Tensor LEED Research Articles

Ordered mixed rows of (Pb + Sn) and (Pb + Sb) on Cu(001): A coadsorption study and structure determination using low energy electron diffraction

The surface alloys of (Pb, Sn) and (Pb, Sb) on Cu(001) obtained via the coadsorption method were independently determined using tensor low-energy electron diffraction (LEED). Both surface alloys were obtained under the p(4 × 4) phase. The determined structures consisted of mixed rows of the adsorbates in a one-dimensional chain arrangement. These mixed rows are buckled differently in each row. The best-fit structure indicates that the Sn or Sb atoms are located on the four-fold hollow sites while the Pb atoms are displaced toward the vacancies. The bond lengths of Sn or Sb atoms with Cu underneath of the alternative mixed chains are reduced by 4.8% or 5.9%, respectively, compared to the bulk values. We consider that, the large reductions of these bond lengths and the tendency of the adsorbates to make alloy at the surface stabilize the structure. The other structural parameters, bonding effects of the surrounding Cu atoms with the adsorbates, and similar characteristics of the elements located in the same period of periodic table are discussed in detail. On the basis of the structural characterization, the mechanisms of the formation of the p(4 × 4) phase are proposed.

LEED I/V determination of the structure of a MoO3 monolayer on Au(111): Testing the performance of the CMA-ES evolutionary strategy algorithm, differential evolution, a genetic algorithm and tensor LEED based structural optimization

The structure of a thin MoO3 layer on Au(111) with a c(4×2) superstructure was studied with LEED I/V analysis. As proposed previously (Quek et al., Surf. Sci. 577 (2005) L71), the atomic structure of the layer is similar to that of a MoO3 single layer as found in regular α-MoO3. The layer on Au(111) has a glide plane parallel to the short unit vector of the c(4×2) unit cell and the molybdenum atoms are bridge-bonded to two surface gold atoms with the structure of the gold surface being slightly distorted. The structural refinement of the structure was performed with the CMA-ES evolutionary strategy algorithm which could reach a Pendry R-factor of ∼ 0.044. In the second part the performance of CMA-ES is compared with that of the differential evolution method, a genetic algorithm and the Powell optimization algorithm employing I/V curves calculated with tensor LEED.

The Adsorption and Decomposition of Formic Acid on Cu{100} and Cu{100}Pt Surfaces using Temperature Programmed Reaction Spectroscopy

The structures formed by adsorbing thin-film platinum, formic acid and oxygen on Cu{100} single crystal are investigated by quantitative low-energy electron-diffraction (LEED) and Temperature Programmed Reaction Spectroscopy (TPRS). Symmetrized Automated Tensor LEED (SATLEED) calculations are used to determine the structure of the formed surface alloys and overlayers. TPRS was used to probe the surface reactivity of the systems studied while surface composition was obtained using Auger Electron spectroscopy (AES). The decomposition of a formate intermediate from a clean Cu{100} surface has been monitored through the use of TPD Spectroscopy. It has also been evidenced that platinum has a destabilising effect on the formate intermediate. The peak temperature (Tp ) for the CO2 desorption spectra from copper-platinum model surfaces, appear around 40 K lower than those from clean copper. This suggests a much less stable surface alloy compared to the clean surface. In activation energy terms, this destabilisation can be expressed as a 13% decrease in the energy required for the formate to decompose. It was also observed that desorption is much more rapid from the copper-platinum than from clean copper surfaces. Keywords: Cu{100}, Cu{100}Pt, Formic Acid, TPRS

Quantum Size Effect Driven Structure Modifications of Bi Films on Ni(111)

The quantum-size effect (QSE) driven growth of Bi film structures on Ni(111) was studied in situ using low energy electron microscopy and selective area low energy electron diffraction (μLEED). Domains with a (3×3), [(3)(1)(-1)(2)], and (7×7) film structure are found with a height of 3, 5, and 7 atomic layers, respectively. A comparison of I/V-μLEED curves with tensor LEED calculations shows perfectly accommodated Fermi wavelengths, indicative that not only the quantized height, but also the film structure is driven by QSE.

Relaxation and thermal vibrations at the NaF(100) surface

Abstract The relaxation and the thermal vibrations of the NaF(100) surface are investigated in the temperature range between 25 K and 230 K by means of low-energy electron diffraction (LEED) and a subsequent I(V) structure analysis based on the tensor LEED approach (TLEED). According to the experiments, the NaF(100) surface is not significantly relaxed and has the ideal truncated bulk structure. The thermal vibrational amplitudes of the ions in the topmost layer are significantly enhanced compared to the bulk by a factor of 1.35 ± 0.15 and are equal within the error-bars for Na + and F − ions. Moreover, the relaxation and the dynamics of the NaF(100) surface are investigated using periodic density functional theory (DFT) calculations using pseudopotentials. In agreement with the experimental findings, the calculated relaxation of the NaF(100) surface is weak with static shifts of the ions of 0.01 A to 0.02 A. In the topmost layer, the Na + ions are predicted to be slightly inward shifted, whereas the F − ions are outward shifted, in accordance to predictions of previous shell-model calculations. A Born Oppenheimer molecular dynamics (BO-MD) simulation of the dynamics at the NaF(100) surface leads to a smaller enhancement of thermal motions of the ions at the surface compared to the experiment.

Surface structure and composition of Pd 8Ni 92(1 1 1): A dynamic LEED study

The Pd 8Ni 92(1 1 1) surface was studied using low-energy electron diffraction (LEED). IV spectra collected close to normal incidence were analysed by dynamical tensor LEED. Composition and interlayer spacings were determined for first four atomic layers. The surface slab is found to be enriched in Pd (30% in total against 8% in bulk) but this extra palladium is irregularly distributed over the four topmost layers. Contrary to previous reports, the palladium amount, as compared to the bulk, is not significantly changed in the surface layer and it is enriched to 20% only in the third layer. The detected contractions of interlayer spacings are all but one ( d 34) within the error limits of no contraction at all. We also discuss some aspects of correlations between the fit parameters.

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.

The geometries of coadsorbate layers of O and H on Ru(0 0 1): How well can quantitative LEED see hydrogen atoms?

Using a CCD LEED system for the collection of IV data with low beam damage, and full dynamical as well as tensor LEED calculations, we have determined the geometries of the (2 × 2)-(O + 3H) and the (2 × 2)-(O + H) coadsorbate structures on Ru(0 0 1). We show that here quantitative LEED can locate the H atoms very well. Not only their sites (hcp in the first, fcc in the second case), but also the Ru–H spacings and changes in the first two substrate layers are clearly determined. We argue that this success is due to the relatively large data range and to the smaller H mobility compared to pure H layers caused by their repulsive lateral interactions with the oxygen atoms.

Determination of a (4 × 4) structure formed on a Cu(0 0 1) surface by adsorption of calcium

The adsorption of calcium (Ca) atoms on a Cu(0 0 1) surface has been studied by low-energy electron diffraction (LEED) at 130, 300 and 400 K. It is found that a (4 × 4) was the only LEED pattern appeared at 400 K while a quasi-hexagonal structure was formed in a wide range of submonolayer coverage at 130 K. At 300 K, the (4 × 4) LEED spots were broad and weak. The (4 × 4) structure formed at 400 K was determined by a tensor LEED I– V analysis. It is a new-type of surface alloys consisting of five substitutional Ca atoms, nine surface Cu atoms, and two atomic vacancies in the unit cell. In spite of a quite large size-difference between Ca (3.94 Å) and Cu (2.55 Å) atoms, all Ca atoms are located at the substitutional sites. Among surface alloys so far reported, the atomic size ratio between Cu and Ca in the (4 × 4), 1.54, is the largest. Optimized structural parameters reveal that large lateral displacements of surface Cu atoms, being enabled by the appearance of the vacancies, allow the formation of the (4 × 4) structure.

The structure and lattice dynamics of RbBr(1 0 0) and RbI(1 0 0) single crystal surfaces: A tensor low energy electron diffraction analysis

The structure and lattice dynamics of RbBr(1 0 0) and RbI(1 0 0) single crystal surfaces cleaved under UHV conditions were investigated by means of low energy electron diffraction (LEED) at temperatures of 156 K and 183 K, respectively. Since RbBr and RbI are insulators the experiments were carried out with a microchannel plate LEED system at very low primary currents (∼5 nA). For both materials four different diffraction orders could be observed. Diffraction patterns were recorded over an energy range from 30 eV to 220 eV in increments of 2 eV and I( V) curves for each spot were extracted. The I( V) curves were analyzed using the tensor LEED approach. For both alkali halide substrates surface structures of (1 × 1) periodicity close to the truncated bulk structure were found. For RbBr, the first interlayer distance is reduced by about 2.2%, where the Rb + cations in the topmost layer are shifted inwards by 0.06(3) Å, and the anions also exhibit an inward shift which is however smaller (0.04(3) Å). The root mean square vibrational amplitudes are enlarged by a factor of 1.3 for Rb + and 1.25 for Br −, respectively. For RbI(1 0 0) the cations of the topmost layer are shifted inwards by 0.07(3) Å and the anions outwards by 0.02(1) Å. The vibrational amplitudes of the ions are not enlarged as for RbBr but close to the corresponding bulk values.

Theory of low-energy electron diffraction for nanomaterials—subclusters, automatedsearches

To enable the determination of detailed structures of nanomaterials, we have previously made thetheory of low-energy electron diffraction (LEED) much more efficient for complex and disorderedsystems, calling it NanoLEED: our cluster approach speeds up the computation to scale asnlogn, rather thanthe standard n3 or n2,with n the number of atoms, for example. Strong multiple scattering may occasionally cause poorconvergence: this is solved here by treating all scattering within subclusters of a few atoms (e.g. aSiH3 group) with accurate matrix inversion. For the structure determination of complexnanostructures, an efficient search method is also essential: for that purpose amodified version of tensor LEED is here adapted to nanostructures, and calledNanoTensorLEED.

Surface alloy model of p(2 × 2)Sb/Cu(0 0 1) from LEED I/V data

We report on the re-determination of the structure of p(2 × 2)Sb/Cu(0 0 1) from measured LEED I/V data. The structure solution is performed in two steps. First an approximate “image” of the surface structure model is found by an application to the measured data of the recently-proposed Phase and Amplitude Recovery and Diffraction Image Generation Method (PARADIGM). This reveals that the p(2 × 2) structure is formed by the protrusion of one atom out of four in the outermost surface layer. Since an Sb atom is larger than one of Cu, this suggests a possibility that the p(2 × 2) structure is formed by Sb substitution for one out of four Cu atoms in the outermost layer. Second, a tensor LEED refinement of the model suggested by the PARADIGM yields the same dominant top-layer substitutional model proposed in the earlier LEED study, but with a much lower reliability factor, making much less likely an admixture of other surface phases.

LEED and PIRS structure analysis of physisorbed molecules on insulators: MonolayerC2D2∕KCl(100)

The structure of the monolayer ${\mathrm{C}}_{2}{\mathrm{D}}_{2}$ adsorbed on a KCl(100) single crystal surface has been investigated by means of polarization infrared spectroscopy (PIRS) in transmission geometry at oblique incidence and low-energy electron diffraction (LEED). Infrared spectra in the region of the ${\ensuremath{\nu}}_{3}$ asymmetric stretch vibration show at $75\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ a singlet absorption at $2395.6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. From the ratio of the integrated absorptions in $s$- and $p$-polarized spectra, a parallel orientation of the molecules with respect to the surface is deduced. The observed LEED diffraction patterns are consistent with a $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ lattice with two glide planes, in agreement with the parallel orientation of the molecules. LEED beam intensities of five inequivalent beams were recorded as a function of electron energy and the resulting $I(V)$ curves were analyzed using the tensor LEED approach. According to the $I(V)$ analysis and the analysis of the PIRS spectra, an adsorption geometry with two energetically equivalent but translationally inequivalent molecules in the surface unit cell is deduced with adsorption sites $3.14\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ above the ${\mathrm{K}}^{+}$ cations. While IR spectroscopy supports a herringbone structure with the molecular axis of nearest neighbors intersecting at 90\ifmmode^\circ\else\textdegree\fi{}, the LEED analysis is consistent with an intersection of 62\ifmmode^\circ\else\textdegree\fi{}. These deviating results for the azimuthal orientations of the molecules on the KCl(100) surface are discussed within the concept of ``split positions'' [Over et al., Phys. Rev. B 52, 16812 (1995)] and the consideration of the in-plane libron dynamics of adsorbed molecules.

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.

Tensor LEED study of the temperature dependent dynamics of the NaCl(100) single crystal surface

The structure and dynamics of the NaCl(100) single crystal surface were investigated at 25, 100, 160, and $230\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by means of low-energy electron diffraction (LEED). At each temperature, diffraction peak intensities of four inequivalent beam orders were recorded as a function of electron energy. The experimental $I(V)$ data were analyzed using the tensor LEED approach. Compared to the unrelaxed bulk geometry, the ${\mathrm{Na}}^{+}$ ions in the topmost layer are displaced inwards by $0.09\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ at all investigated temperatures. Moreover, the root-mean-square vibrational amplitudes of ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{\ensuremath{-}}$ ions in the topmost layer are enhanced significantly in the investigated temperature range. The amplitudes of the ions in the second layer are close to the bulk values. While the amplitudes of the cations in the second layer are larger than those of the anions, ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{\ensuremath{-}}$ in the topmost layer have nearly equal vibrational amplitudes at temperatures above $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, the mean-square amplitudes of vibrations of ions at the surface and in the bulk are in good agreement with previously reported lattice dynamics studies.

The [formula omitted] surface oxide: A LEED, DFT and STM study

Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pd ( 1 0 0 ) - ( 5 × 5 ) R 27 ° – O surface oxide structure consisting, in the most recent model, of a strained PdO(1 0 1) layer on top of the Pd(1 0 0) surface [M. Todorova et al., Surf. Sci. 541 (2003) 101]. Both, DFT simulations using the Vienna Ab initio Simulation Package (VASP) and tensor LEED I( V) analysis of newly acquired LEED experimental data, show that the PdO(1 0 1) model is essentially correct. However, compared to the previous study, there is a horizontal shift of the PdO(1 0 1) layer with respect to the Pd(1 0 0) substrate. The atomic coordinates derived by DFT and LEED ( R P = 0.162) are in excellent agreement with each other. We also present STM images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate.

Theoretical and experimental studies of the structure and dynamics of the CaF 2(1 1 1) surface

The structure and dynamics of the CaF 2(1 1 1) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I( E) curves. According to both LEED and MD, the CaF 2(1 1 1) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F–Ca–F triple layer.

Structural investigation of the c(√2 × 5√2)R45° surface alloy formed by Ti deposition on Cu(0 0 1)

The alloying process of Ti deposited on Cu(0 0 1) was studied by means of XPS, LEIS, XPD and LEED intensity analysis. With the sample held at 570 K, a linear decrease of the Cu LEIS signal as a function of the amount of Ti deposited is observed in the early stages of deposition until a constant value is reached. At the onset of the plateau a c(√2 × 5√2)R45° LEED pattern starts to be visible. XPD and LEED intensity measurements were performed for the c(√2 × 5√2)R45° phase prepared depositing ca. 1.5 monolayer of Ti. The angle-scanned XPD curves measured for the phase c(√2 × 5√2)R45° reveal that Ti atoms substitute Cu atoms in the fcc lattice of the substrate. The polar XPD curves show that at least the first four layers of the substrate are involved in the alloying process. We found that the (3 1 0) plane of the Cu 4Ti alloy (D1 a type-structure) fits, without significant contraction or expansion of the lattice parameters, the c(√2 × 5√2)R45° structure. The intensity versus energy curves of the diffracted beams were calculated on the basis of this structural model using the tensor LEED method. The results of the LEED intensity analysis provide a further evidence of the formation of a slab of Cu 4Ti(3 1 0) layers.

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.