Dynamics Of Surface Diffusion Research Articles

Structure and Dynamics of Adsorbed Dopamine on Solvated Carbon Nanotubes and in a CNT Groove.

Advanced carbon microelectrodes, including many carbon-nanotube (CNT)-based electrodes, are being developed for the in vivo detection of neurotransmitters such as dopamine (DA). Our prior simulations of DA and dopamine-o-quinone (DOQ) on pristine, flat graphene showed rapid surface diffusion for all adsorbed species, but it is not known how CNT surfaces affect dopamine adsorption and surface diffusivity. In this work, we use molecular dynamics simulations to investigate the adsorbed structures and surface diffusion dynamics of DA and DOQ on CNTs of varying curvature and helicity. In addition, we study DA dynamics in a groove between two aligned CNTs to model the spatial constraints at the junctions within CNT assemblies. We find that the adsorbate diffusion on a solvated CNT surface depends upon curvature. However, this effect cannot be attributed to changes in the surface energy roughness because the lateral distributions of the molecular adsorbates are similar across curvatures, diffusivities on zigzag and armchair CNTs are indistinguishable, and the curvature dependence disappears in the absence of solvent. Instead, adsorbate diffusivities correlate with the vertical placement of the adsorbate’s moieties, its tilt angle, its orientation along the CNT axis, and the number of waters in its first hydration shell, all of which will influence its effective hydrodynamic radius. Finally, DA diffuses into and remains in the groove between a pair of aligned and solvated CNTs, enhancing diffusivity along the CNT axis. These first studies of surface diffusion on a CNT electrode surface are important for understanding the changes in diffusion dynamics of dopamine on nanostructured carbon electrode surfaces.

Phase-field modeling of the morphological evolution of ringlike structures during growth: Thermodynamics, kinetics, and template effects

Understanding and controlling the morphology of crystals growing from prepatterned shapes, as in selective-area epitaxy, is fundamental for the development of novel device architectures. Here, an in-depth analysis of the faceted growth of ring-shaped crystal structures is presented. This is a prototypical case for assessing the role of thermodynamic and kinetic driving forces, as they induce different faceting between the outer convex region and the inner concave one. The well-established concepts of equilibrium crystal shape, set by surface energy minimization, and its kinetic counterpart, determined by orientation-dependent growth rates, provide a qualitative indication of the outcome of growth experiments. However, they are insufficient to capture the whole evolution pathway. A phase-field growth model including deposition, incorporation, and surface diffusion dynamics is considered, and extensive two- and three-dimensional simulations are performed. A continuum transition in the crystal morphology is recognized by varying the magnitude of the anisotropic surface energy vs the incorporation times. The in-depth analysis of a realistic case, mimicking experiments in the literature, highlights the prominent role of kinetics and of material redistribution from the outer to the inner perimeter of the ring. Finally, the templating effect of the initial pattern is investigated by considering polygonal profiles with different orientations, showing their effect on the facet formation and evolution.

A High‐Speed True Random Number Generator Based on a CuxTe1−x Diffusive Memristor

Herein, a true random number generator (TRNG) based on a CuxTe1−x diffusive memristor (DM) using its threshold switching (TS) behavior is reported. The intrinsic stochasticity of the TS behavior contributes to the randomness of the TRNG system. The switching behavior is discussed through field‐induced nucleation theory and surface diffusion dynamics. Demonstrating the performance of TRNG as a hardware security application, the DM‐based TRNG passes all 15 National Institute of Standards and Technology randomness tests without any post‐processing step, even in high‐temperature conditions. Moreover, a nonlinear‐feedback shift register is implemented for a high‐speed TRNG, producing the highest rate among the reported volatile‐memristor‐based TRNGs.

Sunburst pattern by kinetic segregation in core-shell nanowires: A phase-field study

Compositional segregation is commonly observed during the growth of III-V alloys in core-shell nanowires. Nanometer-thin stripes, enriched in one of the alloy components, are observed along the six <1 1 2> directions perpendicular to the [1 1 1]-wire axis, departing from the core to the outer shell edges in between the {1 1 0} faceted sidewalls. While it has been well established that the phenomenon occurs because of the different mobility of the alloy components, the actual mechanisms by which it happens are yet unclear. A phase-field model, coupling deposition and surface diffusion dynamics, is here developed to inspect the simultaneous evolution of the shell morphology and composition during growth. Both surface energy anisotropy and orientation-dependent growth rates are taken into account to identify their different role. Simulations reveal that the observed segregation is mainly triggered by the enhanced growth rate at the facet edges, while surface anisotropy keeps the stripes thin. Polarity effects are also included to differentiate the behavior between <1 1 2>A and <1 1 2>B orientations, so to reproduce the experimental observation of a 3-fold symmetry.

In-Situ X-Ray Diffraction Study of Pt Oxidation during Oxygen Reduction Reaction

ORR is one of the most studied electrochemical reactions due to its tremendous fundamental and practical importance. Oxygen is a common, readily accessible oxidizing agent and, therefore, the Pt ORR cathode is part of many energy conversion devices, e.g. fuel cells. Unfortunately, slow kinetics of the ORR negatively affects the performance and it is currently one of the main bottleneck in large scale fuel cells commercialization. It is partly caused by the presence of surface Pt oxides, which slow the reaction rate and trap reaction intermediates on the surface. The oxide formation and reduction is also known to cause dissolution of the Pt catalyst, which further degrades the performance. Even though the electrochemical formation of surface oxides on platinum surface has been extensively studied in the past, there are still many questions unanswered, mainly about the detailed structure of the oxide and its growth mechanism [1 and references therein]. Most of the studies were performed in the absence of O2, the fuel cell oxidant, and therefore they are less relevant to the fuel cell operation, as gaseous O2 can modify the oxidation potentials and mechanism. Given the above, further fundamental understanding of the Pt oxidation mechanism and its atomistic picture is clearly needed in order to determine the role of surface oxides in ORR and its effect on the fuel cell performance.Here we show the results of in-situ study of electrochemical oxide formation on Pt(111) and how it is influenced by presence of O2 during ORR. Furthermore, we show that surface reorganization processes taking place during the oxidation/reduction cycle are governed by the ad-atom surface diffusion dynamics and closely resemble the dynamics found under the vacuum conditions. The place exchange process associated with the initial stages of oxidation is followed dynamically during cyclic voltammetry (CV) and potential step experiments in the presence and absence of oxygen. Detailed analysis at two potentials shows that the reconstruction is consistent with a place exchange process between Pt and O atoms, in which the exchanged Pt atoms are directly above their original positions in the Pt(111) lattice. The reconstruction initiates with the CV peak at 1.06 V vs RHE, even though repeated cycling to 1.15 V leads to no changes in the CV. Adding O2 to the electrolyte does not have any significant effect on the oxidation behavior, in contrast to some literature reports, and the O2 accelerated Pt dissolution is not caused by the negative shift in the oxidation potential. Furthermore, the ORR current decreases before oxidation, implying that the presence of the surface oxide is not the limiting factor in the ORR and the high ORR overpotential is solely due to the slow ORR mechanism on an unreconstructed surface.The surface restructuring upon electrochemical oxidation/reduction shows a characteristic ripening behavior where Pt islands grow and become more prominent and homogeneous in size with increasing number of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle number. The structural evolution of the Pt surface morphology strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. This finding shows that the electrolyte does not need to be included in the ab-initio investigations of the Pt surface structure change during ORR. Figure 1

Enhanced atom mobility on the surface of a metastable film.

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

BiotecVisions 2013, March

BiotecVisions 2013, March

Configurational disorder effects on adatom mobilities on Ti1−xAlxN(001) surfaces from first principles

We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain potential energy maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti0.5Al0.5N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared in order to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.

Tuning the local frictional and electrostatic responses of nanostructured SrTiO3—surfaces by self-assembled molecular monolayers

Exploiting the capability of preparing nanostructured bifunctional terminated SrTiO(3) substrates (SrO and TiO(2)), the surface properties have been locally tuned by employing a double bottom-up strategy which combines the use of chemically nanopatterned substrates with molecular self-assembly. The dynamics of surface diffusion that allows SrO and TiO(2) chemical-termination nano-patterning of the SrTiO(3) is first addressed. Second, termination-dependent heterogeneous nucleation is used to demonstrate that stearic acid selectively grows on the TiO(2) terminated terraces. This adsorption improves the frictional properties and modifies the surface contact potential. The possibility of simultaneously tailoring at the nanoscale different surface properties of widely employed oxide substrates is promising for building up new devices useful for emerging applications.

Kinetic Hierarchies and Macroscopic Limits for Crystalline Steps in $1+1$ Dimensions

We apply methods of kinetic theory to study the passage from particle systems to nonlinear partial differential equations (PDEs) in the context of deterministic crystal surface relaxation. Starting with the near-equilibrium motion of N line defects (“steps”) with atomic size a, we derive coupled evolution equations (“kinetic hierarchies”) for correlation functions $F_n^a$, which express correlations of n consecutive steps. We investigate separately the evaporation-condensation and the surface diffusion dynamics in $1+1$ dimensions when each step interacts repulsively with its nearest neighbors. In the limit $a\to0$ with $Na=\mathcal{O}(1)$, where a is appropriately nondimensional, the first equations of the hierarchies reduce to known evolution laws for the surface slope profile. The remaining PDEs take the form of simple continuity equations, which we solve exactly and, thereby, connect continuous limits of $F_n^a$ with the slope profile. In addition, we construct a particular example of $F_n^a$ asymptotically for small but finite a by regularization of measures. In the limit $a\to0$, this construction yields singular correlation functions.

Chemical beam epitaxy growth of self-assembled InAs/InP quantum dots

We have used transmission electron microscopy and low temperature photoluminescence to study the growth by chemical beam epitaxy of self-assembled InAs/InP quantum dots as a function of growth interruption time. As the deposited InAs layer is given time to migrate on the semiconductor surface a number of different features appear. The InAs layer first forms a rough quantum well with monolayer fluctuations in width before forming elongated InAs islands. These islands are elongated along the [01-1] direction due to surface diffusion dynamics. These then break up into individual quantum dots, and slowly increase in height. These dots are found to be approximately square with their sides aligned along the 〈100〉 directions and lateral dimensions of 30–40 nm.

Simulation of epitaxial growth under ion-beam sputtering

Models of thin-film growth under ion-beam sputtering in a vacuum are reviewed. The direct molecular-dynamics simulation of atom flux deposition, as well as the simulation of the single-atom dynamics on the Ag/Cu(001) surface, are discussed. The component mixing effect at the film-substrate interface due to impact substitution is shown to take place for incident ion energies between 3 and 10 eV. Exchange processes are statistically studied, and their probabilities are estimated. Atomic mechanisms and the energy characteristics of surface diffusion near atomic steps are considered for the Cu/Cu(001) system. The dynamics of surface diffusion is investigated by the method of molecular statics. With step roughness taken into account, the activation barriers lower. Texture formation in a growing film under ion bombardment is analyzed in terms of the kinetic Monte Carlo model. This model allows us to explain the enhanced evolution of the axial texture at low substrate temperatures by introducing a concept of competitive growth of variously oriented grains under ion bombardment

Diffusion and electromigration on disordered surfaces

We study a one-dimensional disordered solid-on-solid model in which neighboring columns are shifted by quenched random phases. The static height-difference correlation function displays a minimum at a nonzero temperature. The model is equipped with volume-conserving surface diffusion dynamics, including a possible bias due to an electromigration force. In the case of Arrhenius jump rates a continuum equation for the evolution of macroscopic profiles is derived and confirmed by direct simulation. Dynamic surface fluctuations are investigated using simulations and phenomenological Langevin equations. In these equations the quenched disorder appears in the form of time-independent random forces. The disorder does not qualitatively change the roughening dynamics of near-equilibrium surfaces, but in the case of biased surface diffusion with Metropolis rates it induces a new roughening mechanism, which leads to an increase of the surface width as \(\).

Axisymmetric Surface Diffusion: Dynamics and Stability of Self-Similar Pinchoff

The dynamics of surface diffusion describes the motion of a surface with its normal velocity given by the surface Laplacian of its mean curvature. This flow conserves the volume enclosed inside the surface while minimizing its surface area. We review the axisymmetric equilibria: the cylinder, sphere, and the Delaunay unduloid. The sphere is stable, while the cylinder is long-wave unstable. A subcritical bifurcation from the cylinder produces a continuous family of unduloid solutions. We present computations that suggest that the stable manifold of the unduloid forms a separatrix between states that relax to the cylinder in infinite time and those that tend toward finite-time pinchoff. We examine the structure of the pinchoff, showing it has self-similar structure, using asymptotic, numerical, and analytical methods. In addition to a previously known similarity solution, we find a countable set of similarity solutions, each with a different asymptotic cone angle. We develop a stability theory in similarity variables that selects the original similarity solution as the only linearly stable one and consequently the only observable solution. We also consider similarity solutions describing the dynamics after the topological transition.

Quantitative theory of current-induced step bunching on Si(111)

We use a one-dimensional step model to study quantitatively the growth of step bunches on Si(111) surfaces induced by a direct heating current. Parameters in the model are fixed from experimental measurements near 900 $\ifmmode^\circ\else\textdegree\fi{}$C under the assumption that there is local mass transport through surface diffusion and that step motion is limited by the attachment rate of adatoms to step edges. The direct heating current is treated as an external driving force acting on each adatom. Numerical calculations show both qualitative and quantitative agreement with experiment. A force in the step down direction will destabilize the uniform step train towards step bunching. The average size of the step bunches grows with electromigration time $t$ as ${t}^{\ensuremath{\beta}}$, with $\ensuremath{\beta}\ensuremath{\approx}0.5$, in agreement with experiment and with an analytical treatment of the steady states. The model is extended to include the effect of direct hopping of adatoms between different terraces. Monte Carlo simulations of a solid-on-solid model, using physically motivated assumptions about the dynamics of surface diffusion and attachment at step edges, are carried out to study two-dimensional features that are left out of the present step model and to test its validity. These simulations give much better agreement with experiment than previous work. We find a step bending instability when the driving force is along the step edge direction. This instability causes the formation of step bunches and antisteps that is similar to that observed in experiment.

Spatial probability distribution of adsorbate atoms.

In order to access the dynamics of surface diffusion or of surface chemical reactions it is necessary to determine the spatial probability distributions of the adsorbed atoms or molecules rather than their average positions. Here we demonstrate how such a distribution can be determined from scanned-energy mode photoelectron diffraction data using the maximum entropy method. As a test case we have chosen the distribution of the carbon atom in the methoxy species adsorbed on Ni(111). Both simulated and experimental data are used.

Molecular-Dynamics Simulation of the Surface Diffusion ofn-Alkanes on Pt(111)

Abstract In the present study, the equilibrium adsorption and the dynamics of surface diffusion in a model of ethane and n-butane on a Pt(111) surface were simulated with molecular dynamics. At low temperatures, we found that both admolecules adsorb in a specific binding site. Through analysis of the trajectories, several features of the dynamics were resolved. At low temperature, we observed that diffusion occurs through a nearest-neighbor hopping mechanism involving both lateral rotation and axial translation. At high temperatures, the admolecule makes multiple-site hops and nonlocalized long flights. The temperature dependence of the diffusion coefficients was analyzed and was found to exhibit good Arrhenius behavior. The apparent diffusion coefficients follow trends seen in related experimental studies. In the case of ethane, a comparison between the diffusion barrier measured in the molecular-dynamics simulations and the theoretical barrier predicted by transition-state theory indicates that the simu...

Theoretical considerations of energetics, dynamics, and structure at interfaces

We review several of our recent studies investigating the structure and dynamics of two kinds of interfaces: solid–gas and solid–liquid types. For the former, we examine the energetics of some metal–vacuum interfaces and the dynamics of surface diffusion. Metal–vacuum interfaces are studied in the context of pseudopotential theory where the varied contributions to surface energy can be relatively easily categorized and interpreted while still providing quantitative accuracy. We examine the roles of the Madelung energy, surface dipole layer, Hartree energy, and electron response to the relaxation process. While qualitative behavior such as oscillatory multilayer relaxation can be seen even from the simplest ‘‘electrostatic’’ picture, quantitative predictions require both the complete theory and acknowledgment of the full three dimensionality of the system. Results are shown for the (111), (100), and (110) faces of aluminum. In exploring the dynamics of surface diffusion, we discuss a molecular dynamics (MD) simulation of model systems of lead monomers and dimers on a copper (110) surface (where there is a negligible size mismatch between dimer and substrate) and find that dimer diffusion rates can be two to three times those of the monomer. We show typical time evolutions of relevant dimer properties as well as vibrational densities of states and find that a librational or ‘‘wagging’’ mode of dimer motion acts as a ‘‘door-way state’’ for the diffusive jump and associate the elevation of the diffusion rate with its existence. We again employ MD to study solid–liquid interfaces both in equilibrium and under circumstances of rapid melting and recrystallization such as those found during laser annealing. We examine the (111), (100), and (110) faces of a fcc crystal and display profiles versus depth at various times of such properties as number density, temperature, potential energy, and diffusion constant, as well as growth rates. In particular, the close-packed (111) face enjoys a continuous growth process, uniform across the surface and mediated by an extensive interfacial region of liquid layering over the solid, while on the open, ‘‘rougher’’ (110) face the layering is suppressed and growth proceeds laterally across the exposed face in steps.

Molecular dynamics of surface diffusion. I. The motion of adatoms and clusters

The motion of isolated adatoms and small clusters on a crystal surface is investigated by a novel and efficient simulation technique. The trajectory of each atom is calculated by molecular dynamics, but the exchange of kinetic energy with the crystal lattice is included through interactions with a ’’ghost’’ atom. This atom represents surface atoms of the lattice and is subjected to random and dissipative forces that are related by the fluctuation–dissipation theorem. The diffusion process is characterized by measurements of the velocity autocorrelation function, mean square displacement, directional correlations between hops, and the mean displacement per hop. In addition, the rate of evaporation of single adatoms and the rate of dissociation of clusters are discussed. The diffusion of an isolated adatom is found to be somewhat faster than that predicted by the classical rate theory for an activated process. This effect is a result of diffusion jumps of several atomic diameters that occur preferentially at high temperatures. But Arrhenius behavior is observed over the entire range of temperatures below the melting point. Dimers and larger clusters are found to diffuse more slowly than individual atoms, but with a smaller apparent activation energy. These results do not exhibit the high-temperature anomalies that have been inferred from some experimental data on surface mass transport. In a subsequent paper the method is extended to treat mass transport in the layer of adatoms and clusters that results from a dynamic equilibrium with the vapor phase.