Atomic Rows Research Articles

Straight sections of step edges on a NiAl(110) curved single crystal surface used to calculate an approximation of step formation energy

Curved crystals may feature a smooth transition between different vicinal surfaces. Using one curved single crystal to study different vicinal surfaces requires less experimental time than using several single flat crystals. Here, we study step distributions on the (110) plane of a curved NiAl single-crystal surface, which consists of alternating Ni and Al atom rows. We use scanning tunneling microscopy under UHV conditions at room temperature and our home-built Python-based analysis script to obtain statistical information on kink and straight sections along step-edge distributions from STM images. We perform this analysis mainly to study this single crystal’s kink distributions and step termination We propose a new method to estimate the step formation energy based on step edge analysis and statistical mechanics. With this method, we find an approximation of the step formation energy for NiAl(110).

Sliding grain boundary formations and their atomic and electronic structures in 1T’-WTe2

Understanding the complexity of grain boundaries between domains is essential for controlling material properties. While grain boundaries in two-dimensional (2D) materials have revealed a few cases of unique features with chemical reactivity and electronic structures, the intriguing case of one-dimensional (1D) grain boundaries still remains relatively unexplored, in particular, for non-hexagonal structures. Here, sliding grain boundary formation in 1T’-WTe2 has been investigated at the atomic scale. We found that the grain boundary keeps W-Te zigzag atomic rows in one direction. The asymmetric 1D sliding grain boundary formations exhibited an angle of 38° relative to the W-Te zigzag atomic rows, strain near the sliding grain boundary formation, and fluctuations in the local density of states (LDOS). The electronic structure in the asymmetric 1D sliding grain boundary formation shows two-line features in LDOS mapping. The structural models under the directional constraint were constructed with two symmetry operations, sliding and 180°-rotation, which agree well with the experimental results. Calculation of formation energy for the models suggested that the grain boundary formation was formed by 180°-rotated domains meeting during their growth along with sliding. The understanding of the sliding grain boundary formation provides a promising path to chemical applications such as hydrogen evolution reactions.

Surface band structure of the reconstructed Ir(001)-[formula omitted] surface

It is well known that the atomic structure of the Ir(001)-(5 × 1) surface consists of a quasi-hexagonal topmost layer on top of quadratic substrate layers. In the present work, we investigate how the electronic structure of Ir(001)-(1 × 1) is modified by the (5 × 1) reconstruction. For this purpose, we perform a first-principles density functional theory (DFT) calculation for semi-infinite Ir(001)-(1 × 1) and -(5 × 1) surfaces by employing the surface embedded Green’s function technique. It will be shown that surface bands on the (1 × 1) surface are strongly modified upon reconstruction. At the same time, we will demonstrate that one-dimensional (1D) surface bands localized in atomic rows in the [110] direction characterizing the buckled hexagonal layer emerge on the reconstructed surface.

Alteration of the inter-chain distance between Au atomic wires on Si(111) induced by Tl adsorption

Thallium (Tl) deposition onto the Au/Si(111)5 × 2 reconstruction followed by annealing at about 300 °C has been found to result in the formation of the surface structure having 4 × 2 periodicity. Similar to the most reconstructions in the (Au, Tl)/Si(111) system, Au and Tl do not intermix uniformly. While Au chains preserve their inner structure, Tl atoms being highly mobile at room temperature continuously migrate over the Si chains. Thallium donates electrons into the surface, that results in the two consequences: (i) filling the originally partially filled surface-state band, thus converting the metallic surface into an insulating state; (ii) transforming Si chains, which separate Au atomic rows, from the honeycomb-like to the zigzag according to the electron counting rule. The latter alters the inter-chain distance within the array of Au atomic wires. Au and/or Si chains contain point defects, which are masqueraded by the mobile Tl atoms producing peculiar “wavy” STM images.

Structure of monolayer iron nitride islands on Cu(001) revisited

We revisited the structure of monolayer iron nitride islands grown on Cu(001) using the well-established procedure, i.e. by bombarding the substrate with low-energy N+ ions, depositing Fe at room temperature and annealing in ultra-high vacuum (UHV). The geometric parameters of the obtained iron nitride were matching those of the “Fe2N” phase described in the literature and constituting the middle-plane-cut of the γ′-Fe4N unit cell. However, the atomic structure was found to exhibit an out-of-plane corrugation not reported by other authors so far. Moreover, the islands had an Fe1.3N stoichiometry and were characterized by a much higher work function value than the one predicted theoretically for the Fe2N phase. Based on these findings, we propose a new structural model for monolayer iron nitride on Cu(001), in which an additional N atom resides on top of every second Fe atom in each atomic row of the Fe2N. We also show that the Fe1.33N islands may be reduced to species with Fe2N stoichiometry through the exposure to O2 at room temperature and subsequent annealing in UHV. Thus, direct formation of the Fe2N phase under specific experimental conditions, as indicated by other authors, may be also possible.

Distribution of Electron Density in Self-Assembled One-Dimensional Chains of Si Atoms.

Scanning tunneling microscopy measurements of height profiles, along the chains of Si atoms on the terrace edges of a perfectly ordered Si(553)-Au surface, reveal an STM bias-dependent mixed periodicity with periods of one, two and one and a half lattice constants. The simple linear chain model usually observed with STM cannot explain the unexpected fractional periodicity in the height profile. It was found that the edge Si chain stands for, in fact, a zigzag structure, which is composed of two neighboring rows of Si atoms and was detected in the STM experiments. Tight-binding calculations of the local density of states and charge occupancy along the chain explain the voltage-dependent modulations of the STM profiles and show that oscillation periods are determined mainly by the surface and STM tip Fermi energies.

Simulation of the Dynamics of Supersonic N-Crowdions in fcc Lead and Nickel

In the case where an interstitial atom is located in a close-packed atomic row of the crystal lattice, it is called a crowdion. Crowdions play an important role in the processes of mass and energy transfer resulting from irradiation, severe plastic deformation, ion implantation, plasma and laser processing, etc. In this work, supersonic N-crowdions (N=1, 2) in fcc lattices of lead and nickel are studied by the method of molecular dynamics. Modeling shows that the propagation distance of a supersonic 2-crowdion in lead at a high initial velocity is less than that of a supersonic 1-crowdion. In other fcc metals studied, including nickel, supersonic 2-crowdions have a longer propagation distance than 1-crowdions. The relatively short propagation distance of supersonic 2-crowdions in lead is due to their instability and rapid transformation into supersonic 1-crowdions. This feature of the dynamics of supersonic N-crowdions in lead explains its high radiation-shielding properties.

Effect of Dislocation Character on the CRSS

The yield strength of a crystalline structural material is a fundamental mechanical property predominantly governed by the critical stress for dislocation slip. This Critical Resolved Shear Stress (CRSS) is strongly influenced by the character of the dislocation (e.g., screw, edge, or mixed) as shown in previous experimental studies. Existing analytical approaches for CRSS prediction assume an atomic row description of the slip plane and do not account for Wigner-Seitz (WS) cell area at each discrete lattice site. Further, inadequate consideration of the material's elastic anisotropy and the presumed dislocation “core-width” level precludes correct CRSS determination. This study proposes a predictive model applied to Face Centered Cubic (FCC) materials addressing these shortcomings in predicting glide stress of a dissociated dislocation. The core-width is rigorously determined from the minimization of total energy comprised of continuum strain energy (ESTRAIN) and atomistic misfit energy (EMISFIT) of the dislocation's core. The ESTRAIN is obtained from dislocation strain-fields calculated using the fully-anisotropic Eshelby-Stroh formalism. The EMISFIT is determined from the Generalized Stacking Fault Energy (GSFE) landscape of the slip plane. Previous EMISFIT calculations are restricted to slipped rows in ‘simple’ cubic lattices which do not represent the slip-planes in FCC crystals. The developed model is used to predict CRSS for a wide range of metallic materials correcting the overprediction of experimental CRSS levels. The results unveiled the remarkable dependence of CRSS on the dislocation character, revealing the non-trivial dependence on GSFE parameters. Thus, this study addresses a major void in structure-property prediction for structural materials.

Atomic structure and peculiarities of the electronic properties of Br layers on Ag(111) at different coverages

The structure and electronic properties of Br layers on Ag(111) at different coverage were studied by low-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy (STS) and density functional theory (DFT) calculations. The Br layers are found to form a hexagonal (3×3)R30∘ structure for coverage ⁓ 0.3 monolayer (ML) and a striped (61−12) structure for coverage ⁓ 0.4 ML. STS measurements reveal suppressed differential conductance for both structures at voltages ranging between − 1.2 and 1.4 V, and surprisingly intense peaks at 1.5 V and 2.0 V. The position of the high-intensity peak in the spectra for the striped structure shows lateral variations within the unit cell and is determined by the Br coverage. In our structural model for the striped structure, supported by DFT calculations, the hexagonal (3×3)R30∘ structure transforms to the striped structure due to uniaxial inhomogeneous compression of Br atomic rows. The local work function for the Br/Ag(111) surface, estimated from dz/dV(V) measurements, is 5.0 ± 0.2 eV, which is higher than for the Ag(111) surface (4.4 ± 0.1 eV). This suggests an increase in the electrostatic dipole barrier on Ag(111) due to Br adsorption. Combined with the suppressed local density of states around the Fermi level observed by STS, these results clarify the usefulness of Br layers in reducing the coupling between Ag(111) electrons and adsorbates.

New approaches to study of mismatched interfaces structure on low-index surfaces by molecular dynamics simulation

We used molecular dynamics simulations to study the structure of mismatched Al/Ni and Ag/Cu interfaces, considering low-index surfaces of the substrate. Embedded Atom Potentials are being employed to mimic atomic interactions between the different elements. The effect of the substrate orientation on the adlayer structure is investigated. The results showed that, on the (111) substrate, the adlayer presents a coincidence site lattice characterized by a rotation (labeled α) with respect to the substrate. This coincidence site lattice may contain dislocations and twin boundary that can be produced by the occurrence of a twin spirals (with opposite directions). To accommodate the adlayer deposition on the substrate, a slight shuffling of some atoms occurs in the substrate and leads to a rectangular structures producing a dislocation. Furthermore, on the (001) substrate, the accommodation of the triangular structure on the square lattice (substrate) is produced by a shift of some atomic rows producing a dislocation. But, the agglomeration of several square structures of 2d shape gives rise to a polycrystal delimited by a disclination forming grain boundaries and vacancies. However, the inward adsorption on the (110) substrate restricts the space of extension of adsorbed atoms, producing linear cracks. The relaxation of atoms adsorbed on these cracks change the linear shape of vacancies towards a staircase structure.

A study of step defects on NiAl(110) using a curved single crystal surface

In contrast to common flat single crystal surfaces, curved surfaces provide the opportunity to study a large range of vicinals using only one single crystal. Here, we employ a curved NiAl single crystal surface to study step defects on the (110) plane using scanning tunneling microscopy under UHV conditions and at room temperature. The direction of curvature is chosen to align with the alternating Al and Ni atom rows, and we image the surface at various positions along its curvature. With very few exceptions, we find only monoatomic step heights to occur with local step densities consistent with the expected values along the macroscopically curved surface. We use our home-built Python-based analysis script to analyze STM images and obtain statistical information on step and kink distributions, a.o. it allows us to determine the kink formation energy.

Formation of carbon propeller-like molecules from starphenes under electron irradiation.

Formation of carbon propeller-like molecules (CPLMs) from starphenes on a graphene substrate under electron irradiation with about 100% yield is observed in molecular dynamics simulations using the REBO-1990EVC_CH potential and CompuTEM algorithm. A CPLM consists of three carbon atomic chains connected to the central hexagon and is formed as a result of the spontaneous breaking of bonds between zigzag atomic rows in starphene arms after hydrogen removal by electron impacts. In the absence of the substrate, the CPLM yield is slightly decreased due to sticking between forming chains, while the formation time is increased threefold. The increase of the kinetic electron energy from 45 to 80 keV has no effect on the CPLM formation. Density functional theory (DFT) calculations performed show the stability of CPLMs with respect to the formation of new bonds between carbon atoms in the chains. DFT calculations using the accurate hybrid B3LYP functional provide an insight into the electronic structure of these new molecules.

Ferroelectric control of magnetic coupling of monolayer MnBr[formula omitted] in semiconducting multiferroic van der Waals heterostructure

Multiferroic van der Waals (vdW) heterostructures composed of vdW intrinsic magnets and ferroelectrics hold great significance in energy-efficient vdW spintronic devices. In this work, using first-principles calculations, we investigate the electronic structure and magnetic properties of monolayer MnBr2 as well as the electronic structure and magnetoelectric coupling properties of vdW heterostructure formed by sandwiching antiferromagnetic (AFM) monolayer MnBr2 between two intrinsically ferroelectric monolayer α-In2Se3 (In2Se3/MnBr2/In2Se3 vdW heterostructure). It is found that the ground state of monolayer MnBr2 is AFM state and the magnetic structure is ferromagnetic (FM) stripes of width two atom rows within the layer with AFM coupling between neighboring stripes, in which the AFM and FM couplings originate from superexchange interaction mediated by the pσ and pπ orbitals of the intervening Br in Anderson’s mechanism, respectively. Intriguingly, the ground state of the sandwiched monolayer MnBr2 in the In2Se3/MnBr2/In2Se3 vdW heterostructure can be reversibly switched between AFM and FM states by electrically reversing the ferroelectric polarization direction of α-In2Se3 and the In2Se3/MnBr2/In2Se3 vdW heterostructures are semiconducting for all polarized configurations. Our results present new opportunities for exploring energy-efficient vdW spintronic devices based on semiconducting multiferroic vdW heterostructures.

VO Cluster-Stabilized H2O Adsorption on a TiO2 (110) Surface at Room Temperature.

We probe the adsorption of molecular H2O on a TiO2 (110)-(1 × 1) surface decorated with isolated VO clusters using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and temperature-programmed desorption (TPD). Our STM images show that preadsorbed VO clusters on the TiO2 (110)-(1 × 1) surface induce the adsorption of H2O molecules at room temperature (RT). The adsorbed H2O molecules form strings of beads of H2O dimers bound to the 5-fold coordinated Ti atom (5c-Ti) rows and are anchored by VO. This RT adsorption is completely reversible and is unique to the VO-decorated TiO2 surface. TPD spectra reveal two new desorption states for VO stabilized H2O at 395 and 445 K, which is in sharp contrast to the desorption of water due to recombination of hydroxyl groups at 490 K from clean TiO2(110)-(1 × 1) surfaces. Density functional theory (DFT) calculations show that the binding energy of molecular H2O to the VO clusters on the TiO2 (110)-(1 × 1) surface is higher than binding to the bare surface by 0.42 eV, and the resulting H2O-VO-TiO2 (110) complex provides the anchor point for adsorption of the string of beads of H2O dimers.

Reaction Modes on a Single Catalytic Particle: Nanoscale Imaging and Micro-Kinetic Modeling.

The kinetic behavior of individual Rh(hkl) nanofacets coupled in a common reaction system was studied using the apex of a curved rhodium microcrystal (radius of 0.65 μm) as a model of a single catalytic particle and field electron microscopy for in situ imaging of catalytic hydrogen oxidation. Depending on the extent of interfacet coupling via hydrogen diffusion, different oscillating reaction modes were observed including highly unusual multifrequential oscillations: differently oriented nanofacets oscillated with differing frequencies despite their immediate neighborhood. The transitions between different modes were induced by variations in the particle temperature, causing local surface reconstructions, which create locally protruding atomic rows. These atomic rows modified the coupling strength between individual nanofacets and caused the transitions between different oscillating modes. Effects such as entrainment, frequency locking, and reconstruction-induced collapse of spatial coupling were observed. To reveal the origin of the different experimentally observed effects, microkinetic simulations were performed for a network of 105 coupled oscillators, modeling the individual nanofacets communicating via hydrogen surface diffusion. The calculated behavior of the oscillators, the local frequencies, and the varying degree of spatial synchronization describe the experimental observations well.

(Invited) Mechanism of Interlayer Transport on a Growing Metal Surface: 2D vs. 3D Growth

The atomic scale transitions corresponding to diffusion and interlayer transport of metal adatoms on the low energy, close packed surfaces of several transition metals are studied using density functional theory calculations within the generalized gradient approximation. Minimum energy paths and estimates of activation energy are calculated for processes that influence whether the crystal grows layer-by-layer, i.e. 2D growth, or whether new islands nucleate on top of existing islands resulting in 3D growth. In some cases, the path of lowest activation energy for the descent of an adatom, i.e. interlayer transport, turns out to take place near, but not at, a kink site on a step. The energy barrier for the adatom to subsequently round the corner and enter the kink site is significantly higher. This near-kink-descent mechanism promotes the formation of a new row of step atoms and leads to the introduction of additional kink sites, thereby opening up new low activation energy paths for descent and promoting 2D growth. The sites adjacent and above the step edge can provide large binding energy for the adatom, and form a trough along which the adatom can migrate before descending, thereby increasing the probability that an adatom finds a kink site. These features of the energy landscape for an adatom point to the possibility of re-entrant layer-by-layer growth mode and have, for example, been found for Pt(111) and Au(111) surfaces.

Highly efficient energy and mass transfer in bcc metals by supersonic 2-crowdions

During irradiation, bombardment with energetic particles, or plasma treatment, structural transformations occur in materials, in which point defects play a very important role. Highly mobile crowdions contribute to the transfer of mass and energy in the crystal lattice. Static and slowly moving crowdions are well studied, in contrast to crowdions moving at a speed exceeding the speed of sound. Here, for the first time, dynamics of supersonic N-crowdions (N=1,2) is investigated in bcc metals (tungsten and vanadium) with the help of molecular dynamics simulations. N-crowdions are excited by imparting sufficiently high initial velocities to N neighboring atoms located in the same close-packed atomic row along this row. Both supersonic 1- and 2-crowdions carry one interstitial atom each. Modeling shows that much less energy is required to excite a 2-crowdion compared to a 1-crowdion. Energy dissipation for a 2-crowdion is much slower than for a 1-crowdion suggesting that the supersonic 2-crowdion is an efficient mass transfer mechanism in bcc metals. The distance traveled by supersonic 1- and 2-crowdions in W is much greater than that in V. The supersonic crowdion in W transforms into a moving subsonic crowdion, and in V it transforms into a standing subsonic crowdion carrying a localized high-amplitude oscillation mode. These differences can be explained by the features of interatomic interactions in W and V.

Atomic-Scale Photon Mapping Revealing Spin-Current Relaxation.

A nanoscopic understanding of spin-current dynamics is crucial for controlling the spin transport in materials. However, gaining access to spin-current dynamics at an atomic scale is challenging. Therefore, we developed spin-polarized scanning tunneling luminescence spectroscopy (SP STLS) to visualize the spin relaxation strength depending on spin injection positions. Atomically resolved SP STLS mapping of gallium arsenide demonstrated a stronger spin relaxation in gallium atomic rows. Hence, SP STLS paves the way for visualizing spin current with single-atom precision.

Predicting and explaining crystallographic orientation relationships of exsolved precipitates in garnet using the edge‐to‐edge matching model

AbstractRelative alignments of mineral exsolutions and their host crystals can be described by crystallographic orientation relationships (COR). Exsolved phases in garnet from high‐grade metamorphic rocks and igneous rocks may have COR, but the complexity of COR distributions has thus far restricted their use to identifying exsolved phases. Classification of COR also remains mineral‐specific, leaving doubt as to what information COR preserve. To test how COR may be standardized, we calculated mismatch of low‐index crystallographic planes (d‐value ratios) and crystallographic directions (rows of atoms) between precipitates and garnet and defined search criteria for structural alignments likely to be energetically favourable. We analysed published electron backscatter diffraction (EBSD) data for apatite, rutile, ilmenite, corundum, and quartz precipitates in garnet (ntot = 1,296) for the presence of these alignments. Our method predicts between 88% and 98% of observed alignments across the studied minerals and requires only calculations using unit cell parameters. We further show that each exsolved mineral forms COR predicted by the edge‐to‐edge matching model which was developed to describe unambiguous exsolution textures in alloys. Edge‐to‐edge matching aligns atoms at the host–precipitate interface by parallelism or near‐parallelism of crystallographic planes of similar spacing and parallelism of crystallographic directions (rows of atoms) of similar length on the edges of those planes. Edge‐to‐edge matches likely facilitate coherent to semi‐coherent interfaces by lowering surface free energy and strain energy, stabilizing precipitates. These matches are defined by criteria applicable to all minerals, making them an ideal tool for classifying, discovering and interpreting COR of diverse precipitates in garnet. This approach may also predict COR in other geological mineral pairs (e.g., exsolved feldspars). We find that edge‐to‐edge matching may explain the stability of the needle‐shaped morphologies commonly observed for exsolution textures in garnet. Edge‐to‐edge matching COR distributions can be tested as proxies of the state of the host rock at the time of exsolution to evaluate factors such as temperature, cooling rate, degree of undercooling, and/or strain. Patterns of edge‐to‐edge matching COR may be paired with geothermobarometry and/or petrochronology to provide a powerful new tool for studying the histories of granulite and eclogite facies metamorphic and igneous rocks.

Mechanism of Interlayer Transport on a Growing Au(111) Surface: 2D vs. 3D Growth

The atomic scale transitions corresponding to diffusion and interlayer transport of a Au adatom on the low energy, close packed Au(111) surface are studied using density functional theory calculations within the generalized gradient approximation. Minimum energy paths and estimates of activation energy are calculated for processes that influence whether the crystal grows layer-by-layer, i.e. 2D growth, or whether new islands tend to nucleate on top of existing islands resulting in 3D growth. Kinks on island edges turn out to provide paths for adatom descent with lower activation energy than straight steps. The energy barrier for an adatom to round the corner and enter a kink site is significantly higher. A descent mechanism that places an adatom near but not at a kink site can therefore promote the formation of a new row of step atoms and lead to the introduction of additional kink sites, thereby opening up new low activation energy paths for descent and promotion of 2D growth. The sites adjacent and above the step edge provide large binding energy for the adatom, especially at the B-type step, and form a trough along which the adatom can migrate before descending, thereby increasing the probability that an adatom finds a kink on the B-type step. These features of the energy landscape representing the interaction of a Au adatom with the surface point to the possibility of a re-entrant layer-by-layer growth mode of the low energy, close packed surface of the gold crystal.