Global Structure Search Research Articles

Versatile Titanium Silicide Monolayers with Prominent Ferromagnetic, Catalytic, and Superconducting Properties: Theoretical Prediction.

On the basis of global structure search and density functional theory calculations, we predict a new class of two-dimensional (2D) materials, titanium silicide (Ti2Si, TiSi2, and TiSi4) monolayers. They are proved to be energetically, dynamically, and thermally stable and own excellent mechanical properties. Among them, Ti2Si is a ferromagnetic metal with a magnetic moment of 1.37 μB/cell, while TiSi2 is an ideal catalyst for the hydrogen evolution reaction with a nearly zero free energy of hydrogen adsorption. More importantly, electron-phonon coupling calculations suggest that TiSi4 is a robust 2D phonon-mediated superconductor with a transition temperature of 5.8 K, and the transition temperature can be enhanced up to 11.7 K under a suitable external strain. The versatility makes titanium silicide monolayers promising candidates for spintronic materials, hydrogen evolution catalysts, and 2D superconductors.

Global structure search for molecules on surfaces: Efficient sampling with curvilinear coordinates.

Efficient structure search is a major challenge in computational materials science. We present a modification of the basin hopping global geometry optimization approach that uses a curvilinear coordinate system to describe global trial moves. This approach has recently been shown to be efficient in structure determination of clusters [C. Panosetti et al., Nano Lett. 15, 8044-8048 (2015)] and is here extended for its application to covalent, complex molecules and large adsorbates on surfaces. The employed automatically constructed delocalized internal coordinates are similar to molecular vibrations, which enhances the generation of chemically meaningful trial structures. By introducing flexible constraints and local translation and rotation of independent geometrical subunits, we enable the use of this method for molecules adsorbed on surfaces and interfaces. For two test systems, trans-β-ionylideneacetic acid adsorbed on a Au(111) surface and methane adsorbed on a Ag(111) surface, we obtain superior performance of the method compared to standard optimization moves based on Cartesian coordinates.

Global structure search and physical properties of Os2C

The crystal structures of Os2C were extensively investigated using the structure search method from the first-principles calculations. In contrast to the P63/mmc phase previously proposed as the ground state at ambient pressure, an energetically favorable structure with space group P-6m2 was found more stable at ambient condition. The structural stabilities of the new phase are confirmed by the phonon dispersion and elastic constants. Further calculations indicate that the newly predicted P-6m2 phase is ultra-incompressible with a high bulk modulus of 387 GPa and has a larger ideal shear strength than the P63/mmc phase.

Mn2C monolayer: a 2D antiferromagnetic metal with high Néel temperature and large spin-orbit coupling.

To realize antiferromagnetic spintronics in the nanoscale, it is highly desirable to identify new nanometer-scale antiferromagnetic metals with both high Néel temperature and large spin-orbit coupling. In this work, on the basis of first-principles calculation and particle swarm optimization (PSO) global structure search, we demonstrate that a two-dimensional Mn2C monolayer is an antiferromagnetic metal with a Mn magnetic moment of ∼3μB. Mn2C monolayer has an anti-site structure of MoS2 sheet with carbon atoms hexagonally coordinated by neighboring Mn atoms. Remarkably, the in-plane carrier mobility of 2D Mn2C is highly anisotropic, amounting to about 47 000 cm(2) V(-1) s(-1) in the a' direction, which is much higher than that of MoS2 monolayer. The Néel temperature of Mn2C monolayer is high up to 720 K. Due to strong spin-orbit coupling in plane, the magnetic anisotropy energy of Mn2C monolayer is larger than those of pure metals, such as Fe, Co, and Ni. These advantages render 2D Mn2C sheet with great potential applications in nanometer-scale antiferromagnetic spintronics.

Design of ternary alkaline-earth metal Sn(ii) oxides with potential good p-type conductivity

Novel ternary alkaline-earth metal Sn(ii) oxides with potential good p-type conductivity are discovered with first-principles global optimization structure searches.

Pressure-induced phase transitions of lead iodide

High-pressure phase transitions of lead iodide have been proposed up to 200 GPa through the theoretically swarm-intelligent global structure searches.

A universal chemical potential for sulfur vapours.

The unusual chemistry of sulfur is illustrated by the tendency for catenation. Sulfur forms a range of open and closed S n species in the gas phase, which has led to speculation on the composition of sulfur vapours as a function of temperature and pressure for over a century. Unlike elemental gases such as O2 and N2, there is no widely accepted thermodynamic potential for sulfur. Here we combine a first-principles global structure search for the low energy clusters from S2 to S8 with a thermodynamic model for the mixed-allotrope system, including the Gibbs free energy for all gas-phase sulfur on an atomic basis. A strongly pressure-dependent transition from a mixture dominant in S2 to S8 is identified. A universal chemical potential function, μS(T,P), is proposed with wide utility in modelling sulfurisation processes including the formation and annealing of metal chalcogenide semiconductors.

Global Materials Structure Search with Chemically Motivated Coordinates.

Identification of relevant reaction pathways in ever more complex composite materials and nanostructures poses a central challenge to computational materials discovery. Efficient global structure search, tailored to identify chemically relevant intermediates, could provide the necessary first-principles atomistic insight to enable a rational process design. In this work we modify a common feature of global geometry optimization schemes by employing automatically generated collective curvilinear coordinates. The similarity of these coordinates to molecular vibrations enhances the generation of chemically meaningful trial structures for covalently bound systems. In the application to hydrogenated Si clusters, we concomitantly observe a significantly increased efficiency in identifying low-energy structures and exploit it for an extensive sampling of potential products of silicon-cluster soft landing on Si(001) surfaces.

First-Principles Study on Electronic and Optical Properties of Graphene-Like Boron Phosphide Sheets

Two-dimensional semiconducting materials with moderate band gap and high carrier mobility have a wide range of applications for electronics and optoelectronics in nanoscale. On the basis of first-principles calculations, we perform a comprehensive study on the electronics and optical properties of graphene-like boron phosphide (BP) sheets. The global structure search and first-principles based molecular dynamic simulation indicate that two-dimensional BP sheet has a graphene-like global minimum structure with high stability. BP monolayer is semiconductor with a direct band gap of 1.37 eV, which reduces with the number of layers. Moreover, the band gaps of BP sheets are insensitive to the applied uniaxial strain. The calculated mobility of electrons in BP monolayer is as high as 106 cm2/(V·s). Lastly, the MoS2/BP van der Waals heterobilayers are investigated for photovoltaic applications, and their power conversion efficiencies are estimated to be in the range of 17.7%–19.7%. This study implies the potential applications of graphene-like BP sheets for electronic and optoelectronic devices in nanoscale.

Molecular Precursor Induced Surface Reconstruction at Graphene/Pt(111) Interfaces

Inspired by experimental observations of Pt(111) surfaces reconstruction at the Pt/graphene (Gr) interfaces with ordered vacancy networks in the outermost Pt layer [e.g., Otero, G., et al. Phys. Rev. Lett. 2010, 105, 216102 ], the mechanism of the surface reconstruction is investigated by van der Waals corrected density functional theory in combination with particle-swarm optimization algorithm and ab initio atomistic thermodynamics. Our global structural search finds a more stable reconstructed structure than that which was reported before. With correction for vacancy formation energy, we demonstrate that the experimental observed surface reconstruction occurred at the earlier stages of graphene formation: (1) reconstruction occurred when C60 adsorption (before decomposition to form graphene) for C60 precursor or (2) reconstruction occurred when there were (partial) hydrogens remain in the hydrogenated precursors of C2H4 and planar C60H30. The reason is attributed to the fact that the energy gain, from t...

Structural exploration of two-dimensional materials

Two-dimensional (2D) materials with atomic thickness have attracted intensive research interests for their novel physical properties and promising applications in various fields. In general, 2D materials can be obtained physically or chemically from layered or nonlayered materials, such as graphene, phosphorene, boron-nitride sheet, transitional metal disulfide monolayer, silicene, and zinc oxide monolayer, etc. However, it is still an interesting question to explore and design 2D materials just starting from the elements. In recent years, global structure search has been well developed to predict stable structure of materials just based on the potential energy surface. In particular, many new 2D materials with atomic thicknewss can be predicted with this useful method. Here, we review rencent progress of structural prediction of 2D materials with global structure search, as well as exsiting challenges in this field. We also discuss the other theoretical method to explore and design new 2D materials with chemical knowledge.

High-temperature superconductivity in heavily N- or B-doped graphene

A two-dimensional honeycomb lattice of graphene, if heavily doped with electrons or holes, has been predicted to possess a wealth of fascinating properties including high-temperature superconductivity. Although such a material is possible with high concentration of N or B substitution, its experimental realization has been hindered due to its dynamic instability. Using density functional theory combined with a global structural search and phonon dispersion calculations, we show that an ordered $50%$ N- or B-doped graphene can be made energetically and dynamically stable by simultaneous doping carriers and applying biaxial tensile strain; carrier doping moves the system toward aromaticity while tensile strain reduces adverse effects associated with electrostatic interaction. Electron-phonon coupling calculations show that the N- or B-doped graphene is superconducting with critical temperature reaching above the melting point of nitrogen in the case of $50%$ N-doped graphene. In addition, the ideal strength of N-doped graphene is even higher than that of pure graphene.

Photoelectron spectroscopy of B4O4 (-): Dual 3c-4e π hyperbonds and rhombic 4c-4e o-bond in boron oxide clusters.

Gas-phase anion photoelectron spectroscopy (PES) is combined with global structural searches and electronic structure calculations at the hybrid Becke 3-parameter exchange functional and Lee-Yang-Parr correlation functional (B3LYP) and single-point coupled-cluster with single, double, and perturbative triple excitations (CCSD(T)) levels to probe the structural and electronic properties and chemical bonding of the B4O4 (0/-) clusters. The measured PES spectra of B4O4 (-) exhibit a major band with the adiabatic and vertical detachment energies (ADE and VDE) of 2.64 ± 0.10 and 2.81 ± 0.10 eV, respectively, as well as a weak peak with the ADE and VDE of 1.42 ± 0.08 and 1.48 ± 0.08 eV. The former band proves to correspond to the Y-shaped global minimum of Cs B4O4 (-) ((2)A″), with the calculated ADE/VDE of 2.57/2.84 eV at the CCSD(T) level, whereas the weak band is associated with the second lowest-energy, rhombic isomer of D2h B4O4 (-) ((2)B2g) with the predicted ADE/VDE of 1.43/1.49 eV. Both anion structures are planar, featuring a B atom or a B2O2 core bonded with terminal BO and/or BO2 groups. The same Y-shaped and rhombic structures are also located for the B4O4 neutral cluster, albeit with a reversed energy order. Bonding analyses reveal dual three-center four-electron (3c-4e) π hyperbonds in the Y-shaped B4O4 (0/-) clusters and a four-center four-electron (4c-4e) π bond, that is, the so-called o-bond in the rhombic B4O4 (0/-) clusters. This work is the first experimental study on a molecular system with an o-bond.

Planar dicyclic B6S6, B6S6(-), and B6S6(2-) clusters: boron sulfide analogues of naphthalene.

Inorganic analogues of hydrocarbons or polycyclic aromatic hydrocarbons (PAHs) are of current interest in chemistry. Based upon global structural searches and B3LYP and CCSD(T) calculations, we present herein the perfectly planar dicyclic boron sulfide clusters: D2h B6S6 (1, (1)Ag), D2h B6S6(-) (2, (2)B3u), and D2h B6S6(2-) (3, (1)Ag). These are the global minima of the systems, being at least 0.73, 0.81, and 0.53 eV lower in energy, respectively, than their alternative isomers at the CCSD(T) level. The D2h structures feature twin B3S2 five-membered rings, which are fused together via a B2 unit and terminated by two BS groups. Bonding analyses show that the closed-shell B6S6(2-) (3) cluster possesses 10 delocalized π electrons, closely analogous to the bonding pattern of the aromatic naphthalene C10H8. The B6S6(-) (2) and B6S6 (1) species are readily obtained upon removal of one or two π electrons from B6S6(2-) (3). The results build a new analogous relationship between boron sulfide clusters and their PAH counterparts. The B6S6(-) (2) monoanion and B6S6(2-) (3) dianion can be effectively stabilized in neutral LiB6S6 and Li2B6S6 salts, respectively.

Endohedral and exohedral metalloborospherenes: M@B40 (M=Ca, Sr) and M&B40 (M=Be, Mg).

The recent discovery of the all-boron fullerenes or borospherenes, D(2d) B40(-/0), paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B40 (M=Ca, Sr) and exohedral M&B40 (M=Be, Mg). Extensive global structural searches indicate that Ca@B40 (1, C(2v), (1)A1) and Sr@B40 (3, D(2d), (1)A1) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B40 (5, C(s), (1)A') and Mg&B40 (7, C(s), (1)A') favor exohedral borospherene geometries with a η(7)-M atom face-capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of the borospherene structural motif. The metalloborospherenes are characterized as charge-transfer complexes (M(2+)B40(2-)), where an alkaline earth metal atom donates two electrons to the B40 cage. The high stability of endohedral Ca@B40 (1) and Sr@B40 (3) is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D(2d) B40 borospherene.

Endohedral and Exohedral Metalloborospherenes: M@B40 (M=Ca, Sr) and M&B40 (M=Be, Mg)

AbstractThe recent discovery of the all‐boron fullerenes or borospherenes, D2d B40−/0, paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B40 (M=Ca, Sr) and exohedral M&B40 (M=Be, Mg). Extensive global structural searches indicate that Ca@B40 (1, C2v, 1A1) and Sr@B40 (3, D2d, 1A1) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B40 (5, Cs, 1A′) and Mg&B40 (7, Cs, 1A′) favor exohedral borospherene geometries with a η7‐M atom face‐capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of the borospherene structural motif. The metalloborospherenes are characterized as charge‐transfer complexes (M2+B402−), where an alkaline earth metal atom donates two electrons to the B40 cage. The high stability of endohedral Ca@B40 (1) and Sr@B40 (3) is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D2d B40 borospherene.

Porous Boron Nitride with Tunable Pore Size.

On the basis of a global structural search and first-principles calculations, we predict two types of porous boron-nitride (BN) networks that can be built up with zigzag BN nanoribbons (BNNRs). The BNNRs are either directly connected with puckered B (N) atoms at the edge (type I) or connected with sp(3)-bonded BN chains (type II). Besides mechanical stability, these materials are predicted to be thermally stable at 1000 K. The porous BN materials entail large surface areas, ranging from 2800 to 4800 m(2)/g. In particular, type-II BN material with relatively large pores is highly favorable for hydrogen storage because the computed hydrogen adsorption energy (-0.18 eV) is very close to the optimal adsorption energy (-0.15 eV) suggested for reversible hydrogen storage at room temperature. Moreover, the type-II materials are semiconductors with width-dependent direct bandgaps, rendering the type-II BN materials promising not only for hydrogen storage but also for optoelectronic and photonic applications.

On the structures and bonding in boron-gold alloy clusters: B6Aun− and B6Aun (n = 1−3)

Photoelectron spectroscopy and density-functional theory are combined to investigate the electronic and structural properties of a series of B-Au alloy clusters: B6Aun(-) and B6Aun (n = 1-3). Rich spectral features are observed for each species, and vibrational structures are resolved for numerous detachment transitions of B6Au(-) and B6Au2(-). Electron affinities of B6Aun (n = 1-3) are evaluated to be 2.70 ± 0.03, 2.91 ± 0.02, and 3.21 ± 0.05 eV, respectively. Global structural searches are performed for both the anions and their neutrals. The calculated electronic binding energies are compared with experimental measurements to establish the anion global-minimum structures and their corresponding neutral states. The ground-state structures of these clusters are shown to be planar or quasi-planar with an elongated B6 core, to which the first and second Au atoms are bonded terminally and the third Au in a bridging position. All three anion clusters are π antiaromatic. Natural bond orbital analyses show that the B-Au bonding is highly covalent, providing new examples for the Au∕H analogy in Au alloy clusters.

An effective structure prediction method for layered materials based on 2D particle swarm optimization algorithm

A structure prediction method for layered materials based on two-dimensional (2D) particle swarm optimization algorithm is developed. The relaxation of atoms in the perpendicular direction within a given range is allowed. Additional techniques including structural similarity determination, symmetry constraint enforcement, and discretization of structure constructions based on space gridding are implemented and demonstrated to significantly improve the global structural search efficiency. Our method is successful in predicting the structures of known 2D materials, including single layer and multi-layer graphene, 2D boron nitride (BN) compounds, and some quasi-2D group 6 metals(VIB) chalcogenides. Furthermore, by use of this method, we predict a new family of mono-layered boron nitride structures with different chemical compositions. The first-principles electronic structure calculations reveal that the band gap of these N-rich BN systems can be tuned from 5.40 eV to 2.20 eV by adjusting the composition.

Accelerated cluster structure search using electron diffraction data in a genetic algorithm

We present a new method to accelerate the convergence behavior of a genetic algorithm (GA) for global cluster structure search by adding experimentally determined gas phase electron diffraction information in terms of a profile factor as additional criterion for evaluating trial structures into the objective function of the GA. This algorithm has been applied on two test systems: in case of B18+ the superior ability of locating the global minimum in a narrow side potential energy funnel is shown. In the second test case, Au13-, the robustness of the modified GA against systematic errors in the energy calculation is demonstrated.