The atomic arrangement of Au atoms segregated to the (110) surface of a Ni--0.8 at. % Au alloy was studied with low-energy ion scattering (LEIS) in combination with low-energy electron diffraction (LEED). The LEED results have been published elsewhere [E. G. McRae and R. A. Malic, Surf. Sci. 177, 53 (1986)]. From this LEED work and a preliminary analysis of the present data a model for the atomic structures of Au atoms on the Ni(110) surface has been presented. In the present paper, the LEIS data were analyzed quantitatively by comparison with computer simulations. Most features of the models proposed earlier were confirmed by this analysis. We also used the (static) embedded-atom method (EAM) to calculate the positions of Au atoms on Ni(110) for various coverages. After introducing these positions in the computer simulations of the LEIS data, an even better agreement between experiment and calculation was found. At low coverages of less than 0.5 ML, the Au atoms were found to be located in substitutional Ni sites in the first Ni layer. At a Au coverage of 1.28 ML, the Au atoms are at various heights above the Ni surface, and are hexagonally packed within a 7\ifmmode\times\else\texttimes\fi{}7 unit cell with c(2\ifmmode\times\else\texttimes\fi{}4) subunits. For coverages slightly lower than 1.28 ML several other structures are predicted by our EAM calculations. These structures have indeed been found in scanning tunneling microscopy work. In addition, we discuss the observation of one-dimensional melting of the Au overlayer at a temperature of approximately 700 \ifmmode^\circ\else\textdegree\fi{}C, as observed with LEED and LEIS.

The arrangement of Au atoms segregated to the (110) surface of a Ni-0.8at%Au alloy was studied with low energy ion scattering (LEIS) in combination with low energy electron diffraction (LEED). The results of the LEED analysis have been published earlier [1]. From this analysis it is known that at the very low coverage of around 0.1 monolayer (ML) a 1 X 1 symmetry exists. At high coverages (approximately 1 ML) a 7 X 7 overlayer structure with c(2 X 4) subunits was observed. At intermediate coverages also other symmetries were observed such as (5 X 1) and c(2 X 2). By comparing angular LEIS scans with the results of computer simulations, we were able to determine part of the Au configurations giving rise to these observed symmetries. At low coverage the Au atoms were found to occupy near-substitutional sites in the Ni top layer. No Au atoms were observed on top of the Ni surface, nor in the second Ni layer. The 7 X 7 reconstructed surface is formed by a hexagonal, incommensurate, Au overlayer on top of the Ni (110) structure. A complete 7 X 7 structure would give rise to a coverage of 1.28 ML of Au atoms.

The neutralization behavior of low-energy Ne+ ions scattered from a compositionally ordered Cu3Au(100) surface has been studied over a range of incident energy E0 from 2.4 to 10 keV. Ion fractions of Ne scattered from Cu atoms in the first, or first two, atom layers exhibited a sharp increase setting in at an E0 of 4-5 keV, reaching 70% at 10 keV for first-layer scattering. Inelastic energy losses, up to 130 eV, and Auger electron emission from Ne scattered from Cu, were also observed at incident energies above 4 keV. Ne scattered from the Au atoms on the same Cu3Au(100) surface showed only the usual velocity-dependent Auger and resonance neutralization. An explanation of the Cu results is given in terms of Ne 2s vacancy creation during the close collision of Ne, which is neutralized on the inward path, followed by autoionization on the outward path after scattering into the vacuum. Conversely, Ne cannot approach Au closely enough to form an appropriate inner-shell vacancy. This is due to the higher Coulombic repulsion created by the greater charge of the Au nucleus.

The nature of the surface phase transitions associated with the bulk compositional order-disorder transition in Cu3Au crystal is discussed from the experimental standpoint. The available facts about the order of the surface transition, the accompanying change of surface-layer compositions, and the ordering kinetics are assembled. It is shown that seemingly disparate properties of the (100) and (110) surfaces can be correlated with each other, and with corresponding properties of the bulk crystal, by considering the surface structures arising from type I domain walls.

Low energy electron diffraction and low energy ion scattering observations on Cu3Au(110) reveal a correlation between surface compositional ordering and the average compositions in the first and second surface atom layers. The surface undergoes a broadened, discontinuous 2 × 1 → 1 × 1 transition at least 6 K below the bulk compositional-disordering transition at 660 K, while near 660 K the values of Au atom fractions in the first (second) atom layers approach 0.35 (0.35) starting from 0.45 (0.20) below 400 K.

Using low energy ion scattering (LEIS) we have found that CO adsorbs in on-top Ni sites of the ordered NiAl(110) surface at 100 K. On the clean surface an outward relaxation of Al atoms relative to Ni atoms in the first layer of the solid, which had previously been measured by LEED, MEIS and LEIS was observed using LEIS in the impact collision ion scattering spectroscopy (ICISS) mode. This relaxation is attenuated when CO is adsorbed. 1.9 keV He + and 5 keV Ne + beams were employed in the time-of-flight (TOF) technique which acquires a spectrum with a sufficiently small ion dose to damage only a negligible fraction of the surface. Experimental requirements for He scattering in the TOP mode are discussed, viz., the use of channeling or low-incidence angles to suppress the deep layer scattering yield.

Preferential sputtering of Cu-Ni alloys has been investigated using the Ion Scattering Spectroscopy (ISS). We have observed the preferential sputtering of Ni in low Ni alloys and of Cu in low Cu alloys when sputtering them with 0.8 and 3.0 keV Ne+ beams. Current models which consider mass and surface binding energy predict the behavior for Cu but not for Ni.

The structure of the Au segregated Ni(110)-0.8%Au surface has been studied by scanning tunneling microscopy. The segregated Au layer forms a (7×4) structure with a c(2×4) subunit. At various coverages of Au overlayer, commensurate and incommensurate Au structures were observed. At a coverage of I monolayer, islands and facets assciated with incommensurate Au atoms were observed.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation R. Hsiao, C. J. McMahon, E. W. Plummer, T. M. Buck; Summary Abstract: A multitechnique study of surface segregation of Sn on the (111) surface of an Fe–1.3 at. % Sn crystal. J. Vac. Sci. Technol. A 1 July 1987; 5 (4): 887. https://doi.org/10.1116/1.574334 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science & Technology A Search Advanced Search |Citation Search

The structure of segregated Au layers on the Ni(110)--0.8 at. % Au surface has been studied by scanning tunneling microscopy. The observed structures, including (7\ifmmode\times\else\texttimes\fi{}4) with a c(2\ifmmode\times\else\texttimes\fi{}4) subunit, are shown to be consistent with the observed low-energy electron diffraction pattern. Two different kinds of Au structures, commensurate atomic chains and incommensurate closed-packed atomic rows, were observed. A model for the nucleation and growth of the segregated layer is proposed.

We report on a comprehensive study of the neutralization of (75-180)-keV He ions scattered from the UHV-cleaned and Cs-covered Si(100) surface, using surface-sensitive channeling techniques. It is shown that ions are neutralized exclusively at the solid surface on the ion's outward path. Angular depth and work-function dependence results are discussed. A model is proposed which includes resonant transitions to a broadened He $n=2$ quantum level, and is compared with experimental data.

A clean Fe (001) unreconstructed surface has been analyzed using Low Energy Ion Scattering with time of flight energy analysis. The intensity of scattered 9.5 KeV Ne+ ions and neutrals was measured as a function of elevation and azimuth of the incoming beam while keeping a fixed scattering angle of 90 °. The observed intensity variations can be described by considering the shadowing and blocking of the second and third atomic layers. A simple Firsov potential was used to calculate the shadow cone dimensions. The good agreement between the observed and calculated variations in scattered intensity demonstrates the utility of this relatively simple technique for preliminary analysis of surface structure.

The total spectra, ions plus neutrals, of 5.0 keV and 9.5 keV Ne+ ions backscattered from an ordered Cu3Au(100) surface were measured by the time-of-flight technique (LEIS-TOF). Good agreement between these results and computer simulations was obtained. From detailed analysis of the trajectories comprising the simulated spectra it is inferred that layers deeper than the first three contribute a secondary structure to the backscattered spectra.

We report on a comprehensive study of the neutralization of 75–180 keV He ions scattered from the UHV cleaned “2 × 1” Si (100) surface using surface sensitive channeling techniques. It is shown that ions are neutralized exclusively at the solid surface on the ion's outward path. Angular and depth dependence results are discussed.

The time-of-flight version of low energy ion scattering LEIS (TOF), avoids uncertainties in quantitative analysis due to neutralization in scattering. However, some other factors to consider are: a) accuracy of the scattering cross-sections employed, b) enhancement of scattering yields from sub-surface layers by focusing effects, c) dependence of detector efficiency on scattered energy, and d) background subtraction in integration of single-scattering peaks. We have investigated these problems using 2.4–10 keV Ne+ beams scattered from a Cu3Au (100) ordered surface in conjunction with computer simulation of the energy spectra and have found c) to be the most important potential source of error. Correction factors are derived from measurements of scattering yield vs E0.

With use of low-energy ${\mathrm{Ne}}^{+}$ scattering and low-energy electron diffraction both long-range order and Au segregation have been found at the ${\mathrm{Cu}}_{3}$Au(001) surface. The Au concentrations in the first and second layers are essentially constant at 0.5 and 0, respectively, for $T<~400\ifmmode^\circ\else\textdegree\fi{}$C, beyond which they approach each other. Calculations of shadowing for the ordered surface agree with experiment, e.g., for shadowing of Cu atoms in the second and third layers by Au atoms in the first layer.

By performing simultaneous detection of forward-scattered Ne+ ions and photons emitted by Ne and Ni excited atoms when a Ni(110) surface was bombarded by 4 keV Ne+ ions at near-grazing incidence, we studied the ion and photon yields as a function of the azimuthal angle. The variations observed in the yield anisotropies show the different degrees of influence of the surface structure on outer-shell electronic processes during ion-surface collisions.