Step Kink Research Articles

The Creation of a Domain Structure Using Ultrashort Pulse NIR Laser Irradiation in the Bulk of MgO-Doped Lithium Tantalate

The fabrication of stable, tailored domain patterns in ferroelectric crystals has wide applications in optical and electronic industries. All-optical ferroelectric poling by pulse laser irradiation has been developed recently. In this work, we studied the creation of the domain structures in MgO-doped lithium tantalate by focused irradiation with a femtosecond near-infrared laser. Cherenkov-type second harmonic generation microscopy was used for domain imaging of the bulk. We have revealed the creation of enveloped domains around the induced microtracks under the action of the depolarization field. The domain growth is due to a pyroelectric field caused by a nonuniform temperature change. The domains in the bulk were revealed to have a three-ray star-shaped cross-section. It was shown that an increase in the field excess above the threshold leads to consequential changes in domain shape from a three-ray star to a triangular and a circular shape. The appearance of comb-like domains as a result of linear scanning was demonstrated. All effects were considered in terms of a kinetic approach, taking into account the domain wall motion by step generation and kink motion driven by excess of the local field over the threshold. The obtained knowledge is useful for the all-optical methods of domain engineering in ferroelectrics.

An Ab Initio Study for Oxygen Adsorption Behavior on Polar GaN Surfaces

Theoretical investigations for the adsorption behavior of O atoms on both Ga‐polar (+c) surface and N‐polar (c) GaN surfaces using ab initio calculations to clarify the influence of adsorption behavior on oxygen incorporation are presented. The calculations demonstrate that the O atom is stabilized by forming GaOH bond on the flat +c plane GaN(0001) surface, while the topmost N atom is replaced by the O atom on the flat c plane GaN(000) surface. The calculations using the nudged elastic band method also reveal that the energy barriers for oxygen incorporation on the flat c plane surface are comparable to that on the flat +c plane surface. Furthermore, the adsorption energy on the c plane surface drastically increases on the vicinal surface with step edges and kinks, indicating that step edges and kinks enhance the incorporation of O atoms on the c plane surfaces. The calculated results offer better understanding of the atom‐scale mechanisms for the orientation dependence of oxygen concentrations in GaN.

Ab initio study for adsorption behavior on AlN(0001) surface with steps and kinks during metal-organic vapor-phase epitaxy

We present our systematic theoretical study by performing ab initio calculations to clarify the behavior of adsorption for constituent atoms such as Al and N on a vicinal AlN(0001) surface with step edges and kinks during metal-organic vapor-phase epitaxy (MOVPE). The calculations reveal that the surface reconstruction affects the adsorption of Al and N adatoms near the kinks and step edges. Furthermore, we find the incorporation of an Al adatom at the kink and that of N adatoms not only at the kink but also in the terrace regions. The calculated results give some insights for an atomic-scale understanding of the step-flow growth during the MOVPE growth of AlN.

Antisolvent crystallization of ammonium dihydrogen phosphate (ADP) from aqueous solutions containing Al(III) and Fe(III) impurities by addition of ethanol

Experimental results of a study of the effect of concentration of Al(III) and Fe(III) ions on the metastable zone width (MSZW) of aqueous ADP saturated solutions, as determined by the maximum antisolvent content Δxmax in them for spontaneous three-dimensional (3D) nucleation, by feeding ethanol antisolvent to them at different rates RA and the nucleation and growth of crystals from these solutions are described and discussed. Crystal structure and segregation coefficients of impurity in the crystals were also studied by X-ray diffraction analysis and microwave plasma atomic absorption spectrometry, respectively. It was observed that: (1) the value of Δxmax for spontaneous 3D nucleation increases with an increase in ethanol feeding rate RA as well as impurity concentration ci, (2) the morphology and size of the ADP crystals changes with an increase in impurity concentration ci and antisolvent feeding rate RA, and (3) the effective segregation coefficients keff of the impurities in ADP crystals at a constant RA of ethanol feeding decreases with an increase in impurity concentration ci in the solution. The experimental data of the dependence of Δxmax on RA for solutions containing different impurity concentration ci were analyzed according to the approach based on the classical theory for 3D nucleation to calculate values of the preexponential factor A and the effective interfacial energy γeff of the theory, those of the morphology of ADP crystals from consideration of adsorption of impurity particles on the surfaces of growing crystals, and those of the dependence of segregation coefficient keff of the impurities on their concentration ci in the solution using a model based on competitive adsorption of solute and impurity molecules. It is argued that the formation of 3D nuclei of ADP during MSZW measurements and growth of these ADP nuclei as large crystals from aqueous solutions containing impurities and segregation of impurities in the crystals may be explained from consideration of different rates of ligand exchange of the surface-adsorbed complexes formed by interaction between univalently-charged cation complex present in aqueous solution and the H2PO4− ions of kinks in steps.

Boosting the H2–D2 Exchange Activity of Dilute Nanoporous Ti–Cu Catalysts through Oxidation–Reduction Cycle–Induced Restructuring

The use of nanoporous metals as catalysts has attracted significant interest in recent years. Their high‐curvature, nanoscale ligaments provide not only high surface area but also a high density of undercoordinated step edge and kink sites. However, their long‐term stability, especially at higher temperatures, is often limited by thermal coarsening and the associated loss of surface area. Herein, it is demonstrated that the nanoscale morphology of nanoporous Cu can be regenerated by applying oxidation/reduction cycles at 250 °C. Specifically, the morphological evolution and H2 dissociation activity of hierarchical nanoporous Cu catalysts doped with Ti during structural rearrangement triggered by oxidative and reductive atmospheres at elevated temperatures are studied. In addition to coarsening of the structure at elevated temperatures, oxidation at 400 °C causes an expansion of the ligaments. Subsequent reduction at 400 °C leads to the formation of particles and a drop in the H2 dissociation activity compared the fresh catalyst. However, performing the redox cycle at 250 °C reverses coarsening and boosts the H2 dissociation activity for the hydrogen–deuterium (H2–D2) reaction. Herein, the possibility to reverse coarsening is demonstrated, thereby mitigating the loss of activity frequently observed in nanoporous catalysts.

Tip-induced domain growth on the non-polar cut of lithium niobate with various stoichiometry deviations

Appearance of wedge-like domains during local switching by a biased tip of a scanning probe microscope (SPM) was studied in non-polar cut plates of lithium niobate (LN) with the spatial distribution of the deviation of Li concentration from stoichiometry composition (ΔcLi) created by vapor transport equilibration. The voltage dependences of the domain sizes were measured in the areas of LN plates with various values of ΔcLi. It was shown that the domain length increased linearly with the voltage, while the domain base width demonstrated a square root voltage dependence. The obtained dependence of the base width was attributed to the spatial distribution of the polar component of the external field produced by a biased SPM tip. The obtained results were considered in terms of the kinetic approach to domain growth. The growth of wedge-like domains was attributed to step generation and kink motion. The velocities of the base growth and kink motion were defined by the excess of the local value of superposition of the polar components of an external field produced by a biased tip, depolarization field, and screening fields over the threshold values. The average length of the elementary steps at the domain walls revealed from the domain length to width ratio demonstrated the square root voltage dependence. It was revealed that the domain length and the width of the base inversely depended on ΔcLi. The obtained dependences of the domain growth parameters on the composition allow for improving the periodical poling technique used for the fabrication of the nonlinear optical devices.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111).

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C1-C4) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C1-C3 alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C1-C4 carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol-1 per CH2) and similar for those adsorbed at step edge (4.8 kJ mol-1 per CH2) and Au terrace sites (4.9 kJ mol-1 per CH2).

CO organization at ambient pressure on stepped Pt surfaces: first principles modeling accelerated by neural networks.

Step and kink sites at Pt surfaces have crucial importance in catalysis. We employ a high dimensional neural network potential (HDNNP) trained using first-principles calculations to determine the adsorption structure of CO under ambient conditions (T = 300 K and P = 1 atm) on these surfaces. To thoroughly explore the potential energy surface (PES), we use a modified basin hopping method. We utilize the explored PES to identify the adsorbate structures and show that under the considered conditions several low free energy structures exist. Under the considered temperature and pressure conditions, the step edge (or kink) is totally occupied by on-top CO molecules. We show that the step structure and the structure of CO molecules on the step dictate the arrangement of CO molecules on the lower terrace. On surfaces with (111) steps, like Pt(553), CO forms quasi-hexagonal structures on the terrace with the top site preferred, with on average two top site CO for one multiply bonded CO, while in contrast surfaces with (100) steps, like Pt(557), present a majority of multiply bonded CO on their terrace. Short terraced surfaces, like Pt(643), with square (100) steps that are broken by kink sites constrain the CO arrangement parallel to the step edge. Overall, this effort provides detailed analysis on the influence of the step edge structure, kink sites, and terrace width on the organization of CO molecules on non-reconstructed stepped surfaces, yielding initial structures for understanding restructuring events driven by CO at high coverages and ambient pressure.

Domain shapes in bulk uniaxial ferroelectrics

We considered the experimentally observed variety of domain shapes in crystals of lithium niobate and lithium tantalate family. The shapes were classified, and their formation was discussed in terms of the universal kinetic approach to the domain growth using step generation and kink motion processes basing on analogy with crystal growth. The screening retardation resulting in step generation at the vertices of the polygonal domains under the action of the spatially nonuniform local electric field was considered. The domain shape field dependence was explained by simulated field distribution and difference between the threshold fields for step generation and kink motion.

Large Enantiospecificity of Step–kink Metal Surfaces: Contributions from the Backbone and Side Chain of α-Amino Acids

Designing step–kink metal surfaces with large enantiospecificity for multiple enantiomers is an important goal for the pharmaceutical industry. Here, we use density functional theory simulations to systematically study the enantiospecificity of the (531) surfaces of Ag, Cu, Pd, and Pt for α-amino acids. This leads us to propose a step–kink metal surface, Pt(531), which presents considerable enantiospecificity for a variety of α-amino acids, such as alanine, α-aminobutyric acid, valine, leucine, phenylalanine, serine, cysteine, and 3-aminoalanine. Further, we explored the role that the molecular backbone and the side chain play in the enantiospecific binding, and found that the backbone dominates the enantiospecificity of step–kink surface for α-amino acids, especially for nonpolar ones.

Tip-induced domain growth in the non-polar cuts of SBN:Ce single crystals

The local switching by conductive tip of scanning probe microscope was studied in the non-polar cuts of Ce-doped Sr0.61Ba0.39Nb2O6 single crystals after creation of the single-domain state. The switched domains possessed the egg-shaped heads and wedge-like tails. The dependences of lengths of the domain head and tail and width of the domain head on the voltage and pulse duration were derived. The start voltage for growth of the domain tail was revealed. The fast relaxation of the domain head and slow relaxation of the domain tail were observed. The model of the forward domain growth by step generation and kink motion was used for explanation of the experimental results. The obtained knowledge can be used for the domain engineering in ferroelectrics.

Thermodynamic model for metalorganic vapor-phase epitaxy of N-polar group-III nitrides in step-flow growth mode: Hydrogen, competitive adsorption, and configuration entropy

A thermodynamic model for metalorganic vapor-phase epitaxy (MOVPE) of the N-polar $(000\overline{1})$ binary group-III nitrides (AlN, GaN, and InN) in the step-flow growth mode is proposed based on the Burton, Cabrera, and Frank (BCF) theory. The coverages of the group-III adatoms are thermodynamically evaluated under competitive adsorption with hydrogen, which is used as a carrier gas or dissociated from the ${\mathrm{NH}}_{3}$ source gas during MOVPE. The chemical potentials of the group-III and H adatoms on N-polar group-III nitride surfaces are modeled using the respective bond energies with the surface N atoms of the nitride and the vibrational frequencies of the adatoms. The coverages of the coadsorbed group-III and H adatoms are calculated using the Langmuir adsorption isotherm with these chemical potentials. The configuration entropy of the group-III adatoms bridges the gap between the thermodynamic model and the BCF theory. The coverage of the group-III adatoms plays a role like partial pressure of the group-III gas in the thermodynamics. The equilibrium coverage of the group-III adatoms and the equilibrium pressure of the ${\mathrm{NH}}_{3}$ gas are evaluated from the conditions of Gibbs energy balance between the sources (group-III adatom and ${\mathrm{NH}}_{3}$ gas molecule) and products (group-III nitride and $3/2\phantom{\rule{0.28em}{0ex}}{\mathrm{H}}_{2}$ gas molecules) and of speed balance between group-III and N incorporation into step kinks. Fair agreements with the experimentally optimized growth conditions for MOVPE of N-polar GaN and InN are obtained by this method. Among the examined binary group-III nitrides, AlN growth is hardly affected by ${\mathrm{H}}_{2}$ gas pressure, GaN growth is controlled well by ${\mathrm{H}}_{2}$ gas pressure, and InN growth is strongly inhibited by ${\mathrm{H}}_{2}$ gas. A criterion for selecting the ${\mathrm{NH}}_{3}$/group-III flow ratio for maximum products/cost and minimum waste of the materials is demonstrated using the growth model and the estimated growth parameters. The offcut angle dependence of the growth rate on the vicinal substrates is also investigated.

Kink Distance and Binding Energy of Colloidal Crystals

Particle interaction is a critical parameter for growth kinetics of colloidal crystals. The equilibrium concentration and step free energy are strongly dependent on the particle interaction. The growth mechanism of the attractive system of colloidal crystals is similar to that of vapor or solution growth, where crystals grow by incorporating diffusing ad-particles on the terrace into kinks of steps (Terrace-Step-Kink model). Here, we have applied the theory of crystal growth to experimentally determine the binding energy that originates from the particle interaction. We focus on the relationship between kink distance and bond energy, which is described by BCF (Burton-Cabrera-Frank) theory. The value of the step free energy determined by the kink distance measurement agrees well with the values obtained from other measurements, including nucleation rate and critical radius. The obtained value also accounts well for the change in step velocity of two-dimensional islands, which is due to the Gibbs–Thomson ef...

In-Situ Formation of Fe Nanoparticles from FeOOH Nanosheets on γ-Al2O3 as Efficient Catalysts for Ammonia Synthesis

The preparation process of industrial Fe-based catalysts for ammonia synthesis involves energy-intense and sophisticated treatments. It is thus highly desirable to develop new catalysts with sufficient catalytic performance within smart and more economic approaches. Herein, γ-Al2O3-supported FeOOH nanosheets are employed for the first time as ammonia synthesis catalysts. To monitor the potential of these new materials, unsupported FeOOH and K-promoted catalysts are additionally investigated for comparison. While no activity is observed from the unsupported catalyst, the activity of γ-Al2O3-supported catalysts shows an inverse relationship with respect to the amount of Fe. Upon correlation of the catalytic performance with the final state of catalysts, the activity is closely related to the particle size of metallic Fe. Higher activity and lower apparent activation energy can be reached for smaller nanoparticles that are likely to contain more step–kink structures serving as active sites. The best catalyti...

Antimalarials inhibit hematin crystallization by unique drug–surface site interactions

In malaria pathophysiology, divergent hypotheses on the inhibition of hematin crystallization posit that drugs act either by the sequestration of soluble hematin or their interaction with crystal surfaces. We use physiologically relevant, time-resolved in situ surface observations and show that quinoline antimalarials inhibit β-hematin crystal surfaces by three distinct modes of action: step pinning, kink blocking, and step bunch induction. Detailed experimental evidence of kink blocking validates classical theory and demonstrates that this mechanism is not the most effective inhibition pathway. Quinolines also form various complexes with soluble hematin, but complexation is insufficient to suppress heme detoxification and is a poor indicator of drug specificity. Collectively, our findings reveal the significance of drug-crystal interactions and open avenues for rationally designing antimalarial compounds.

Role of step stiffness and kinks in the relaxation of vicinal (001) with zigzag [110] steps

We present a kinetic Monte Carlo study of the relaxation dynamics and steady state configurations of 〈110〉 steps on a vicinal (001) simple cubic surface. This system is interesting because 〈110〉 (fully kinked) steps have different elementary excitation energetics and favor step diffusion more than 〈100〉 (nominally straight) steps. In this study we show how this leads to different relaxation dynamics as well as to different steady state configurations, including that 2-bond breaking processes are rate determining for 〈110〉 steps in contrast to 3-bond breaking processes for 〈100〉−steps found in previous work [Surface Sci. 602, 3569 (2008)]. The analysis of the terrace-width distribution (TWD) shows a significant role of kink-generation-annihilation processes during the relaxation of steps: the kinetic of relaxation, toward the steady state, is much faster in the case of 〈110〉−zigzag steps, with a higher standard deviation of the TWD, in agreement with a decrease of step stiffness due to orientation. We conclude that smaller step stiffness leads inexorably to faster step dynamics towards the steady state. The step-edge anisotropy slows the relaxation of steps and increases the strength of step-step effective interactions.

The role of acid sites induced by defects in the etherification of HMF on Silicalite-1 catalysts

Defects in Silicalite-1 (structure type MFI) generate acid sites in the form of (i) hydroxyl nests present inside the channels or (ii) Brønsted–Lewis acid pairs, associated with terrace step kink sites present on the external surface of zeolite crystals. The amount of the sites has been changed by controlling the thermal treatment and pH value during preparation, or by post-synthesis treatments (ionic-exchange, silylation). These sites have been quantified by NMR, XRD and FT-IR pyridine adsorption/desorption at room temperature and 150°C, and correlated with the catalytic etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol, a relevant reaction to produce diesel additives. The results show that the surface acidity of Silicalite-1 materials presents an heterogeneous distribution of Lewis and Brønsted acid sites with different degrees of strength. The amount and distribution of such active centers determine the catalytic performance in HMF etherification. It is also shown that adsorption of water, produced in situ by ethanol condensation, has a promoter effect at lower concentration and high temperature, due to the dissociation of S2R siloxanes bonds, and formation of Lewis and Brønsted acid centers with medium-high strength, responsible for the activation pathways of HMF conversion to 5-(ethoxymethyl)furfan-2-carbaldehyde (EMF) and ethyl-4-oxopentanoate (EOP), respectively.

Weakly-bound hydrogen on defected Pt(111)

Abstract Step edges and kinks, abundant on multi-faceted nanoparticles, are catalytically active sites. Weakly-bound atomic H, at either topmost surface or subsurface sites, would be important for low-temperature hydrogenation in platinum-based catalysts. Here we report experimental results for such H atoms on Pt(111). Saturation-adsorbed atomic H from molecular H 2 on the defect-free Pt(111) surface indeed gave only a single-peaked H 2 desorption (β 2 ) at 285 K. Instead, defected Pt(111) surfaces rendered triple peaks (β 1 to β 3 ) including a prominent feature (β 1 ) at as low as 205 K in addition to another desorption (β 3 ) at 360 K. This β 1 –H state was inhibited and created by pre- and post-adsorbed CO, respectively. We attribute the β 1 –H 2 desorption to H atoms trapped at interstitial sites beneath surface defects on the basis of: (1) its desorption at a very low temperature in addition to two other peaks from terrace- and defect-adsorbed H; (2) its and total H uptakes by far larger than the surface defect density; (3) its desorption amount up to ~ 3.6 times that of the β 3 desorption from defects; (4) its complete inhibition by a small pre-coverage of CO; and (5) the complete β 3 -to-β 1 H conversion, while the β 1 –H state remaining intact, by postdosed CO. Our proposed mechanism is that the derelaxation (upward lifting) of the H- or CO-bound Pt lattice atoms at (step) defects, as a result of strong H–H and even stronger H–CO lateral repulsions under (near) saturation surface coverages, opens a low-barrier path for H diffusion into the subsurface.

Thermodynamics of surface defects at the aspirin/water interface.

We present a simulation scheme to calculate defect formation free energies at a molecular crystal/water interface based on force-field molecular dynamics simulations. To this end, we adopt and modify existing approaches to calculate binding free energies of biological ligand/receptor complexes to be applicable to common surface defects, such as step edges and kink sites. We obtain statistically accurate and reliable free energy values for the aspirin/water interface, which can be applied to estimate the distribution of defects using well-established thermodynamic relations. As a show case we calculate the free energy upon dissolving molecules from kink sites at the interface. This free energy can be related to the solubility concentration and we obtain solubility values in excellent agreement with experimental results.

Fundamental Controls of Dissolution Rate Spectra: Comparisons of Model and Experimental Results

Real-time microscopic observations of reacting mineral surfaces have provided a significant advance in the understanding of mineral dissolution and growth. The kinetics of surfaces are often dominated by the distribution of defects, e.g., screw dislocations, that provide a critical means of sustaining step movement and kink creation that are fundamental to dissolution and growth processes. However, in natural settings, the discontinuities created by physical grain boundaries and contacts may constitute a critical contribution to “whole rock” reactivity, a point that has been recognized in the past. Because of the complex relationship between mineral surface characteristics such as area, roughness, and reactivity, this problem requires a modeling approach that examines the relationship between rate and surface morphology. This approach is relevant to the question of whether a critical size exists that is governed by surfaces kinetics, below which the reactivity of grains is dominated by contributions from boundary sites. We discuss these results in light of experimental observations of the statistical distribution of rates using the concept of rate spectra.