of Sn in the surface layer of Pt(1 1 1) reduces the binding energy of CO by a few kcal/mol. IRAS data show two C-O stretching frequencies, ∼2100 and ∼1860 cm⁻¹, corresponding to atop and bridge bonded species, respectively. Bridge bonded stretching frequencies are only observed for Pt(1 1 1) and (2 × 2) Sn/Pt(1 1 1) alloy surfaces. A slight coverage dependence of the vibrational frequencies is observed for the three surfaces. High pressure IRAS experiments over a broad temperature range show no indication of bridge bonded CO on any of the three surfaces. Direct CO adsorption on Sn sites is not observed over the measured temperature and pressure ranges.     

Water monolayer and multilayer adsorption on Ni(1 1 1)

Water adsorbed on Ni(1 1 1) forms an ordered, hydrogen bonded ice structure with a (2√7 × 2√7)R19° unit cell. The 2√7 wetting structure forms as islands and persists up to saturation of the first layer. Adsorption of a fraction of a monolayer more water into a second layer destroys the 2√7 registry and creates a disordered ice film. Gas adsorption measurements indicate that the wetting layer is completely covered by a second layer of water before thicker multilayer ice forms. As the second layer is completed the film orders to form an incommensurate crystalline ice film with a hexagonal LEED pattern, oriented to the Ni close packed rows. This ordered, incommensurate structure persists as the ice multilayer grows thicker.     

Structural analysis of the c(4 × 2) reconstruction in Si(0 0 1) and Ge(0 0 1) surfaces by low-energy electron diffraction

The c(4 × 2) structures in (0 0 1) surfaces of Si and Ge have been studied by low-energy electron diffraction (LEED). Using a proper cleaning method for the Si surface, we were able to observe clear c(4 × 2) LEED patterns up to incident energy of ∼400 eV as well as the Ge surface. Extensive experimental intensity-voltage curves allowed us to optimize the asymmetric dimer model up to the eighth layer (including the dimer layer) in depth in the dynamical LEED calculation. Optimized structural parameters are almost the same for the Si and Ge except for the height of the buckled-up atom of the asymmetric dimer. For the Ge surface, the structural parameters are in excellent agreement with those obtained by a previous theoretical calculation. The tilt angle and bond length of the dimer are 18 ± 1 (19 ± 1)° and 2.4 ± 0.1 (2.5 ± 0.1) Å for the Si(0 0 1) (Ge(0 0 1)), respectively.     

Mixed water/OH structures on Pd(1 1 1)

Chemisorbed O and water react on Pd(1 1 1) at low temperatures to form a mixed OH/H₂O layer with a (√3 × √3)R30° registry. Reaction requires at least two water molecules to each O before the (2 × 2)O islands are consumed, the most stable OH/water structure being a (OH + H₂O) layer containing 0.67 ML of oxygen, formed by the reaction 3H₂O + O → 2(H₂O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (√3 × √3)R30° − (OH + H₂O) phase cannot be formed by O/H reaction and is distinct from the (√3 × √3)R30° structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(1 1 1), preventing this phase catalyzing the low temperature H₂/O₂ reaction which only occurs at higher temperatures.