Dynamical LEED Research Articles

Reliability of detailed LEED structural analyses: Pt(111) and Pt(111)-p(2×2)-O

As surface structures are being examined in more detail than ever before, the reliability of structural details becomes an important issue. To discuss necessary components of a high quality, reliable dynamical LEED studies, detailed dynamical LEED analyses of the clean Pt(111) and the Pt(111)-p(2×2)-O structures have been carried out utilizing different computer programs and search methods applied to a common set of experimental LEED I- V data. We have investigated the effects of various non-structural parameters, in particular those involved in the construction of the phase shifts, on the resulting structures of the clean Pt(111) and the Pt(111)-p(2×2)-O surfaces. The use of relativistic Pt phase shifts is found to be important in order to determine the adsorption structure accurately. For clean Pt(111), we find the top interlayer spacing is noticeably expanded by 0.025 ± 0.01 A ̊ with a low R p-factor value of 0.15. For the Pt(111)-p(2×2)-O system we find an R p-factor of 0.18. O atoms in the p(2×2)-O overlayer are found to adsorb in fcc-hollow sites and induce buckling in both the first and second metal layers. In addition, there is an expansion of the first metal-metal interlayer spacing and a small contraction of the second.

The surface structure and segregation profile of Ni 50Pd 50(100): a dynamical LEED study

The (100) surface of a single crystal Ni 50Pd 50 alloy was studied using low energy electron diffraction. IV-spectra collected at three angles of incidence were analyzed by dynamical LEED calculations using the average t-matrix approximation. The compositions of the first three atomic layers and the first two interlayer spacings were determined. An oscillatory segregation profile was observed, with the surface layer enriched in Pd (composition 20% Ni), the second layer enriched in Ni (composition 100% Ni), and the third layer slightly enriched in Pd (composition 36% Ni). Both the first and second interlayer spacings were found to be contracted relative to the interplanar spacings of the bulk alloy, the first by 2.7% of the bulk value and the second by 3.9% of the bulk value.

The [formula omitted] structure: adsorbate induced relaxations

The adsorption geometry and the adsorbate induced relaxations of p (2 × 2 ) ordered oxygen on Ni (111) have been reinvestigated by a dynamical LEED analysis in the energy range between 30 and 387 eV. 14 non-equivalent beams have been recorded with a total energy range of 2830 eV. Vertical distances, buckling and lateral shifts including rotations have been tested for the adsorbate and the first 2 substrate layers. Top and the three-fold coordinated hcp and fcc sites were tested as adsorption sites. While the top site can be excluded already by tests on the non-buckled surface the discrimination between hcp and the strongly favoured fcc sites is only possible if buckling is included in the model calculations. Optimal agreement between calculated and experimental curves ( R p = 0.226) was obtained for a Ni-O nearest neighbour distance of 1.83 Å. Significant buckling up to 0.09 Å was found in the first 2 substrate layers, but the average Ni-Ni layer distance is only slightly (0.02 Å) expanded between the first and second substrate layer, and corresponds to the bulk value (2.03 Å) for the other layers. Adsorbate induced lateral shifts (0.02 ± 0.04 Å) for nearest neighbours of O) turned out to be at the limit of detectability already in the first substrate layer. No indication could be found for adsorbate induced rotational shifts in the topmost substrate layers.

Dynamical LEED analyses of the Pt(111)-p(2× 2)-NO and the Ni(111)-c(4 × 2)-2NO structures: substrate relaxation and unexpected hollow-site adsorption

Dynamical LEED analyses of ordered structures of NO molecules chemisorbed on Pt(111) and on Ni(111) conclusively yield threefold-coordinated hollow-site adsorption and reasonable bond lengths. Both metal surfaces exhibit molecular induced substrate relaxations; those in Ni(111) are greater than those in Pt(111). The Pt(111)-p(2 × 2)-NO structure has NO adsorbed in a threefold fee site with a small buckling of the first metal layer. NO of the Ni(111)-c(4 × 2)-2NO structure is adsorbed in threefold fee and hep sites with a 0.16 Å buckling of the first metal layer. The hollow-site adsorption on both metal surfaces invalidates long-standing bridge and top-site assignments based on vibrational spectroscopy measurements, calling that approach into question. The threefold hollow-site adsorption determined for these systems also contrasts with the lower coordinated bridge and top sites generally found for CO molecules on the same metal surfaces.

Crystallography of epitaxial face centered tetragonal Co/Cu(100) by low energy electron diffraction

A crystallographic LEED I/V characterization of thin Co/Cu(100) films is presented. Results from dynamical LEED calculations are compared with experiments via Pendry's reliability factor. The first Co layer grows with a small, but noticeable, amount of Co atoms in the second layer. For thicker films up to at least 10 ML, Co is found to grow on Cu layer by layer in a tetragonally distorted structure. The film adopts the lateral Cu spacing with an in-plane lattice expansion and a contraction of the vertical interplanar distances. By subsequent Cu on Co deposition, lateral and vertical Cu spacings are kept, reproducing the initial substrate structure.

LEED structural analysis of the (001) surface of the ordered fcc Pt 3Ti alloy

The surface structure of the (001) plane of the ordered fcc Pt 3Ti alloy was studied by dynamical LEED analysis. The model that gives the best theory-experiment agreement corresponds to bulk truncation, with the outermost plane being pure platinum. The results also indicate a contraction of the first interplanar distance with respect to the bulk value and a slight upward buckling of Ti atoms in the second outermost plane.

He-diffraction investigations of a molecular chemisorption system: Ni(110)/(2×1)CO

We present the first measurements of complete He-diffraction spectra together with quantitative intensity analyses for a molecular chemisorption system, the 2 × 1-saturation phase of carbon monoxide on Ni(110). Relating the corrugation function to calculated surface charge densities confirms the previously reported structural model determined by dynamical LEED and yields for the outermost lying O-atoms of the CO-molecules a lateral distance of 1 Å relative to the close packed metal rows. From the initial decrease of the specular intensity at low coverages a scattering cross section for dilute CO on Ni(110) of 110 Å was obtained in good agreement with results on several other surfaces.

He-diffraction investigations of a molecular chemisorption system: Ni(110)/(2 × 1)CO

We present the first measurements of complete He-diffraction spectra together with quantitative intensity analyses for a molecular chemisorption system, the 2 × 1-saturation phase of carbon monoxide on Ni(110). Relating the corrugation function to calculated surface charge densities confirms the previously reported structural model determined by dynamical LEED and yields for the outermost lying O-atoms of the CO-molecules a lateral distance of 1 Å relative to the close packed metal rows. From the initial decrease of the specular intensity at low coverages a scattering cross section for dilute CO on Ni(110) of 110 Å was obtained in good agreement with results on several other surfaces.

Scattering phases in low energy electron diffraction from spot profile analysis and from multiple scattering theory

Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) on pseudomorphic submonolayers of Fe(110) on W(110) provides experimental access to phase shift differences between wavelets scattered from Fe top-layer terraces and from the uncovered W-surface, respectively. These phase shifts are alternatively determined within a dynamical LEED calculation and agree surprisingly well with the experimental results.

Calculation of Surface‐Induced Core‐Level Shifts for Covalent Semiconductors C, Si, Ge, and α‐Sn. II. (100) 2 × 1 Surfaces

AbstractThe theory developed for the surface‐induced core‐level shift is applied to (100) 2 × 1 surfaces of group IV materials. The initial‐state potential effect is related to ion charges (of the ground state), changes of the atomic hybridization state, and the polarity of chemical bonds accompanying the atomic configuration due to reconstruction. The many‐body relaxation mechanism is neglected. The atomic configurations are taken from total energy minimizations, dynamical LEED or ion scattering studies. In the case of Si and Ge the results are compared with experimental shifts. A tendency is observed for the confirmation of symmetric dimer formation during the reconstruction.

A He-diffraction study of the low-coverage (1 × 2)-phase of hydrogen on Rh(110)

He-diffraction investigations of the chemisorption of hydrogen on Rh(110) confirm the previous LEED observations concerning the formation of five different ordered phases with increasing coverage. He-diffraction intensity analyses were performed for the clean surface as well as for the low coverage (1 × 2)H-phase. Comparison of the best-fit corrugation of the (1 × 2)H-phase with surface charge density contours calculated by overlapping ionic densities yields height and lateral location of the adsorbed H-atoms relative to the close-packed rows of substrate atoms in good agreement with the conclusions based on dynamical LEED calculations.

Calculation of Surface‐Induced Core‐Level Shifts for Covalent Semiconductors C, Si, Ge, and α‐Sn I. (111) 2 × 1 Surfaces

AbstractA theory is developed for the surface induced core‐level shift. Since the many body relaxation mechanism can be neglected only the initial‐state potential effect is considered. Starting from tight‐binding assumptions the shift is related to ion charges accompanying the reconstruction, the hybridization state, the polarity of chemical bonds, and the atomic configuration. The atomic configurations are taken from total energy minimizations, dynamical LEED or ion scattering studies. The other input parameters are calculated self‐consistently. The developed theory is applied to the (111) 2 × 1 surface of group IV materials. Various fundamental reconstruction models — buckling, π‐bonded chains, and π‐bonded molecules — are studied. The shifts resulting for eight atoms per surface elementray cell and the associated core electron line shapes seem to favour the chain model in the (111) 2 × 1 case. The molecule model cannot be completely ruled out only from core‐level shift calculations because of its homopolar bond character.

The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

Disordered asymmetrical surface structures of clean Mo(100)-“(1 × 1)” and of “c(2 × 2)” sulfur on Mo(100) from dynamical LEED analyses

Disordered asymmetrical surface structures of clean Mo(100)-“(1 × 1)” and of “c(2 × 2)” sulfur on Mo(100) from dynamical LEED analyses

Disordered asymmetrical surface structures of clean Mo(100)-“(1 × 1)” and of “c(2 × 2)” sulfur on Mo(100) from dynamical leed analyses

The structures of the clean “(1 × 1)” and the “c(2 × 2)” sulfur-covered molybdenum (100) surfaces have been studied by low-energy electron diffraction (LEED) intensity analyses. It is found that the atoms in the top layer of both these surfaces probably reside at asymmetrical adsorption sites, destroying the perfect (1 × 1) and c(2 × 2) periodicities normally deduced from the LEED patterns. For clean room-temperature Mo(100), a slight preference is found for the topmost atoms to be displaced 0.13 ± 0.05 Å away from the centers of hollow sites of the second metal layer. This displacement occurs in the four equivalent 〈11〉 surface directions (diagonally in the square surface lattice), possibly randomly. The clean surface exhibits first, second and third interlayer spacing relaxations of − 6% ± 1.5%, 2% ± 2% and 0.5% ± 3%, respectively, relative to the bulk interlayer spacing. In the “c(2 × 2)” sulfur overlayer, the sulfur atoms are found to reside away from the center of hollow sites by 0.20 ± 0.05 Å. This displacement occurs in the four equivalent 〈10〉 surface directions (parallel to the sides of the square metal lattice cells), possibly randomly. The sulfur-Mo interlayer spacing is 1.03 ± 0.02 Å, while the spacings between metal layers have relaxed closer to their bulk value, which a top metal-metal spacing still contracted by 3% ± 3%. The asymmetrical site gives rise to three different bond lengths between sulfur and molybdenum atoms: 2.33 and 2.58 Å for Mo atoms in the topmost metal layer, and 2.56 Å for Mo atoms in the second metal layer. In this case, no lateral Mo relaxations were investigated.

Oxygen induced reconstruction of a close-packed surface: A LEED IV study on Ru(001)-p(2 × 1)O

A LEED IV study was carried out on the apparent (2 × 2) structure of dissociated oxygen on the hexagonally close-packed Ru(001) surface at saturation coverage. Four non-equivalent integral order beams and six non-equivalent half order beams were measured at normal incidence. Main results of the comparison with dynamical LEED calculations are: The (2 × 2) structure at half monolayer coverage is due to incoherent mixing of 3 domains of a p(2 × 1) structure. Oxygen chemisorption causes buckling and pairing in at least the first two substrate layers parallel to the mirror plane left for the (2 × 1) unit cell. This result is the first direct evidence that also close-packed surfaces can be reconstructed by strongly chemisorbed adlayers. Although amplitudes of atomic shifts are below 0.15 Å both in lateral and vertical directions, both vertical and even lateral shifts significantly improve the agreement of fits. The oxygen atom still occupies a site close to the hcp threefold coordinated site of the unreconstructed surface, at a vertical distance to the outermost Ru layer of 1.2 ± 0.02 Å. Vertically, both first and second Ru layers stay at their bulk positions, but are buckled in addition. The changes in effective Ru radii are attributed to charge redistribution between Ru surface atoms and oxygen.

C(0001)-(√3 × √3)R30°-Cs structure determination: R-factor analysis

A dynamical LEED analysis is applied to study the atomic structure of the Cs/C(0001)-(√3×√3)R30° surface. The - r-factor for every diffraction beam as well as the trend of the - r( d) function are used to search for the right surface model and to determine the correct geometric parameters. A model incorporating (CsAABAB) stacking with Cs atoms chemisorbed in hollow sites 2.7 Å above the top graphite plane, and the two top graphite layers shear-shifted to AA stacking and expanded to a separation of 3.85 Å, is the most probable structure.

Spin-polarized LEED investigation of the magnetized Ni(111) surface

Experimentally-determined exchange and spin-orbit induced scattering asymmetries A ex and A so for spin-polarized electrons diffracted from a magnetized Ni(111) surface are reported together with the results of fully relativistic dynamical LEED calculations. Comparison between theory and experiment shows generally good agreement and suggests that use of a model scattering potential incorporating an energy-dependent exchange correlation and localized absorption inside the muffin-tin spheres is to be preferred. Such comparisons also indicate that the surface is ferromagnetic with a top-layer magnetization comparable to that of the bulk.

Total energy minimization for surfaces of covalent semiconductors C, Si, Ge, and α-Sn: I. (111)2×1 surfaces

A semi-empirical tight-binding approach to the total energy of a surface system is presented. The resulting energy functional includes the electron-electron and ion-ion interaction in a reasonable way. It incorporates a self-consistent treatment of energies and charge transfers and is therefore capable of handling complex surface reconstruction geometries. The energy gain due to reconstruction and the resulting equilibrium surface geometry follow from the minimization of the total energy functional with respect to the atomic coordinates. The method is applied to the (111)2×1 surfaces of diamond, silicon, germanium, and α-Sn. Three reconstruction models are studied: the Haneman buckling model, the Pandey π-bonded chain geometry, and the Chadi molecule structure. Among the models suggested for the 2×1 reconstructed (111) surfaces the π-bonded chain model has to be favoured from the energetical point of view. The energy minimization in the framework of the buckling model only yields the ideal bulk-terminated surface structure. The molecule reconstruction does not give rise to any positive energy gain. For the π-bonded chain model the deepest energy is found for slightly buckled but undimerized chains. Chemical trends are discussed. The minimum-energy equilibrium geometrical structures obtained for this model are compared with atomic configurations extracted from other total energy calculations, medium-energy ion scattering and dynamical LEED. Overall agreement is stated. However there are also characteristic differences with respect to the magnitude of the tilts in the individual atomic layers, the relaxation of these layers and the strain resulting in these geometries. The atomic structures obtained experimentally are tested by means of the presented total energy functional.

THE Cs/C(0001)-(2×2) SURFACE STRUCTURE ANALYSIS BY LEED DYNAMICAL CALCULATION

The I-V curves of a series of possible models of Cs/C(0001)—(2×2) surface are calculated by dynamical LEED theory with RSP and RFS methods. The dalculated curves are selected with experimental results and the best structure is chosen by R-factor calculation. The model of Cs intercalating graphite layers forming intercalate structure and the model of Cs atoms adsorbed on top position of graphite surface are ruled out by R-factor examination. The most probable structure is that Cs atoms adsorbed on hollow position of graphite layer with a interlayer distance of 2.80 ? between Cs laver and graphite, and substrate has the same, structure as that of graphite bulk structure.