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Controllable large-scale synthesis of two-dimensional materials (2DMs) such as graphene is a prerequisite for industrial applications. Chemical vapor deposition (CVD) is currently the most widespread synthesis method as it is efficient and easy to automatize. The process itself is quite complex and poorly understood, but it is generally believed to involve a number of distinct steps such as hydrocarbon decomposition into surface-bound intermediates, diffusion on the catalytic substrate, generation of nucleation points and, finally, graphene growth. In situ monitoring and tailoring of such a complex procedure is beneficial for understanding the growth kinetics and, eventually, for controlling the graphene growth. Herein, we report on a novel metrology system based on in situ reflectance spectroscopy that has been developed for real-time monitoring of surface changes during graphene growth on Cu foils at high operating temperatures. The implementation of this technique for extracting kinetic parameters of the growth process is presented. Furthermore, a microkinetic model of graphene growth based on density-functional theory (DFT) and the hindered translator / rotator model for enthalpy and entropy corrections is constructed and used to obtain a microscopic understanding of the apparent activation energy and related rate-determining steps in graphene growth.

Accurate and transferable models of reaction kinetics are of key importance for chemical reactors on both laboratory and industrial scale. Usually, setting up such models requires a detailed mechanistic understanding of the reaction process and its interplay with the reactor setup. We present a data driven approach which analyzes the influence of process parameters on the reaction rate to identify locally approximated effective rate laws without prior knowledge and assumptions. The algorithm we propose determines relevant model terms from a polynomial ansatz employing well established statistical methods. For the optimization of the model parameters special emphasize is put on the robustness of the results by taking not only the quality of the fit but also the distribution of errors into account in a multi-objective optimization. We demonstrate the flexibility of this approach based on artificial kinetic data sets from microkinetic models. This way, we show that the kinetics of both the classical HBr reaction and a prototypical catalytic cycle are automatically reproduced. Further, combining our approach with experimental screening designs we illustrate how to efficiently explore kinetic regimes by using the example of the catalytic oxidation of CO.

Based on density functional theory, we present a detailed investigation into the on-surface adsorption of atomic oxygen at all three low-index Pd facets in the low-coverage regime. Relying on one consistent computational framework allows for a systematic comparison with respect to surface symmetry, while discerning trends in the adsorption geometries, energies, work functions, and electron densities. We overall find a persisting degree of O-Pd hybridization that is accompanied by minimal charge transfer from the substrate to the adsorbate, thereby resulting in comparable binding energies and diffusion barriers at the three surfaces. Small differences in reactivity are nevertheless reflected in subtle variations of the underlying electronic structure which do not, however, follow the expected order according to atom packing density.

The correlation between in vitro dissolution rates and the efficiency of drug formulations establishes an opportunity for accelerated drug development. Using in silico methods to predict the dissolution rates bears the prospect of further efficiency gains by avoiding the actual synthesis of candidate formulations. Here, we present a computational protocol that achieves such prediction for molecular crystals at low undersaturation. The protocol exploits the classic spiral dissolution model to minimize the number of material parameters that require explicit molecular simulations. Comparison to available data for acetylsalicylic acid and alpha lactose monohydrate indicates a tunable accuracy within one order of magnitude.

The interaction strength of N 2 O and CO 2 molecules with different Lewis-acidic sites within MFU-4 l –type metal-organic frameworks was studied via gas sorption measurements and density-functional theory calculations. MFU-4 l comprising Zn–Cl units shows only physisorption of both gases. Introduction of Li into the parent MFU-4 l framework leads to a remarkable increase of binding strength of both N 2 O and CO 2 showing considerable Lewis acidity of Li I centers. Cu I -MFU-4 l shows even stronger binding of N 2 O, as compared to Li-MFU-4 l , whereas CO 2 doesn't bind to Cu I centers. Preferential binding of N 2 O to Cu I centers was also confirmed by in situ synchrotron X-ray powder diffraction measurements. These results show that Cu I -MFU-4 l can be considered as a material for selective N 2 O adsorption.

We combine first-principles atomistic thermodynamics with a Wulff–Kaichew construction to determine the equilibrium shape of Ag particles supported on α-Al 2 O 3 (0 0 0 1) under gas-phase conditions representative for ethylene epoxidation. The formation of three-dimensional truncated octahedra is predicted for both the Al-terminated (AlO 3 Al-R) and the fully hydroxylated O-terminated (H 3 O 3 Al-R) alumina surface, albeit with largely different contact angles. Overall, this is fully consistent with the common description of hemispherical Ag microcrystals on this support, but at variance with the cubic Ag nanoparticles reported recently by Christopher and Linic (2008). This result suggests the latter particles to be stabilized kinetically. We establish a framework from which strategies to thermodynamically stabilize cubic particles can be explored. The Wulff–Kaichew construction is proposed to provide a straightforward criterion that may be used as a suitable descriptor in the computational screening of surface additives that would stabilize the arguably highly selective Ag(1 0 0) facets.

The CHARMM, general Amber and OPLS force fields are evaluated for their suitability in simulating the molecular dynamics of the dissolution of the hydrophobic, small-molecule active pharmaceutical ingredients aspirin, ibuprofen, and paracetamol in aqueous media. The force fields are evaluated by comparison with quantum chemical simulations or experimental references on the basis of the following capabilities: accurately representing intra- and intermolecular interactions, appropriately reproducing crystal lattice parameters, adequately describing thermodynamic properties, and the qualitative description of the dissolution behavior. To make this approach easily accessible for evaluating the dissolution properties of novel drug candidates in the early stage of drug development, the force field parameter files are generated using online resources such as the SWISS PARAM servers, and the software packages ACPYPE and Maestro. All force fields are found to reproduce the intermolecular interactions with a reasonable degree of accuracy, with the general Amber and CHARMM force fields showing the best agreement with quantum mechanical calculations. A stable crystal bulk structure is obtained for all model substances, except for ibuprofen, where the reproductions of the lattice parameters and observed crystal stability are considerably poor for all force fields. The heat of solution used to evaluate the solid-to-solution phase transitions is found to be in qualitative agreement with the experimental data for all combinations tested, with the results being quantitatively optimum for the general Amber and CHARMM force fields. For aspirin and paracetamol, stable crystal–water interfaces were obtained. The (100), (110), (011) and (001) interfaces of aspirin or paracetamol and water were simulated for each force field for 30 ns. Although generally expected as a rare event, in some of the simulations, dissolution is observed at 310 K and ambient pressure conditions.

Kinetic Monte Carlo (kMC) simulations have emerged as a key tool for microkinetic modeling in heterogeneous catalysis and other materials applications. Systems, where site-specificity of all elementary reactions allows a mapping onto a lattice of discrete active sites, can be addressed within the particularly efficient lattice kMC approach. To this end we describe the versatile kmos software package, which offers a most user-friendly implementation, execution, and evaluation of lattice kMC models of arbitrary complexity in one- to three-dimensional lattice systems, involving multiple active sites in periodic or aperiodic arrangements, as well as site-resolved pairwise and higher-order lateral interactions. Conceptually, kmos achieves a maximum runtime performance which is essentially independent of lattice size by generating code for the efficiency-determining local update of available events that is optimized for a defined kMC model. For this model definition and the control of all runtime and evaluation aspects kmos offers a high-level application programming interface. Usage proceeds interactively, via scripts, or a graphical user interface, which visualizes the model geometry, the lattice occupations and rates of selected elementary reactions, while allowing on-the-fly changes of simulation parameters. We demonstrate the performance and scaling of kmos with the application to kMC models for surface catalytic processes, where for given operation conditions (temperature and partial pressures of all reactants) central simulation outcomes are catalytic activity and selectivities, surface composition, and mechanistic insight into the occurrence of individual elementary processes in the reaction network., Comment: 21 pages, 12 figures

We investigate transport effects in in situ studies of defined model catalysts using a multi-scale modeling approach integrating first-principles kinetic Monte Carlo simulations into a fluid dynamical treatment. We specifically address two isothermal flow setups: i) a channel flow with the gas-stream approaching the single crystal from the side, as is representative for reactor scanning tunneling microscopy experiments; and ii) a stagnation flow with perpendicular impingement. Using the CO oxidation at RuO2 (110) as showcase we obtain substantial variations in the gas-phase pressures between the inlet and the catalyst surface. In the channel geometry the mass transfer limitations lead furthermore to pronounced lateral changes in surface composition across the catalyst surface. This prevents the aspired direct relation between activity and catalyst structure. For the stagnation flow the lateral variations are restricted to the edges of the catalyst. This allows to access the desired structure-activity relation using a simple model., 22 pages, 7 figures

We use Density-Functional Theory to investigate the structure and stability of a methylthiolate self-assembled monolayer (SAM) on Ag (1 1 1). Focusing on the role of substrate vacancies, we indeed find various structures with a reduced Ag-atom density in the outermost substrate layer that are significantly more stable than an unreconstructed SAM/substrate interface. The most stable structure exhibits a very small rumpling of the methylthiolate layer and a mean height of the anchoring S atoms above the surface that are both in excellent agreement with experiment. Previously reported discrepancies thus result from an insufficient configurational screening.

The conceptual idea of degree of rate control (DRC) approaches is to identify the "rate limiting step" in a complex reaction network by evaluating how the overall rate of product formation changes when a small change is made in one of the kinetic parameters. We examine two definitions of this concept by applying it to first-principles kinetic Monte Carlo simulations of the CO oxidation at RuO2(110). Instead of studying experimental data we examine simulations, because in them we know the surface structure, reaction mechanism, the rate constants, the coverage of the surface and the turn-over frequency at steady state. We can test whether the insights provided by the DRC are in agreement with the results of the simulations thus avoiding the uncertainties inherent in a comparison with experiment. We find that the information provided by using the DRC is non-trivial: It could not have been obtained from the knowledge of the reaction mechanism and of the magnitude of the rate constants alone. For the simulations the DRC provides furthermore guidance as to which aspects of the reaction mechanism should be treated accurately and which can be studied by less accurate and more efficient methods. We therefore conclude that a sensitivity analysis based on the DRC is a useful tool for understanding the propagation of errors from the electronic structure calculations to the statistical simulations in first-principles kinetic Monte Carlo simulations., Comment: 27 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

We return to the geometry optimization problem of Lennard-Jones clusters to analyze the performance dependence of "cut and splice" genetic algorithms (GAs) on the employed population size. We generally find that admixing twinning mutation moves leads to an improved robustness of the algorithm efficiency with respect to this a priori unknown technical parameter. The resulting very stable performance of the corresponding mutation+mating GA implementation over a wide range of population sizes is an important feature when addressing unknown systems with computationally involved first-principles based GA sampling., Comment: 5 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

A parallel implementation of a nonlinear pseudo-spectral MHD code for the simulation of turbulent dynamos in spherical geometry is reported. It employs a dual domain decomposition technique in both real and spectral space. It is shown that this method shows nearly ideal scaling going up to 128 CPUs on Beowulf-type clusters with fast interconnect. Furthermore, the potential of exploiting single precision arithmetic on standard x86 processors is examined. It is pointed out that the MHD code thereby achieves a maximum speedup of 1.7, whereas the validity of the computations is still granted. The combination of both measures will allow for the direct numerical simulation of highly turbulent cases (1500 < Re < 5000), which have been previously impractical, given today’s computational speed. © 2008 Elsevier B.V. All rights reserved.

Combining high-resolution core-level spectroscopy (HRCLS), scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations we reanalyze the Pd(100)-(SQRT(5) x SQRT(5) R27^o)-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work (M. Saidy et al., Surf. Sci. 494, L799 (2001)), is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al.

NNumerous experiments in ultra-high vacuum as well as (T ¼ 0K ,p ¼ 0) theoretical studies on surfaces have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite temperature and high pressure. Accordingly, modern surface science has realized that bridging the so-called pressure gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations. In this paper we stress that there is still much to learn and important questions to be answered concerning the complex atomic and molecular processes which occur at surfaces and actuate catalysis and corrosion, although significant advances in this exciting field have been made over the years. We demonstrate how synergetic effects between theory and experiment are leading to the next step, which is the development of simple concepts and understanding of the different modes of the interaction of chemisorbed species with surfaces. To a large extent this is being made possible by recent developments in theoretical methodology, which allow to extend the ab initio (i.e., starting from the selfconsistent electronic structure) approach to poly-atomic complexes with 10,000 and more atoms, time scales of seconds, and involved statistics (e.g., ab initio molecular dynamics with 10,000 and more trajectories). In this paper we will 1. sketch recent density–functional theory based hybrid methods, which bridge the length and time scales from those of electron orbitals to meso- and macroscopic proportions, and 2. present some key results on properties of surfaces, demonstrating their role in corrosion and heterogeneous catalysis. In particular we discuss • the influence of the ambient gas phase on the surface structure and stoichiometry, • adsorbate phase transitions and thermal desorption, and • the role of atoms’ dynamics and statistics for the surface chemical reactivity.

The microscopic process of the formation of oxides on metal surfaces is barely known or understood. Usingdensityfunctional theory we studied the oxidation of Ruð 0001 Þ: from the initial oxygen adsorption, subsequent O incorporation into the metal, aggregation of sub-surface islands, to the transition to the oxide film. Along the atomistic pathway several metastable precursor configurations are identified. It is argued that their properties and the metastabilities in the surface-oxide formation process will have important consequences for the discernment and molecular modelingof catalysis. 2002 Elsevier Science B.V. All rights reserved.

The present status of theories for interpreting experimental ballistic electron emission microscopy (BEEM) data is reviewed. Current formalisms may be divided into two broad classes: one-electron theories, where carriers do not exchange energy with other excitations in the solid, and scattering approaches, where such losses are considered. While the former theories have been formulated with the help of Green’s functions (GFs), the latter have relied more on simulation by Monte-Carlo techniques. For the one-electron approach, we discuss why the originally suggested free propagation of carriers (e.g., ballistic electrons) does not oAer a consistent interpretation of the experimental database and should be replaced instead by considering the coherent propagation of electrons interacting with the periodic potential in the metal base. Bridging towards the scattering formalisms, it is shown how GFs incorporating a complex self-energy are still a feasible approach, when only a single inelastic source of scattering (e.g., electron‐electron (e‐e) interaction) is operative. Within this one-electron scheme, the importance of an accurately computed transmission coeAcient at the metal-semiconductor interface is stressed, when aiming to obtain absolute BEEM currents. Analyzing results from scattering techniques, it is argued that this coeAcient should be modified to take into account the back-injection of electrons from the semiconductor into the metal. A general expression for BEEM currents is given that can be used to simulate results in real-space, reciprocal-space or energy-space (spectroscopy with BEEM). Some experimental results are discussed in relation to the theories presented. ” 2001 Elsevier Science Ltd. All rights reserved.

Using a decimation technique, and imposing electrostatic self-consistency, we compute the surface electronic structure of various CoSi 2 (111)/Si(111) phases. The projected band structures and LDOS indicate a richness of surface related features. For the (1×1)Co-rich termination excellent agreement with experimental data and a recent DFT investigation is obtained. Strongly localized surface states on the high chain atoms of the (2×1) Pandey-chain like reconstruction are identified as causing the experimentally observed surface topography induced contrast in Ballistic Electron Emission Microscopy (BEEM) images of such films.

We describe the FORTRAN-90 program BEEM v2.1 for the computation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES). Based on a LCAO Hamiltonian, the equilibrium oneparticle Green’s functions of a semi-infinite crystal are obtained using a decimation technique. By means of Keldysh’s formalism, the STM tip is coupled to this crystal, and expressions for BEEM currents in real and reciprocal space are obtained. Finally, a Dyson-like equation and a self-consistent charge neutrality condition are applied to incorporate reconstructed or relaxed layers in the surface region. Running demos for Au and CoSi2 are provided. © 2000 Published by Elsevier Science B.V. All rights reserved.

With the time-differential-perturbed-angular-correlation method (TDPAC) we measured the efg for the probe nucleus 111 Cd in the system 111 Cd Sb with small amounts of inactive In-metal. For the quadrupole frequency we observed a relatively strong dependence of the In-concentration. The temperature dependence of the quadrupole interaction follows for In-concentrations larger than 0.1 % of weight a well-known T 3 2 relation. For smaller concentrations we got strong deviations. For the concentration of 0.5 % In we also did an isochronous annealing measurement. The resulting curve is in analogy to that one for radiation induced lattice defects in cubic lattices.

With the time-differential perturbed-angular-distribution method (TDPAD) using the second excited level in 19 F we measured the efgs at the 19 F site in different fluorine compounds. The data can be interpreted quite sufficiently by the model of Townes and Dailey and the bond switching model of Das and Hahn.