Low-energy Electron Diffraction Calculations Research Articles

Bromine Adsorption and Thermal Stability on Rh(111): A Combined XPS, LEED and DFT Study.

This study addresses a fundamental question in surface science: the adsorption of halogens on metal surfaces. Using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (XPS), temperature-programmed XPS, low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we investigated the adsorption and thermal stability of bromine on Rh(111) in detail. The adsorption of elemental bromine on Rh(111) at 170 K was followed in situ by XPS in the Br 3d region, revealing two individual, coverage-dependent species, which we assign to fcc hollow- and bridge-bound atomic bromine. In addition, we find a significant shift in binding energy upon increasing coverage due to adsorbate-adsorbate interactions. Subsequent heating shows a high thermal stability of bromine on Rh(111) up to above 1000 K, indicating strong covalent bonding. To complement the XPS data, LEED was used to study the long-range order of bromine on Rh(111): we observe a (√3×√3)R30° structure for low coverages (≤0.33 ML) and a star-shaped compression structure for higher coverages (0.33-0.43 ML). Combining LEED and DFT calculations, we were able to visualize bromine adsorption on Rh(111) in real space for varying coverages.

Vectorial spin polarization detection in multichannel spin-resolved photoemission spectroscopy using an Ir(001) imaging spin filter

We report on spin- and angular resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over $10^3$ compared to standard single channel detectors. Furthermore, we review the spin filter preparation, and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

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

Graphene/Ni(111) surface structure probed by low-energy electron diffraction, photoelectron diffraction, and first-principles calculations

The structural properties of graphene/Ni(111) are investigated by a combination of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), angle-scanned photoelectron diffraction (PED), and first-principles calculations. XPS data indicate that graphene interacts strongly with the topmost Ni layer. Diffraction data show that graphene is deposited commensurably with the underlying Ni surface atoms. Twelve different graphene preparations were analyzed by LEED and one by PED. Considering the relative position between the carbon and Ni atoms, five experimental sets indicate a top-fcc structure, three show a bridge-top structure, and four have a mixed structure. The analysis of the PED experiment suggests the top-fcc termination. Our first-principles calculations show that the total energies of the top-fcc and bridge-top structures are nearly degenerate, which corroborates the observed bistability of those phases. Moreover, by comparing the structural parameters obtained by the three methods (LEED, PED, and ab initio calculations), excellent agreement is achieved.

Novel genetic algorithm search procedure for LEED surface structure determination

Low Energy Electron Diffraction (LEED) is one of the most powerful experimental techniques for surface structure analysis but until now only a trial-and-error approach has been successful. So far, fitting procedures developed to optimize structural and nonstructural parameters—by minimization of the R-factor—have had a fairly small convergence radius, suitable only for local optimization. However, the identification of the global minimum among the several local minima is essential for complex surface structures. Global optimization methods have been applied to LEED structure determination, but they still require starting from structures that are relatively close to the correct one, in order to find the final structure. For complex systems, the number of trial structures and the resulting computation time increase so rapidly that the task of finding the correct model becomes impractical using the present methodologies. In this work we propose a new search method, based on Genetic Algorithms, which is able to determine the correct structural model starting from completely random structures. This method—called here NGA-LEED for Novel Genetic Algorithm for LEED—utilizes bond lengths and symmetry criteria to select reasonable trial structures before performing LEED calculations. This allows a reduction of the parameter space and, consequently of the calculation time, by several orders of magnitude. A refinement of the parameters by least squares fit of simulated annealing is performed only at some intermediate stages and in the final step. The method was successfully tested for two systems, Ag(1 1 1)(4 × 4)-O and Au(1 1 0)-(1 × 2), both in theory versus theory and in theory versus experiment comparisons. Details of the implementation as well as the results for these two systems are presented.

Surface investigation of Au monolayers on Pd(100)

Details of the growth and of the surface structure of 1, 2, and 3 monolayers of Au on Pd(100) have been investigated by reflection high-energy electron diffraction (RHEED), quantitative low-energy electron diffraction (LEED)-I(V) analysis and density-functional theory calculations (DFT). Despite the 4.7% misfit between Au- and Pd-lattice parameters, no change on the lateral lattice parameter was observed up to a thickness of 10 ML by RHEED, evidencing pseudomorphous growth. Up to 3 ML a ($1\ifmmode\times\else\texttimes\fi{}1$) LEED pattern is observed, while at higher coverages a ($7\ifmmode\times\else\texttimes\fi{}1$) superstructure is formed. The interlayer distances of the last atomic layers of $n$-ML-Au/Pd(100) ($n=1$, 2, 3) as determined by quantitative LEED and DFT calculations are compared and show only a reasonable agreement. However, DFT results indicate that surface alloying is energetically favorable, and a better match with the experimentally determined interlayer distances is obtained with surface alloy formation.

Structural analysis of the c(4 × 2) reconstruction in Si(0 0 1) and Ge(0 0 1) surfaces by low-energy electron diffraction

The c(4 × 2) structures in (0 0 1) surfaces of Si and Ge have been studied by low-energy electron diffraction (LEED). Using a proper cleaning method for the Si surface, we were able to observe clear c(4 × 2) LEED patterns up to incident energy of ∼400 eV as well as the Ge surface. Extensive experimental intensity–voltage curves allowed us to optimize the asymmetric dimer model up to the eighth layer (including the dimer layer) in depth in the dynamical LEED calculation. Optimized structural parameters are almost the same for the Si and Ge except for the height of the buckled-up atom of the asymmetric dimer. For the Ge surface, the structural parameters are in excellent agreement with those obtained by a previous theoretical calculation. The tilt angle and bond length of the dimer are 18 ± 1 (19 ± 1)° and 2.4 ± 0.1 (2.5 ± 0.1) Å for the Si(0 0 1) (Ge(0 0 1)), respectively.

A molecular T-matrix approach to calculating Low-Energy Electron Diffraction intensities for ordered molecular adsorbates

We present a novel approach to calculating Low-Energy Electron Diffraction (LEED) intensities for ordered molecular adsorbates. First, the intra-molecular multiple scattering is computed to obtain a non-diagonal molecular T-matrix. This is then used to represent the entire molecule as a single scattering object in a conventional LEED calculation, where the Layer Doubling technique is applied to assemble the different layers, including the molecular ones. A detailed comparison with conventional layer-type LEED calculations is provided to ascertain the accuracy of this scheme of calculation. Advantages of this scheme for problems involving ordered arrays of molecules adsorbed on surfaces are discussed.

Angle-resolved low-energy ion scattering studies of quasicrystalline Al–Pd–Mn

We present angle-resolved low-energy ion scattering data that provide information about the composition of the topmost surface layers and atomic arrangement for a clean, 5-fold surface of a single-grain of icosahedral Al–Pd–Mn. The results show that the surface of this material has 5-fold periodicity and has a surface composition with a higher Al content than the bulk material (Al-86, Pd-13, Mn-1 versus Al-71, Pd-20, Mn-9 atom%). A predominant neighbor atom distance of 7.6±0.5 Å is calculated from the results, consistent with scanning probe measurements and bulk models of Al–Pd–Mn quasicrystals. We calculate a nearest neighbor distance of 3.0±0.2 Å. We also find that a Pd-rich layer exists just below an Al-rich outer layer, in agreement with dynamical low-energy electron diffraction calculations.

Probing unoccupied bulk bands via the cross section of quantum well states in thin films

The photoemission cross section of quantum well states in thin Cu(100) films on Co(100) has been analyzed in normal emission as a function of photon energy. Assuming conservation of the perpendicular wave vector k⊥, the cross section maxima can be assigned to direct transitions from the (initial) quantum well state to bulk-like final states. The value of k⊥ is obtained from the sharp k⊥ distribution of specific quantum well states, which is centered about discretized k⊥ values. Our data are compared with free-electron-like models and a low energy electron diffraction calculation of high-energy states in bulk Cu.

Segregation and chemical ordering in the surface layers of Pt 25Co 75(111): a LEED/STM study

Segregation and chemical ordering on Pt 25Co 75(111) are studied by quantitative low-energy electron diffraction (LEED) analysis and scanning tunnelling microscopy (STM). Although LEED patterns do not show any sign of superstructure, LEED calculations undoubtedly point to a surface that contains about the same amount of both species and reveal a significant short-range chemical ordering (down to the third layer). Pt and Co surface sites are locally arranged with a (1×2) unit cell, in the manner of the ordered tetragonal L1 0 phase. More direct evidence is given by STM images that exhibit parallel Pt and Co monoatomic chains a few lattice constants long and an apparent height difference of about 0.2 Å for Pt and Co sites. LEED shows that the Pt sublattice in the top layer actually resides 0.1 Å above the Co one. Otherwise the surface is bulk-like, with weak relaxations of interlayer distances. The use of a chemically ordered model for the LEED analysis, in which sublattice occupancies are optimized, results in a remarkable improvement of the fit with experiment as compared to a totally random distribution; however, most interestingly, it does not modify the average layer concentration profile versus depth (55, 5 and 35 at.% Pt, respectively, for the three outermost layers). The distortions needed for the tetragonal L1 0 phase with respect to the fcc L1 2 phase explain why chemical order does not extend over larger domains. Finally, both techniques yield complementary pictures and quite consistent results as to the top layer content and chemical order.

Recent studies on diamond surfaces

The surface properties of diamond have been studied by ultra-violet photoemission spectroscopy (UPS), Kelvin probing and low energy electron diffraction (LEED). The atomic level structure of diamond surfaces was determined by LEED intensity vs. energy [I(E)] measurements in combination with Tensor LEED calculations. The LEED analysis of the C(100)-(2×1)-H surface revealed the formation of symmetrical dimers on the top carbon layer. For the C(100)-(1×1)-O surface, quantitative LEED analysis indicated a structural model where oxygen occupied the bridge site on the surface. Systematic investigations were carried out using UPS and a Kelvin probe measurement to reveal the effect of alkali metal fluoride overlayers on the work function of the diamond surfaces. LiF and RbF have been found to act as effective dipole layers to lower the surface work function and induce a negative electron affinity.

Water adsorption structures on flat and stepped Ru(0001) surfaces

Water adsorption and desorption (H2O and D2O) were studied on vicinal Ru(0001) surfaces with nominal (101̄l) (l=8, 12, and 22) orientations employing low-energy electron diffraction (LEED), thermal desorption spectroscopy, and work-function change measurements (ΔΦ). LEED measurements and theoretical calculations from clean Ru(101̄l) confirmed the existence of terraces with average regular spacing and steps of one atomic height. Due to the hcp structure of Ru the terrace width and the atomic arrangement of the step alternate from one terrace to the next. This is confirmed by LEED measurements and calculations showing a triple-spot splitting of the reflexes. For water adsorption, due to the presence of steps, new desorption states with higher desorption temperature (220–350 K) are observed, which are missing on the flat Ru(0001) surface. LEED experiments demonstrate that the ordered water overlayers found on Ru(0001) are absent on the stepped surfaces. The isotope effect for D2O (the so-called A1 desorption state found for H2O at 212 K is missing for D2O) is also observed on the stepped surfaces indicating that this effect is due to a short-range interaction rather than a longer-range interaction in the water bilayer.

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.

A layer-by-layer study of CdTe(110) surface Debye temperature and thermal vibrations by low energy electron diffraction

The Debye temperature and the thermal vibrations of the first two surface layers of the CdTe(110) have been studied by low energy electron diffraction (LEED). The full dynamic LEED calculations were performed using the LEEDFIT code that allows one to carry out optimizations of the Debye temperature and vibrational amplitudes of each atom in each surface layer.

SCANNING TUNNELING MICROSCOPY INVESTIGATION OF THE Si(103)- (1 × 1)–In SURFACE

In view of the special importance of the IV(103)- (1× 1)–III surface structures to the III/IV interfacial systems, in this paper the atomic structure of the Si(103)-(1× 1)–In surface is studied by means of scanning tunneling microscopy. The model that contains an indium and a silicon adatom in a unit cell, which has passed the test of low energy electron diffraction calculations, is confirmed to be correct. The dangling bond of the silicon adatom is found to be essentially empty.

Surface structure ofC(100)−(2×1)−Hstudied by a quantitative LEED analysis

The structure of the hydrogen terminated, $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed diamond (100) surface has been investigated by low-energy electron-diffraction (LEED) intensity versus energy $[I(E)]$ measurements in combination with tensor LEED calculations. It has been found that the surface corresponds to the formation of symmetric dimers on the top C layer, with a dimer length of 1.60 \AA{}. The top layer shows slight inward relaxation; the interlayer spacing between the first and second C layers reduces to 0.81 \AA{}, which corresponds to an \ensuremath{\sim}7% contraction compared to the bulk value. The structural details of the first four carbon layers have been determined and are compared to those given by theoretical calculations.

Electron inelastic mean free path, electron attenuation length, and low-energy electron-diffraction theory

Low-energy electron diffraction (LEED) theory is used for describing the electron transport in crystalline solids with the purpose of determining the electron attenuation length. The inelastic scattering of the primary electron in the electron gas of the material is introduced into the LEED theory in terms of the electron inelastic mean free path derived by Tanuma, Powell, and Penn from the Lindhard dielectric function and optical data [Surf. Interface Anal. 17, 911 (1991)]. The theorem of flux reversal for electrons in situations of inward and outward propagation is deduced from local inversion symmetry and specific boundary conditions at the sources. The theory is applied to 50\char21{}400 eV electrons incident on the three low-index surfaces of copper, and a fair agreement is found with a previous Monte Carlo simulation of the electron transport in amorphous copper. In addition to the inelastic electron-electron gas scattering, the inelastic electron-phonon scattering has a significant effect on the attenuation length in a crystalline material. The temperature parameter, necessary in a LEED calculation, does not occur in current Monte Carlo simulations. Common scattering potential models, at low energy, for LEED and for Auger electron spectroscopy and x-ray photoemission spectroscopy are discussed.

Lattice Relaxation Driven Reorientation Transition inNin/Cu(100)

The magnetic anisotropy energy of NinyCus100d is calculated in terms of the spin-polarized fully relativistic Korringa-Kohn-Rostoker method including surface relaxation by using 2D structure constants originally described for low-energy electron diffraction calculations. Investigating different relaxations, an explanation for the reorientation transition from in-plane to perpendicular can be given. For a relaxation of 25.5% (cya ­ 0.945) this reorientation occurs at about seven layers of Ni and yields second order terms to the magnetic anisotropy energy that are in excellent agreement with experiment. [S0031-9007(98)08322-7]

Structural studies of sulfur-passivated GaAs (100) surfaces with LEED and AFM

We present the results of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and atomic force microscopy (AFM) analysis of sulfur-passivating layers on the GaAs (100) surface. The GaAs surfaces were passivated with both inorganic [(NH 4) 2S x ] and organic [octadecylthiol (ODT)] S-based compounds. We prepared the inorganic sulfur-passivated GaAs(100) surfaces with a wet chemical treatment using (NH 4) 2S solution. This was followed by thermal annealing of the treated sample in an ultra-high vacuum (UHV). After ex-situ and in-situ treatments, the surface resulted in a (2×1) LEED pattern. The LEED data ( I– E curves) were recorded and compared with dynamical LEED calculations for different structural models for the sulfur-passivated GaAs (100) surface. The results showed that the sulfur-passivated (2×1) surface structure is an arsenic–sulfur dimer on a Ga-terminated substrate. The ex-situ AFM results also revealed a (2×1) structure for the inorganic passivation and a very smooth surface for the organic ODT in the ethanol-treated sample.