Coverage-dependent structural phase transformations in the adsorption of pentacene on an aperiodically modulated Cu film.

Surface ordering of pentacene molecules adsorbed on an aperiodic Cu surface has been studied with density functional theory (DFT) and scanning tunnelling microscopy as a function of coverage. Below 0.73 ML (5.3 × 1013 molecules cm-2), the adsorbate structure is row-like with the molecular axes aligned with the rows in the Cu structure. Between this coverage and 1 ML (7.3 × 1013 molecules cm-2), a structural phase with a checkerboard structure is seen. At this coverage region, the molecules are very close to each other which leads to unusual bending. At higher coverages, a further phase transition to a high-density row structure is seen for most of the film. DFT with van der Waals functionals is employed to study how the molecule-molecule and molecule-surface interactions evolve as a function of coverage.

The atomic structure of low-index surfaces of the intermetallic compound InPd.

The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ∼580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ∼660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (≥750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and "global" measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature.

Molecular dynamics simulation of radiation damage in CaCd6 quasicrystal cubic approximant up to 10 keV.

Due to the peculiar nature of the atomic order in quasicrystals, examining phase transitions in this class of materials is of particular interest. Energetic particle irradiation can provide a way to modify the structure locally in a quasicrystal. To examine irradiation-induced phase transitions in quasicrystals on the atomic scale, we have carried out molecular dynamics simulations of collision cascades in CaCd6 quasicrystal cubic approximant with energies up to 10 keV at 0 and 300 K. The results show that the threshold energies depend surprisingly strongly on the local coordination environments. The energy dependence of stable defect formation exhibits a power-law dependence on cascade energy, and surviving defects are dominated by Cd interstitials and vacancies. Only a modest effect of temperature is observed on defect survival, while irradiation temperature increases lead to a slight increase in the average size of both vacancy clusters and interstitial clusters.

Atomic structure of an FeCrMoCBY metallic glass revealed by high energy x-ray diffraction.

Amorphous bulk metallic glasses with the composition Fe48Cr15Mo14C15B6Y2have been of interest due to their special mechanical and electronic properties, including corrosion resistance, high yield-strength, large elasticity, catalytic performance, and soft ferromagnetism. Here, we apply a reverse Monte Carlo technique to unravel the atomic structure of these glasses. The pair-distribution functions for various atomic pairs are computed based on the high-energy x-ray diffraction data we have taken from an amorphous sample. Monte Carlo cycles are used to move the atomic positions until the model reproduces the experimental pair-distribution function. The resulting fitted model is consistent with ourab initiosimulations of the metallic glass. Our study contributes to the understanding of functional properties of Fe-based bulk metallic glasses driven by disorder effects.

Atomic arrangements in an amorphous CoFeB ribbon extracted via an analysis of radial distribution functions.

We discuss the atomic structure of amorphous ferromagnetic FeCoB alloys, which are used widely in spintronics applications. Specifically, we obtain the pair-distribution functions for various atomic pairs based on high-energy x-ray diffraction data taken from an amorphous Co20Fe61B19specimen. We start our reverse Monte Carlo cycles to determine the disordered structure with a two-phase model in which a small amount of cobalt is mixed with Fe23B6as a second phase. The structure of the alloy is found to be heterogeneous, where the boron atoms drive disorder through the random occupation of the atomic network. Our analysis also indicates the presence of small cobalt clusters that are embedded in the iron matrix and percolating the latter throughout the structure. This morphology can explain the enhanced spin polarization observed in amorphous magnetic materials.

Low-energy electron diffraction and density functional theory study of potassium adsorbed on Pb(1 0 0).

Alkali metal adsorption systems provide important models for chemisorption. Low-energy electron diffraction experiments and density functional theory calculations were carried out for the adsorption of potassium on Pb(1 0 0). The stable structure for all submonolayer coverages was found to be the commensurate c(2 × 2) structure, with potassium atoms located in substitutional sites in the top substrate layer. This structure is temperature activated and occurs for adsorption or annealing of the film above 200 K. This finding is consistent with an earlier theory that proposed that for substrates with low energies of vacancy formation, substitutional structures can be the most stable. The structural and vibrational parameters deduced from the experiment are in agreement with the calculated values, and these values fit well into and add to the database of alkali metal adsorption properties.

Surface relaxation of Cu(5 1 1).

The multilayer relaxation of the stepped Cu(5 1 1) surface has been studied by quantitative low-energy electron diffraction and analyzed using the CLEED program package. Relaxations with respect to the bulk interlayer spacing of 0.6934 Å are -9.5%, -10.4%, +8.2% and -1.8% for the first four interlayer spacings, respectively (negative sign corresponds to contraction). The relaxation sequence (- - + -…) is thus in agreement with the theoretical prediction. The deeper relaxations are damped in a non-uniform manner and the lateral relaxations are smaller than 2% of the lateral spacing. This result agrees well with theoretical studies of the same surface. The Pendry R-factor for the favored structure is 0.21.

Correlation of electron self-energy with geometric structure in low-energy electron diffraction.

In low-energy electron diffraction (LEED) studies of surface geometries where the energy dependence of the intensities is analyzed, the in-plane lattice parameter of the surface is usually set to a value determined by x-ray diffraction for the bulk crystal. In cases where it is not known, for instance in films that are incommensurate with the substrate, it is desirable to fit the in-plane lattice parameters in the same analysis as the perpendicular interlayer spacings. We show that this is not possible in a conventional LEED I(E) analysis because the inner potential, which is typically treated as an adjustable parameter, is correlated with the geometrical structure. Therefore, without having prior knowledge of the inner potential, it is not possible to determine the complete surface structure simply from LEED I(E) spectra, and the in-plane lattice parameter must be determined independently before the I(E) analysis is performed. This can be accomplished by establishing a more precise experimental geometry. Further, it is shown that the convention of omitting the energy dependency of the real part of the inner potential means geometrical LEED results cannot be trusted beyond a precision of approximately 0.01 Å.

The evolution of the electronic structure at the Bi/Ag(111) interface studied using photoemission spectroscopy.

The growth of Bi on Ag(111) induces different surface structures, including (√3 × âˆš3)R30° surface alloy, Bi-(p × âˆš3) overlayer and Bi(110) thin film, as a function of increasing Bi coverage. Here we report the study of electronic states of these structures using core level and valence band photoemission spectroscopy at room temperature. The sp-derived Shockley surface state on Ag(111) is rapidly quenched upon deposition of Bi, due to the strong variation of the in-plane surface potential in the Ag(2)Bi surface alloy. The Bi 4f core levels of the (√3 × âˆš3)R30° alloy and Bi(110) thin film are shifted to lower binding energy by ~0.6 eV and ~0.3 eV compared with the Bi bulk value, respectively. Mechanisms inducing the core level shifts are discussed as due to a complex superposition of several factors. As Bi coverage increases and a Bi(110) overlayer forms on Ag(111), a new state is observed at ~0.9 ML arising from electronic states localized at the Ag-Bi interface. Finally the change of work function as a function of coverage is discussed on the basis of a charge transfer model.

LEED I-V and DFT structure determination of the (√3 × âˆš3)R30° Pb-Ag(111) surface alloy.

The deposition of 1/3 of a monolayer of Pb on Ag(111) leads to the formation of PbAg(2) surface alloy with a long range ordered (√3 × âˆš3)R30° superstructure. A detailed analysis of this structure using low-energy electron diffraction (LEED) I-V measurements together with density functional theory (DFT) calculations is presented. We find strong correlation between experimental and calculated LEED I-V data, with the fit between the two data sets having a Pendry's reliability factor of 0.21. The Pb atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional PbAg(2) surface alloy, as expected, with the Pb atoms residing approximately 0.4 Å above the Ag atoms due to their size difference. DFT calculations are in good agreement with the LEED results.

The uniaxially aperiodic structure of a thin Cu film on fivefold i-Al-Pd-Mn.

A thin film of copper on the fivefold surface of Al-Pd-Mn forms a structure that is uniaxially commensurate with the aperiodic structure of the substrate. This structure has been analyzed using low-energy electron diffraction and is found to consist of a vicinal surface of a body-centered tetragonal (bct) (100) structure. This bct(100) structure has lattice parameters of a = 2.88 Å, b = 2.55 Å and c = 2.88 Å, with the vicinal surface making an angle α of 13.28° relative to the a-b plane. This structure provides an explanation for the delayed ordering observed during the growth of the film. Simple conditions are derived for which the growth of ordered one-dimensionally quasiperiodic thin films on quasicrystals may be favorable. This finding is relevant to the use of quasicrystals as a means of matching interfaces in thin film systems.

Surface geometry of C(60) on Ag(111).

The geometry of adsorbed C(60) influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a C(60) monolayer, Ag(111)-(2 square root of 3 x 2 square root of 3) 30 degrees -C(60), and related density functional theory calculations. The stable monolayer has C(60) molecules in vacancies that result from the displacement of surface atoms. C(60) bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C(60) monolayers on close-packed metal surfaces.