Ab-Initio Simulation Laboratory

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Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 x 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)’5.5 x 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates similar to 20 Cu atoms with similar to 20 Si atoms being expelled from the corresponding 7 x 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu2Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 x 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 x 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images. (C) 2009 Elsevier B.V. All rights reserved.

In this study, the 13-atom cluster structures of alkaline metals, alkaline-earth metals, boron group metals, carbon group metals, and 3d, 4d, and 5d transition metals in the periodic table are investigated by density functional theory with three kinds of exchange-correlation (XC) functionals: (i) local-density approximation (LDA); (ii) generalized gradient approximation (GGA) with Perdew-Wang 91; and (iii) generalized gradient approximation with Perdew-Burke-Ernzerhof. The dependence on pseudopotentials (PPs) with and without semicore electrons is also examined. The relative energies of five selected high-symmetry three-dimensional and four low-symmetry layer-type isomers for each element of interest are calculated and studied. Among the 44 metallic 13-atom clusters, our results show that the two GGA XC functionals have a great consistency; LDA and GGA results also reveal a great consistency, apart from the Cr, Mn, Fe, Co, Ni, and Rh 13-atom clusters, for which the results show a significant difference. Meanwhile, for most of the elements, the calculations with and without semicore PPs also produce consistent results, except for Cr, Mo, and V, which require a careful treatment of semicore states in the PPs.

To assess the accuracy of exchange-correlation approximations within density functional theory (DFT), diffusion quantum Monte Carlo (DMC) and DFT methods are used to calculate the energies of isomers of three covalently bonded carbon and boron clusters (C(20), B(18), and B(20)), and three metallic aluminum and copper clusters (Al(13), Al(55), and Cu(13)). We find that local and semilocal DFT methods predict the same energy ordering as DMC for the metallic clusters but not for the covalent clusters, implying that the DFT functionals are inadequate in such systems. In addition, we find that DFT fails to describe energy reductions arising from Jahn-Teller distortions..

The clean-off reaction of AgO added rows by CO on Ag(110) and Au/Ag(110) bimetallic surfaces was studied by scanning tunneling microscopy (STM) and compared with density functional theory (DFT). This combined study of a model system illustrated the complexity of catalytic enhancement in bimetallic systems. By analyzing in situ time-lapsed STM image series, we found that CO oxidation on a Au-enriched Ag(110) surface leads to an exponential depletion of oxygen with time and a reaction rate that is synergistically enhanced by the presence of Au. First principles calculations indicate that the local atomic configuration around the active reaction sites at the chain ends and the preference of An atom substitution into the subsurface second Ag layer are of critical importance. By calculating CO adsorption energies and reaction barriers for plausible reaction pathways, a detailed description of the CO oxidation reaction emerges, For the optimal reaction pathway, a large (similar to 0.09 eV) barrier reduction and a small barrier of similar to 0.01 eV were found for the Eley-Rideal (ER) mechanism. In contrast, a small (similar to 0.03 eV) barrier reduction and a moderate barrier of similar to 0.23 eV were obtained for the Langmuir-Hinshelwood (LH) mechanism. The ER transitional state was also found to be lower in energy. We conclude that, irrespective of whether the ER mechanism is actually rate dominating, it is definitively enhanced.

Whether photoemission probes surface or bulk properties has long been a topic of interest and debate. This work employs angle-resolved photoemission to map the electronic structure of Ag films of varying thicknesses prepared on Si(111). As expected, the discrete quantum-well states or subbands observed at small thicknesses merge into a continuum as the film thickness approaches the bulk limit. However, a number of discrete states remain isolated within gaps or pockets in the bulk continuum. While these Ag surface states have been predicted previously by calculations, most are experimentally identified herein only for the first time. Copyright (C) EPLA, 2009

The adsorption and desorption of stilbene on Ag/Ge(111)-(root 3 x root 3)R30 degrees (Ag/Ge(111)-root 3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (I)FT). Both trans- and cis-stilbenes form a (2 x 1) overlayer structure on Ag/Ge(111)-root 3 at a coverage of similar to 1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than I ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-root 3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-root 3 is proposed and discussed.

Experimentally, self-organized Ag planar clusters have been observed on the periodic template found on the Pb quantum islands, which are grown on the Si(111) surface. These planar clusters register a remarkable abundance variation at some specific atomic numbers and possess enhanced stability. They are thus denoted as two-dimensional magic Ag nanoclusters (or nanopucks). In this work, detailed calculations based on ab initio density functional theory are made to illuminate how the size and shape effects related to electronic confinement influence the sequence of these two-dimensional Ag nanostructures. The simulation results demonstrate that the evolution of a sequence of planar magic Ag clusters is strongly correlated with their electronic structures. Meanwhile, the role of substrate in the formation of magic Ag clusters is also examined. The symmetry and size of the periodic pattern on the substrate have helped to build up the distinguishable geometric structures in experiment. Further analysis of the related electronic and geometrical properties of these clusters not only explains the occurrence and sequence of the magic numbers but also helps to elucidate the mechanism of their formation.

Using density functional calculations, we demonstrate a catalytic reaction path with activation barriers of less than 0.5 eV for CO oxidation on the neutral and unsupported icosahedral nanoclusters of Au(55), Ag(55), and Au(25)Ag(30). Both CO and O(2) adsorb more strongly on these clusters than on the corresponding bulk surfaces. The reaction path consists of an intermediate involving OOCO complex through which the coadsorption energy of CO and O(2) on these clusters is expected to play an important role in the reaction. Based on the studies for the Au and Ag nanoclusters, a model alloy nanocluster of Au(25)Ag(30) was designed to provide a larger coadsorption energy for CO and O2 and was anticipated to be a better catalyst for CO oxidation from energetic analysis. (C) 2008 American Institute of Physics.

Electrical conduction mechanism in ultrathin Pb (111) films formed on the Si(111)root 3x root 3-Pb surface has been investigated by means of in situ conductivity measurements, angle-resolved photoemission spectroscopy, and first-principles calculations. To investigate the origin of the bilayer oscillation observed in the present conductivity measurement, we perform some simulations based on the calculated band structure. They reveal that the density of states near the Fermi level cannot explain the bilayer oscillation, therefore, exclusively assigning it to the relaxation time. Surface roughness during the bilayer film growth seems to play a crucial role in the bilayer oscillation of the relaxation time.

We report first-principles calculations of Pb (100) films up to 22 monolayers to study variations in the surface energy and work function as a function of film thickness. An even-odd oscillation is found in these two quantities, while a jelliumlike model for this s-p metal predicts a periodicity of about three monolayers. This unexpected result is explained by considering a coherent superposition of contributions from quantum-well states centered at both the Gamma and M points in the two-dimensional Brillouin zone, demonstrating the importance of crystal band structure in studying the quantum size effect in metal thin films.

Using scanning-tunneling microscopy and first-principles total-energy calculations, we have considered the structural properties of the so-called doped clusters formed by depositing additional 0.05 monolayer of In onto the 4x3-periodicity magic-cluster array in the In/Si(100) system. Low-temperature STM observations have revealed that most of the doped clusters have an asymmetric shape. According to the total-energy calculations, these clusters have plausibly Si6In8 composition. In such a cluster, one of the In atoms is mobile and can hop between four equivalent sites within a cluster. The hopping between sites, located in the different 2ax3a halves of the cluster, is characterized by the barrier of about 0.7 eV, and this hopping becomes frozen at 55 K. In contrast, the hopping between the neighboring sites within the same cluster half persists up to very low temperatures, as the barrier height here is an order of magnitude lower. Due to the above structural properties, the doped asymmetric Si6In8 cluster can be treated as a promising switch, logic gate, or memory cell of the atomic-scale size.

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.

We have used ab initio methods to study the possible transition between icosahedral (ico) and cuboctahedral (fcc) structures in lead nanoclusters of sizes up to 309 atoms. Spontaneous fcc-to-ico transition in Pb-13 was observed in the ab initio molecular dynamics (MD) simulations at various temperatures. The transition path can be described predominantly by an angular variable s, which can, generally be applied to the similar transitions in clusters of larger sizes and was observed to follow the Mackay model. We have calculated the two-dimensional energy surface that describes the transition in Pb-13 and found a barrierless fcc-to-ico transition path, which is consistent with the observed spontaneous transition in the ab initio MD simulations. The atomic displacements in the transition were identified as one of the vibrational eigenmodes of these two Pb-13 clusters. For clusters of larger sizes (Pb-n, where n = 55, 147, and 309), the possible transitions following similar paths were determined not to be barrierless and the sizes of the barriers were determined by the ab initio elastic band method.

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Hydrogenation-induced metal-semiconductor transitions usually occur in simple systems based on rare earths and/or magnesium, accompanied by major reconstructions of the metal host (atom shifts >2 Angstrom). We report on the first such transition in a quaternary system based on a transition element. Metallic LaMg2Ni absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 which has a nearly unchanged metal host structure (atom shifts <0.7 Angstrom). The transition is induced by a charge transfer of conduction electrons into tetrahedral [NiH4](4-) complexes having closed-shell electron configurations.

Using ab initio density-functional theory, the self-diffusion of adatom and dimer on fcc(100) metal surfaces are studied. For adatom diffusion, we find that the exchange mechanism is favored for Al, Ir, Ni, Pd, Pt and Au, while the hopping mechanism is favored for Rh, Cu, and Ag. Except for Ir/Ir(100), the exchange diffusion energy has a surprising large size-effect and decreases as the surface unit cell increases. This is due to the long-ranged strain-field created at the exchange transition state, which needs a larger cell to relax. The hopping diffusion energy, on the other hand, has a very small size-effect and keeps approximately the same value for various surface unit cells. For self-diffusion on lr(100), the formation of covalent bonds are found at the exchange transition state, and thus the exchange diffusion energy has a little size-effect. Our results also indicate that the exchange mechanism is energetically more favorable for dimer diffusion on fcc(100) surface whenever it is favored for adatom diffusion on fcc(100) surface.

Previous energetic considerations have led to the belief that carbon nanotubes (CNTs) of 4 Angstrom in diameter are the smallest stable CNTs. Using high-resolution transmission electron microscopy, we find that a stable 3 Angstrom CNT can be grown inside a multiwalled carbon nanotube. Density functional calculations indicate that the 3 Angstrom CNT is the armchair CNT(2,2) with a radial breathing mode at 787 cm(-1). Each end can be capped by half of a C(12) cage (hexagonal prism) containing tetragons.

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.

Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N=5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N=6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties.

The structure of a type of surface magic cluster is determined by a combination of scanning tunneling microscopy, density-functional calculations, and dynamical low energy electron diffraction. The diffraction method is applicable because these clusters created through hierarchical self-organization of Ga deposited onto a Si(111)-7x7 surface have identical size and structure and form an ordered array with exact translational symmetry. The unprecedented detailed structure information provided by the diffraction measurement is consistent with direct microscopic imaging and theoretical calculations.

We have studied the properties of quantum well states in supported Ag(100) films on the Fe substrate by first-principles density-functional calculations. The energies of these quantum well states as a function of thickness N are examined in terms of the characteristic phase shift of the electronic wave function at the interface. These energy-dependent phase shifts are determined numerically for both the film-substrate and film-vacuum interfaces. It is also found that the substrate has a major effect on film stability, enhancing the stability of the N=5 film and reversing that of the N=2 film.

We present first-principles calculations that provide a detailed diffusion picture of an adsorbed Si atom on the Si(111)-(7x7) surface. Several diffusion paths for the adsorbed Si atom are established by mapping out the total energy as a function of its positions on the surface. For diffusion between the faulted and unfaulted halves, the energy barriers range from 0.96 to 1.21 eV, while remarkable low-energy barriers from 0.3 to 0.7 eV are discovered within the faulted and unfaulted regions.

Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.

Three-dimensional atomic images of a Pt(111) surface are obtained by direct inversion of multiple low-energy Kikuchi electron-diffraction patterns. The images are in the backscattering direction. and the positions of the images are consistent with those expected from the atomic structure near the Pt(111) surface. The strong electron scattering of the Pt atoms causes no observable problems in the Kikuchi electron holography.

The work function of atomically uniform Ag films grown on Fe(100) is measured as a function of film thickness. It shows layer-resolved variations as a result of quantum confinement of the valence electrons. A first-principles calculation reproduces the observed variations except for very thin films (one and two monolayers), and the differences can be attributed, in part, to strain effects caused by the lattice mismatch between Ag and Fe. These results illustrate the close interaction between interface effects and surface properties.

We have carried out first-principles calculations of Pb (111) films up to 25 monolayers to study the oscillatory quantum size effects exhibited in the surface energy and work function. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. It is found that a quantitative description of these quantum size effects requires a full consideration of the crystal band structure.

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

We find that electron diffraction patterns can be directly inverted to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either holography or a Patterson inversion scheme is used. For diffraction patterns which were generated from a localized emitter source or predominantly by an inelastic-scattering feature like low-energy Kikuchi electrons, holography inversion is needed. The information obtained from Kikuchi electron holography includes the building blocks on the surface and their relative position to the atoms below the surface layer. On the other hand, for diffraction patterns generated predominantly by an inelastic-scattering feature like low-energy electron diffraction (LEED), a Patterson inversion is needed. The information obtained from the Patterson transform of the LEED I(E) curves is the relative positions of surface atoms to the atoms in underlying layers; no intra-layer information can be extracted with this method. High-fidelity and artifact-free three-dimensional atomic structures obtained by inversion of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The results from the two inversion methods are complementary and can be used to construct or to discriminate the surface atomic structural models. The future of these direct methods by inverting diffraction patterns is discussed.

A comprehensive atomic model for the reconstructed surface of Si3N4 thin layer grown on Si(lll) is presented. Kikuchi electron holography images clearly show the existence of adatoms on the Si3N4(0001)/Si(111)-(8 x 8) surface. Compared with the nb initio calculations, more than 30 symmetry-inequivalent atomic pairs in the outmost layers are successfully identified. Scanning tunneling microscopy (STM) images show diamond-shaped unit cells and nine adatoms in each cell. High-resolution STM images reveal extra features and are in good agreement with the partial charge density distribution obtained from total-energy calculations.

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.

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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

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.

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.

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.

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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

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.