Tensor Low-energy Electron Diffraction Analysis Research Articles

Position of segregated Al atoms and the work function: Experimental low energy electron diffraction intensity analysis and first-principles calculation of the (√3×√3)R30° superlattice phase on the (111) surface of a Cu–9at.%Al alloy

The position of segregated Al atoms in the ordered (√3×√3)R30° superlattice phase on the (111) surface of a Cu–9at.%Al alloy was precisely determined by tensor low energy electron diffraction (LEED) analysis. Work function values of the superlattice phase, the alloy without Al segregation, and Cu (111) were measured and compared. Both the position of segregated Al atoms and the work function values were computed using first-principles calculations. The conclusion from the tensor LEED analysis that the segregated Al atoms, occupying substitutional positions in bulklike lattice sites within the top layer, were located outward by 6% with respect to the plane of the top layer Cu agreed well with the calculations. The results that the work function of the alloy surface with substitutional Al segregation was very close to that without Al segregation were obtained from both the experiments and the calculations. The effect of segregation of atoms and the position of segregants on the work function value was also 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.

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 MnBi 4 clusters are arranged in the top Cu layer instead of MgBi 4. 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.

Equivalent ordered-mixed-surface-structures of p(4 × 4)- p4 gm formed on Cu(0 0 1) by coadsorptions of Bi + Mg and Sb + Mg

Two coadsorption systems of Bi + Mg and Sb + Mg on Cu(0 0 1) have been studied by means of low-energy electron diffraction (LEED). A (4 × 4) LEED pattern with glide planes was observed in either case at ∼400 K. The two (4 × 4) structures were determined by a tensor LEED analysis. Both (4 × 4) structures are equivalent, and consist of a grid network of Mg 2Bi 2 (Mg 2Sb 2) plane clusters on a partially Bi (Sb) substituted Cu(0 0 1) layer. Structural parameters show that not only the formation of the clusters but also the arrangement of the clusters into the grid network contributes to the stabilization of this ordered-mixed-surface-structure.

Ordered mixed surface structures formed by coadsorption of dissimilar metal atoms on Cu(0 0 1)

We have found that various ordered mixed surface structures are formed by coadsorption of two dissimilar metal atoms on Cu(0 0 1) at room tepmerature, using low-energy electron diffraction (LEED) I– V analysis. As coadsorbates, we employed Mg, Bi, Li and K, and surface structures formed by the coadsorption systems of (Mg, Li), (Mg, K) and (Mg, Bi) are presented. A tensor LEED analysis provided detailed geometries of the coadsorbates and the substrate surface. It was found that the surface structures in the above three coadsorption sytems exhibit the restructuring of the Cu(0 0 1) surface. The phase separation into individual adsorbates does not take place, implying that some additional stabilization arises. We demonstrate two origins for the stabilization of the ordered mixed surface structures on Cu(0 0 1). Structures and features formed by the individual adsorption of Mg, Bi, Li and K atoms on Cu(0 0 1) are described first, then those of (2√2×√2)R45°-Mg,Li, (√5×√5)R26.7°-Mg,K, c(2×2)-Mg,Bi, and c(6×4)-Mg,Bi structures formed by the coadsorption are presented. We consider on the basis of the determined structural parameters the question why ordered mixed surface structures are formed instead of the phase separation.

The monolayer structure of CO2 adsorbed on the NaCl(100) single crystal surface: A tensor low-energy electron diffraction analysis

For the first time a detailed experimental analysis of all structural parameters of the p(2×1) monolayer phase of CO2 physisorbed at 75 K on a NaCl(100) single crystal substrate has been performed. The system was investigated by means of low-energy electron diffraction (LEED) at primary beam currents in the nA range. Intensities of six beams, including two of fractional order, were recorded at normal incidence in the energy range between 70 and 250 eV. These served as experimental data set for a TLEED-based I(V) analysis, in which the observed glide-plane symmetry as well as the linearity of the physisorbed molecules were fully considered. The substrate was represented by (a) an ideally bulk-terminated surface and (b) the relaxed surface which was reported before for the clean NaCl(100) surface. For both substrate models satisfactory agreement between experimental and theoretical I(V) curves was attained with Pendry R factors RP of 0.23 and 0.22, respectively, and similar molecular best-fit structures. Thereafter, the LEED experiment supports the herringbone adsorbate lattice proposed by theoretical studies as well as earlier experiments, with adsorption sites close to the sodium cations. However, the vertical distance of the molecules to the surface is significantly smaller than predicted by potential calculations, the lower oxygens in the tilted molecules being (a) 2.44±0.05 Å and (b) 2.46±0.04 Å above the geometrical means of the topmost layer in the different surface models, respectively. The corresponding molecular tilt angles are 62±2° and 61±2° with respect to the surface normal, in good agreement with the value resulting from the polarization dependence of infrared spectra of the ν3 monolayer vibration.

Determination of ( [formula omitted])R26.7° structures formed on Cu(0 0 1) by coadsorption of Bi and K(Cs): on-top site adsorption of K(Cs)

By using low-energy electron diffraction (LEED), we observed a ( 5 × 5 )R26.7° pattern in the coadsorption of Bi and K (or Cs) on Cu(0 0 1) at room temperature. Both ( 5 × 5 )R26.7°-Bi,K and -Bi,Cs structures were determined by a tensor LEED analysis. The two structures are ordered mixed phases and similar to each other: Bi atoms are located at surface fourfold hollow sites and K (or Cs) atoms occupy on-top sites of the substrate surface Cu atoms. Significant rumpling of 0.13 Å (-Bi,K) or 0.19 Å (-Bi,Cs) is found in the top Cu layer. It is suggested that the on-top site occupation of K or Cs atoms is stabilized by additional bonds with surrounding Bi atoms. The Hamilton-ratio tests suggest that K atoms displace laterally from the exact on-top position, but that Cs atoms are upright on the on-top sites.

An ordered surface alloy formed by attractive interaction between coadsorbates: c(2×2) on Cu(0 0 1) by Mg and Bi

The coadsorption of dissimilar metals, Mg and Bi, on Cu(0 0 1) at room temperature was studied using low energy electron diffraction (LEED). Five LEED patterns were observed at various coverages: c(2×2), c(4√2×2√2) R45°, c(6×4), c(6×6) and p(4×4). The c(2×2) structure was determined by a tensor LEED analysis. Mg atoms substitute for every second Cu atom in the top layer, and Bi atoms are located at the on-top sites of the remaining surface Cu atoms. The structure parameters demonstrate that Bi atoms pull up Mg atoms located in the substitutional sites, indicating a direct, attractive interaction between coadsorbates. Reasons for the formation of the c(2×2) structure are discussed in comparison with the surface structures formed by the individual adsorption of Mg and Bi.

AN ORDERED MIXED STRUCTURE FORMED BY RESTRUCTURING TYPE COADSORPTION OF Na AND K ON Ag(001)

Coadsorption of Na and K on Ag(001) at room temperature has been studied by low energy electron diffraction (LEED). A 3 × 3 LEED pattern was observed irrespective of the order of adsorption. For this formation, it is necessary to deposit Na and K atoms with appropriate coverage. We have determined the 3 × 3 structure by a tensor LEED analysis. It is a restructured surface and is very similar to the previously determined 3 × 3 structure formed on Ag(001) by pure Na adsorption. In the coadsorption, Na and K atoms occupy preferable sites selectively, and construct an ordered mixed structure on Ag(001). That is, small Na atoms are located in the missing row sites, while large K atoms sit on the hollow sites of four-Ag-atom islands. The reason for the site selectivity of Na and K atoms in the mixed 3 × 3 structures is discussed.

An Ordered Mixed Structure Formed by Restructuring Type Coadsorption of Na and K on Ag(001)

Coadsorption of Na and K on Ag(001) at room temperature has been studied by low energy electron diffraction (LEED). A 3 × 3 LEED pattern was observed irrespective of the order of adsorption. For this formation, it is necessary to deposit Na and K atoms with appropriate coverage. We have determined the 3 × 3 structure by a tensor LEED analysis. It is a restructured surface and is very similar to the previously determined 3 × 3 structure formed on Ag(001) by pure Na adsorption. In the coadsorption, Na and K atoms occupy preferable sites selectively, and construct an ordered mixed structure on Ag(001). That is, small Na atoms are located in the missing row sites, while large K atoms sit on the hollow sites of four-Ag-atom islands. The reason for the site selectivity of Na and K atoms in the mixed 3 × 3 structures is discussed.

Determination of a (2√2×√2)R45° structure formed by coadsorption of Li and Mg on a Cu([formula omitted]) surface

The coadsorption of Li and Mg on Cu(001) at room temperature was studied by low-energy electron diffraction (LEED). A (2√2×√2)R45° structure was observed and determined by a tensor LEED analysis. It is an ordered mixed phase, in which Mg atoms are located on fourfold substitutional sites, while Li atoms occupy surface hollow sites, instead of the separation into c(2×2)-Mg and (2×1)-Li phases formed in the individual adsorption. The (2√2×√2)R45° structure is comprised of alternative atomic-rows of substitutional Mg atoms and hollow-site Li atoms aligned along the [010] direction. The stability of this structure is discussed in terms of adsorption energy of Li and Mg atoms and the displacement of surface Cu atoms.

Ordered mixed surface structures formed on Cu(0 0 1) by coadsorption of dissimilar metals: (2√2×√2)R45° by Mg and Li, and (√5×√5)R26.7° by Mg and K(Cs)

The coadsorption of Mg and alkali-metals (Li, K or Cs) on Cu(0 0 1) at room temperature has been studied by low-energy electron diffraction (LEED). A (2√2×√2)R45° structure is formed by Mg and Li at coverages both about 0.25 ML, and which has been determined by a tensor LEED analysis. It is an ordered mixed phase, in which Mg atoms are located in fourfold substitutional sites and Li atoms occupy fourfold surface hollow sites, forming alternative one-dimension-like atomic rows along the [0 1 0] direction. On the other hand, a (√5×√5)R26.7° structure is formed by the coadsorption of Mg with either K or Cs at coverage about 0.2 ML each. These structures have also been determined: Mg atoms are in fourfold substitutional sites, while K (Cs) atoms are on surface hollow sites, forming square arrangements of Mg and K (Cs). It is found that the top layer Cu atoms in both ordered structures are displaced laterally, and that Mg atoms are located at deeper positions in fourfold substitutional sites than in a c(2×2) substitutional structure formed by individual Mg adsorption. The reason for the formation of the ordered mixed surface structures is discussed.

Structure of ultrathin Fe films on Cu(100) prepared by pulsed laser deposition

The structures of ultrathin Fe films on Cu(100) prepared by pulsed laser deposition (PLD) and conventional thermal deposition (TD) are compared by means of low-energy electron diffraction (LEED). Below 5 ML coverage, PLD films exhibit up to four times higher integer-order LEED spot intensities, i.e., improved film quality, and show small but significant structural changes as compared to TD films. A quantitative tensor LEED analysis of the 4-ML PLD film reveals a similar $5\ifmmode\times\else\texttimes\fi{}1$ superstructure to that found previously for TD films, but with an enhanced surface buckling and a flat Cu bulklike iron interface layer. The latter is attributed to the incorporation of Cu atoms in the Fe interface caused by sputter effects in PLD. Above 6-ML coverage, both deposition methods yield comparable LEED $I(E)$ spectra, revealing similar surface structures. The proposed structural models allow a consistent interpretation of the strongly altered magnetic properties observed for PLD-grown iron films.

Tensor low-energy electron diffraction analysis of the surface structure of NaCl(100) thin films grown on Pd(100) and Pt(111)

Thin films of NaCl were molecularly deposited and subsequently ordered on Pd(100) and Pt(111). These films exhibited the same adsorbate–substrate interaction on Pd(100) and Pt(111) as evidenced by the lone multilayer desorption peak observed in the temperature programmed desorption spectra of NaCl on these two substrates. The ordered structures of the films were produced by exposing a heated substrate to a flux of NaCl emanating from a Knudsen cell and investigated through the use of low-energy electron diffraction (LEED). On Pd(100), the adsorbed multilayer yielded a (1×1) NaCl(100) LEED pattern. The (1×1) pattern was seen as the result of the film's thickness being greater than the sampling depth of the elastically scattered electrons used in the LEED experiment, thus removing the contribution of the substrate's lattice to the observed pattern. For the (1×1) pattern of NaCl(100) on Pd(100), it was found that the NaCl film grew along the [010]-type surface directions of Pd(100) to take advantage of a near 1:1 lattice match. On Pt(111), the ordered NaCl multilayer gave a (1×1) NaCl(100) LEED pattern also, but, unlike Pd(100), one of the overlayer's unit cell vectors lies parallel to that of the substrate's with the other rotated by 90° from the previous one to preserve the geometry of the NaCl(100) lattice. Multiple intensity versus electron energy ( IV) data sets were gathered for the NaCl(100) multilayer film on both Pd(100) and Pt(111). An analysis of the I– V curves of the multilayer NaCl(100) pattern on both substrates showed that the multilayer has the same structure on both substrates to a depth sampled by the electrons in the experimental energy range. Data from both systems were used in a fully dynamical LEED calculation. In the final structure, which refined down to a Pendry R-factor ( R P) of 0.16, the largest deviation from the ideally terminated NaCl(100) structure was the movement of the surface Na + towards the bulk, thereby causing a 0.12±0.03 Å corrugation of the surface layer. This shift of the surface cations was not followed by a corresponding displacement of the underlying anions, so the first interlayer spacing, d(Cl 2–Na 1), was reduced to 2.72±0.03 Å from its bulk value of 2.82 Å. Deviations from the bulk were smaller in subsequent layers, e.g. the second interlayer spacing, d(Na 3–Cl 2), was 2.77±0.03 Å, and there was essentially no buckling of the second layer.

Tensor LEED analysis for the Ni(111)-(√7 × √7)R19.1°-P surface structure: comparison with other √7 systems

A crystallographic analysis is reported using low-energy electron diffraction (LEED) in the tensor LEED approach for the (√7 × √7)R19.1° structure formed by 3/7 monolayer of phosphorus at the Ni(111) surface. This surface has a novel structure in which each phosphorus atom bonds to seven neighbouring Ni atoms, four in the top layer and three in the second layer, at an average distance close to 2.39 Å. Formally this reconstruction involves three neighbouring Ni atoms in a triangular arrangement per unit mesh of the original unreconstructed surface being replaced by three phosphorus atoms. A discussion is included of the structural relaxations that occur in this surface as the demands of Ni–P bonding are balanced against those for Ni–Ni and P–P contributions. Comparisons are made with the √7 reconstructions observed for related systems, including those for P/Rh(111), S/Pd(111) and S/Cu(111). Copyright © 1999 John Wiley & Sons, Ltd.

Surface structure of cesium adsorption on the Si(001)2×1surface

The detailed surface structure of Cs adsorption on the Si(001) $2\ifmmode\times\else\texttimes\fi{}1$ surface was investigated by means of tensor low-energy electron diffraction analysis. Various adsorption sites were considered including the recently proposed possibility of asymmetric dimer reconstruction of the Si top layer. We confirm the structure model with double-layer adsorbates and symmetric Si dimers underneath for the saturation coverage Cs adsorption. The major structural parameters within this structure are discussed in terms of the covalent bonding between the adsorbates and Si surface atoms.

(n×n) SURFACE STRUCTURES FORMED COMMONLY ON Cu(001), Ag(001) AND Ni(001) BY ALKALI-METAL ADSORPTION

We have studied structures formed on Ag(001) by Na adsorption at 300 K and on Ni(001) by Li deposition at 370 K by means of low energy electron diffraction (LEED). Structure sequences (2×1)→(3×3)→(4×4)→(4×2)→(2×1) and c (2×2)→(4×4)→(5×5) are found with increasing coverages for Na / Ag (001) and Li / Ni (001), respectively. The (3×3) for Na / Ag (001) is determined by tensor LEED analysis to be identical to the previously determined (3×3) structure formed for Li / Cu (001). We suggest that the (4×4) structures for Na / Ag (001) and Li / Ni (001) are also identical to the previously determined (4×4) for Li / Cu (001), and that the (5×5) for Li / Ni (001) is analogous to the (3×3) and (4×4) structures.