XPS and RBS investigations of Si–Er–O interactions on a Si(1 0 0)-2x1 surface

The interactions among erbium, oxygen and silicon atoms on a Si(1 0 0)-2x1 reconstructed surface have been studied by means of X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Erbium and oxygen were deposited at 600 °C on the Si surface and their behavior has been observed after different thermal processes. It was found that at 600 °C, the formation of a stable surface complex Er–O–Si is obtained together with Si oxidation; after an 800 °C annealing, the amount of oxygen bound to Si decreases and the remaining O atoms are mainly bonded to Er. An abrupt change was observed after 900 and 1000 °C annealings, which bury the Er atoms about 60 Å below the substrate surface. Our results give some hints to hypotise the O diffusion towards the Si bulk.     

InP(001)-(2 x 1) surface: a hydrogen stabilized structure

The InP(001)(2 x 1) surface has been reported to consist of a semiconducting monolayer of buckled phosphorus dimers. This apparent violation of the electron counting principle was explained by effects of strong electron correlation. Combining first-principles calculations with reflectance anisotropy spectroscopy and LEED experiments, we find that the (2 x 1) reconstruction is not at all a clean surface: it is induced by hydrogen adsorbed in an alternating sequence on the buckled P dimers. Thus, the microscopic structure of the InP growth plane relevant to standard gas phase epitaxy conditions is resolved and shown to obey the electron counting rule.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

LEED crystallographic analysis for the structure formed by 2 ML of O at the Zr(0001) surface

A tensor LEED analysis is reported for the Zr(0001)-(1 × 1)-O surface which involves oxygen at a total coverage of 2 monolayers. The structure is indicated to have two layers of O: one forms an overlayer in which the O atoms bond to hollow sites of three-fold coordination on the regular metal surface, while the other layer has the O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB... corresponds to the first three layers of anion-terminated cubic ZrO₂, although some lateral compression is needed for superposition on the regular hcp Zr structure. The absorption of O in the tetrahedral holes results in a significant expansion in the first-to-second Zr---Zr interlayer spacing to about 3.44 Å from the bulk vaue of 2.57 Å. The O---Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.21 Å for the O atoms in tetrahedral hole sites. Comparisons are made with the structures of the corresponding 0.5 and 1 ML surfaces formed by the O/Zr(0001) system.     

Atomic structure of the polar NiO(111)- p(2x2) surface

Using grazing-incidence x-ray diffraction, the p(2x2) surface structures of the single crystal NiO(111) and a 5 monolayer thick NiO(111) film on Au(111) were both shown to exhibit locally the theoretically predicted octopolar reconstruction, with some important differences. The single crystal exhibits a single Ni termination with double steps. The thin film exhibits both possible terminations (O and Ni) and single steps. These surfaces were found to be nonreactive with respect to hydroxylation.     

Surface structure of TiO2(011)-(2x1)

A combined experimental and first principles study of the (2x1)-reconstructed rutile TiO2(011) surface is presented. Our results provide evidence that the surface structure is described by a model that includes onefold coordinated (titanyl) oxygen atoms giving rise to double bonded Ti=O species. These species should play a special role in the enhanced photocatalytic activity of the TiO2(011) surface.     

Structural investigations of the Yb  Si(111) - 2x1, 5x1 and 3x1 overlayers

The YbSi interface has been investigated in the sub-monolayer regime employing Ion Scattering Spectroscopy (ISS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). Three different structures, YbSi(111) 2x1, YbSi(111) 5x1, and YbSi(111) 3x1, have been established by heat treatments of the interfaces. The structures consist of a stable overlayer of Yb atoms on the Si(111) surfaces. The distance of the Yb atoms to the uppermost layer of Si atoms has been estimated by comparing the YbSi ISS intensity ratio with the predictions of a model based on classical scattering theory and a Thomas-Fermi-Moliére potential. The height of the Yb atoms relative to the substrate toplayer was found to be 1.9 ± 0.3→.     

Leed debye-waller analysis of vibrational hybridization for p(2 × 2)-O/Pd(100)

The temperature dependence of the 75 eV specular LEED beam intensity was measured between ～100–300°C and ~50–200°C for clean p(1 × 1) and quarter-monolayer O-covered p(2 × 2) Pd(100), respectively. The experiments were performed in a near-normal-incidence geometry to increase sensitivity to vibrations perpendicular to the surface, which include the high-energy, HRELS-active O-stretch mode. The measurements indicate that the effective meansquare displacements were ～40% larger for the clean surface than for the bulk, in qualitative agreement with expectation, and were increased another 20% for the O-chemisorption structure. This observed increase in the Debye—Waller effect upon chemisorption is interpreted to suggest hybridization between the substrate phonon continuum and the high-energy, O-stretch modes. As a result of the hybridization oxygen would have a vibrational component within the substrate continuum that would dominate the magnitude of the oxygen mean-square displacements. A bulk impurity model due to Mannheim is invoked for numerical simulation of such hybridization effects and to serve as an analogy with the surface problem. It is shown that this analogy can be extended to describe resonant modes on a clean surface, and the force-constant increases necessary to create local modes at step defects, in agreement with more realistic treatments. It is shown also that vibrational hybridization can significantly influence the choice of the appropriate adsorbate Debye temperature — an important contrast parameter — that enters into dynamical LEED calculations.     

Structural and optical properties of the Ge(111)-(2 x 1) surface

We study the two lowest-energy isomers of the Ge(111)-(2 x 1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the silicon case, the stablest isomer differs from the standard "buckled Pandey chains" reconstruction. This conclusion is supported by accurate total-energy results.     

Photoemission study of the (2 × 2) structure formed by H2O adsorption on the Zr(0 0 0 1) surface

We have studied adsorption of water on the Zr(0 0 0 1) surface at sub-monolayer coverage by means of LEED and photoemission spectroscopy. An ordered (2 × 2) structure is formed after adsorption of 0.6-1.4 Langmuirs at 473 K. The sharpest LEED pattern was observed at an exposure of 1.2 L implying a coverage of 0.5 ML of oxygen. The same exposure at 293 K gives only a weak and diffuse (2 × 2) pattern. In addition, the sharp (2 × 2) pattern obtained at 473 K can be reversibly weakened by cooling to 293 K and subsequently sharpened by heating. For the sharp (2 × 2) structure, valence band spectra indicated dissociation of water and showed a peak composed mainly of O 2p derived states with two components at 6.0 eV and 6.6 eV binding energy. On cooling to 293 K, the O 2p peak became narrower and a new state appeared at 7.9 eV. Two components of the O 1s core level were resolved for the (2 × 2) structure, assigned to oxide and hydroxyl groups. The hydrogen on the surface of Zr(0 0 0 1) resulting from the dissociation of water and from bulk segregation strongly influenced the formation of the (2 × 2) structure of oxygen, and caused a temperature instability of the structure.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.