Applications of a tight-binding total-energy method for transition and noble metals: Elastic constants, vacancies, and surfaces of monatomic metals

Evaluation of Sampling Algorithms Used for Bayesian Uncertainty Quantification of Molecular Dynamics Force Fields

New Bayesian parameter estimation methods have the capability to enable more physically realistic and reliable molecular dynamics (MD) simulations by providing accurate estimates of uncertainties of force-field (FF) parameters and associated properties. However, the choice of which Bayesian parameter estimation algorithm to use has not been widely investigated, despite its impact on the effective exploration of parameter space. Here, using a case example of the Embedded Atom Method (EAM) FF parameters, we investigated the ramifications of several of the algorithm choices. We found that Ensemble Slice Sampling (ESS) and Affine-Invariant Ensemble Sampling (AIES) demonstrate a new level of superior performance, culminating in more accurate parameter and property estimations with tighter uncertainty bounds, compared to traditional methods such as Metropolis-Hastings (MH), Gradient Search (GS), and Uniform Random Sampler (URS). We demonstrate that Bayesian Uncertainty Quantification with ESS and AIES leads to significantly more accurate and reliable predictions of the FF parameters and properties. The results suggest that ESS and AIES should be used to obtain more accurate parameter and uncertainty estimations while providing deeper physical insights.

Fast and accurate prediction of material properties with three-body tight-binding model for the periodic table

Parametrized tight-binding models fit to first-principles calculations can provide an efficient and accurate quantum mechanical method for predicting properties of molecules and solids. However, well-tested parameter sets are generally only available for a limited number of atom combinations, making routine use of this method difficult. Furthermore, many previous models consider only simple two-body interactions, which limits accuracy. To tackle these challenges, we develop a density functional theory database of nearly 1 000 000 materials, which we use to fit a universal set of tight-binding parameters for 65 elements and their binary combinations. We include both two-body and three-body effective interaction terms in our model, plus self-consistent charge transfer, enabling our model to work for metallic, covalent, and ionic bonds with the same parameter set. To ensure predictive power, we adopt a learning framework where we repeatedly test the model on new low-energy crystal structures and then add them to the fitting data set, iterating until predictions improve. We distribute the materials database and tools developed in this paper publicly.

Body-Ordered Approximations of Atomic Properties

We show that the local density of states (LDOS) of a wide class of tight-binding models has a weak body-order expansion. Specifically, we prove that the resulting body-order expansion for analytic observables such as the electron density or the energy has an exponential rate of convergence both at finite Fermi-temperature as well as for insulators at zero Fermi-temperature. We discuss potential consequences of this observation for modelling the potential energy landscape, as well as for solving the electronic structure problem.

Multiscale Modeling of Bio-Nano Interactions of Zero-Valent Silver Nanoparticles

Understanding the specifics of interaction between the protein and nanomaterial is crucial for designing efficient, safe, and selective nanoplatforms, such as biosensor or nanocarrier systems. Routing experimental screening for the most suitable complementary pair of biomolecule and nanomaterial used in such nanoplatforms might be a resource-intensive task. While a range of computational tools are available for prescreening libraries of proteins for their interactions with small molecular ligands, choices for high-throughput screening of protein libraries for binding affinities to new and existing nanomaterials are very limited. In the current work, we present the results of the systematic computational study of interaction of various biomolecules with pristine zero-valent noble metal nanoparticles, namely, AgNPs, by using the UnitedAtom multiscale approach. A set of blood plasma and dietary proteins for which the interaction with AgNPs was described experimentally were examined computationally to evaluate the performance of the UnitedAtom method. A set of interfacial descriptors (log P^NM, adsorption affinities, and adsorption affinity ranking), which can characterize the relative hydrophobicity/hydrophilicity/lipophilicity of the nanosized silver and its ability to form bio(eco)corona, was evaluated for future use in nano-QSAR/QSPR studies.

Oxygen-vacancy-induced magnetism in anti-perovskite topological Dirac semimetal Ba3SnO

The thermodynamic, structural, magnetic and electronic properties of the pristine and intrinsic vacancy-defect-containing topological Dirac semimetal Ba₃SnO are studied using first-principles density functional theory calculations. The thermodynamic stability of Ba₃SnO has been evaluated with reference to its competing binary phases Ba₂Sn, BaSn and BaO. Subsequently, valid limits of the atomic chemical potentials derived from the thermodynamic stability were used for assessing the formation of Ba, Sn and O vacancy defects in Ba₃SnO under different synthesis environments. Based on the calculated defect-formation energies, we find that the charge-neutral oxygen vacancies are the most favourable type of vacancy defect under most chemical environments. The calculated electronic properties of pristine Ba₃SnO show that inclusion of spin-orbit coupling in exchange-correlation potentials computed using generalized gradient approximation yields a semimetallic band structure exhibiting twin Dirac cones along the Γ-X path of the Brillouin zone. The effect of spin-polarization and spin-orbit coupling on the physical properties of intrinsic vacancy defects containing Ba₃SnO has been examined in detail. Using Bader charges, electron localization function (ELF), electronic density of states (DOS) and spin density, we show that the isolated oxygen vacancy is a magnetic defect in anti-perovskite Ba₃SnO. Our results show that the origin of magnetism in Ba₃SnO is the accumulation of unpaired charges at the oxygen vacancy sites, which couple strongly with the 5d states of the Ba atom. Owing to the metastability observed in earlier theoretically predicted magnetic topological semimetals, the present study reveals the important role of intrinsic vacancy defects in giving rise to magnetism and also provides opportunities for engineering the electronic structure of a Dirac semimetal.

Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators

The magnetic structures of MnBi₂Te₄(Bi₂Te₃)_n can be manipulated by tuning the interlayer coupling via the number of Bi₂Te₃ spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi₄Te₇ stays at A-type antiferromagnetic (AFM) phase with a decreasing Néel temperature and an increasing saturation field. In sharp contrast, MnBi₆Te₁₀ experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed saturation field under pressure. First-principles calculations reveal the essential role of intralayer exchange coupling from lattice compression in determining these magnetic properties. Such magnetic phase transition is also observed in 20% Sb-doped MnBi₆Te₁₀ because of the in-plane lattice compression.

Direct correlation of oxygen adsorption on platinum-electrolyte interfaces with the activity in the oxygen reduction reaction

The oxygen reduction reaction (ORR) on platinum catalysts is essential in fuel cells. Quantitative predictions of the relative ORR activity in experiments, in the range of 1 to 50 times, have remained challenging because of incomplete mechanistic understanding and lack of computational tools to account for the associated small differences in activation energies (<2.3 kilocalories per mole). Using highly accurate molecular dynamics (MD) simulation with the Interface force field (0.1 kilocalories per mole), we elucidated the mechanism of adsorption of molecular oxygen on regular and irregular platinum surfaces and nanostructures, followed by local density functional theory (DFT) calculations. The relative ORR activity is determined by oxygen access to platinum surfaces, which greatly depends on specific water adlayers, while electron transfer occurs at a similar slow rate. The MD methods facilitate quantitative predictions of relative ORR activities of any platinum nanostructures, are applicable to other catalysts, and enable effective MD/DFT approaches.

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy

We review the state-of-the-art in the theory of dissociative chemisorption (DC) of small gas phase molecules on metal surfaces, which is important to modeling heterogeneous catalysis for practical reasons, and for achieving an understanding of the wealth of experimental information that exists for this topic, for fundamental reasons. We first give a quick overview of the experimental state of the field. Turning to the theory, we address the challenge that barrier heights (E_b, which are not observables) for DC on metals cannot yet be calculated with chemical accuracy, although embedded correlated wave function theory and diffusion Monte-Carlo are moving in this direction. For benchmarking, at present chemically accurate E_b can only be derived from dynamics calculations based on a semi-empirically derived density functional (DF), by computing a sticking curve and demonstrating that it is shifted from the curve measured in a supersonic beam experiment by no more than 1 kcal mol^-1. The approach capable of delivering this accuracy is called the specific reaction parameter (SRP) approach to density functional theory (DFT). SRP-DFT relies on DFT and on dynamics calculations, which are most efficiently performed if a potential energy surface (PES) is available. We therefore present a brief review of the DFs that now exist, also considering their performance on databases for E_b for gas phase reactions and DC on metals, and for adsorption to metals. We also consider expressions for SRP-DFs and briefly discuss other electronic structure methods that have addressed the interaction of molecules with metal surfaces. An overview is presented of dynamical models, which make a distinction as to whether or not, and which dissipative channels are modeled, the dissipative channels being surface phonons and electronically non-adiabatic channels such as electron-hole pair excitation. We also discuss the dynamical methods that have been used, such as the quasi-classical trajectory method and quantum dynamical methods like the time-dependent wave packet method and the reaction path Hamiltonian method. Limits on the accuracy of these methods are discussed for DC of diatomic and polyatomic molecules on metal surfaces, paying particular attention to reduced dimensionality approximations that still have to be invoked in wave packet calculations on polyatomic molecules like CH₄. We also address the accuracy of fitting methods, such as recent machine learning methods (like neural network methods) and the corrugation reducing procedure. In discussing the calculation of observables we emphasize the importance of modeling the properties of the supersonic beams in simulating the sticking probability curves measured in the associated experiments. We show that chemically accurate barrier heights have now been extracted for DC in 11 molecule-metal surface systems, some of which form the most accurate core of the only existing database of E_b for DC reactions on metal surfaces (SBH10). The SRP-DFs (or candidate SRP-DFs) that have been derived show transferability in many cases, i.e., they have been shown also to yield chemically accurate E_b for chemically related systems. This can in principle be exploited in simulating rates of catalyzed reactions on nano-particles containing facets and edges, as SRP-DFs may be transferable among systems in which a molecule dissociates on low index and stepped surfaces of the same metal. In many instances SRP-DFs have allowed important conclusions regarding the mechanisms underlying observed experimental trends. An important recent observation is that SRP-DFT based on semi-local exchange DFs has so far only been successful for systems for which the difference of the metal work function and the molecule's electron affinity exceeds 7 eV. A main challenge to SRP-DFT is to extend its applicability to the other systems, which involve a range of important DC reactions of e.g. O₂, H₂O, NH₃, CO₂, and CH₃OH. Recent calculations employing a PES based on a screened hybrid exchange functional suggest that the road to success may be based on using exchange functionals of this category.

Nonthermal phase transitions in metals

It is well known that sufficiently thick metals irradiated with ultrafast laser pulses exhibit phonon hardening, in contrast to ultrafast nonthermal melting in covalently bonded materials. It is still an open question how finite size metals react to irradiation. We show theoretically that generally metals, under high electronic excitation, undergo nonthermal phase transitions if material expansion is allowed (e.g. in finite samples). The nonthermal phase transitions are induced via an increase of the electronic pressure which leads to metal expansion. This, in turn, destabilizes the lattice triggering a phase transition without a thermal electron-ion coupling mechanism involved. We find that hexagonal close-packed metals exhibit a diffusionless transition into a cubic phase, whereas metals with a cubic lattice melt. In contrast to covalent solids, nonthermal phase transitions in metals are not ultrafast, predicative on the lattice expansion.

Theoretical studies of the vibrational properties of octahedrane (C12H12): A polyhedral caged hydrocarbon molecule

The vibrational properties of octahedrane (C₁₂H₁₂) are calculated using density-functional theory employing two different computational methods: an all-electron Gaussian orbital approach and a Naval Research Laboratory-tight-binding scheme (NRL-TB) coupled with molecular dynamics (NRL-TBMD). Both approaches yield vibrational densities of states for octahedrane that are in good general agreement with each other. NRL Molecular Orbital Library can also provide accurate infrared and Raman spectra which can be analyzed and compared with experimental results, while NRL-TBMD can be conveniently scaled up for larger finite-temperature simulations. This latter approach is used in our paper to produce a theoretical prediction for a stable room temperature structure of octahedrane.

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.

A bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles

We describe a bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles. In this method, we apply a many-body scaling function (in similar manner as in the environment-modified total energy based tight-binding method) to the DFT-derived diatomic AO interaction potentials (like in the conventional orbital-based density-functional tight binding approach) strictly according to atomic valences acquired naturally in a bulk structure. This modification, (a) facilitates all atom orbital-based electronic structure calculations of charge carrier dynamics in nanoscale structures with a molecular acceptor, and (b) allows to closely match high-level density functional calculation data (previously adjusted to the available experimental findings) for bulk structures. To advance practical application of the BA-LCAO approach we parameterize the Hamiltonian of wurtzite CdSe by fitting its band structure to a high-level DFT reference, corrected for experimentally measured band edges. Here, unlike in conventional DFTB approach, we: (1) use hydrogen-like AOs for the basis as exact atomic eigenfunctions, while orbital energies of which are taken from experimentally measured ionization potentials, and (2) parameterize the many-body scaling functions rather than the atomic wavefunctions. Development of this approach and parameters is guided by our goals to devise a method capable of simultaneously treating the problems of (i) interfacial electron/hole transfer between finite, variable size nanoparticles and electron scavenging molecules, and (ii) high-energy electronic transitions (Auger transitions) that mediate multi-exciton decay in quantum dots. Electronic structure results are described for CdSe quantum dots of various sizes. © 2018 Wiley Periodicals, Inc.

Chemomechanical Origin of Hydrogen Trapping at Grain Boundaries in fcc Metals

Hydrogen embrittlement of metals is widely observed, but its atomistic origins remain little understood and much debated. Combining a unique identification of interstitial sites through polyhedral tessellation and first-principles calculations, we study hydrogen adsorption at grain boundaries in a variety of face-centered cubic metals of Ni, Cu, γ-Fe, and Pd. We discover the chemomechanical origin of the variation of adsorption energetics for interstitial hydrogen at grain boundaries. A general chemomechanical formula is established to provide accurate assessments of hydrogen trapping and segregation energetics at grain boundaries, and it also offers direct explanations for certain experimental observations. The present study deepens our mechanistic understanding of the role of grain boundaries in hydrogen embrittlement and points to a viable path towards predictive microstructure engineering against hydrogen embrittlement in structural metals.

The CO oxidation mechanism on the W(111) surface and the W helical nanowire investigated by the density functional theory calculation

Two CO oxidation reactions (CO + O2 → CO2 + O and CO + O → CO2) were considered in the Eley-Rideal (ER) reaction mechanism. These oxidation processes on the W(111) surface and the W helical nanowire were investigated by the density functional theory (DFT) calculation. The stable adsorption sites of O2 and O as well as their adsorption energies were obtained first. In order to understand the catalytic properties of the W helical nanowire, the Fukui function and local density of state (LDOS) profiles were determined. The nudged elastic band (NEB) method was applied to locate transition states and minimum energy pathways (MEPs) of CO oxidation processes on the W helical nanowire and on the W(111) surface. In this study, we have demonstrated that the catalytic ability of the W helical nanowire is superior to that of the W(111) surface for CO oxidation.

Investigation of mechanical properties and thermal stability of the thinnest tungsten nanowire by density functional theory

The most stable structure of the thinnest tungsten (W) nanowire with the radius of 1.9 Å was predicted by the simulated annealing basin-hopping method (SABH) with the tight-binding (TB) potential and the penalty algorithm.

How to derive tight-binding spd potentials? Application to zirconium

We propose here a general methodology to derive tight-binding potentials accounting for spd hybridization in transition metals, dealing simultaneously with electronic structure and energy properties. This methodology is illustrated for zirconium which is largely used for technological applications, in particular in the nuclear industry, and whose modelling is known to be complex and challenging. Such potentials are very promising. Their fits have a clear physical meaning with a limited amount of parameters and their complexity can be adjusted as a function of the problem under consideration.

Effects of surface diffusion on high temperature selective emitters

Using morphological and optical simulations of 1D tantalum photonic crystals at 1200K, surface diffusion was determined to gradually reduce the efficiency of selective emitters. This was attributed to shifting resonance peaks and declining emissivity caused by changes to the cavity dimensions and the aperture width. Decreasing the structure's curvature through larger periods and smaller cavity widths, as well as generating smoother transitions in curvature through the introduction of rounded cavities, was found to alleviate this degradation. An optimized structure, that shows both high efficiency selective emissivity and resistance to surface diffusion, was presented.

Charting the complete elastic properties of inorganic crystalline compounds

The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

Growth morphology of thin films on metallic and oxide surfaces

In this work we briefly review recent investigations concerning the growth morphology of thin metallic films on the Mo(110) and Ni3Al(111) surfaces, and Fe and copper phthalocyanine (C32H16N8Cu) on the Al2O3/Ni3Al(111) surface. Comparison of Ag, Au, Sn, and Pb growth on the Mo(110) surface has shown a number of similarities between these adsorption systems, except that surface alloy formation has only been observed in the case of Sn and Au. In the Pb/Mo(110) and Pb/Ni3Al(111) adsorption systems selective formation of uniform Pb island heights during metal thin film growth has been observed and interpreted in terms of quantum size effects. Furthermore, our studies showed that Al2O3 on Ni3Al(111) exhibits a large superstructure in which the unit cell has a commensurate relation with the substrate lattice. In addition, copper phthalocyanine chemisorbed weakly onto an ultra-thin Al2O3 film on Ni3Al(111) and showed a poor template effect of the Al2O3/Ni3Al(111) system. In the case of iron cluster growth on Al2O3/Ni3Al(111) the nucleation sites were independent of deposition temperature, yet the cluster shape showed a dependence. In this system, Fe clusters formed a regular hexagonal lattice on the Al2O3/Ni3Al(111).

Structural and electronic properties of tungsten nanoclusters by DFT and basin-hopping calculations

The structural and electronic properties of small tungsten nanoclusters W_n (n = 2–16) were investigated by density functional theory (DFT) calculations.

Development of a ReaxFF potential for Pt–O systems describing the energetics and dynamics of Pt-oxide formation

A ReaxFF force field description of Pt–O systems has been developed, validated and applied to oxygen diffusion on Pt(111).

Uniaxial tension-induced fracture in gold nanowires with the dependence on size and atomic vacancies

The size effect dominates the rupture of gold nanowires, which is also related to atomic vacancies in a single-layer crystalline plane.

The effect of long-range order on the elastic properties of Cu3Au

Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C(11) and C(12) decrease, whereas C(44) remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.

Thermodynamically consistent force fields for the assembly of inorganic, organic, and biological nanostructures: the INTERFACE force field

The complexity of the molecular recognition and assembly of biotic-abiotic interfaces on a scale of 1 to 1000 nm can be understood more effectively using simulation tools along with laboratory instrumentation. We discuss the current capabilities and limitations of atomistic force fields and explain a strategy to obtain dependable parameters for inorganic compounds that has been developed and tested over the past decade. Parameter developments include several silicates, aluminates, metals, oxides, sulfates, and apatites that are summarized in what we call the INTERFACE force field. The INTERFACE force field operates as an extension of common harmonic force fields (PCFF, COMPASS, CHARMM, AMBER, GROMACS, and OPLS-AA) by employing the same functional form and combination rules to enable simulations of inorganic-organic and inorganic-biomolecular interfaces. The parametrization builds on an in-depth understanding of physical-chemical properties on the atomic scale to assign each parameter, especially atomic charges and van der Waals constants, as well as on the validation of macroscale physical-chemical properties for each compound in comparison to measurements. The approach eliminates large discrepancies between computed and measured bulk and surface properties of up to 2 orders of magnitude using other parametrization protocols and increases the transferability of the parameters by introducing thermodynamic consistency. As a result, a wide range of properties can be computed in quantitative agreement with experiment, including densities, surface energies, solid-water interface tensions, anisotropies of interfacial energies of different crystal facets, adsorption energies of biomolecules, and thermal and mechanical properties. Applications include insight into the assembly of inorganic-organic multiphase materials, the recognition of inorganic facets by biomolecules, growth and shape preferences of nanocrystals and nanoparticles, as well as thermal transitions and nanomechanics. Limitations and opportunities for further development are also described.

Magnetism: the driving force of order in CoPt, a first-principles study

CoPt equiatomic alloy orders according to the tetragonal L1(0) structure which favors strong magnetic anisotropy. Conversely, magnetism can influence the chemical ordering. We present here ab initio calculations of the stability of the L1(0) and L1(2) structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters.

Magnetic and electronic properties of bulk and clusters of FePt L1(0)

An efficient tight-binding model including magnetism and spin-orbit interactions is extended to metallic alloys. The tight-binding parameters are determined from a fit to bulk ab initio calculations of each metal and rules are given to obtain the heteroatomic parameters. The spin and orbital magnetic moments as well as the magneto-crystalline anisotropy are derived. We apply this method to bulk FePt L1(0) and the results are compared with success to ab initio results where available. Finally this model is applied to a set of FePt L1(0) clusters and physical trends are derived.

Quantum-confined nanowires as vehicles for enhanced electrical transport

Electrical transport in semiconductor nanowires taking quantum confinement and dielectric confinement into account has been studied. A distinctly new route has been employed for the study. The fundamental science underlying the model is based on a relationship between the quantum confinement and the structural disorder of the nanowire surface. The role of surface energy and thermodynamic imbalance in nanowire structural disorder has been described. A model for the diameter dependence of energy bandgap of nanowires has been developed. Ionized impurity scattering, dislocation scattering and acoustic phonon scattering have been taken into account to study carrier mobility. A series of calculations on silicon nanowires show that carrier mobility in nanowires can be greatly enhanced by quantum confinement and dielectric confinement. The electron mobility can, for example, be a factor of 2-10 higher at room temperature than the mobility in a free-standing silicon nanowire. The calculated results agree well with almost all experimental and theoretical results available in the literature. They successfully explain experimental observations not understood before. The model is general and applicable to nanowires from all possible semiconductors. It is perhaps the first physical model highlighting the impact of both quantum confinement and dielectric confinement on carrier transport. It underscores the basic causes of thin, lowly doped nanowires in the temperature range 200 K ≤ T ≤ 500 K yielding very high carrier mobility. It suggests that the scattering by dislocations (stacking faults) can be very detrimental for carrier mobility.

Conductance fluctuations in gold point contacts: an atomistic picture

This paper concerns an experimental and theoretical study of the transition between two consecutive conductance plateaus as obtained in breaking gold contact experiments. The experimental measurements performed at 100 K with a scanning tunneling microscope and variable elongation speeds show that the transitions between consecutive plateaus can appear in the conductance traces as an abrupt conductance step, a smooth quasicontinuous change or as large amplitude conductance fluctuations. The theoretical calculations based on a non-orthogonal tight-binding Hamiltonian have shown that for a given deformation there are several structures having close and competing energies.We discuss the relation between the temperature, sampling frequency, stretching speed and energy barriers which can explain the conditions for the observation of the three kinds of conductance traces.

An order-N electronic structure theory with generalized eigenvalue equations and its application to a ten-million-atom system

A linear algebraic theory called the 'multiple Arnoldi method' is presented and realizes large-scale (order-N) electronic structure calculations with generalized eigenvalue equations. A set of linear equations, in the form of (zS - H)x = b, are solved simultaneously with multiple Krylov subspaces. The method is implemented in a simulation package ELSES (www.elses.jp) with tight-binding-form Hamiltonians. A finite-temperature molecular dynamics simulation is carried out for metallic and insulating materials. A calculation with 10(7) atoms was realized by a workstation. The parallel efficiency is shown up to 1024 CPU cores.

Thermodynamic imbalance, surface energy, and segregation reveal the true origin of nanotube synthesis

Extensive analyses of thermodynamic imbalance, surface energy, and segregation of nanotubes on nanoparticle surfaces are performed. A model for surface energy i developed. In addition, nanotube growth both by vapor-phase and solid-phase mechanisms is described. Segregation of the nanotube species to the periphery of the nanoparticle, the creation of an amorphous shell at this periphery, a droplet created in this shell, and the mediation of this droplet for supersaturation and nucleation of the nanotube species may be the true causes of nanotube growth.

Shock-induced breaking in the gold nanowire with the influence of defects and strain rates

Defects in metallic nanowires have raised concerns about the applied reliability of the nanowires in nanoelectromechanical systems. In this paper, molecular dynamics simulations are used to study the deformation and breaking failure of the [100] single-crystal gold nanowires containing defects at different strain rates. The statistical breaking position distributions of the nanowires show mechanical shocks play a critical role in the deformation of nanowires at different strain rates, and deformation mechanism of the nanowire containing defects is based on a competition between shocks and defects in the deformation process of the nanowire. At low strain rate of 1.0% ps(-1), defect ratio of 2% has changed the deformation mechanism because micro-atomic fluctuation is in an equilibrium state. However, owing to strong symmetric shocks, the sensitivity of defects is not obvious before a defect ratio of 25% at high strain rate of 5.0% ps(-1).

Tight-Binding Hamiltonians for Carbon and Silicon

We demonstrate that our tight-binding method - which is based on fitting the energy bands and the total energy of first-principles calculations as a function of volume - can be easily extended to accurately describe carbon and silicon. We present equations of state that give the correct energy ordering between structures. We also show that quantities that were not fitted, such as elastic constants and the band structure of C60, can be reliably obtained from our scheme.

Interatomic Potentials for Al and Ni From Experimental Data and AB Initio Calculations

New embedded-atom potentials for Al and Ni have been developed by fitting to both experimental data and the results of ab initio calculations. The ab initio data were obtained in the form of energies of different alternative computer-generated crystalline structures of these metals. The potentials accurately reproduce basic equilibrium properties of Al and Ni such as the elastic constants, phonon dispersion curves, vacancy formation and migration energies, stacking fault energies, and surface energies. The equilibrium energies of various alternative structures not included in the fitting database are calculated with these potentials. The results are compared with predictions of total-energy tight-binding calculations for the same structures. The embedded-atom potentials correctly reproduce the structural stability trends, which suggests that they are transferable to different local environments encountered in atomistic simulations of lattice defects.

An introduction to “Computational Crystallography”

The currently available methods for the computation of structures and their properties are reviewed. After a brief introduction into some common technical aspects, the capabilities and limitations of the most commonly used approaches are discussed. Examples are given to show the state of the art in Computational “Crystallography”, and possible future developments are outlined