Step Kink Research Articles

The impact of YSZ surface steps on structure and morphology of La 2Zr 2O 7 islands growing on YSZ( [formula omitted]) surfaces by vapour–solid reaction

La 2Zr 2O 7 (LZO) islands of well-defined crystallographic orientation are growing on YSZ(1 0 0) single crystal surfaces, when the latter are exposed to La 2O 3 vapours at elevated temperature. The initial stage of this topotaxial vapour–solid reaction is investigated by HRTEM, AFM, and XRD, specifically considering the influence of steps on the YSZ(1 0 0) surface onto the LZO nucleation process. It is shown how this influence results in a particular structure and morphology of the LZO islands. Compared with the previously studied cases of LZO island formation on flat surface regions and at the rims of surface pits, new features are observed. These are discussed in terms of the preferred nucleation of the islands at step kinks, their specific growth kinetics under the influence of the step edges, and the partial relaxation of stresses arising in the growing islands by the deformation of the step edges.

Computer simulation of calcite growth inhibition: A study of monophosphonate interaction with calcite

We have performed atomistic surface simulations of the interaction of monophosphonate growth inhibitor ions with the planar and obtuse stepped { 10 1 ¥ 4 } surfaces of calcium carbonate. We show that phosphonate inhibitor ions have a smaller adsorption energy on the planar { 10 1 ¥ 4 } surface compared to the obtuse stepped { 10 1 ¥ 4 } surface in accordance with experiment; and that the binding energy of the inhibitor with the surface is dominated by electrostatic forces. We find that replacement processes which simulate the irreversible incorporation of monophosphonate ions at terrace and step sites are energetically more favourable than those calculated for the diphosphonate ion. The inhibition mechanism proposed to operate for the deprotonated monophosphonate/calcite system is via the binding or incorporation of monophosphonate ions to the calcite obtuse step sites and kink sites thus slowing step flow and thereby destroying and/or delaying the formation of potential kink sites and step assembly.

Evidence for a common molecular basis for sequence recognition of N3-guanine and N3-adenine DNA adducts involving the covalent bonding reaction of (+)-CC-1065.

The antitumor antibiotic (+)-CC-1065 can alkylate N3 of guanine in certain sequences. A previous high-field 1H NMR study on the (+)-CC-1065d[GCGCAATTG*CGC]2 adduct (* indicates the drug alkylation site) showed that drug modification on N3 of guanine results in protonation of the cross-strand cytosine [Park, H.-J.; Hurley, L. H. J. Am. Chem. Soc. 1997, 119, 629.]. In this contribution we describe a further analysis of the NMR data sets together with restrained molecular dynamics. This study provides not only a solution structure of the (+)-CC-1065(N3-guanine) DNA duplex adduct but also new insight into the molecular basis for the sequence-specific interaction between (+)-CC-1065 and N3-guanine in the DNA duplex. On the basis of NOESY data, we propose that the narrow minor groove at the 7T8T step and conformational kinks at the junctions of 16C17A and 18A19T are both related to DNA bending in the drugDNA adduct. Analysis of the one-dimensional 1H NMR (in H2O) data and rMD trajectories strongly suggests that hydrogen bonding linkages between the 8-OH group of the (+)-CC-1065 A-subunit and the 9G10C phosphate via a water molecule are present. All the phenomena observed here in the (+)-CC-1065(N3-guanine) adduct at 5'-AATTG* are reminiscent of those obtained from the studies on the (+)-CC-1065(N3-adenine) adduct at 5'-AGTTA*, suggesting that (+)-CC-1065 takes advantage of the conformational flexibility of the 5'-TPu step to entrap the bent structure required for the covalent bonding reaction. This study reveals a common molecular basis for (+)-CC-1065 alkylation at both 5'-TTG* and 5'-TTA*, which involves a trapping out of sequence-dependent DNA conformational flexibility as well as sequence-dependent general acid and general base catalysis by duplex DNA.

Experimental absolute step and kink formation energies on Pb( [formula omitted]) vicinal surfaces

Three-dimensional crystallites of Pb, equilibrated at various temperatures and imaged by scanning tunneling microscopy at temperature, have provided the basis for a quantitative determination of absolute step free energies and kink formation energies of vicinal Pb(1 1 1) surfaces. The anisotropy of large (1 1 1) facets was evaluated as a function of temperature. Products of radius times local curvature for A- and B-steps of these facets were calculated and their dependence on temperature fitted by theoretical expressions. Absolute values of step energies at 0 K are f 1A(0)=131 meV and f 1B(0)=117 meV, with the corresponding kink energies of ε kA=40.0 meV and ε kB=60.3 meV.

Effect of Fe(III) ions on the growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

The effect of concentration of Fe(III) ions and solution supersaturation on growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals is studied at a constant temperature of 30°C and predefined supersaturations up to 8%. It was observed that, in contrast to the usually observed decrease in face growth rates of crystals with an increase in the concentration of an impurity, Fe(III) ions lead to an increase in the growth rates of different faces of AO crystals. The experimental data of the dependence of face growth rates R on supersaturation σ at different concentrations c i of Fe(III) were analysed from the standpoint of BCF surface diffusion theory and its modified version in the power-law form, spiral growth theory involving cooperating dislocation source and multiple nucleation model. It was found that the cooperating screw dislocation and multiple nucleation models represent the experimental data on growth kinetics from pure and impure solutions most satisfactorily. Analysis of the data according to these models revealed that the free energy of a growing face decreases with an increase in the concentration of Fe(III) ions, suggesting that the impurity facilitates the formation of kinks in growth steps. It was also found that the values of the differential heat of adsorption of Fe(III) impurity, obtained in the present study, are similar to those observed in the case of growth systems in which impurities lead to a decrease in the growth rate.

Step edges on galena (100): Probing the basis for defect driven surface reactivity at the atomic scale

Scanning tunneling microscopy (STM) images of the PbS (100) surface with a step and several kinks were obtained with atomic resolution. These images show an increased tunneling current at step edge sites and an apparent deformation of the lattice near the step. The experimental images are compared with theoretical ab initio calculations for which we developed a hybrid method of constant current and constant height mode STM image simulation. With these calculations, we find that the apparent deformation is mainly an electronic effect rather than relaxation of atoms. In addition, with the help of these calculations, we can identify the changes of individual terrace-like and step-like orbitals that are observed using the STM in terms of the energy, density and shape of these states. This detailed knowledge of the electronic behavior of the PbS surface near a step can be used as a basis for explaining adsorption, acid/base, and redox behavior on PbS terraces and at steps, and the differences between the two.

Step-bunching transitions on vicinal surfaces with attractive step interactions

We study vicinal crystal surfaces within the terrace–step–kink model on a discrete lattice. Including both a short-ranged attractive interaction and a long-ranged repulsive interaction arising from elastic forces, we discover a series of phases in which steps coalesce into bunches of n b steps each. The value of n b varies with temperature and the ratio of short- to long-range interaction strengths. For bunches with large number of steps, we show that, at T=0, our bunch phases correspond to the well-known periodic groove structure first predicted by Marchenko. An extension to T>0 is developed. We propose that the bunch phases have been observed in very recent experiments on Si surfaces, and we advance a conjecture explaining the exponent β≈0.5 describing the shape of the observed phase transition curves. Further experiments are suggested. Within the context of a mapping of the model to a system of bosons on a 1D lattice, the bunch phases appear as quantum n-mers.

Atomically resolved edges and kinks of NaCl islands on Cu(111): Experiment and theory

Atomically resolved dynamic force microscopy (DFM) images of step and kink sites of NaCl films grown on the Cu(111) surface are presented. Combining experimental results with an atomistic modeling of DFM imaging, we study the mechanism of contrast formation and extract more information about the tip and NaCl film structure. The experimental results and theoretical modeling systematically demonstrate the enhanced interaction of step and kink sites of one kind with the tip. This is explained by the enhanced gradient of the electrostatic potential at low-coordinated surface sites, and considerable displacements of the step edge and kink atoms from their sites due to the interaction with the tip upon approach. The theoretical analysis predicts that the silicon tip is effectively an insulator, and that the NaCl island cannot be thicker than two monolayers. We discuss the shape and chemical structure of the tip and the mechanism of damping during DFM imaging.

On the supersaturation and impurity concentration dependence of segregation coefficient in crystals grown from solutions

A general expression relating effective segregation coefficient k eff of impurities with their concentration c i in solution and growth supersaturation σ is proposed, considering surface coverage due to impurity adsorption and dependence of accumulation and depletion of solvated host molecules at kinks on supersaturation σ. The published experimental data on the dependence of k eff in some fairly and sparingly soluble compounds (potassium sulphate, zinc potassium sulphate hexahydrate, potassium dihydrogen phosphate, potassium acid phthalate, ammonium oxalate monohydrate and gypsum) on c i and σ are analysed from the standpoint of the model. It was found that the new expression describes the reported experimental data satisfactorily. An analysis of the published experimental data according to the proposed model also reveals that the nature of plots of k eff against σ and σ/ c i is associated with the accumulation and depletion of solute molecules at kinks of steps on the F faces of a crystal.

Scanning tunnelling microscopy study of palladium on stepped Cu(210) surfaces: chemical contrast and room-temperature tip-induced motion

The chemical contrast and tip-induced motion of palladium on stepped Cu(210) surfaces at room temperature have been studied by scanning tunnelling microscopy (STM). Contrast inversion is shown to be useful for chemical-sensitive imaging and features such as fringing can be resolved. Step edges and kinks are shown to be thermodynamically more stable palladium adsorption sites. Tip-induced motion of palladium adatoms on Cu(210) at 300K has been demonstrated with a threshold gap resistance of 20–50MΩ. Annealing the palladium-covered surface to 600–700K results in the ejection of copper atoms to form crystalline copper islands which lie across the substrate steps with their [110] axes aligned to the [001] substrate axis.

Effects of step–step interactions on the fluctuations of an individual step on a vicinal surface and its wavelength dependence

We relate properties of an anisotropic continuum model of a two-dimensional vicinal surface to those of a model with fluctuating and interacting steps. We show that analysis of the fluctuations of an individual step in the array provides information about the length scale on which the surface has reached equilibrium and can be used to estimate fundamental step parameters from locally equilibrated surfaces. Monte Carlo simulations of a stable vicinal surface using the terrace–step–kink model agree with the theoretical predictions. We further apply this analysis to steps on an unstable surface during reconstruction-induced faceting and show that it can be used to determine whether the faceting process is in the nucleation or spinodal decomposition regime.

Magnetic ordering in Co films on stepped Cu(100) surfaces

Ultrathin films of Co were grown on Cu(100) and characterized by nanometer resolution secondary electron microscopy, Auger electron spectroscopy, and the surface magneto-optic Kerr effect. An unexpected out-of-plane remanence was detected in many films. The anisotropy of atoms near defects along the Co/vacuum interface calculated via the Néel model indicates that atoms at the bottom corner of a step edge are canted out-of-plane. Full three-dimensional micromagnetics simulations which incorporate site specific anisotropy (including step edges, kinks, and voids) have been performed. Simulations with unidirectional arrays of [11_0] steps, such as vicinal surfaces, do not exhibit out-of-plane remanence. Simulations with facets consisting of connected [110] and [11_0] steps exhibit out-of-plane remanence of 0.03. This is lower than the experimental value of 0.11.

Grundsätzliche Probleme der Korrosion von Legierungen

Phenomenological equations are given for the steady state dissolution of alloys and their composition at the surface. The slow component with the smallest rate constant essentially determines the steady state dissolution rate and is enriched in the alloy surface. The phenomenological equations are compared to experimental results both for active and passive alloys. In ideal cases, phenomenological rate constants are independent of composition. Rate constants of pure slow components being much smaller than rate constants of pure fast components are enhanced by alloying with the fast component. Thus, corrosion protection by alloying often is much less efficient than predicted by the ideal case. Deviations from the ideal case can be understood, if dissolution proceeds at kinks in steps on low index planes of a crystalline alloy, or from equivalent sites in glassy metal alloys. Thus, the dissolution rates depend on the rate constants for dissolution from a kink and on the surface concentration of kinks. The surface concentration of kinks is determined by processes parallel to the dissolution from kinks, e.g. by dissolution of atoms from densely packed steps or by 2-dimensional nucleation in low index planes. The probability of such processes grows with the electrode potential and the concentration of a fast component. The resulting increase of the surface concentration of kinks occupied mainly by the slow component enhances its phenomenological rate constant. Steady states are approached at room temperature much slower than expected from diffusion coefficients of solid alloys extrapolated from high temperatures. Consequently, at room temperature changes of composition penetrate deeply into the bulk. The apparent increase of the interdiffusion coefficients is due to vacancy injection connected to the processes of kink generation. Selective corrosion and passivation often are accompanied by large surface stresses which may result in plastic deformation or fracture.

Surface Diffusion Induced Growth of Solid Solution over a Free Crystal Surface

It has been shown that interdiffusion along a free vicinal crystal surface can lead, at comparably low temperatures, to layered growth of solid solution over the surface without the involvement of bulk diffusion. The alloy concentration distribution along the surface, as well as the normal rate of solution growth has been calculated. The formation of a layer of the solid solution has been experimentally observed on a vicinal (111) surface of Ni single crystal as a result of surface interdiffusion between Ni and a thin film of Au deposited on a part of the Ni surface. The surface diffusion coefficients, as well as other parameters responsible for the exchange rate between the adatoms and kinks of elementary steps have been measured in the temperature range 550–700°C.

Fundamental aspects of the corrosion of alloys

Phenomenological equations are given for the steady state dissolution of alloys and their composition at the surface. The slow component with the smallest rate constant essentially determines the steady state dissolution rate and is enriched in the alloy surface. The phenomenological equations are compared to experimental results both for active and passive alloys. In ideal cases, phenomenological rate constants are independent of composition. Rate constants of pure slow components being much smaller than rate constants of pure fast components are enhanced by alloying with the fast component. Thus, corrosion protection by alloying often is much less efficient than predicted by the ideal case. Deviations from the ideal case can be understood, if dissolution proceeds at kinks in steps on low index planes of a crystalline alloy, or from equivalent sites in glassy metal alloys. Thus, the dissolution rates depend on the rate constants for dissolution from a kink and on the surface concentration of kinks. The surface concentration of kinks is determined by processes parallel to the dissolution from kinks, e.g. by dissolution of atoms from straight steps or by two-dimensional nucleation in low index planes. The probability of such processes grows with the electrode potential and the concentration of a fast component. The resulting increase of the surface concentration of kinks occupied mainly by the slow component enhances its phenomenological rate constant. Steady states are approached at room temperature much slower than expected from diffusion coefficients of solid alloys extrapolated from high temperatures. Consequently, at room temperature changes of composition penetrate deeply into the bulk. The apparent increase of the interdiffusion coefficients is due to vacancy injection connected to the processes of kink generation. Selective corrosion and passivation often are accompanied by large surface stresses which may result in plastic deformation or fracture.

AFM Study on Morphology Evolution of Zinc Electrodeposits.

Nano-size growth of zinc electrodeposit is studied by using atomic force microscopy (AFM). The (0001)η and {1010}η of hexagonal plate crystals are aligned parallel to the steel substrate with Sn++ as additive. An AFM cantilever is placed on these planes in order to focus the nano-size growth image on these planes. The (0001)η consists of both 30 nm of granular crystals and alignments of these granular crystals, i.e. steps. These steps grow laterally as a result of the embedment of zinc ion clusters to the kinks of steps. The {1010}η and initial deposit also consist of these granular crystals. The initial zinc deposits are made up of the edges of hexagonal platelet crystals oriented ten to twenty degree from the substrate. The (0001)η of hexagonal platelet crystals becomes parallel to the substrate as the deposit grows with Sn++ as additive. The {1010}η, on the other hand, becomes parallel to the substrate without Sn++ as additive. These changes in the texture are caused by the locations of nucleation sites of the steps on the (0001)η.

Some exactly solved models of steady crystal growth

Crystal growth at the molecular level can be described by statistical models for the surface structure and for the capture and escape of molecules. For some models the steady growth can be analysed exactly, leading to exact expressions for growth rate and surface characteristics. These are expressed in terms of the dimensions and angles of the surface and the rates of molecular events. Two-dimensional models describe the growth of a single layer on the surface of a three-dimensional crystal and approximate the growth of lamellar polymer crystals. The well known growth regimes for polymer crystals can be derived exactly. A two-dimensional model that can generate a rougher edge and a porous structure provides some insights into fast growth and into deposition of thin films from the vapour. A terrace–step–kink growth model in three dimensions can be exactly solved. For some models that have not been solved exactly, one can find conditions on the rates of molecular capture and escape that ensure steady, stable crystal growth.

Schwoebel barriers on Si(111) steps and kinks

Motivated by our previous work using the Stillinger–Weber potential, which shows that the [2̄11] step on the relaxed 1×1 Si(111) has a Schwoebel barrier of 0.61±0.07 eV, we calculate here the same barrier corresponding to two types of kinks on this step—one with rebonding between upper and lower terrace atoms (type B) and the other without (type A). From the binding energy of an adatom, without additional relaxation of other atoms, we find that the Schwoebel barrier must be less than 0.39 eV (0.62 eV) for the kink of type A (type B). Such a bound is argued to be a robust feature following from the presence of rebonding at the step edge or kink site. From the true adatom binding energy we determine the Schwoebel barrier to be 0.15±0.07 eV (0.50±0.07 eV) for the kink of Type A (B). The decrease in the Schwoebel barrier is roughly consistent with our previous estimates of its upper bound of 0.05 eV. However, as the true binding energy plots show discontinuities due to significant movement of atoms at the kink site, we speculate on the possibility of multi-atom processes having smaller Schwoebel barriers.

Effects of impurities on crystal growth processes

The mechanisms of adsorption of impurities during the growth of bulk crystals are surveyed. The impurities considered are foreign substances deliberately added to or inherently present in a growth medium. After a brief introduction to different growth models, the nature of impurities and types of impurity effects on growth kinetics, adsorption of impurities on F faces at relatively low and high impurity concentrations is first discussed. Here mechanisms of adsorption at kinks in steps and surface terrace in the presence of low concentrations of one impurity and two competitive impurities are presented. Then the concepts of adsorption of impurities involving the formation of two-dimensional impurity layer and three-dimensional impurity clusters as growth promotors on a growing face are developed. Thereafter the mechanism of adsorption of impurities on S and K faces and the determination of the adsorption mechanism on a growing face from the kinetic data are discussed. Finally, impurity effects on surface morphology of bulk crystals, and growth morphodroms of alkali halides are briefly reviewed.

Interactions of Br with Si(111)-7 x 7: Chemisorption, step retreat, and terrace etching.

The dissociative chemisorption of ${\mathrm{Br}}_{2}$ on Si(111)-7\ifmmode\times\else\texttimes\fi{}7 and the effects of spontaneous etching have been studied with scanning tunneling microscopy as a function of temperature, flux, and fluence. At room temperature, the Si surface retains the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction, and bias-dependent imaging reveals Br bonding to adatom and rest-atom dangling-bond sites. For 700\ensuremath{\le}T\ensuremath{\le}900 K, terrace etching involves Si removal from adatom sites and conversion to a 1\ifmmode\times\else\texttimes\fi{}1 periodicity that is stabilized by Br. In this temperature range, bilayer step flow etching dominates and Si removal is fastest along 〈11\ifmmode\bar\else\textasciimacron\fi{}0〉. Regrowth structures derived from six-membered Si rings terminated by Br appear near the bilayer steps. They are more common near steps that descend along [1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}2] than those that descend along [12\ifmmode\bar\else\textasciimacron\fi{}1] or [2\ifmmode\bar\else\textasciimacron\fi{}11], a distribution that reflects differences in the atomic scale bonding at the steps. Step flow continues at 1000 K but terrace pitting is also activated. This produces triangular bilayer pits bounded by 〈11\ifmmode\bar\else\textasciimacron\fi{}0〉 edges. Analysis yields the ratio of the rates of formation of terrace pits and step kink formation, giving a difference in activation energies for these processes of 0.8 eV. Flux-dependent studies at 1000 K show that pit sizes and densities vary dramatically, an effect related to the mean Br content on the terrace. No such dependence was observed at 900 K because pits could not be formed and the terraces were inactive once converted to 1\ifmmode\times\else\texttimes\fi{}1. At 1100 K, etching produces disordered vacancy clusters in the adatom layer. The presence of small ordered domains amidst randomly distributed adatoms is attributed to facile local removal. In all cases removal proceeds in a layer-by-layer fashion because of the striking anisotropy in etching energetics.