Methane activation on Ni(1 1 1): Effects of poisons and step defects

We have, theoretically and experimentally, investigated the dissociation of methane on the terraces and steps of a Ni(1 1 1) surface. Using Density Functional Theory (DFT) total energy calculations combined with Ultra High Vacuum (UHV) experiments, we find that the steps exhibit a higher activity than the terraces. We have, furthermore, investigated how carbon and sulfur present on the surface will deactivate the steps, leaving only the terraces active. We find the intrinsic sticking probabilities of methane on the steps and terraces at 500 K to be 2.8 × 10⁻⁷ for the steps and 2.1 × 10⁻⁹ for the terraces, in complete agreement with our calculated difference in activation energy of 17 kJ/mol.     

Facet-edge fluctuations with periphery diffusion kinetics

We investigate the novel scaling of the steps bounding a facet surrounded by a rough region. The hindered, asymmetric fluctuations can be associated with the emergence of a dominant non-linear term in the Hamiltonian governing the step fluctuations. We explore the crossover from unhindered to hindered fluctuations, calculating the growth exponent, β, with Monte Carlo simulation within the TSK model. The hindered behavior is found in the simulations when the facet-edge step is separated by fewer than six atomic spacings from the second step. Actual fluctuations are larger than in this calculation, particularly at higher temperatures, making the hindered behavior easier to observe. In addition, we discuss the possibility that volume conservation effects in nanoscale structures may cause similar confinement in non-conserved fluctuations.     

Characterization of crystal growth using a spiral nucleation model

Classical concepts of two-dimensional nucleation and spiral growth are used together with recent findings on the dynamics of dislocation spirals to derive a new crystal growth model. Initial growth nuclei result from the organization of adsorbed molecules in spirals around surface dislocations. The energetic barrier for the activation of the spiral nuclei is considerably lower than the admitted by classical two dimensional nucleation models. Stable nuclei evolve into bigger growth hillocks in supersaturated media through the incorporation of adsorbed units into their steps. The displacement velocity of steps during solution and vapour growth is calculated by different kinetic approaches, taking into consideration the distinct role of surface diffusion in each process, and avoiding known limitations of conventional theories. A generalized expression is obtained relating the crystal growth rate with main variables such as supersaturation, temperature, crystal size, surface topology and interfacial properties. At the end of the paper, the crystallization kinetics of sucrose measured at 40 °C is interpreted in the light of the new perspectives resulting from the proposed model. The application example illustrates how to estimate interfacial and topological properties from the experimental crystal growth results.     

Calculation of anisotropic strain-engineered patterns of monolayer islands

The self-organization of monolayer epitaxy islands in presence of anisotropies in surface stress, applied stress, and lattice mismatch between the film and substrate materials is investigated. The fundamental nature of island interactions is addressed in the context of a model wherein the system free energy consists of the excess energy and strain energy of atomic surface steps. It is shown the anisotropy can change the character of island interactions. An energy-reducing kinetic relation is adopted to evolve an initially random morphology towards a generally metastable minimum energy state. It is found the self-organization of islands into a regular array requires both the repulsion between islands and tendency for islands to aligned in a particular direction. Small anisotropies provide the required repulsion but not the tendency for islands to align and large anisotropies provide the necessary alignment but cause islands to attract. Modest levels of anisotropy provide the most favorable conditions of self-organization.     

Decay of multilayer holes on SrTiO3(0 0 1)

We have studied the decay kinetics of nanoscale multilayer holes on SrTiO₃(0 0 1) surfaces, using variable temperature scanning tunneling microscopy. We have performed real time observation of the decay of multilayer holes with diameters of 10 nm order at 750 °C. We have found that the hole decays, filling layer by layer from the bottom while expanding the periphery of the hole. We have performed numerical simulations of hole decay based on a step flow model. The observed decay kinetics is found to be diffusion limited with local mass conservation.     

Bridging the gap: From cellular automata to differential equation models for pedestrian dynamics

Cellular automata (CA) and ordinary differential equation (ODE) models compete for dominance in microscopic pedestrian dynamics. There are two major differences: movement in a CA is restricted to a grid and navigation is achieved by moving directly in the desired direction. Force based ODE models operate in continuous space and navigation is computed indirectly through the acceleration vector. We present the Optimal Steps Model and the Gradient Navigation Model, which produce trajectories similar to each other. Both are grid-free and free of oscillations, leading to the conclusion that the two major differences are also the two major weaknesses of the older models.     

Propagation of a reaction front accompanied by island formation: CO/Au/Ni(1 1 1)

Recent high-pressure scanning tunneling microscopy studies, performed at room temperature, have explicitly demonstrated the specifics of the CO-mediated removal of Ni atoms from the topmost layer of an Au/Ni(1 1 1) surface alloy. After an incubation period, the reaction is found to start at step edges. On each edge, a large fraction of Ni atoms is removed from the terrace in certain areas, whereas other areas are nearly intact after a given time. With increasing time, the former areas begin to overlap and the reaction front becomes somewhat more homogeneous. The Au atoms remaining behind the front form nm-sized islands. Here, we present Monte Carlo simulations reproducing all these observations.     

Carbide induced reconstruction of monatomic steps on Ni(1 1 1) - A density functional study

We present density functional calculations for carbon adsorption at the two types of monatomic steps on a Ni(1 1 1) surface. We show that it is thermodynamically favourable to make a carbon induced clock-type reconstruction at the close-packed step with a [1 1 1] step geometry, which creates fourfold sites at the step-edge. It is furthermore possible to extend the carbide with the clock reconstructed geometry onto the upper terrace with a net energy gain compared to adsorption of carbon on unreconstructed close-packed steps or terrace sites on Ni(1 1 1). Our findings explain the fact that carbide islands start to grow preferentially on the close-packed steps as has been observed using scanning tunneling microscopy.     

Immobilization of protein molecules on step-controlled sapphire surfaces

We have studied effects of surface morphology on immobilization of protein molecules using step-controlled sapphire surfaces. Preferential adsorption of avidin molecules on the step edges was observed on the single-stepped sapphire surface. A randomly-stepped sapphire surface was found to be suitable for high-density immobilization of protein molecules. These results indicate atomic scale structures of the substrate surface influence the adsorption efficiency of the proteins. By using an atomic force microscopy (AFM) equipped with a biotin-modified cantilever, we have confirmed that the immobilized avidin molecules on the substrates keep their biological activity. This means that the ligand-receptor interaction can be detected using the phase image mode of a standard AFM.     

Sub-layer growth of MnHg(SCN)4(C2H6OS)2 crystal

Growth steps and 2D nuclei are observed by AFM on the {0 0 1} faces of MnHg(SCN)₄(C₂H₆OS)₂ (MMTD) crystals. Measurements of the heights of steps and nuclei show the lowest value is equal to c/4. According to the interplanar distance modification established by Donney and Harker, the lowest height should be c/2. Appearance of the sub-layer growth is correlative with the crystal structure of MMTD.