Ab-Initio Simulation Laboratory

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We invert high-energy (E greater-than-or-equal-to 400 eV) photoelectron and Auger-electron interference patterns to construct 3D images of surface and interface atoms. A new scheme is introduced to correct the phase shift of the image. Image reconstruction is demonstrated for Si(111) square-root 3 x square-root 3-B, a system in which multiple-scattering effects are small and all source waves are equivalent. Using diffraction results from multiple-scattering slab calculations, we achieve a spatial resolution of 1.0-1.3 angstrom, thus qualifying the technique as a direct structural tool.

We have calculated the multiple-scattering x-ray photoemission spectroscopy angular profiles of hcp Co(0001). Layer-by-layer emission contributions are presented, and the focusing directions are identified. Angular transformation of the pattern is performed to obtain real-space images of the nearest-neighbor atoms above the emitters.

Total and single-differential cross sections for electron-impact ionization are calculated in a relativistic formulation for ions in the hydrogen isoelectronic sequence: HI, He II, C VI, Ne X, Fe XXVI, and Ag XLVII. Transition amplitudes are evaluated in the two-potential distorted-wave approximation. Sets of different asymptotic charges are used to study the mutual screening of the primary and secondary electrons. Relativistic effects are investigated by taking the nonrelativistic limit and are found to increase the cross sections. Thomson’s scaling law along the isoelectronic sequence is also studied.

A multiple-energy phase-summing method is applied to diffuse low-energy electron diffraction intensity spectra to obtain three-dimensional images of surface atoms. In this demonstration, calculated DLEED intensity spectra from a multiple scattering method are directly inverted to produce high-fidelity 3D images of near-neighbor atoms measured from an adatom. No prior knowledge of adsorption site, bond length, bond angle, or type of atom is needed. The images are essentially free from artifacts and have a spatial resolution of approximately 1 angstrom when viewed along any cut-plane. These results demonstrate that holographic DLEED has the potential of being an accurate and direct structural tool for low-density disordered adsorption systems.

We present a method for spatially resolved imaging of energy-dependent photoelectron diffraction. Energy-dependent photoelectron-diffraction spectra are individually Fourier transformed to three-dimensional vector space. The complex transformed amplitudes are summed over a span of phi angles or over a span of polar angles. The images are, respectively, well resolved in the radial and azimuthal directions, or in the radial and polar directions. The intersections of these real-space maps fix the atomic coordinates. We show how the intensity loci from single and multiple scattering paths are separately resolved and how most multiple scattering contributions are eliminated. By varying the collection angles, atoms in different regions relative to the emitter, e.g., surface or bulk atoms, are imaged. One can also use the photon's A vector to enhance the near-pi backscattering geometry. We compare this method with another direct method: extended x-ray-absorption fine structure.

Field evaporation of silicon as positive and negative ions in the field ion microscope and scanning tunneling microscope configurations is investigated with the charge-exchange model using atomic potentials from an empirical potential due to Tersoff [Phys. Rev. B 37, 6991 (1988)] and an environment dependent potential developed by Bolding and Andersen [Phys. Rev. B 41, 10568 (1990)]. For the geometry of the field ion microscope, Si+ should be the observable ion species. In the close-spaced electrode geometry of the scanning tunneling microscope, Si2- should be the favored ion species since it requires the lowest evaporation field.

Energy- and angle-dependent photoelectron cross sections from surface and bulk W(110) 4f7/2 core levels are measured and compared with dynamical multiple scattering calculations. The agreement between experimental and theoretical results is found to be significantly better than corresponding previous studies, permitting a determination of the first layer atomic plane distance: d12 = 2.26 +/- 0.05 angstrom. Forward-scattering enhancements along bond directions are observed under selected scattering conditions. In all cases, final-state multiple scattering accounts for the principal energy and angle dependencies in the cross section. Typical variation of bulk and surface 4f photoelectron intensities with kinetic energy or emission angle resulting from final-state effects is observed to be a factor of 2. This result suggests that previous core-level spectra for stepped W(110) surfaces have been incorrectly interpreted.

We demonstrate that two-dimensionally resolved diffuse low-energy electron diffraction intensities can be measured with sufficient accuracy and at multiple energies to allow direct inversion for a low coverage (5%) disordered K/Ni(100) surface. The data inversion reveals three-dimensional coordinates of atoms with atom images whose full width at half maximum is less than 1 angstrom in all spatial directions. By varying the angle of incidence, first layer and second layer near-neighbor Ni atoms are separately imaged. This is the first demonstration of multiple-energy internal-source electron holography using measured elastically backscattered electrons.

We demonstrate a direct surface structural tool with high resolution of approximately 1 angstrom in all directions by direct Fourier transformation of measured Kikuchi patterns using a multiple-energy phase-summing method. In this method, with an integral over continuous energy spectra in each direction, both the forward- and backward-scattering oscillations are selected for Fourier transformation by varying the energy range and size of the grid. High-fidelity and artifact-free three-dimensional images of Ag atoms for (100) and (111) single-crystal surfaces are obtained.

We propose in this paper a theoretical model to investigate surface self-diffusion of single adatoms on the face-centered-cubic metals. Calculations are performed on the channeled (110), densely packed (111) and loosely packed (001) surfaces of iridium at temperature T = 800 K. Three realistic model potentials, Embedded Atom method, Sutton-Chen and Rosato-Guillope-Legrand potentials, are applied to describe the interatomic interaction of the adatom/substrate systems. These potentials all involve a few empirical fittings of bulk properties of solid which incorporate with many-body effects. With these potentials, conventional molecular dynamics (MD) is employed to obtain trajectories of the atoms. On the (111) plane, via the Einstein relation, the estimated random walk exponential prefactors and activation energies do exhibit Arrhenius behavior, which are in reasonably good agreement with the experimental results. On the (001) and (110) faces, a number of theoretical evidences for atomic diffusion by exchange mechanism of the adatom with a surface atom are presented, which are again in fairly good agreement with the experiments. In addition, an examination of the exchange diffusion characteristics on several systems (Cu, Rh and Pt) is also presented.

We obtained highly resolved and artifact-free 3D holographic images reconstructed from measured Kikuchi electron (quasi-elastic electron) diffraction patterns with contributions from different emitters. Direct inversion of Kikuchi patterns with glancing incidence geometry shows clear images of the surface dimer and the bulk atoms of Si(001)(2 x 1) surface. This observation demonstrates the applicability of electron-emission holography to complicated systems that contain more than one emitter. This work also demonstrates the surface sensitivity of Kikuchi electron holography.

Direct inversion of measured Kikuchi and simulated photoelectron diffraction patterns shows clear images of the neighboring atoms within the range of the electron mean free path. More than ten nearby atoms are obtained for the Ag(100), Si(100) and (2X1) Na/Si(100) systems by the integral-energy phase-summing method. The key point in removing artifacts is a correct role of background subtraction. When this is achieved, the three-dimensional images are essentially high fidelity and artifact free. This demonstrates that electron-emission holography can be used as a direct local structural probe.

Single differential and total cross sections of the electron-impact ionization of the hydrogen atom are calculated numerically in the two-potential distorted-wave approximation for excess energies from 0 to 1 eV. Partial-wave contributions to the cross sections are also investigated. The near-threshold cross section is parametrized by the power gamma and the proportionality constant a(0) for models with various asymptotic charges, and the dependence of a(0) on the asymptotic charge is studied. The validity range of the threshold power law is also discussed.

Direct inversion of measured multiple-energy Kikuchi electron patterns from a Si(001) (2 x 1) surface with glancing and normal-incidence geometry shows clear images of the surface dimer and the bulk atoms. The three-dimensional artifact-free real-space images of the atoms contributed from different local emitters are resolved clearly. The observations demonstrate that Kikuchi electron holography has the surface sensitivity and can reveal the atomic structures of complicated multiemitter systems. By changing the collecting angle of Kikuchi electrons, one can selectively image the atoms behind the emitter in the backward direction; thus the surface and the bulk information can be obtained with different collecting angles. Therefore, the potential of Kikuchi electron holography to solve the local atomic structure of the unknown surfaces is high.

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Following our previous work, we carry out a more detailed study of dissociation dynamics of diatomic ions field-evaporated from a metallic tip of field-ion microscopy, We have found that the partial fractal behavior of the dissociation probability near threshold field strength and the multiple peaks in the time-of-flight spectrum can be attributed to different combinations of vibrational and rotational cycles before ions dissociate, But more clear revelation of the origin of the fractal behavior comes from investigating the uniform-field limit, where we show that two types of chaos exist, one is associated with the rotational saddle orbit and the other associated with the moving potential barrier of a DC field-distorted anharmonic oscillator, The homoclinic tangles associated with these saddles give rise to the sensitive dependence of dynamics on initial conditions.

We report the first atomically resolved images of ordered Au trimers on Si(111)root 3X root 3R30 degrees-Au using wave-front reconstruction of scanned-energy glancing-angle Kikuchi electron spectra. Each Au image has a resolution (full width at half magnitude) of less than 1 Angstrom. The images indicate that Au trimers art ordered and nonrotated within the surface plane and with respect to the second-layer Si plane providing direct evidence of the conjugate honeycomb-chained-trimer model for the Au-root 3 system.

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We used the embedded-atom method potential to study the structures, adsorption energies, binding energies, migration paths, and energy barriers of the Ir adatom and small clusters on fcc Ir (100), (110), and (111) surfaces. We found that the barrier for single-adatom diffusion is lowest on the (111) surface, higher on the (110) surface, and highest on the (100) surface. The exchange mechanisms of adatom diffusion on (100) and (110) surfaces are energetically favored. On all three Ir surfaces, Ir-2 dimers with nearest-neighbor spacing are the most stable. On the (110) surface, the Ir-2 dimer diffuses collectively along the (110) channel, while motion perpendicular to the channel walls is achieved by successive one-atom and correlated jumps. On (111) surface, the Ir-2 dimer diffuses in a zigzag motion on hcp and fee sites without breaking into two single atoms. On the (100) surface, diffusion of the Ir-2 dimer is achieved by successive one-atom exchange with the substrate atom accompanying by a 90 degrees rotation of the Ir-2 dimer. This mechanism has a surprisingly low activation energy of 0.65 eV, which is 0.14 eV lower than the energy for single adatom exchange on the (100) surface. Trimers were found to have a one-dimensional (1D) structure on (100) and (110) surfaces, and a 2D structure on the (111) surface. The observed abrupt drop of the diffusion barrier of tetramer, I-gamma 4 on the Ir (111) surface was confirmed theoretically.

The structural bases on the metal/semiconductor interfaces, such as gold trimers on the Au/Si (111)(root 3 x root 3)R30 degrees surface and antimony trimers on the Sb/Si(111)(root 3 x root 3)R30 degrees surface, can be imaged directly with a simple inversion of low-energy (<600 eV) Kikuchi-electron patterns (Kikuchi-electron holography-KEH). The relative positions of the building blocks (trimers) on the adsorbates to the substrate atoms are also determined. This short-range-order KEH tool, which provides the 3D Patterson function, can be viewed as a twin of grazing-incidence X-ray diffraction. Using direct structural information obtained by KEH, one can greatly reduce the tested models in a complete trial-and-error structural-determination process.

The presence of sulfur atoms on the Pd(100) surface is known to hinder the dissociative adsorption of hydrogen. Using density-functional theory and the full-potential linear augmented plane-wave method, we investigate the potential energy surface (PES) of the dissociative adsorption of H-2 On the sulfur covered Pd(100) surface. The PES is changed significantly compared to the dissociation on the clean Pd(100) surface, particularly for hydrogen close to the S atoms. While the hydrogen dissociation at the clean Pd(100) surface is nonactivated, for the (2 x 2) sulfur adlayer (coverage Theta (S) = 0.25) the dissociation of H-2 is inhibited by energy barriers. Their heights strongly depend on the distance between the hydrogen and sulfur atoms leading to a highly corrugated PES. The largest barriers are in the vicinity of the sulfur atoms due to the strong repulsion between sulfur and hydrogen. Still the hydrogen dissociation on the (2x2) sulfur covered Pd(100) surface is exothermic. Thus the poisoning effect of sulfur adatoms for H-2 dissociation at low sulfur coverage (Theta(S) less than or equal to 0.25) is mainly governed by the formation of energy barriers, not by blocking of the adsorption sites. For the c(2 x 2) sulfur adlayer (Theta(S)= 0.5), the PES for hydrogen dissociation is purely repulsive. This is due to the fact that for all different possible adsorption geometries the hydrogen molecules come too close.to the sulfur adatoms before the dissociation is completed.

Photoelectron-diffraction studies of Nb(001) have been performed to determine the first-interlayer contraction using Mg K alpha radiation (h nu = 1253.6 eV) as an excitation source. Photoemission intensities of the 3d(5/2) core level were measured as a function of the polar angle along several azimuths on the single-crystal surface. The 202.3-eV (205.0-eV) binding energy for the 3d(5/2) (3d(3/2)) core level was well resolved in the photoemission spectra, where the peak intensity could be easily evaluated by curve-fitting processes. Large oscillations of the 3d(5/2) intensity as a function of the polar angle due to forward-focusing were observed. Based on multiple-scattering calculations for several first-interlayer spacings ranging from the bulk value to 16% contraction, the best agreement with experiment was obtained for a (13 +/- 5)% contraction of the first-interlayer spacing.

We report calculations of the dissociative adsorption of H-2 at Pd (100) covered with 1/4 monolayer of sulfur using quantum dynamics as well as molecular dynamics and taking all six degrees of freedom of the two H atoms fully into account. The ab-initio potential-energy surface (PES) is found to be very strongly corrugated. In particular, we discuss the influence of tunneling, zero-point vibrations due to the localization of the wave function of the nuclei when narrow valleys of the PES are passed, steering of the approaching H-2 molecules towards low-energy barrier configurations, and the important role of subsurface absorbates for the hydrogen dissociation. It is shown that "established" concepts derived from low-dimensional dynamical studies are not necessarily valid in a high-dimensional treatment. (C) 1998 Elsevier Science B.V. All rights reserved.

Atomic structures of the reconstructed Si(113) surfaces were studied by using Kikuchi electron holography (KEH). Three-dimensional images show clearly the characteristics of the puckering model for both Si(113)(3x2) and (3x1) surfaces. The KEH results support the puckering model. Based on bur studies, the tetramers are puckering alternatively in the (3x2) surface. Whereas in (3X1) structures, there are two domains, within each of them, tetramers buckled uniformly, but the overall directions are opposite. When doped with H atoms on a (3x2) surface, the asymmetric tetramers change into symmetric form. [S0163-1829(99)51116-8].

More than 50 symmetry-inequivalent emitter-scatterer (E-S) pairs were observed for the Si(111)-(7 x 7) surface using Kikchi electron holography (KEH) with various incident/detection configurations. For different configurations, the E-S pairs in the backscattering direction are preferentially enhanced. Thus, one can obtain detailed structural information by changing the incident/detection direction, which will be helpful in sorting out the correct surface structure model.

The adatoms, dimers and rest atoms in the three outermost atomic layers of the Si(111)-(7 x 7) surface are directly imaged with glancing Kikuchi electron holography. The applicability of Kikuchi electron holography to complicated multiple-emitter surfaces is evident. The three-dimensional relative positions of atoms on the Si(111)-(7 x 7) surface are in good accordance with the LEED-optimized DAS model.

We have observed simple oscillations in three-dimensional (3D) patterns of electron thermal diffuse scattering (separated from electron-electron energy loss) measured on a Si(001) surface. We interpret these oscillations as coherent interference within a small cluster of atoms in which vibrational correlation within the nearest neighbors (NN) is dominant. A 3D Patterson function analysis of the oscillation reveals the atomic structure of the Si(001) surface consisting of NN pairs including dimers. This finding provides a promising new clue to determine the structures of bulk and the surface of solids.

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Trimer is the smallest cluster that can have a one-dimensional or a two-dimensional structure on fee (111) surface. Using first-principles density-functional-theory calculations, the structural and dynamical properties of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has four close-packed (compact) triangular configurations, two linear configurations, and some other noncompact triangular configurations. The close-packed triangular trimers are more stable than the noncompact triangular trimers as well as the linear trimers. For the dynamics of Al trimer on Al(111) surface, the diffusion processes are much more complicated than the adatom and dimer diffusions. There are three different kinds of diffusion mechanisms: concerted translations and rotation of compact triangular trimers (the energy barrier

A Patterson-like scheme is proposed for direct inversion of the conventional low-energy electron diffraction (LEED) intensity versus energy I(E) curves, which is in contrast with the previously suggested holographic scheme. Using the Si(111)-(7 X 7) and Si(113)-(3 X 2) surfaces as examples, high quality three-dimensional images, with a resolution better than 0.5 Angstrom, of both surface atoms and bulk atoms are obtained from the direct Patterson inversion of LEED-I(E) curves with the integral-energy phase-summing method.

Several long-range superstructures have been observed with the scanning tunneling microscopy on the reconstructed Pt(100) surface at room temperature. They are present in strained domains and involve both the Fermi electrons and the concomitant lattice distortions. A first-principles calculation shows that the top layer expanded similar to 18% on average and the Fermi surface for a single hexagon layer displays some nesting portions, which can be related to the wave vectors of the observed superstructures. Thus, these superstructures existing in the local domains of the reconstructed surface have the likely origin of incipent charge density waves.

A simple oscillatory intensity variation in medium-energy electron diffraction found recently [Abukawa ei al., Phys. Rev. Lett. 82, 335 (1999)] was termed correlated thermal diffuse scattering (CTDS). The potential of CTDS as a direct surface structural tool has been fully explored for the Si(001)2 X 1 surface at 300 K in a very-grazing-incidence condition. Nearly 2 pi solid-angle, three-dimensional (3D) CTDS patterns were measured for an energy range of 500-2000 eV. The 3D Patterson functions obtained by Fourier inversion of the measured CTDS patterns clearly revealed the building blocks of the Si(001)2 X 1 surface, i.e., the bond orientations and lengths of the buckled Si dimers, within an accuracy of 1 degrees and 0.1 Angstrom, respectively.

We use ab initio density-functional theory supplemented with the embedded-atom method to study the self-diffusion of small clusters on the (111) surface of eight fee metals. A zigzag motion is found to be important in the dimer and tetramer diffusions. The dimer diffuses by a zigzag and concerted motion. The trimer diffuses by a concerted three-atom motion. The tetramer diffuses through a zigzag motion where only two atoms move simultaneously in each step. Thus, instead of increasing, the migration energy is lowered (or stays constant) for the tetramer as compared to that for the trimer. This novel break of the upwards trend in migration energy is predicted to be a general phenomenon.

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Trimer is the smallest cluster that can have a one-dimensional or two-dimensional structure on surfaces, and it can diffuse and transform between these structures. Using first-principles density-functional theory (DFT) calculations, the structural and dynamical behaviors of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has three different kinds of structure conformations (groups with similar configurations): close-packed (compact) triangular trimers, non-compact triangular trimers, and linear trimers. The close-packed triangular trimers are more stable than the non-compact triangular trimers and the linear trimers, while most of the non-compact triangular trimers are as stable as the linear trimers. For the dynamics of Al trimer on Al(111) surface, there are three different kinds of diffusion mechanisms: (1) concerted translations and rotation of compact triangular trimers (the highest energy barrier by DFT calculation

Using ab initio calculations, we fmd that the calculated energy barrier for exchange diffusion of Ni adatoms on Ni(100) surfaces shows a surprisingly large dependence on the size of the surface unit cell. It decreases from 1.39 to 0.78 eV when the cell size changes from (2 x 2) to (6 x 6). This is due to the long-ranged strain field created by the transition state for atomic exchange, which needs a larger cell to relax. The hopping diffusion energy, on the other hand, shows only a very small size effect and remains approximately constant at 0.82-0.86 eV, independently of the cell size. Our results indicate that Ni diffusion on Ni(100) occurs by the exchange mechanism and this is consistent with recent experiments. Previous results obtained using (3 x 3) or (4 x 4) unit cells did not converge sufficiently well to yield correct conclusions.