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See detailPotential energy landscape of monolayer-surface systems governed by repulsive lateral interactions: The case of (3 x 3)-I-Pt(111)
Tkatchenko, Alexandre UL; Batina, Nikola; Galvan, Marcelo

in PHYSICAL REVIEW LETTERS (2006), 97(3),

Combined density functional theory (DFT) and Monte Carlo (MC) approach is applied to study the potential energy landscape of four iodine atoms adsorbed on the Pt(111) surface in a (3x3) unit cell. Three ... [more ▼]

Combined density functional theory (DFT) and Monte Carlo (MC) approach is applied to study the potential energy landscape of four iodine atoms adsorbed on the Pt(111) surface in a (3x3) unit cell. Three critical points were identified: (3x3)-sym and (3x3)-asym, corresponding to structures well known from experimental studies, while the third one (3x3)-zigzag is a new structure not reported before. An interaction model fitted to DFT calculations allows us to explain the difference between arrangements of iodine monolayer in vacuum, air, and solution environments as a result of different repulsion regimes. [less ▲]

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See detailAdsorption of Ar on graphite using London dispersion forces corrected Kohn-Sham density functional theory
Tkatchenko, Alexandre UL; von Lilienfeld, O. Anatole

in PHYSICAL REVIEW B (2006), 73(15),

Using Kohn-Sham (KS) density functional theory, the adsorption of Ar on graphite has been computed with various conventional exchange-correlation functionals. While the local density approximation yields ... [more ▼]

Using Kohn-Sham (KS) density functional theory, the adsorption of Ar on graphite has been computed with various conventional exchange-correlation functionals. While the local density approximation yields a reasonable estimate of equilibrium distance and energy, three generalized gradient approximated functionals fail. Extending the KS Hamiltonian by an empirical nonlocal and atom-centered potential enables quantitative predictions. The adsorption on the on-top, hollow, and bridge sites has been investigated, and it is found that the London dispersion corrected calculations prefer the hollow site which is in agreement with other studies. Furthermore, the adsorption effect of several submonolayer coverages and of the graphitic bulk has been studied. [less ▲]

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See detailRole of high-order Fourier terms for stability of monolayer-surface structures: Numerical simulations
Tkatchenko, Alexandre UL

in Physical Review B - Condensed Matter and Materials Physics (2006), 74(23),

The role of high-order atom-surface Fourier terms is analyzed for the monolayer with coverage θ= 3 7 on (111) surface in cells with variable number of adsorbate atoms, allowed to relax to obtain the ... [more ▼]

The role of high-order atom-surface Fourier terms is analyzed for the monolayer with coverage θ= 3 7 on (111) surface in cells with variable number of adsorbate atoms, allowed to relax to obtain the global minimum in each of the unit cells. A Fourier expansion with one or two shells of reciprocal cell vectors is used and three different models for the lateral interactions in the monolayer are tested, from purely repulsive to a real HFD-B2 potential. It is found that the simple commensurate (7x7) R19.1° three-atom structure is the most stable only when the contribution of the second Fourier term is included. In contrast to the conventional view, higher corrugation of the single-term Fourier model favors incommensurability. Evidence is collected that the high-order Fourier terms are mandatory for the stabilization of commensurate structures of an infinite monolayer. © 2006 The American Physical Society. [less ▲]

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See detailClassification of hexagonal adlayer arrangements by means of collective geometrical properties
Tkatchenko, Alexandre UL; Batina, N.

in Journal of Chemical Physics (2006), 125(16),

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between θ= 1 3 and θ=1 on the hexagonal substrate, with atomic radius of ... [more ▼]

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between θ= 1 3 and θ=1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on (111) surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results (i.e., scanning-tunneling microscopy images of atomic adlayers). © 2006 American Institute of Physics. [less ▲]

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See detailAnalytic theory of hexagonal monolayer interacting with hexagonal substrate
Tkatchenko, Alexandre UL

in PHYSICAL REVIEW B (2006), 74(3),

Analytic theory of the infinite rigid hexagonal monolayer interacting with hexagonal substrate is developed. The interaction potential is described by a Fourier series with up to six shells of the ... [more ▼]

Analytic theory of the infinite rigid hexagonal monolayer interacting with hexagonal substrate is developed. The interaction potential is described by a Fourier series with up to six shells of the reciprocal cell vectors. It is rigorously shown that energy of minimum structures is directly related to their symmetry, assuming a decaying behavior of Fourier coefficients. Preliminary comparison with real systems of halogens and alkalis adsorbed on metal surfaces is encouraging and indicates that the adsorbate-substrate interaction is one of the main driving forces in the monolayer formation. The need of detailed ab initio calculations to obtain the Fourier coefficients for different complex adsorption systems is emphasized. Our results indicate the importance of accurate description of the adsorbate-substrate interactions for quantitative theory of monolayer epitaxy on well-ordered surfaces. [less ▲]

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See detailUnequal-sphere packing model for simulation of the uniaxially compressed iodine adlayer on Au(111)
Tkatchenko, Alexandre UL; Batina, Nikola

in JOURNAL OF PHYSICAL CHEMISTRY B (2005), 109(46), 21710-21715

A simple unequal-sphere packing (USP) model, based on pure geometrical principles, was applied to study the centered-rectangular iodine c(p x root 3)R30 degrees adlayer on the Au(111) surface, well-known ... [more ▼]

A simple unequal-sphere packing (USP) model, based on pure geometrical principles, was applied to study the centered-rectangular iodine c(p x root 3)R30 degrees adlayer on the Au(111) surface, well-known from surface X-ray structure (SXS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) experiments. To reproduce the exact patterns observed in experiments, two selective conditions-minimum average adsorbate height and minimum adlayer roughness-were imposed. As a result, a series of adlayer patterns with c(p x root 3)R30 degrees symmetry (2.3 < p < 3), with precise structural details, including atomic registry and identification of the p-bisector as the most likely trajectory for the iodine adatom movement during the so-called uniaxial compression phenomenon, were identified. In addition, using the same model, the difference between the iodine adlayer arranged in hexagonal and centered-rectangular c(p x root 3)R30 degrees patterns, as in the case of Pt(111) and Au(111) surfaces, was investigated. Qualitative and quantitative comparison shows that iodine adatoms in these two arrangements differ significantly in atomic registry, distance from the substrate, and the adlayer corrugation. Our findings could be of special interest in the study of the nature of the iodine adatom bonding to different substrates (i.e., Au vs Pt). [less ▲]

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See detailDetailed characterization of (3X3) iodine adlayer on Pt(111) by unequal-sphere packing model
Tkatchenko, Alexandre UL; Batina, Nikola

in JOURNAL OF CHEMICAL PHYSICS (2005), 122(9),

A simple unequal-sphere packing model is applied to study the iodine (3x3) adlayer on the Pt(111) surface. By using a newly introduced parameter, defined as the average adsorbate height, three ... [more ▼]

A simple unequal-sphere packing model is applied to study the iodine (3x3) adlayer on the Pt(111) surface. By using a newly introduced parameter, defined as the average adsorbate height, three characteristic adlattices, (3x3)-sym, (3x3)-asym, and (3x3)-lin, have been selected and characterized in great detail, including the exact adatom registry. The (3x3)-sym iodine adlattice, observed in many experimental studies appears to be, on average, the closest one to the substrate surface. A special contour plot of average adsorbate height vs X and Y positions of the (3x3) iodine unit cell indicates the existence of two local minima which are related to preferential formation of (3x3)-sym and (3x3)-asym iodine adlattices. Our model gives good agreement with experimental findings, and explains the mechanism of preferential appearance of (3x3)-sym and (3x3)-asym structures. (C) 2005 American Institute of Physics. [less ▲]

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See detailUnequal-sphere packing model for the structural arrangement of the well-ordered adsorbate-substrate system
Tkatchenko, Alexandre UL; Batina, Nikola

in Physical Review. B: Condensed Matter and Materials Physics (2004), 70(19), 1-8

In order to understand the well-ordered adsorbate-substrate systems at atomic level, a method is developed based on the simulation of packing arrangements for layers of unequal spheres, in three ... [more ▼]

In order to understand the well-ordered adsorbate-substrate systems at atomic level, a method is developed based on the simulation of packing arrangements for layers of unequal spheres, in three-dimensional space. The model, based on geometrical principles, is developed for fee structure consisting of two hexagonal ordered layers. During simulation, adsorbate spheres were accommodated in different positions, forming a great variety of structures, in dependence of the intersphere distance of the upper layer spheres. Using the average height of the adsorbate layer on the flat substrate as a determinant parameter, several specific structures have been selected as the most probable: (√3 x √3)R30°, (√7 x √7)R19.1°, and (3 x 3). Indeed, they correspond to typical accommodations of the iodine adatoms on the Pt(111) surface, earlier found in experimental studies, which clearly supports the validity of our model. The model developed in our study could completely and satisfactorily describe the accommodation process of the iodine adlayer on the Pt(111) surface. This methodology could be of great help for interpretation of scanning tunneling microscopy images, better understanding of adlayer structures, and design of adsorbate-substrate systems with exciting properties. [less ▲]

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