Growth of long range ordered pentacene/benzenethiol/Cu(100) heterostructure

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) with detection of both neutrals and ions is applied to structural analysis of oxygen adsorbed on a W(211) surface. The site position for oxygen in the high-dose [\ensuremath{\Theta}=1.5 monolayers (ML), saturation coverage] p(1\ifmmode\times\else\texttimes\fi{}2) low energy electron diffraction (LEED) pattern is determined; a preliminary study of oxygen in the low-dose (\ensuremath{\Theta}=0.5 ML) p(2\ifmmode\times\else\texttimes\fi{}1) LEED pattern is also presented. Both Ar and Ne backscattering (BS) and oxygen recoiling spectra, induced by pulsed 4-keV ${\mathrm{Ar}}^{+}$ and 5-keV ${\mathrm{Ne}}^{+}$ primary-ion beams, are monitored as a function of polar beam incident angle \ensuremath{\alpha}, surface azimuthal angle \ensuremath{\delta}, scattering angle \ensuremath{\theta}, and recoiling angle \ensuremath{\varphi}. Plots of BS (or recoil) intensities in (\ensuremath{\alpha},\ensuremath{\delta}) space provide scattering (or recoiling) structural contour maps and three-dimensional scattering (or recoiling) structural plots which are representative of the adsorption sites of oxygen on the W(211) surface; the symmetry of the adsorption sites is determined from these plots.Measurements of BS and recoil intensities as a function of \ensuremath{\alpha} along different azimuths \ensuremath{\delta} provide experimental values of the critical incident angles ${\ensuremath{\alpha}}_{c}$,sh${}^{\mathrm{i}}$ for shadowing and the critical ejection angles ${\ensuremath{\beta}}_{c}$ (or ${\ensuremath{\alpha}}_{c}$,bl) for blocking by neighboring atoms. Trajectory simulations and calculations of the shadowing and blocking cones obtained from potentials calibrated for this system are used to determine the oxygen-adsorption-site coordinates. The results show that oxygen is dissociatively adsorbed within the troughs along the [1 1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}] direction. For the saturation p(1\ifmmode\times\else\texttimes\fi{}2) structure, the oxygen is in threefold sites formed by two first- and one second-layer W atoms; these are the only sites which are consistent with all of the experimental data. The coordinates of these sites relative to the W coordinates are determined to an accuracy of \ifmmode\pm\else\textpm\fi{}0.1 A\r{}. The oxygen-tungsten chemisorption bond length is determined as 1.83 A\r{} to the two first-layer W atoms and 2.17 \AA{} to the second-layer W atom; this reflects the unsaturated and fully saturated valencies of the first- and second-layer W atoms, respectively. These trough sites are occupied for coverages up to more than one monolayer of oxygen. For the low-dose p(2\ifmmode\times\else\texttimes\fi{}1) structure, the data indicate occupancy of the same threefold sites, but with different coordinates. The data are consistent with shifting of the relaxed clean W surface structure to the bulk truncated structure upon ${\mathrm{O}}_{2}$ chemisorption. The sensitivity of TOF-SARS to details of adsorbate structure, the use of both BS and recoil data in a complementary manner, and the ability to extract adsorbate structural information with only simple calculations are presented.

Growth and crystalline structure of Co layers on Cu(0 0 1)

Structures of sulfur on TiO 2( [formula omitted]) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

Growth and surface structure of vanadium oxide on anatase (0 0 1)

$\mathrm{CO}$ adsorption on $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ has been studied by temperature programmed desorption (TPD), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). Thin layers (1 to 3 ML) of Ce on $\mathrm{Pt}(111)$ form upon annealing to $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ a surface alloy which most likely is Pt terminated. The $\mathrm{CO}$ overlayer structure on $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ at saturation coverage is argued to be an intermediate between $(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(4\ifmmode\times\else\texttimes\fi{}2)$ superstructures. This intermediate structure exhibits a $c(4\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern in which some of the spots appear together as ordered triangles. The thermal desorption spectra for the $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ surface alloy are shifted down by a temperature of about $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ compared to the $\mathrm{CO}$ desorption of pure $\mathrm{Pt}(111)$, most likely due to a change in the $d$-band due to hybridization between Ce and Pt states, which results in a weakening of the resonant interaction between $\mathrm{CO}$ $5\ensuremath{\sigma}$ and Pt $5d$ states.

Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and temperature programmed thermal desorption (TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well-ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature < 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off-normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures < 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.

CuPc molecules adsorbed on Au(1 1 0)-(1 × 2): growth morphology and evolution of valence band states

A RF H‐plasma exposure was used to clean the surface of patterned wafers. The areal coverage of to bare Si was 4 to 1, and the patterns were long strips, small squares, and large open regions. The plasma‐surface etching was monitored by residual gas analysis (RGA). The RGA spectra indicated etching of the Si surface at temperatures below 400°C and no detectable by‐products due to interactions with the regions for temperatures <450°C. The patterned surfaces were characterized with low energy electron diffraction (LEED) (from the bare Si regions) and atomic force microscopy (AFM). The LEED patterns indicate and surface symmetries at 300 and 450°C, respectively. The sharpness of the LEED patterns as well as the reconstruction indicated that the H‐plasma cleaned the bare Si regions. In addition, AFM measurements indicated that the Si and surface rms roughnesses do not vary significantly due to the H‐plasma exposure. It can be concluded from the RGA and AFM data that the remote H‐plasma process at 450°C cleaned the surface and did not significantly react with either the Si or regions.

The interaction of atomic hydrogen/deuterium with the Ag(111) surface was investigated with high-resolution electron loss spectroscopy (HREELS), thermal desorption spectroscopy, low-energy electron diffraction (LEED) and the results correlated with the work function change and an absolute coverage measurement. Dissociation of molecular hydrogen is an activated process on Ag(111) but atomic hydrogen chemisorbs and at a temperature of ∼185 K, recombines and desorbs as H2. Upon the adsorption of atomic hydrogen at 100 K, a sharp (2×2) LEED pattern is observed in the coverage range 0.25∼0.5 ML and is followed by combined (2×2)+(3×3) patterns with an increase in coverage of 0.1 ML. The saturation coverage is determined to be Θsat=0.6±0.1 ML. In contrast to other H/fcc(111) systems, the intensities of the fractional order diffraction beams are quite large, suggesting an H-induced reconstruction. The work function increases monotonically with H-exposure to a maximum value of Δφmax=+0.32 eV at saturation. Specular HREELS vibrational data show a single loss at 106 meV for hydrogen (76 meV for deuterium), similar to the normal vibrational mode observed for the H/Ag(110) system. A rather low activation energy (8.7±1.0 kcal/mol) for recombinative desorption is obtained from the low coverage thermal desorption spectra (Θ&amp;lt;0.1 ML).

Hydrogen adsorption on a Pt(110)−(1 × 2) surface

Low energy electron diffraction (LEED) intensities have been measured for the (110) and (111) faces of GaP. The LEED intensities of the (110) face have been analyzed using a dynamical multiple-scattering model of the diffraction process. The intralayer multiple scattering is treated exactly, while for the interlayer multiple scattering, the renormalized-forward-scattering method is used. Comparison of the calculated and observed LEED intensities suggests that both the Ga and the P atoms on the (110) face may exhibit a contracted outermost layer spacing with a rippled geometry. The surface layer is compressed by about 5% such that the top layer spacing is reduced by 0.1 Å. The P atoms protrude from the surface, whereas the Ga atoms are displaced inward such that no nearest-neighbor bond lengths are altered. LEED intensity analysis of the first order beams indicates that the bond-rotational angle ω is approxiimately 27 °. The quality and stability of the LEED patterns of the Gap (110) and (111) faces have been compared with the LEED patterns of the GaAs (110) and (111) faces. It is concluded that the Gap (110) face is less stable than the GaAs (110) face, but the Gap (111) face is far more stable than the GaAs (111) face. Phosphorus atoms start vaporizing from the GaP (111) face at an annealing temperature of 650 °C. Continued heating at 650 °C for 10 min resulted in a complete vaporization of phosphorus atoms and left a pure gallium surface layer on the GaP (111) face. A clear 1×1 threefold symmetry LEED pattern was observed on this Ga and Gap (111) surface. This indicates that the gallium atoms crystallize on the Gap (111) face and it appears reasonable to conclude that a good Schottky contact would be relatively easy to obtain on the GaP (111) face.

High-resolution Si 2p core-level and low-energy electron diffraction studies of the Ca/Si(1 1 1)-(3 × 2) surface

Origin of 3×3 diffraction on the Sn 1− xSi x/Si(1 1 1) [formula omitted] surface

High-energy ion scattering, x-ray photoemission spectroscopy, and low-energy electron diffraction (LEED) were used to study the interfacial reaction of very thin Ti films on Al(110) surfaces. The Al surface peak areas in the backscattering spectra of 0.96 MeV He+ ions incident along the [1̄1̄0] direction decreased sharply during the deposition of the first four monolayers of Ti. This suggests a growth model in which an ordered Ti overlayer shadows the Al surface atoms. This model is supported by electron diffraction measurements where sharp diffraction spots persisted with the rectangular symmetry of the substrate. Throughout this coverage regime, photopeak intensities due to Al were attenuated strongly with Ti coverage. With additional Ti deposition the Al surface peak intensities increase while the Al photopeak intensities remain unchanged, indicating a reaction at the Ti/Al interface that exposes additional Al surface atoms to the incident ion beam. Ion scattering spectra taken with the He+ ion beam incident along a random direction with respect to the Al substrate exhibit increased Ti peak areas indicating Ti shadowing. A LEED pattern was observed throughout the experiment suggesting the formation of an ordered overlayer. We present a growth model to explain these observations.

A LEED study of the FeTiO 3(0001) surface following annealing in O 2 partial pressures