MgO/Ag(001) interface structure and STM images from first principles

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.

MgO/Ag(001) interface structure and STM images from first principles

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.

Interfacial interactions and structure of organic-inorganic nanohybrids

Understanding the interfacial interactions and structure is important to better design and application of organic-inorganic nanohybrids. This paper presents our recent molecular dynamic studies on organoclays and polymer nanocomposites, including the layering behavior of organoclays, structural and dynamic properties of dioctadecyldimethyl ammoniums in organoclays, and interfacial interactions and structure of polyurethane nanocomposites. The results demonstrate that the layering behaviors of organoclays are closely related to the chain length of quaternary alkyl ammoniums and cation exchangeable capacity of clays. In addition to typical layered structures such as monolayer, bilayer and pseudo-trilayer, a pseudo-quadrilayer structure was also observed in organoclays modified with dioctadecyldimethyl ammoniums (DODDMA). In such a structure, alkyl chains do not lie flat within a single layer but interlace, and also jump to the next layer or even the next nearest layer. Moreover, the diffusion constants of nitrogen and methylene atoms increase with the temperature and methelene towards the tail groups. For polyurethane nanocomposite, the van der Waals interaction between apolar alkyl chains and soft segments of polyurethane predominates the interactions between organoclay and polyurethane. Different from most bulk polyurethane systems, there is no distinct phase-separated structure for the polyurethane.

Molecular dynamics simulations of the hydrophobin SC3 at a hydrophobic/hydrophilic interface

Hydrophobins are small (similar to 100 aa) proteins that have an important role in the growth and development of mycelial fungi. They are surface active and, after secretion by the fungi, self-assemble into amphipathic membranes at hydrophobic/hydrophilic interfaces, reversing the hydrophobicity of the surface. In this study, molecular dynamics simulation techniques have been used to model the process by which a specific class I hydrophobin, SC3, binds to a range of hydrophobic/ hydrophilic interfaces. The structure of SC3 used in this investigation was modeled based on the crystal structure of the class II hydrophobin HFBII using the assumption that the disulfide pairings of the eight conserved cysteine residues are maintained. The proposed model for SC3 in aqueous solution is compact and globular containing primarily P-strand and coil structures. The behavior of this model of SC3 was investigated at an air/water, an oil/water, and a hydrophobic solid/water interface. It was found that SC3 preferentially binds to the interfaces via the loop region between the third and fourth cysteine residues and that binding is associated with an increase in a-helix formation in qualitative agreement with experiment. Based on a combination of the available experiment data and the current simulation studies, we propose a possible model for SC3 self-assembly on a hydrophobic solid/water interface.

Structural characterization of Co‐Re superlattices

Co‐Re superlattices were prepared with nominal periodicities of 65–67 Å and varying bilayer composition. The structural characterization was made by x‐ray diffraction and Rutherford backscattering spectrometry (RBS). First, second, and third order satellites are observed in the x‐ray diffractogram at 2θ values and with intensities close to those predicted by simulation. This confirms the coherence of the superlattice. RBS measurements combined with RUMP simulations give information on interface sharpness and the absolute thicknesses of the Co and Re layers. Discrepancies between the experimental and simulated diffractograms are found for Co thicknesses below 18 Å.

Atomistic-continuum modeling of ultrafast laser-induced melting of silicon targets

In this work, we present an atomistic-continuum model for simulations of ultrafast laser-induced melting processes in semiconductors on the example of silicon. The kinetics of transient non-equilibrium phase transition mechanisms is addressed with MD method on the atomic level, whereas the laser light absorption, strong generated electron-phonon nonequilibrium, fast heat conduction, and photo-excited free carrier diffusion are accounted for with a continuum TTM-like model (called nTTM). First, we independently consider the applications of nTTM and MD for the description of silicon, and then construct the combined MD-nTTM model. Its development and thorough testing is followed by a comprehensive computational study of fast nonequilibrium processes induced in silicon by an ultrashort laser irradiation. The new model allowed to investigate the effect of laser-induced pressure and temperature of the lattice on the melting kinetics. Two competing melting mechanisms, heterogeneous and homogeneous, were identified in our big-scale simulations. Apart from the classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to the large negative volume of melting of silicon, the material contracts upon the phase transition, relaxes the compressive stresses, and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. A series of simulations for a range of absorbed fluences allowed us to find the threshold fluence value at which homogeneous liquid nucleation starts contributing to the classical heterogeneous propagation of the solid-liquid interface. A series of simulations for a range of the material thicknesses showed that the sample width we chosen in our simulations (800 nm) corresponds to a thick sample. Additionally, in order to support the main conclusions, the results were verified for a different interatomic potential. Possible improvements of the model to account for nonthermal effects are discussed and certain restrictions on the suitable interatomic potentials are found. As a first step towards the inclusion of these effects into MD-nTTM, we performed nanometer-scale MD simulations with a new interatomic potential, designed to reproduce ab initio calculations at the laser-induced electronic temperature of 18946 K. The simulations demonstrated that, similarly to thermal melting, nonthermal phase transition occurs through nucleation. A series of simulations showed that higher (lower) initial pressure reinforces (hinders) the creation and the growth of nonthermal liquid nuclei. For the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with a face-centered cubic crystal structure. The results of this study, therefore, have important implications for interpretation of experimental data on the kinetics of melting process of semiconductors.

Chromium nanostructures formed by dewetting of heteroepitaxial films on W(100)

In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer onW(100).We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands’ shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of ∼1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets ±5◦ off of the substrate normal bound by (210) planes at ∼26◦. The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal.We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology

Electrochemical behavior of cobalt oxide coatings on cold-rolled steel in alkaline sodium sulfate

The electrochemical behavior of a coating of cobalt oxide on cold-rolled steel in alkaline sodium sulfate was Studied using the electrochemical techniques of open-circuit potential measurements and electrochemical impedance spectroscopy. The coating was prepared at different annealing temperatures ranging from 350 to 750 degreesC and characterized by SEM, EDX and XRD. Below 550 degreesC the composition of the coating was basically of Co3O4. At 750 degreesC CoO was formed and big cracks appeared on the film exposing an inner layer of iron oxides. Analysis of the EIS data is very difficult because of the complexity of the interface structure. It can be inferred that the charge transfer resistance of the coatings prepared at 350 and 450 C were higher than those for the coatings prepared at temperatures above 550 degreesC. (C) 2002 Published by Elsevier B.V. Ltd.

Educação ambiental e direito ambiental: desvelando relações em práticas político-pedagógicas de ONGs ambientalistas

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Between the scales: Water from different perspectives

In der vorliegenden Arbeit werden verschiedene Wassermodelle in sogenannten Multiskalen-Computersimulationen mit zwei Auflösungen untersucht, in atomistischer Auflösung und in einer vergröberten Auflösung, die als "coarse-grained" bezeichnet wird. In der atomistischen Auflösung wird ein Wassermolekül, entsprechend seiner chemischen Struktur, durch drei Atome beschrieben, im Gegensatz dazu wird ein Molekül in der coarse-grained Auflösung durch eine Kugel dargestellt.rnrnDie coarse-grained Modelle, die in dieser Arbeit vorgestellt werden, werden mit verschiedenen coarse-graining Methoden entwickelt. Hierbei kommen hauptsächlich die "iterative Boltzmann Inversion" und die "iterative Monte Carlo Inversion" zum Einsatz. Beides sind struktur-basierte Ansätze, die darauf abzielen bestimmte strukturelle Eigenschaften, wie etwa die Paarverteilungsfunktionen, des zugrundeliegenden atomistischen Systems zu reproduzieren. Zur automatisierten Anwendung dieser Methoden wurde das Softwarepaket "Versatile Object-oriented Toolkit for Coarse-Graining Applications" (VOTCA) entwickelt.rnrnEs wird untersucht, in welchem Maße coarse-grained Modelle mehrere Eigenschaftenrndes zugrundeliegenden atomistischen Modells gleichzeitig reproduzieren können, z.B. thermodynamische Eigenschaften wie Druck und Kompressibilität oder strukturelle Eigenschaften, die nicht zur Modellbildung verwendet wurden, z.B. das tetraedrische Packungsverhalten, welches für viele spezielle Eigenschaft von Wasser verantwortlich ist.rnrnMit Hilfe des "Adaptive Resolution Schemes" werden beide Auflösungen in einer Simulation kombiniert. Dabei profitiert man von den Vorteilen beider Modelle:rnVon der detaillierten Darstellung eines räumlich kleinen Bereichs in atomistischer Auflösung und von der rechnerischen Effizienz des coarse-grained Modells, die den Bereich simulierbarer Zeit- und Längenskalen vergrössert.rnrnIn diesen Simulationen kann der Einfluss des Wasserstoffbrückenbindungsnetzwerks auf die Hydration von Fullerenen untersucht werden. Es zeigt sich, dass die Struktur der Wassermoleküle an der Oberfläche hauptsächlich von der Art der Wechselwirkung zwischen dem Fulleren und Wasser und weniger von dem Wasserstoffbrückenbindungsnetzwerk dominiert wird.rn

Edge-to-edge matching model for predicting orientation relationships and habit planes-the improvements

The edge-to-edge matching model has been further developed along with the Cu/Cr system as an example. The conditions for zigzag atom rows to be matching directions are included and the critical value of interatomic spacing misfit along matching directions and the critical value of d-value mismatch between matching planes are proposed in the new version of the model. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Edge-to-edge matching and its applications - Part I. Application to the simple HCP/BCC system

The edge-to-edge matching crystallographic model has been used to predict all the orientation relationships (OR) between crystals that have simple hexagonal close packed (HCP) and body-centered cubic (BCC) structures. Using the critical values for the interatomic spacing misfit along the matching directions and the cl-value mismatch between matching planes, the model predicted all the four common ORs, namely the Burgers OR, the Potter OR, the Pitsch-Schrader OR and the Rong Dunlop OR, together with the corresponding habit planes. Taking the c(H)/a(H) and a(H)/a(B) ratios as variables, where H and B denote the HCP and BCC structures respectively, the model also predicted the relationship between these variables and the four ORs. These predictions are perfectly consistent with the published experimental results. As was the case in the FCC/BCC system, the edge-to-edge matching model has been shown to be a powerful tool for predicting the crystallographic features of diffusion-controlled phase transformations. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Edge-to-edge matching and its applications - Part II. Application to Mg-Al, Mg-Y and Mg-Mn alloys

A model for the crystallography and morphology of diffusion-controlled phase transformations - edge-to-edge matching - has been used to predict the orientation relationships (OR) and habit planes of precipitates Mg17Al12 in Mg-Al alloy, Mg24Y5 in Mg-Y alloy and alpha-Mn in Mg-Mn alloy. Based on the crystal structures and lattice parameters only, the model predicts that the possible ORs between Mg17Al12 and Mg matrix are the near Burgers OR, the Potter OR, the Gjonnes-Ostmoe OR and the Crawley OR. In the Mg-Y alloy, the OR between Mg24Y5 precipitates and the Mg matrix is predicted to be the Burgers OR only. The model also predicts that there are no reproducible ORs between alpha-Mn and Mg in the Mg-Mn alloy. Combining the edge-to-edge matching model and W. Zhang's Deltag approach, the habit plane and side facets of the precipitate for each OR can be determined. All the predicted ORs and the corresponding habit planes in Mg-Al and Mg-Y alloys agree very well with the experimental results. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Edge-to-edge matching - The fundamentals

The basis of the present authors' edge-to-edge matching model for understanding the crystallography of partially coherent precipitates is the minimization of the energy of the interface between the two phases. For relatively simple crystal structures, this energy minimization occurs when close-packed, or relatively close-packed, rows of atoms match across the interface. Hence, the fundamental principle behind edge-to-edge matching is that the directions in each phase that correspond to the edges of the planes that meet in the interface should be close-packed, or relatively close-packed, rows of atoms. A few of the recently reported examples of what is termed edge-to-edge matching appear to ignore this fundamental principle. By comparing theoretical predictions with available experimental data, this article will explore the validity of this critical atom-row coincidence condition, in situations where the two phases have simple crystal Structures and in those where the precipitate has a more complex structure.

Structure development in octadecyl trimethylammonium templated silicate films grown at the air/water interface

The mechanism of growth of silicate films at the air/liquid interface has been investigated in situ by a series of grazing incidence diffraction experiments using a 20 x 25 cm(2) imaging plate as the detector. C(18)TAX (X = Br- or Cl-) has been used as the film templating surfactant. The formation of a layered phase, prior to growth of the hexagonal mesophase in C(18)TABr templated films. has been seen. This layered structure has a significantly shorter d spacing compared to the final hexagonal film (43 versus 48 Angstrom, respectively). The correlation lengths associated with the development of the hexagonal in-plane diffraction spots are much longer in-plane than perpendicular to the air/liquid interface (300 Angstrom versus 50 Angstrom). This implies that the film forms via the growth or aggregation of islands that are initially only a micelle or two thick. which then grow down into the solution.