Stable Atomic Configurations Research Articles

On the reliability of using reverse Monte Carlo simulations to construct the atomic structure model of metallic glasses

Reverse Monte Carlo (RMC) simulations have been widely utilized to generate three-dimensional structural models for amorphous materials. One practical case is the modeling of atomic-level structure in metallic glasses, based on, for example, X-ray scattering and extended x-ray absorption fine structure (EXAFS) data. However, to what extent this approach can faithfully mimic the atomic structure of real metallic glasses has not been quantitatively evaluated. Using the “known” configurations from molecular dynamics simulations as the benchmark, we evaluate the reliability of RMC modeling with the total structure factor and EXAFS as the input for RMC fitting. The results indicate that the RMC-generated configuration lacks accuracy in reconstructing the local atomic packing, which tends towards the most random (and thus more energetically unfavored) state that can reproduce the reference data. To help guide towards a physically stable and meaningful atomic configuration, we advise the incorporation of additional constraints and validation check points.

Beyond single-atom catalysts: Exploration of Cu dimer and trimer for CO2 reduction to methane

We extend the single-atom catalysts (SACs) concept to metal-dimer and trimer catalysts anchored on graphene and explore their characteristic performance in catalyzing CO2RR into methane using first-principles calculations. In designing the catalyst, we first identify highly energetically stable Cu dimer and trimer atomic configurations in graphene host and then compute the Gibbs free energy changes in the reaction pathway from carbon dioxide to methane. Remarkably, compared to Cu SAC, the limiting potential is 0.60 V for the Cu-trimer and 1.22 V for Cu-dimer, which is lower than 1.82 V for SAC. The Cu dimer is suggested to be the optimal choice because it demonstrates reasonably high activity to CO2RR and high selectivity against HER than SAC. The methodology study suggested that the adsorption term plays the key role in the whole CO2RR process. The present work points out a new strategy for designing high-performance catalysts beyond single atoms for CO2RR.

New van der Waals Heterostructures Based on Borophene and Rhenium Sulfide/Selenide for Photovoltaics: An Ab Initio Study

One of the urgent tasks of modern materials science is the search for new materials with improved optoelectronic properties for various applications of optoelectronics and photovoltaics. In this paper, using ab initio methods, we investigate the possibility of forming new types of van der Waals heterostructures based on monolayers of triangulated borophene, and monolayers of rhenium sulfide (ReS), and rhenium selenide (ReSe2), and predict their optoelectronic properties. Energy stable atomic configurations of borophene/ReS2 and borophene/ReSe2 van der Waals heterostructures were obtained using density functional theory (DFT) calculations in the Siesta software package. The results of calculating the density of electronic states of the obtained supercells showed that the proposed types of heterostructures are characterized by a metallic type of conductivity. Based on the calculated optical absorption and photocurrent spectra in the wavelength range of 200 to 2000 nm, it is found that borophene/ReS2 and borophene/ReSe2 heterostructures demonstrate a high absorption coefficient in the near- and far-UV(ultraviolet) ranges, as well as the presence of high-intensity photocurrent peaks in the visible range of electromagnetic radiation. Based on the obtained data of ab initio calculations, it is predicted that the proposed borophene/ReS2 and borophene/ReSe2 heterostructures can be promising materials for UV detectors and photosensitive materials for generating charge carriers upon absorption of light.

Influence of Al location on formation of silver clusters in mordenite

Formation of zeolite supported Ag 0 clusters depends on a combination of thermodynamically stable atomic configurations, charge balance considerations, and mobility of species on the surface and within pores. Periodic density functional theory (DFT) calculations were performed to evaluate how the location of Al in the mordenite (MOR) framework and humidity control Ag 0 nanocluster formation. Four Al framework sites were studied (T1-T4) and the Al positions in the framework were identified by the shifts in the differential Al⋯Al pair distribution function (PDF). Furthermore, structural information about the Ag 0 nanoclusters, such as dangling bonds, can be identified by Ag⋯Ag PDF data. For Ag 0 formation in vacuum MOR structures with a Si:Al ratio of 5:1 with Al in the T1 position resulted in the most framework flexibility and the lowest Ag 0 nanocluster charge, indicating the best result for formation of charge neutral nanoclusters. When water is present, Al in the T3 and T4 positions results in the formation of the smallest average Ag 0 nanoclusters plus greater expansion of the O-T-O bond angle than in vacuum, indicating easier diffusion of the Ag 0 nanoclusters to the surface. The presence of Al in 4-membered rings and in pairs indicates favorable MOR structures for formation of single Ag atoms, despite the existence of synthesis challenges. Therefore, Al in the T2 position is the least favorable for Ag 0 nanocluster formation in both vacuum and in the presence of water. Al in the T1, T3, and T4 positions provides beneficial effects through framework flexibility and changes in nanocluster size or charge that can be leveraged for design of zeolites for formation of metallic nanoclusters. • Differential pair distribution functions identified Al locations in mordenite. • Al position alters framework flexibility and charge of Ag clusters. • Al in the T3 and T4 positions forms several smaller Ag cluster in water. • Location of Al in mordenite influences the type and size of Ag clusters formed.

A novel ternary bismuthide, NaMgBi: crystal and electronic structure and electrical properties

Abstract A new ternary sodium magnesium bismuthide, NaMgBi, has been synthesized from the constituent metals, and its crystal structure was determined by single-crystal X-ray diffraction. NaMgBi crystallizes in a tetragonal PbFCl-type structure corresponding to the space group P4/nmm, where Z = 2, a = 4.7123(4) and c = 7.8158(7) Å. The structure is composed of layers formed by edge-sharing Bi tetrahedra centered with Mg stacked in the c-axis direction, and these layers sandwich the Na atoms. First-principles computations based on density functional theory calculations have verified that the most stable atomic configuration is the one in which the Na and Mg atoms occupy the 2a and 2c sites, respectively. The electrical resistivity measured for a sintered polycrystalline sample of NaMgBi with a relative density of 70% was found to gradually decrease from 868 to 26.4 mΩ cm upon increasing the temperature from 297 to 506 K, and the Seebeck coefficient decreased from 273 to 180 μV K−1 upon increasing the temperature from 298 to 496 K. Electronic structure calculations have revealed that NaMgBi must be a semiconductor with a small band gap of ∼0.1 eV.

Thermoconformational Behavior of Cellulose Nanofiber Films as a Device Substrate and Their Superior Flexibility and Durability to Glass.

The design and high-throughput manufacturing of thin renewable energy devices with high structural and atomic configurational stability are crucial for the fabrication of green electronics. Yet, this concept is still in its infancy. In this work, we report the extraordinary durability of thin molecular interlayered organic flexible energy devices based on chemically tuned cellulose nanofiber transparent films that outperform glass by decreasing the substrate weight by 50%. The nanofabricated flexible thin film has an exceptionally low thermal coefficient of expansion of 1.8 ppm/K and a stable atomic configuration under a harsh fabrication condition (over 190 °C for an extended period of 5 h). A flexible optoelectronic device using the same renewable cellulose nanofiber film substrate was found to be functionally operational over a life span of 5 years under an intermittent operating condition. The success of this device's stability opens up an entirely new frontier of applications of flexible electronics.

On Two Highly Symmetric Stencils for Crystal Structures of Inorganic Compounds

A crystallographic analysis of hexagonal poudretteite (KNa2B3Si12O30, symmetry P6/mcc) and cubic pollucite (CsAlSi2O6, Ia $$\bar {3}$$ d) has revealed characteristic features of two versions of energetically stable atomic configurations, which are abundant among natural and synthesized inorganic compounds. These are structures with a minimum number of parameters (not fixed by the symmetry of basis-atom coordinates) and minimum volumes V* of independent parts of unit cells. The structures of silicates with a large number of tetrahedral cations exhibit a symmetry of glide planes and screw axes, which provides both stability and close packing of topologically connected fragments (templates) of these structures.

Antiphase Boundaries as Faceted Metallic Wires in 2D Transition Metal Dichalcogenides.

Antiphase boundaries (APBs) in 2D transition metal dichalcogenides have attracted wide interest as 1D metallic wires embedded in a semiconducting matrix, which could be exploited in fully 2D‐integrated circuits. Here, the anisotropic morphologies of APBs (i.e., linear and saw‐toothed APBs) in the nanoscale are investigated. The experimental and computational results show that despite their anisotropic nanoscale morphologies, all APBs adopt a predominantly chalcogen‐oriented dense structure to maintain the energetically most stable atomic configuration. Moreover, the effect of the nanoscale morphology of an APB on electron transport from two‐probe field effect transistor measurements is investigated. A saw‐toothed APB has a considerably lower electron mobility than a linear APB, indicating that kinks between facets are the main factors of scattering. The observations contribute to the systematical understanding of the faceted APBs and its impact on electrical transport behavior and it could potentially extend the applications of 2D materials through defect engineering to achieve the desired properties.

Fedorov Groups of Crystallographic Symmetry As Algorithms of Space and Energy Transformations in Realization of Stable Atomic Configurations

The effect of symmetry ordering of sites of material particles (atoms, chemically strong atomic groups) is shown on characteristic examples. This ordering leads to reduction of the degrees of their freedom and, therefore, minimizes the energy of a system during crystallization. The geometry of Fedorov groups, supplemented with the symmetry energy function, determines the processes occurring in condensed atomic medium (“Kingdom of crystals,” according to E.S. Fedorov). Criteria are proposed for comparative estimation of the stability of structures, including the structural stencils (structure types of tourmaline, apatite, et al.) abundant in nature.

Coupling point defects and potential energy surface exploration

The free and potential energy surfaces (F/PES) of atomistic systems contain important information regarding structural stability that is crucial in the development of new materials and devices. An effective method for navigating the energy surface can help to realize new synthesis pathways, fabrication techniques, and aid in the prediction of complex atomic structures. In most physical systems, an accurate exploration of the most stable atomic configurations in the FES can be computationally prohibitive using traditional, temperature dependent Monte-Carlo and dynamics based techniques due to the large spatial rearrangements and long time scales involved. In this work, we demonstrate the role of point defect (PD) mediated processes in navigating the PES by implementing PD transformations in a conventional PES exploration technique in order to enable shortcuts in finding the global energy minimum. Using the standard minima-hopping method as a means of sampling the PES, we discuss the details of a point defect mediated Minima-Hopping (PDMH) approach. We study the necessity of incorporating classification and biasing techniques during the search, such as a funnel characterization procedure with an accompanying super-basin history list. An example of our method is demonstrated on fullerene clusters starting from known, nearby low energy configurations. The results show that our method can enable pathways toward the global minimum energy configuration where an optimized MH calculation could not. The implications of such a method are discussed along with future areas of development.

Effect of stretching on the initial oxidation of 3C-SiC nanowire by first-principle simulation

To understand the initial oxidation of the stretched perfected 3C-SiC nanowire, the stable atomic configurations of an O atom adsorption on the 3C-SiC (1¯21¯) surface were investigated by first-principle calculation. The results show that the stress-free 3C-SiC (1¯21¯) has 1 × 2 periodicity due to the surface reconstruction. The strongest O atom adsorption happens at B2–3 site in the stretched 3C-SiC (1¯21¯) with the adsorption energy around −5.6 eV, consistent with the results of the stress-free 3C-SiC (1¯21¯). This is because that the relative position between the O atom and the oxidized Si atoms does not change with the stretching. The adsorption energies of the O atoms at T-1, T-5, T-6 and B-6 sites on the surface of the stretched 3C-SiC (1¯21¯) increase, resulting from the decreased stability of the topmost oxidized Si atoms and the outstretched distance between two Si atoms. Therefore, though 3C-SiC nanowires are used as reinforcements under tensile strain in practical applications, the stretched perfected nanowires aren't easilier oxidized than the stress-free ones with retaining their toughening effect.

Biaxial strain modulated the electronic structure of hydrogenated 2D tetragonal silicene.

Silicene-based materials have attracted great attention due to their easier incorporation into silicon-based devices and components. In addition to the reported hydrogenated 2D tetragonal silicene (γ-SiH), we propose two stable atomic configurations of hydrogenated 2D tetragonal silicene (α-SiH and β-SiH) based on first-principles calculation. The calculated results indicate hydrogenation can effectively open the band gap of 2D tetragonal silicene, α-SiH is a semiconductor with a direct band gap of 2.436 eV whereas β-SiH is indirect band gap of 2.286 eV. We also find that the electronic band structure of α-SiH, β-SiH and γ-SiH can be modulated via biaxial strain. By applying biaxial strain in the range of −10% to 12%, the band gap of α-SiH, β-SiH and γ-SiH can be tuned in a range of 1.732–2.585 eV. Furthermore, direct–indirect or indirect–direct transition can be induced under biaxial strain, showing a high degree of flexibility in electronic band structure. The research not only broadens the diversity of hydrogenated 2D tetragonal silicenes, but also provides more possibilities of their applications in spintronic devices.

First principles calculations on the effect of interstitial oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys

The effect of oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys has been studied using first principles calculations. Three stable atomic configurations of Ti–Nb (Ti-12.5, 16.6, and 25 at.% Nb) systems, which can transform from β-phase to α″-phase without changing the local atomic position of Nb atoms, were first identified using the cluster expansion method. Phonon calculations indicated that these structures were stable. Next, the martensitic transformation behavior of Ti–Nb–O system was studied using these structures. We observed a significant lattice distortion around oxygen atoms occupying octahedral interstitial sites that resembles a bcc type of stacking. Our results conclusively revealed that while the oxygen interstitials can oppose the atomic shuffle required for martensitic transformation, they can also cooperatively stabilize the β-phase even at 1 at.% oxygen concentrations by inducing local elastic shear strains. Interestingly, the canceling of these fields can stabilize the β-phase by suppressing the β to α″ transformation which decreases the martensitic start temperature (Ms). Our study revealed that the reduction in Ms is higher at lower Nb concentration. The stabilization of β-phase increases with oxygen concentration.

Evidence of β-antimonene at the Sb/Bi2Se3 interface

We report a study of the interface between antimony and the prototypical topological insulator Bi2Se3. Scanning tunnelling microscopy measurements show the presence of ordered domains displaying a perfect lattice match with bismuth selenide. Density functional theory calculations of the most stable atomic configurations demonstrate that the ordered domains can be attributed to stacks of β-antimonene.

Density functional theory calculation of diffusion mechanism of intrinsic defects in rutile TiO2

Diffusion mechanisms of four intrinsic point defects in rutile TiO2, titanium interstitial (TiI), titanium vacancy (Vti), oxygen interstitial (OI) and oxygen vacancy (VO) are studied in the framework of density functional theory with quantum ESPRESSO suite. Diffusion processes are simulated by defect movement between two stable atomic configurations through using the climbing image nudged elastic band (CI-NEB) method.The initial and final atomic structure in the minimum energy path (MEP) are constructed with 3×3×4 perfect supercell matrix of 216 atoms. Considering that oxygen atoms build up TiO6 octahedron and half of the octahedral centers are occupied by Ti atoms in rutile, interstitial defect is constructed by adding one Ti or O atom to the empty oxygen octahedral center, and vacancy defect is constructed by removing one atom from crystal lattice grid. Structural relaxation is performed before performing the NEB calculation with gamma k point sampling in irreducible Brillouin zone with an energy cutoff of 650 eV. As rutile TiO2 has tetragonal symmetry (P42/mnm), the diffusion channel along the[100] direction is equivalent to the[010] direction. Then, the diffusion paths along the direction parallel to c axis ([001] direction) and perpendicular to the c axis ([100] or[110] direction) are chosen to find the minimum diffusion energy path of TiI and OI. As for VTi and VO, diffusion paths are established from the vacancy site to nearest lattice site of the same kind.Calculation results exhibit significant anisotropy of energy barrier and diffusion mechanism. Of all defect species, TiI diffusion along the[001] direction through interstitial mechanism has the lowest activation barrier of 0.5057 eV. In addition, diffusions along the[100] and[110] direction through kick-out mechanism show higher energy barriers of 1.0024 eV and 2.7758 eV, respectively. Compared with TiI, OI shows small barrier discrepancy between different diffusion directions, which is 0.859 eV along[001] and 0.902 eV along[100] direction. For vacancy defects, diffusion can occur only through the vacancy mechanism. The activation barrier energy of symmetrically inequivalent diffusion path of VO is 0.735 eV along the[110] direction, 1.747 eV along the[001] direction, and 1.119 eV from the TiO6 apex site to the equator site. On the other hand, VTi has two inequivalent paths with much larger diffusion energy barriers:2.375 eV along the[111] direction and 3.232 eV along the[001] direction. In summary, the TiI interstitial diffusion along the[001] direction (parallel to the c axis) has the lowest activation barrier in rutile TiO2, which is in excellent agreement with former experimental and theoretical data.

Overcoming nanoscale friction barriers in transition metal dichalcogenides

We study the atomic contributions to the nanoscale friction in layered $M{X}_{2}$ $(M=\text{Mo}$, W; $X=\text{S}$, Se, Te) transition metal dichalcogenides by combining ab initio techniques with group-theoretical analysis. Starting from stable atomic configurations, we propose a computational method, named normal-modes transition approximation (NMTA), to individuate possible sliding paths from only the analysis of the phonon modes of the stable geometry. The method provides a way to decompose the atomic displacements realizing the layer sliding in terms of phonon modes of the stable structure, so as to guide the selection and tuning of specific atomic motions promoting $M{X}_{2}$ sheets gliding, and to adjust the corresponding energy barrier. The present results show that main contributions to the nanoscale friction are due to few low frequency phonon modes, corresponding to rigid shifts of $M{X}_{2}$ layers. We also provide further evidences that a previously reported Ti-doped ${\mathrm{MoS}}_{2}$ phase is a promising candidate as new material with enhanced tribologic properties. The NMTA approach can be exploited to tune the energetic and the structural features of specific phonon modes, and, thanks to its general formulation, can also be applied to any solid state system, irrespective of the chemical composition and structural topology.

Si Nanoribbons on Ag(110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain Structure.

We report a combined grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and density-functional theory (DFT) study which clearly elucidates the atomic structure of the Si nanoribbons grown on the missing-row reconstructed Ag(110) surface. Our study allows us to discriminate between the theoretical models published in the literature, including the most stable atomic configurations and those based on a missing-row reconstructed Ag(110) surface. GIXD measurements unambiguously validate the pentamer model grown on the reconstructed surface, obtained from DFT. This pentamer atomistic model accurately matches the high-resolution STM images of the Si nanoribbons adsorbed on Ag(110). Our study closes the long-debated atomic structure of the Si nanoribbons grown on Ag(110) and definitively excludes a honeycomb structure similar to that of freestanding silicene.

First-principles study of twin grain boundaries in epitaxial BaSi2 on Si(111)

Epitaxial films of BaSi2 on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi2 epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi2 revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba(II) atoms form hexagons containing central Ba(I) atoms in both the bulk and DB regions. Four possible interface models containing Ba(I)-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba(I) atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba(II) atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m2. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi2.

Atomistic Study for the Vibrational Properties on Σ5 Symmetric Tilt Bicrystal Copper Nanowires

In this study, an atomistic simulation was performed to investigate intrinsic resonance propensity of clamped-clamped copper nanowires with Σ5 (310)/[001] symmetric tilt grain boundary. Grain boundary energy γGB for bicrystal structure was calculated based on an iterative approach. The stable atomic configuration was then doubly clamped and excited via flexural oscillation under varied temperatures. From the result, the appearance of grain boundary significantly alters the resonance properties of Cu nanowires. Greater attenuation in kinetic energy can be observed with increased temperature. Quality factors attains Q ~ 1/T0.7144 and 1/T0.7249 with temperatures from kinetic energy and centroid root mean square spectrum, respectively, where the former seems more reliable to employ at elevated temperatures.

Atomistic Modeling of the Charge Process in Lithium/Air Batteries

We present a combined classical and density functional theory (DFT) based Molecular Dynamics (MD) study of the mechanisms underlying the oxygen evolution reactions during the charging of lithium/air batteries. As models for the Li2O2 material at the cathode we employ small amorphous clusters with a 2:2 Li:O stoichiometry, whose energetically most stable atomic configurations comprise both O atoms and O–O pairs with mixed peroxide/superoxide character, as revealed by their bond lengths, charges, spin moments, and densities of states. The oxidation of Li8O8 clusters is studied in unbiased DFT-based MD simulations upon removal of either one or two electrons, either in vacuo or immersed in dimethyl sulfoxide solvent molecules with a structure previously optimized by means of classical MD. Whereas removal of one electron leads only to an enhancement of the superoxide character of O–O bonds, removal of two electrons leads to the spontaneous dissolution of either an O2 or a LiO2+ molecule. These results are inte...