Nearest-neighbour Atoms Research Articles

WSe2 Monolayer: A Stable Two-Dimensional Heterostructure Material from First-Principles of Simulation Calculations

P-N junctions or heterostructures are commonly used in the fabrication of self-driven photodetectors because they serve critical roles in influencing the electrical characteristics of two-dimensional (2D) materials. For stable 2D heterostructures, planar WSe2 monolayers have attracted a lot of interest. Using first-principles energetic and dynamic calculations, we find that the transition from the indirect band gap of WSe2 bulk phase to the direct band gap of a two-dimensional plane is caused by the formation of p-n hetero junctions. Furthermore, it affects the carrier concentration transport mode and results in a significant orbital spin coupling. There is no virtual frequency in the phonon dispersion curve across the Brillouin zone at zero pressure. The interaction of the nearest neighbour atoms causes the frequency of the two acoustic modes of the G-point to be zero, as well as the frequency of the two optical films to merge. The theoretical analysis can provide support for the transport of two-dimensional planar carriers in electronics and photonics.

Local structure and electron density distribution analysis of tin(II) sulfide using pair distribution function and maximum entropy method

Abstract Tin(II) sulfide (SnS) is a low symmetric orthorhombic double-layered dual bandgap semiconductor. It is low cost, toxic-free and highly abundant on Earth, with multifunctional optical, electronic, magnetic and light conversion applications when doped adequately with impurity. These physical properties can be understood only by the complete understanding of microstructural properties like average structure, electron density distribution inside the unit cell, bonding nature and local structure. In this work, the average and local structure, along with the electron density distribution of a nano crystallite sized single-phase sample of tin(II) sulfide is elucidated with the help of precise X-ray intensity data. The average structural information was extracted using Rietveld refinement analysis and the visual mapping of 3D, 2D and 1D electron density distribution inside the unit cell and its numerical contribution using maximum entropy method (MEM). The bonding between the first inter and intra bonding between Sn and S atoms is 2.65,105 Å and 3.2689 Å with mid bond electron density 0.907 e/Å3 and 0.1688 e/Å3 respectively. The inter-atomic correlations of 1st, 2nd and 3rd nearest neighbour atoms, their bond length, and the crystallite size are reported from pair distribution function (PDF) analysis using low Q-XRD data (Q ∼ 6.5 Å−1). The PDF analysis shows that the first and second nearest Sn–S bonding distance is 2.6064 Å and 3.4402 Å, first is between the Sn and S atoms of the same layer and the other between the Sn and S atoms of the adjacent layers respectively.

Temperature dependence of the correlation displacement functions of atoms under pressure effects for Cu x Ag1−x alloy in EXAFS theory

Reported here are expressions temperature dependence under pressure effects for the mean square displacement, the mean square relative displacement, the correlation displacement function of atoms, and the ratio of the relationship between the quantities due to anharmonicity under pressure effects in extended x-ray absorption fine structure spectra. The expressions are determined using the anharmonic correlated Debye model and the anharmonic Debye model. Complicated calculations due to the many-particle effects and anharmonic properties are replaced by a calculation based on the effective anharmonic potential, including the interaction of absorbing and scattering atoms with their nearest-neighbour atoms. Based on the Debye–Waller factor, the difference between the mean square relative displacement and mean square displacement is analyzed, and their ratios are calculated. This work is applied to face-centered cubic crystals and their alloys. Numerical results for copper (Cu-Cu), silver (Ag-Ag) crystals and copper–silver alloys CuxAg1−x agree with experimental values and other studies.

Intramolecular crossover from unconventional diamagnetism to paramagnetism of palladium ions probed by soft X-ray magnetic circular dichroism

The case of palladium(II) ions in molecular polyoxopalladates highlights the importance of accounting not only for nearest neighbour atoms or ions in order to understand, model or predict magnetic characteristics. Here, using site-specific soft X-ray magnetic circular dichroism (XMCD), the effects of different bond lengths, delocalization of 4d electrons, and 4d spin-orbit coupling on the electronic and magnetic properties are investigated and three different states identified: Conventional diamagnetism in a square-planar O4 coordination environment, paramagnetism caused by four additional out-of-plane oxygen anions, and an unusual diamagnetic state in the diamagnetic/paramagnetic crossover region modified by significant mixing of states and facilitated by the substantial 4d spin-orbit coupling. The two diamagnetic states can be distinguished by characteristic XMCD fine structures, thereby overcoming the common limitation of XMCD to ferro-/ferrimagnetic and paramagnetic materials in external magnetic fields. The qualitative interpretation of the results is corroborated by simulations based on charge transfer multiplet calculations and density functional theory results.

Spin transport in a rippled graphene periodic chain

We discuss the solution of the scattering problem of ballistic electrons on the finite periodic chain. Each element of this chain is composed of a combination of a planar and rippled graphene elements. The analysis is carried out in the effective mass approximation when only the interaction between nearest neighbour atoms is taken into account. We find that, with the increase of number of elements, the chain becomes more transparent for electrons with spin up travelling in one direction and for electrons with spin down travelling in the opposite direction.

RETRACTED ARTICLE: A universal principle for a rational design of single-atom electrocatalysts

Developing highly active single-atom catalysts for electrochemical reactions is a key to future renewable energy technology. Here we present a universal design principle to evaluate the activity of graphene-based single-atom catalysts towards the oxygen reduction, oxygen evolution and hydrogen evolution reactions. Our results indicate that the catalytic activity of single-atom catalysts is highly correlated with the local environment of the metal centre, namely its coordination number and electronegativity and the electronegativity of the nearest neighbour atoms, validated by available experimental data. More importantly, we reveal that this design principle can be extended to metal–macrocycle complexes. The principle not only offers a strategy to design highly active nonprecious metal single-atom catalysts with specific active centres, for example, Fe-pyridine/pyrrole-N4 for the oxygen reduction reaction; Co-pyrrole-N4 for the oxygen evolution reaction; and Mn-pyrrole-N4 for the hydrogen evolution reaction to replace precious Pt/Ir/Ru-based catalysts, but also suggests that macrocyclic metal complexes could be used as an alternative to graphene-based single-atom catalysts.

АТОМЫ УГЛЕРОДА В МЕЖДОУЗЛИЯХ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ ЦЕМЕНТИТА: AB INITIO МОДЕЛИРОВАНИЕ

It was shown in previous works that iron sublattice in the structure of cementite Fe3C has four different types of interstitial sites that may potentially be occupied by carbon atoms, viz. ‘normal’ and ‘distorted’ prismatic (NP, DP) and ‘normal’ and ‘distorted’ octahedral (NO, DO) sites. Distances between their centres and the centres of z nearest-neighbour iron atoms are 1.99–2.04 A (NP sites, z = 6); 1.80–1.87 A (NO sites, z = 6); 1.62 A (DP sites, z = 4) and 1.25 A (DO sites, z = 2). It is usually believed that all carbon atoms are located in NP positions. In this work other possible variants are considered by ab initio calculations using FP-LAPW full-potential method as realized in WIEN2k program package and a cementite supercell containing 16 atoms (4 formula units). It is found that the structure containing all carbon atoms in NP positions has the lowest energy and volume. The structure with all carbon atoms in NO positions has the energy 0.267 eV/f.u. higher and the volume 9.20 % greater. If only one of the four carbon atoms in the supercell is shifted from NP to NO site the energy and volume increase by 0.233 eV/f.u. and 3.59 % correspondingly. The structure with one carbon atoms located in DP site and the other three in DO sites has the energy 0.452 eV and the volume 3.75 % higher than those of cementite with all carbon atoms in NP sites. The structures with all carbon atoms in DP sites and one or all carbon atoms in DO sites are mecha-nically unstable and transform spontaneously to conventional NP structure. Thermodynamic estima-tion based on these data shows that even at temperatures below 1000 K the fraction of carbon atoms that may move from NP sites to other ones may be as great as 20 %. Evaluation of the vacancy formation energy in cementite (supercell of 128 atoms containing 96 atoms of Fe and 32 atoms of C) resulted in the values of 0.50 eV for carbon vacancy and 1.34 (FeG) or 1.60 eV (FeS) for iron vacancy. In all these cases vacancy formation did not result in volume change of the system.

A structural investigation of the interaction of oxalic acid with Cu(110)

The interaction of oxalic acid with the Cu(110) surface has been investigated by a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), soft X-ray photoelectron spectroscopy (SXPS), near-edge X-ray absorption fine structure (NEXAFS) and scanned-energy mode photoelectron diffraction (PhD), and density functional theory (DFT). O 1s SXPS and O K-edge NEXAFS show that at high coverages a singly deprotonated monooxalate is formed with its molecular plane perpendicular to the surface and lying in the [11¯0] azimuth, while at low coverage a doubly-deprotonated dioxalate is formed with its molecular plane parallel to the surface. STM, LEED and SXPS show the dioxalate to form a (3 × 2) ordered phase with a coverage of 1/6 ML. O 1s PhD modulation spectra for the monooxalate phase are found to be simulated by a geometry in which the carboxylate O atoms occupy near-atop sites on nearest-neighbour surface Cu atoms in [11¯0] rows, with a CuO bondlength of 2.00 ± 0.04 Å. STM images of the (3 × 2) phase show some centred molecules attributed to adsorption on second-layer Cu atoms below missing [001] rows of surface Cu atoms, while DFT calculations show adsorption on a (3 × 2) missing row surface (with every third [001] Cu surface row removed) is favoured over adsorption on the unreconstructed surface. O 1s PhD data from dioxalate is best fitted by a structure similar to that found by DFT to have the lowest energy, although there are some significant differences in intramolecular bondlengths.

Athermal migration of vacancies in iron and copper induced by electron irradiation

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.

Structure analyses of Cu nanoclusters in the soft magnetic Fe85.2Si1B9P4Cu0.8 alloy by XAFS and fcc cluster model

Size of the clusters and structure details of fcc Cu clusters in nanocrystalline soft magnetic alloy of Fe85-86Si1-2B8P4Cu1 (NANOMET) are investigated. A linear combination fitting of XAFS data indicates that about 30% of Cu atoms are partitioned in the fcc clusters and the rest in the amorphous matrix. EXAFS of the fcc Cu nanocluster embedded in amorphous matrix is calculated on the basis of a simple fcc structure model using FEFF9. Surface effect of the nanoclusters is considered by counting a fraction of the nearest neighbour atoms in amorphous matrix. Good agreement with the experimental result is obtained for the fcc nanocluster with 9 coordination shells which consists of total 177 atoms within 1.5 nm in a diameter.

Soliton solution in nonlinear lattice with nearest neighbour Born–Mayer interaction

We study the dynamics of one-dimensional uniform lattice with the interatomic Born–Mayer potential. The travelling wave solutions such as solitons are analytically described. The wave propagation in the one-dimensional lattice where nearest neighbour atoms interact via the Born–Mayer potential is considered. The Born–Mayer lattice admits travelling wave type solutions represented by Jacobian elliptic functions and limiting form of such a wave solution is the localized pulse-like form called the solitary wave. This solitary wave has further remarkable properties under collision, leading to the concept of solitons in nonlinear discrete lattices which has been studied.

Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe‐Mn alloys: A first‐principles study

Hydrogen interstitials in austenitic Fe-Mn alloys were studied using density-functional theory to gain insights into the mechanisms of hydrogen embrittlement in high-strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest-neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity.

A novel pitch evaluation of one-dimensional gratings based on a cross-correlation filter

If one-dimensional (1D), p-period and arbitrarily structured grating position-related topographical signals coexist with noise, it is difficult to evaluate the pitch practically using the center-of-gravity (CG) method. The Fourier-transform-based (FT) method is the most precise to evaluate pitches; nevertheless, it cannot give the uniformity of pitches. If a cross-correlation filter—a half period of sinusoidal waveform sequence (pT period), cross-correlates with the signals, the noise can be eliminated if pT ≈ p. After cross-correlation filtering, the distance between any two adjacent waveform peaks along the direction perpendicular to 1D grating lines is one pitch value. The pitch evaluation based on the cross-correlation filtering together with the detection of the peak's position is described as the peak detection (PD) method in this paper. The pitch average and uniformity can be calculated by using the PD method. The computer simulation has indicated that the average of pitch deviations from the true pitch and the pitch variations are less than 0.2% and 0.2% for the sinusoidal and rectangular waveform signals with up to 50% uniform white noise, less than 0.1% and 1% for the sinusoidal and rectangular waveform signals and 0.6% and 2.5% for the triangular waveform signal if three waveform signals are mixed with Gaussian white, binomial and Bernoulli noise up to 50% in standard deviation, one probability and trial probability, respectively. As examples, a highly oriented pyrolytic graphite (HOPG) with a 0.246 nm distance between second nearest neighbour atoms and a 1D grating with 3000 nm nominal pitch are measured by a ultra-high vacuum scanning tunneling microscope (UHV STM) and a metrological atomic force microscope (AFM), respectively. After the position-related topographical signals are cross-correlation filtered, the 0.240 and 3004.11 nm pitches calculated by using the PD method are very close to the 0.240 and 3003.34 nm results evaluated by the FT method.

Spin dynamics in one-dimensional optical lattices

Dynamics for localized spin-wave packets (localized magnons) are studied in a two-component ultra-cold atom system trapped in one-dimensional optical lattices. The centre of the Gaussian wave packet produced in a ferromagnetic chain shows Bloch oscillations under an additional transverse gradient magnetic field, which can be applied to measure the many-body superexchange interaction between the nearest-neighbour atoms.

Nucleation of size calibrated three-dimensional nanodots in atomistic model of strained epitaxy: a Monte Carlo study

We introduce a simple model of strained epitaxy to study the growth of coherent several-monolayers-high nanoislands at low coverage. The size mismatch between the substrate (which is assumed to be rigid and passive) and the growing overlayer is modelled by the hard-sphere interaction between the adatoms. In the case of positive misfit the sphere diameter is larger than the substrate lattice parameter. Elastic interactions are described within the harmonic approximation to the substrate potential in the vicinity of the deposition sites. With additional attractive interaction between the nearest neighbour atoms the model exhibited the growth of miniature nanoislands of several morphologies with their kinetics similar to those found in real quantum dot (QD) systems. The 3D QDs exhibited narrow distributions of their heights, diameters and volumes in qualitative agreement with experimental observations.

High precision electronic charge density determination for L10-ordered γ-TiAl by quantitative convergent beam electron diffraction

Low order structure and Debye–Waller (DW) factors for tetragonal L10-ordered γ-TiAl were measured simultaneously using quantitative convergent beam electron diffraction. The high precision and accuracy (largest error <0.5%) measurements allowed the construction of charge density difference maps from full sets of structure and DW factors, suitable for validation of first principles density functional theory (DFT) calculation results. Comparison of the experimentally determined charge density distribution with theoretical DFT predictions shows excellent qualitative agreement in this study. The three-dimensional charge density representations indicate a large electron charge localisation centred about the tetrahedral site at ¼, ¼, ¼, which is coordinated by two Ti atoms at 0, 0, 0 and ½, ½, 0 and two Al atoms at ½, 0, ½ and 0, ½, ½, respectively. Compared to experimental data, the DFT calculations based on full-potential linearised augmented plane wave (LAPW) method (implemented in WIEN2K) were found to quantitatively overestimate charge density between Ti–Ti second nearest neighbour atoms. Moreover, the results from the DFT method based on the projector-augmented wave (PAW) method and a plane wave basis set (implemented in VASP) were found to differ appreciably from both the experimental and LAPW-DFT results, implying that the PAW approach may not accurately describe the bonding in the intermetallic systems with 3 d electrons, such as γ-TiAl.

Structural Changes of Liquid Pb-Bi Eutectic Alloy

Liquid Pb–Bi eutectic (LBE) alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). The alloy has been characterized in liquid state from melting (125 °C) to 750 °C by mechanical spectroscopy, i.e. internal friction (IF) and dynamic modulus measurements. The experiments have been carried out using hollow reeds of austenitic stainless steel filled with Pb-Bi alloy and sealed at the extremities. Dynamic modulus showed a remarkable drop in the range 350-520 °C. In the same temperature range radial distribution functions (RDFs), determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The phenomenon has been explained as a structural re-arrangement of atoms in the liquid metal.

Ligand-field symmetry effects in Fe(ii) polypyridyl compounds probed by transient X-ray absorption spectroscopy

Ultrafast excited-state evolution in polypyridyl Fe(II) complexes is of fundamental interest for understanding the origins of the sub-ps spin-state changes that occur upon photoexcitation of this class of compounds as well as for the potential impact such ultrafast dynamics have on incorporation of these compounds in solar energy conversion schemes or switchable optical storage technologies. We have demonstrated that ground-state and, more importantly, ultrafast time-resolved X-ray absorption methods can offer unique insights into the interplay between electronic and geometric structure that underpins the photo-induced dynamics of this class of compounds. The present contribution examines in greater detail how the symmetry of the ligand field surrounding the metal ion can be probed using these X-ray techniques. In particular, we show that steady-state K-edge spectroscopy of the nearest-neighbour nitrogen atoms reveals the characteristic chemical environment of the respective ligands and suggests an interesting target for future charge-transfer femtosecond and attosecond spectroscopy in the X-ray water window.

Mechanical Spectroscopy Investigation of Liquid Pb-Bi Alloys

Liquid Pb–Bi eutectic alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system. The alloy has been characterized in liquid state from melting (125 °C) to 650 °C by mechanical spectroscopy. Experiments have been carried out using hollow reeds of austenitic stainless steel filled with the Pb-Bi alloy and sealed at the extremities. From 350 °C to 520 °C modulus shows a remarkable drop accompanied by a broad internal friction maximum. In the same temperature range radial distribution functions, determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The anelastic phenomena have been attributed to a structural re-arrangement of liquid metal. For comparison, other alloys of the Pb-Bi system with hypo-eutectic composition have been investigated.

Ferrimagnetism and disorder of epitaxial Mn2−xCoxVAl Heusler compound thin films

The quaternary full Heusler compound Mn2−xCoxVAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0–2 were prepared by dc and RF magnetron co-sputtering on heated MgO (0 0 1) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn2VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with the help of band structure calculations in the coherent potential approximation. Mn2VAl is very sensitive to disorder involving Mn, because nearest-neighbour Mn atoms couple antiferromagnetically. Co2VAl has B2 order and has reduced magnetization. In the cases with x ⩾ 0.9 conventional ferromagnetism was observed, closely related to the atomic disorder in these compounds.