Atomic Rows Research Articles

Mechanisms of micro-voids formation caused by optical breakdown in KCl single crystals

The phenomenon of optical breakdown has been studied experimentally for KCl single crystals exposed to laser emission focused on the neodymium glass with modulated quality-factor, pulse duration 5·10-8 s, wavelength λ= 1054 nm, and pulse energy of the order 1 J in the regime of local intrinsic absorption of the laser emission by the single crystal. Evaluations of local heat flash energetic constituents and characteristic durations for both local area heating and relaxation processes and following comparison with experimental results have shown that the relaxation process takes place in two stages: the first is fast phasefollowed by crowdion mass transfer with shock wave participation, and the second is slow phase with participation also dislocation mass transfer. The energy losses for heat radiation and thermal conductivity are found to be by orders of value less than the absorption energy flux Iabs that provides fast local heating and plasma formation. From the viewpoint of the mechanics of continua the process under study where the pressure achieves value exceeding the theoretical strength limit for the time less 10-6 s, should be considered as explosion-like or shock process. The general scheme of plastic deformation arising from abovementioned estimations and observations is seemed as follows. In the beginning, under action of the shock wave the crowdions are generated which carry the substance from the high pressure area and move along close-packed atomic rows (<110> type directions in KCl crystals); the void is formed almost completely during the shock wave passing the relaxation zone crosssection. This time is of the order of τrel, i. e. 10-9…10-8 s. After falling temperature and pressure and vapor condensation into liquid, at the end of relaxation process, the void boundaries expand already under liquid melt pressure, and the mass transfer dislocation mechanism comes into action providing additionally some enlarging the void volume. This process continues also after stopping the laser emission, during the crystal cooling down to the melt crystallization in the void and formation of a pore with size observed.

Electronic structure of first and second row atoms under harmonic confinement

AbstractAtoms under pressure undergo a number of modifications of their electronic structure. Good examples are the spontaneous ionization, stabilization of excited‐state configurations, and contraction of atomic‐shells. In this work, we study the effects of confinement with harmonic potentials on the electronic structure of atoms from H to Ne. Dynamic and static correlation is taken into account with coupled cluster with single and double excitations and CASSCF calculations. Because the strength of harmonic confinement cannot be translated into pressure, we envisioned a “calibration” method to transform confinement into pressure. We focused on the effect of confinement on: (a) changes of electron distribution and localization within the K and L shells, (b) confinement‐induced ionization pressure, (c) level crossing of electronic states, and (d) correlation energy. We found that contraction of valence and core‐shells are not negligible and that the use of standard pseudopotentials might be not adequate to study solids under extreme pressures. The critical pressure at which atoms ionize follows a periodic trend, and it ranges from 28 GPa for Li to 10.8 TPa for Ne. In Li and Be, pressure induces mixing of the ground state configuration with excited states. At high pressure, the ground states of Li and Be become a doublet and a triplet with configurations 1s22p and 1s22s2p, respectively, which could change the chemistry of Be. Finally, it is observed that atoms with fewer electrons correlation increases, but for atoms with more electrons, the increasing of kinetic energy dominates over electron correlation.

Features of mass transfer by N-crowdions in fcc Ni3Al lattice

Crowdions being interstitial atoms located in closely packed atomic rows, play an important role in relaxation processes occurring in metals and alloys under severe external impacts effectively transferring mass and energy. Dynamics of crowdions motion is well studied for pure metals, while for intermetallic alloys this issue has not previously been addressed. It was earlier shown that the so-called N crowdion that involves more than one atom in its 1D motion is much more effective in frames of propagation path and excitation energy that 1-crowdion. In the present study, the motion of 1- and 2-crowdions in the Ni3Al intermetallic alloy, was studied by means of the molecular dynamics method. N-crowdion was excited by providing the same value of initial energy to N neighbouring atoms along the close-packed atomic row. It was shown that the studied crowdions can propagate along the biatomic close-packed row, but show strong instability when moving in monatomic close-packed directions. It has been established that 2-crowdions have a mean free path 2-2.5 times longer than 1-crowdions with the same initial velocity. It was also revealed that the mean free path of the nickel atom initiated crowdion than that initiated on the aluminium atom. The results suggest a higher efficiency of 2-crowdions in mass transfer in the Ni3Al intermetallic alloy. In further works, it is proposed to study the possibility of launching 2-crowdions by bombarding the crystal surface with biatomic molecules.

Excitation of Soliton-Type Waves in Crystals of the A3B Stoichiometry

Using the method of molecular dynamics and taking Ni3Al and Pt3Al as examples, crystals of the A3B composition are considered for the possibility of excitation of soliton-type waves in them. The potentials obtained by the embedded-atom method were used to describe interatomic interactions. It is shown that the harmonic external stimulus can excite waves of the soliton type in a Pt3Al crystal, but not in Ni3Al. Such compression–expansion waves are generated because of excitation of discrete breathers with soft nonlinearity that cannot exist in a Ni3Al crystal near the affected region. The detected waves are capable of propagating to thousands of nanometers along the Pt3Al crystal without losses of integrity and speed. The shape of the obtained wave corresponds to the kink solution of the sine-Gordon equation. The aggregate amount of energy transferred by a wave is determined by the number of rows of atoms involved in fluctuations; this may involve dozens and hundreds of electron volts.

Near-equilibrium growth from borophene edges on silver.

Two-dimensional boron, borophene, was realized in recent experiments but still lacks an adequate growth theory for guiding its controlled synthesis. Combining ab initio calculations and experimental characterization, we study edges and growth kinetics of borophene on Ag(111). In equilibrium, the borophene edges are distinctly reconstructed with exceptionally low energies, in contrast to those of other two-dimensional materials. Away from equilibrium, sequential docking of boron feeding species to the reconstructed edges tends to extend the given lattice out of numerous polymorphic structures. Furthermore, each edge can grow via multiple energy pathways of atomic row assembly due to variable boron-boron coordination. These pathways reveal different degrees of anisotropic growth kinetics, shaping borophene into diverse elongated hexagonal islands in agreement with experimental observations in terms of morphology as well as edge orientation and periodicity. These results further suggest that ultrathin borophene ribbons can be grown at low temperature and low boron chemical potential.

Multi-axial and multi-energy channeling study of disorder evolution in ion-irradiated nickel

To better understand defect structure and the evolution of irradiation-induced damage in single crystal Ni, in situ Rutherford backscattering spectroscopy in channeling geometry (RBS/C) is performed along the <100>, <110>, and <211> axes with different probing beam energies. The RBS/C data reveal that damage evolution occurs in three steps. The first step at low doses (up to 0.2 dpa), characterized by a linear increase with dose, is related to the formation of point defects and small clusters. The second step in the intermediate dose range (0.2–1.0 dpa) shows a sublinear increase in disorder to saturation. This sublinear increase is due to the growth of defect clusters resulting from the interaction of irradiation-induced defects with already existing damage from previous ion impacts. The third step at high doses (1.0–32.4 dpa) exhibits a surprising decrease in the disorder level, which may be attributed to defect evolution from black spot defects to large dislocation loops that leads to strain relaxation. In addition, the damage extends much deeper than the predicted depths and is attributed to long-range defect migration effects confirmed by transmission electron microscopy (TEM) observations. Although similar damage evolution trends have been observed along all channeling directions, the disorder accumulation is largest along the <100> and <211> axes than observed along <110> axis. This “preferential” disordering process along <100> and <211> suggests that more defects are shielded by the <110> atomic rows than the two other axes. The co-existence of both uncorrelated displaced lattice atoms and dislocation loops in Ni irradiated at 1 ion/nm2 is revealed by the energy-dependent RBS/C studies along all three axes. In contrast, dislocation loops and stacking fault tetrahedral are simultaneously present in the crystal structure of Ni irradiated at 100 ions/nm2 that is consistent with previous molecular dynamics simulations and TEM observations.

“Dual contrast mode” imaging of anatase TiO2(001) − (1 × 4) reconstructed surface using non-contact atomic force microscopy

Different imaging modes of anatase TiO2(001) − (1 × 4) reconstructed surface obtained with non-contact atomic force microscopy are investigated. In addition to the three typical types of imaging mode (protrusion, hole, and neutral modes) previously reported, we obtained “dual contrast” mode images in which two different tip apex atoms provide contrast. The new imaging mode has two atomic rows that are acquired in different spatial resolutions.

Initial Fe3O4(100) Formation on Fe(100)

The initial oxidation of Fe(100) at 400 °C has been studied by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction, in combination with density functional theory calculations. The first observed well-ordered surface oxide is formed at a coverage of ∼3 oxygen atoms per unreconstructed surface Fe(100) atom. STM shows that this surface oxide is terminated by straight atomic rows exhibiting a p(2 × 1) periodicity. However, already for oxide films with a coverage of ∼4 oxygen atoms (corresponding to one Fe3O4 unit cell thickness), wiggly atomic rows appear similar to the c(2 × 2) reconstructed Fe3O4(100)-surface with the Fe3O4 unit vectors rotated 45° to Fe(100). The wiggly rows are a consequence of subsurface cation iron vacancies, which previously have been observed for bulk surfaces. The formation of subsurface vacancies is supported by the XPS O 1s signature, which is modeled by considering the core-level shifts for all oxygen atoms in the film. Throughout the oxidation series, the microscopy results reveal a layer-by-layer (Frank–van der Merwe) growth.

Initial stages of the epitaxial growth of MnN on the GaAs (001)-(2 × 2) surface: First-principle study

First-principle total-energy calculations have been employed to investigate the epitaxial growth of manganese nitride (MnN) on the non-reconstructed GaAs (001)-(2 × 2) surface. Also, we have studied the surface nitridation process proposed by Gwo and Tokumoto. According to results, the surface nitridation generates a (3 × 3) flat surface that is composed by an array of missing N atomic rows along both [−110] and [110] directions. The initial stages of the epitaxial growth of MnN have been investigated by considering the adsorption and incorporation of Mn atoms. Calculations show that for ¼ ML coverage the most favorable adsorption site of Mn induces an fcc configuration and for the Mn incorporation an hcp-2 arrangement. The surface formation energy (SFE) yields the adsorption of Ga adatoms at the hcp-2 site as a stable configuration; in addition, a surface with 3 Mn atoms replacing 3 Ga atoms of the 1st ML is stable for intermediate and As-rich conditions. When only one MnN ML is considered, three interfaces may be formed with 25%, 75% and 100% of Mn, which yield stable configurations showing the FM/AFM interface. A second MnN ML is considered; in this case, only one structure is stable, and an Mn/MnN interface is formed. Calculations show that the interface exhibits an FM/AFM coupling structure.

Interatomic interactions and structural formations in WFeNi(Ti) and MoFeNi(Ti) layers under intense plastic deformation induced by ball collisions

Abstract WFeNi(Ti) and MoFeNi(Ti) alloy layers were fabricated onto Ti surfaces from W, Mo, and Ni targets through ball collisions initiated by a mechanically vibrated vial. Steel balls were used for processing as the Fe source. The as-fabricated alloy structures could be represented as a heterogeneous multi-modal nanocrystalline structure with elemental flakes embedded within a matrix composed of nanocrystalline grains encapsulated within an amorphous phase. The deformation-induced chemical interactions between dissimilar atoms in the WFeNi(Ti) and MoFeNi(Ti) systems affected the structural formation of the alloy layers. Fe atoms in the WFeNi(Ti) alloy system tended to react with Ni, forming fcc (Fe,Ni) phase, while W atoms dissolved into the fcc (Fe,Ni) lattice. In the MoFeNi(Ti) alloy system, Fe tended to react with Mo over Ni, resulting in the formation of a bcc Fe(NiMo) solid solution. Formation of an amorphous phase was mainly related to compositional changes and distortion of the bcc Fe atomic rows with refractory metal atoms. The W and Mo nanocrystalline grains reduced their size continuously during processing via amorphization and wear effects. The Ti particles absorbed oxygen atoms. Chemical interactions between Ti and oxygen atoms made the Ti and oxygen components passive with regard to other present metallic elements. The formation of atomic clusters was related to the attractive bonding forces between dissimilar atoms that bonded the atoms together. The yield stress value of the as-fabricated layers evaluated via nanoindentation was (2.5 ± 0.4) GPa, which was much higher than the yield stress for tungsten heavy alloys.

Low-energy channel for mass transfer in Pt crystal initiated by molecule impact

Crystal surface bombardment by atoms or molecules, neutral or ionized, occurs both in ambient conditions and in many technological operations, such as surface plasma treatment, ion implantation, etc. Recently, it was established that the impact of a molecule initiates the mass transfer in the one-dimensional Frenkel-Kontorova atomic chain more efficiently than that of a single atom. This is explained by the fact that the atom can initiate only a very sharp, fast-moving crowdion (anti-kink), which requires relatively high energy, while the molecule is able to initiate a less localized crowdion with considerably lower velocity and energy. In the current study, by means of molecular dynamics simulation, for the first time, this phenomenon is studied for a realistic 3D model of platinum crystal. We compare the efficiency of single Pt atom impact and Pt2 molecule impact on the (101) surface of fcc Pt crystal for the initiation of mass transfer in the material by crowdions. It is revealed that in order to generate a crowdion moving inside the crystal, the properly oriented molecule needs an order of magnitude smaller energy than single atom. This considerable reduction of required energy happens when the molecule is oriented perpendicularly to the crystal surface and hits the crystal along a close-packed atomic row. Furthermore, it is revealed for the first time that the molecule with sufficiently large velocity can initiate the so-called supersonic 2-crowdion, which travels longer distances in the crystal than the classical supersonic crowdion having same or even higher energy. Our results can be useful for understanding and prediction the mass transfer during technological applications where bombardment by atomic clusters is employed to modify and improve mechanical or functional properties of surfaces.

Orbital-dependent backflow wave functions for real-space quantum Monte Carlo

We present and motivate an efficient way to include orbital dependent many--body correlations in trial wave function of real--space Quantum Monte Carlo methods for use in electronic structure calculations. We apply our new orbital--dependent backflow wave function to calculate ground state energies of the first row atoms using variational and diffusion Monte Carlo methods. The systematic overall gain of correlation energy with respect to single determinant Jastrow-Slater wave functions is competitive with the best single determinant trial wave functions currently available. The computational cost per Monte Carlo step is comparable to that of simple backflow calculations.

Quantum chemistry with Coulomb Sturmians: Construction and convergence of Coulomb Sturmian basis sets at the Hartree-Fock level

The first discussion of basis sets consisting of exponentially decaying Coulomb Sturmian functions for modelling electronic structures is presented. The proposed basis set construction selects Coulomb Sturmian functions using separate upper limits to their principle, angular momentum and magnetic quantum numbers. Their common Coulomb Sturmian exponent is taken as a fourth parameter. The convergence properties of such basis sets are investigated for second and third row atoms at the Hartree-Fock level. Thereby important relations between the values of the basis set parameters and the physical properties of the electronic structure are recognised. For example, an unusually large limit for the angular momentum quantum number in unrestricted Hartree-Fock calculations can be linked to the breaking of spherical symmetry in such cases. Furthermore, a connection between the optimal, i.e. minimum-energy, Coulomb Sturmian exponent and the average Slater exponents values obtained by Clementi and Raimondi (E. Clementi and D. L. Raimondi, J. Chem. Phys. 38, 2686 (1963)) is made. These features of Coulomb Sturmian basis sets emphasise their ability to correctly reproduce the physical features of Hartree-Fock wave functions.

Возбуждение волн солитонного типа в кристаллах стехиометрии A-=SUB=-3-=/SUB=-B

In the work, using the molecular dynamics method, the crystals of composition А3В are considered, for example Ni3Al and Pt3Al, for the possibility of excitation of soliton-type waves in them. To describe the interatomic interactions, the potentials obtained by the immersed atom method were used. It is shown that under a harmonic external action, excitation of soliton-type waves is possible in a Pt3Al crystal, but not in Ni3Al. The occurrence of such compression-extension waves is due to the excitation near the zone of action of discrete breathers with a soft type of nonlinearity, the existence of which is impossible in a Ni3Al crystal. The detected waves can propagate thousands of nanometers along a Pt3Al crystal without any loss of shape or speed. The shape of the received wave corresponds to the kink solution of the sin-Gordon equation. The total amount of energy carried by the wave is determined by the number of rows of atoms involved in the oscillations, we can talk about tens and hundreds of electron volts

Dynamics and Stability of Subsonic Crowdion Clusters in 2D Morse Crystal

Recently, the concept of supersonic N-crowdions was offered. In molecular dynamics simulations, they can be excited by initial kick of N neighboring atoms located in one close-packed atomic row along this row. In the present study, in 2D Morse crystal, we apply initial kick to M neighboring atoms located in neighboring close-packed atomic rows along these rows. This way, we initiate crowdion clusters called subsonic M-crowdions. It is well known that static 1-crowdion in 2D Morse lattice is unstable; as a result, the interstitial atom leaves the close-packed atomic row and becomes immobile. However, we show that 1-crowdion moving with sufficiently large subsonic velocity remains in the close-packed atomic row. Crowdion clusters with M equal to or greater than 2 appear to be stable even at rest, with growing M transforming into prismatic dislocation loops. It is important to note that stable subsonic M-crowdions (M > 1) remain mobile and they can carry interstitial atoms over long distances.

Growth and orientation relationships of Ni and Cu films annealed on slightly miscut [formula omitted] r-sapphire substrates

Nano-crystalline Ni and Cu films deposited on the r-plane of sapphire (α-Al2O3) develop a 〈1 1 1〉 fiber-texture upon annealing, in which grains grow up to 300 to 500 times larger than the film thickness. Most of the largest grains, which have grown at the expense of others, display one of four preferred orientation relationships (ORs) to the substrate. The four preferred ORs are OR1r = Me(1 1 1)[11¯0]//α-Al2O3(11¯02)[112¯0], OR2r = Me(1 1 1)[11¯0]//α-Al2O3(11¯02)[1¯101] (Me = Ni or Cu), and their twins, which are rotated 60° about the 〈1 1 1〉 axis perpendicular to the substrate. For these ORs, one of the densest 〈1 1 0〉 atomic rows that lies within the Me {1 1 1} interfacial plane, aligns with the direction of the step edges that form at the intersection of the r-plane with one of its neighboring facets on the equilibrium shape of sapphire (i.e. the c- (0 0 0 1), p-{112¯3} or s{101¯1}-planes). One of the ORs is most preferred when the steps at the interface are such that a {1 0 0}-type ledge of the Me {1 1 1} interfacial plane faces the ledge of the sapphire step. This observation provides a useful insight into the origin of the preferred ORs, and confirms the important role of surface steps in texture development. The evolution of Ni films of different thicknesses, ranging from 100 to 560 nm, was analyzed. Grain boundary grooving inhibits grain boundary motion and favors hole formation and dewetting in the thinnest films (100 nm). The crystals left behind after dewetting display ORs which may differ from the preferred ones. Large grains with the preferred ORs can grow at the expense of others when the film thickness is greater than 300 nm.

Coexistence of nanowire-like hex and (1 × 1) phases in the topmost layer of Au(100) surface

Scanning tunneling microscopy and density functional theory calculations were used to study the structure of the Au(100) surface after ion bombardment. The results indicate a development of two phases: the quasi-hexagonal (hex) and the (1 × 1) on the surface. A decrease in the number of surface atoms caused by ion bombardment leads to the development of a coexistence of phases inside the surface layer. Our experimental data and theoretical results rule out a scenario that the unreconstructed (1 × 1) domains under study represent a subsurface atomic layer revealed by ion sputtering. The hex phase is anisotropic, with the quantized width in accord with (6n + 1) formula. In certain conditions, nanowire-like structures consisting of seven atomic rows of alternating height are developed on the surface.

Atomic-Scale Structure and Stress Release Mechanism in Core-Shell Nanoparticles.

Core-shell nanoparticles find applications in catalysts, sensors, and theranostics. The full internal 3D atomic structure, however, cannot be resolved by current imaging and diffraction techniques. We analyzed the atomic positions and stress-release mechanism in a cubic Au-Pd core-shell nanoparticle in approximately 1000 times higher resolution than current experimental techniques using large-scale molecular dynamics simulation to overcome these limitations. The core-shell nanocube of 73 nm size was modeled similarly to solution synthesis by random epitaxial deposition of a 4 nm thick shell of Pd atoms onto a Au core of 65 nm side length using reliable interatomic potentials. The internal structure reveals specific deformations and stress relaxation mechanisms that are caused by the +4.8% lattice mismatch of gold relative to palladium and differential confinement of extended particle facets, edges, and corners by one, two, or three Au-Pd interfaces, respectively. The three-dimensional lattice strain causes long-range, arc-like bending of atomic rows along the faces and edges of the particle, especially near the Au-Pd interface, a bulging deformation of the Pd shell, and stacking faults in the Pd shell at the corners of the particle. The strain pattern and mechanism of stress release were further characterized by profiles of the atomic layer spacing in the principal crystallographic directions. Accordingly, strain in the Pd shell is several times larger in the extended facets than near the edges and corners of the nanoparticle, which likely affects adsorption, optical, and electrochemical properties. The findings are consistent with available experimental data, including 3D reconstructions of the same cubic nanoparticle by coherent diffractive imaging (CDI) and may be verified by more powerful experimental techniques in the future. The stress release mechanisms are representative for cubic core-shell nanoparticles with fcc structure and can be explored for different shapes by the same methods.

Particularities of trichloroethylene photocatalytic degradation over crystalline RbLaTa2O7 nanowire bundles grown by solid-state synthesis route

This is the first report on synthesis and photocatalytic activity for trichloroethylene (TCE) degradation under simulated solar light over RbLaTa2O7 layered perovskites with predominant nanowire or platelet morphologies. SEM images witnessed that the one step thermal treatment at 1200 °C lead to formation of RbLaTa2O7 nanowires with diameter of 80–320 nm and several microns in length associated in bundles and sharp-edged, merged platelets (minor phase). The two-step annealing at 950 °C and 1200 °C resulted in decrease of wires bundle population and increase in that of platelets merged in facetted particles. The RbLaTa2O7 nanowires are made of by well-aligned atomic rows with preferred orientation toward the c-axis, relatively free of defect. High density of hydroxyl groups on the sample calcined in mild conditions (RbLaTa_01) favors the photo mineralization of TCE. In contrast, the activity of RbLaTa_02 annealed in harsh conditions (950 and 1200 °C), poor in surface hydroxyl groups, remained modest. The weak surface basicity directed the reaction mainly to generation of intermediate chlorinated compounds. Pd and Au were supported on RbLaTa2O7 perovskites as an alternative strategy to boost the removal of chlorinated pollutants by combining photocatalytic (mineralization) and catalytic (hydrodechlorination, HDC) processes. The mineralization of TCE to Cl− was drastically hindered in presence of methanol due to quenching of ⋅OH radicals by alcohol. The results suggested that the density of RbLaTa2O7 surface hydroxyl groups is essential for photo mineralization of TCE whereas the surface carbonate is beneficial for the formation of intermediate chlorinated product.

Scenarios of mass transfer in fcc copper: the role of point defects

It is well known that mass transfer in crystalline materials in most cases is assisted by the motion of point defects. Interstitial atoms are highly mobile, because they have relatively small migration energy as compared to vacancies. Crowdion is a type of interstitial defect being an extra atom inserted in a closely packed atomic row and having even smaller migration energy than an interstitial atom occupying a pore. In this work, we study scenarios of collisions of supersonic N-crowdions in Cu by means of molecular dynamics simulation. It was found that 2-crowdions collide leading to the emergence of two new 2-crowdions, which is an example of the soliton-type interaction.