High-throughput computational materials screening of transition metal peroxides.

Semiconductor materials of abnormal stoichiometric ratio often exhibit unique properties, yet it is still a challenge to determine the structures of such materials in an efficient way. Herein, we propose a method for structurally biased screening according to the coordination numbers and the numbers of Wyckoff positions, balancing the atom local environment and the global symmetry of structures. Based on first-principles calculations, we have predicted two metastable peroxides P21/c-ScO2 and Pmmn-TiO3 with more than six coordination points. For these two structures, the most stable intrinsic defect is the oxygen vacancy (VO) at the peroxide anion (O2-2), which induces the absence of antibonding orbital formed by O2-2 near the valence band maximum. With the introduction of VO, the decrease of coordination numbers leads to charge recombination, and results in the appearance of an ordered phase TiO2.5 with stronger Ti-O orbital hybridization. The proposed method presents a promising and feasible approach for the screening of novel compounds.

Energy landscape of Au13: a global view of structure transformation.

It has long been a challenge in physics and chemistry to acquire a global picture of the energy landscape of a specific material, as well as the kinetic transformation process between configurations of interest. Here we have presented a comprehensive approach to deal with the structure transformation problem, along with the illustration of the energy landscape, as exemplified with the case of Au13. A configuration space based on interatomic distances was proposed and demonstrated to have a strong correlation between structure and energy, with application in structure analysis to screen for trial transition pathways. As several representative configurations and their transition pathways ascertained and by projecting on a plane, a visual two-dimensional contour map was sketched revealing the unique energy landscape of Au13. It shows that the 2D and 3D clusters form two funnels in the high-dimensional configuration space, with a transition pathway with a 0.976 eV barrier bridging them.

Motif based high-throughput structure prediction of superconducting monolayer titanium boride.

Two-dimensional boron structures, due to their diverse properties, have attracted great attention because of their potential applications in nanoelectronic devices. A series of TiBn (2 ≤ n ≤ 13) monolayers are efficiently constructed through our motif based method and theoretically investigated through high-throughput first-principles calculations. The configurations are generated based on the motifs of boron dimeric/triangular/quadrilateral fragments and multi-coordinate titanium-centered boron molecular wheels. Besides previously reported TiB4 and TiB9 which were discovered by the global search method, we predict that high symmetry monolayer TiB7 (Cmmm), which is octa-coordinate titanium boride, is dynamically stable. The TiB7 monolayer is a BCS superconductor with a transition temperature Tc of up to 8.3 K. The motif based approach is proved to be efficient in searching stable structures with prior knowledge so that the potentially stable transition metal monolayers can be quickly constructed by using basic cluster motifs. As an efficient way of discovering materials, the method is easily extended to predict other types of materials which have common characteristic patterns in the structure.

Atom Classification Model for Total Energy Evaluation of Two-Dimensional Multicomponent Materials.

The tunable properties of materials originate from variety of structures; however, it is still a challenge to give an accurate and fast evaluation of stabilities for screening numerous candidates. Herein, we propose an atom classification model to describe the multicomponent materials based on the structural recognition, in which the atoms are classified to estimate the total energies. Taking two-dimensional planar C1-xBx and C1-2x(BN)x as examples, we have found that the test error of total energies is about 3 meV per atom. Notably, the distributions of classified atoms demonstrate the evolution of configurations as a function of temperature, providing a clearer picture of phase transition. In addition, our method is universal, which can be flexibly extended to the bulk structures with more components.

Two-Dimensional Anti-Van't Hoff/Le Bel Array AlB6 with High Stability, Unique Motif, Triple Dirac Cones, and Superconductivity.

We report the discovery of a rule-breaking two-dimensional aluminum boride (AlB6-ptAl-array) nanosheet with a planar tetracoordinate aluminum (ptAl) array in a tetragonal lattice by comprehensive crystal structure search, first-principles calculations, and molecular dynamics simulations. It is a brand new 2D material with a unique motif, high stability, and exotic properties. These anti-van't Hoff/Le Bel ptAl-arrays are arranged in a highly ordered way and connected by two sheets of boron rhomboidal strips above and below the array. The regular alignment and strong bonding between the constituents of this material lead to very strong mechanical strength (in-plane Young's modulus Y x = 379, Y y = 437 N/m, much larger than that of graphene, Y = 340 N/m) and high thermal stability (the framework survived simulated annealing at 2080 K for 10 ps). Additionally, electronic structure calculations indicate that it is a rare new material with triple Dirac cones, Dirac-like fermions, and node-loop features. Remarkably, this material is predicted to be a 2D phonon-mediated superconductor with Tc = 4.7 K, higher than the boiling point of liquid helium (4.2 K). Surprisingly, the Tc can be greatly enhanced up to 30 K by applying tensile strain at 12%. This is much higher than the temperature of liquid hydrogen (20.3 K). These outstanding properties may pave the way for potential applications of an AlB6-ptAl-array in nanoelectronics and nanomechanics. This work opens up a new branch of two-dimensional aluminum boride materials for exploration. The present study also opens a field of two-dimensional arrays of anti-van't Hoff/Le Bel motifs for study.

Theoretical investigations on diamondoids (CnHm, n = 10-41): Nomenclature, structural stabilities, and gap distributions.

Combining the congruence check and the first-principles calculations, we have systematically investigated the structural stabilities and gap distributions of possible diamondoids (CnHm) with the carbon numbers (n) from 10 to 41. A simple method for the nomenclature is proposed, which can be used to distinguish and screen the candidates with high efficiency. Different from previous theoretical studies, the possible diamondoids can be enumerated according to our nomenclature, without any pre-determination from experiments. The structural stabilities and electronic properties have been studied by density functional based tight binding and first-principles methods, where a nearly linear correlation is found between the energy gaps obtained by these two methods. According to the formation energy of structures, we have determined the stable configurations as a function of chemical potential. The maximum and minimum energy gaps are found to be dominated by the shape of diamondoids for clusters with a given number of carbon atoms, while the gap decreases in general as the size increases due to the quantum confinement.

Two-Dimensional Semiconducting Boron Monolayers.

The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations. Unexpectedly, we have first found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+px,y and pz orbitals, with which various semiconducting boron monolayers are designed to realize the band gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.

Phonon-mediated superconductivity in Mg intercalated bilayer borophenes.

Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes BxMgBx (x = 2-5). Remarkably, B2MgB2 and B4MgB4 are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (Tc) of 23.2 K and 13.3 K, respectively, while B4MgB4 is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg-B systems with high Tcs.

Geometrical eigen-subspace framework based molecular conformation representation for efficient structure recognition and comparison.

We have developed an extended distance matrix approach to study the molecular geometric configuration through spectral decomposition. It is shown that the positions of all atoms in the eigen-space can be specified precisely by their eigen-coordinates, while the refined atomic eigen-subspace projection array adopted in our approach is demonstrated to be a competent invariant in structure comparison. Furthermore, a visual eigen-subspace projection function (EPF) is derived to characterize the surrounding configuration of an atom naturally. A complete set of atomic EPFs constitute an intrinsic representation of molecular conformation, based on which the interatomic EPF distance and intermolecular EPF distance can be reasonably defined. Exemplified with a few cases, the intermolecular EPF distance shows exceptional rationality and efficiency in structure recognition and comparison.

An intrinsic representation of atomic structure: From clusters to periodic systems.

We have improved our distance matrix and eigen-subspace projection function (EPF) [X.-T. Li et al., J. Chem. Phys. 146, 154108 (2017)] to describe the atomic structure for periodic systems. Depicting the local structure of an atom, the EPF turns out to be invariant with respect to the choices of the unit cell and coordinate frame, leading to an intrinsic representation of the crystal with a set of EPFs of the nontrivial atoms. The difference of EPFs reveals the difference of atoms in local structure, while the accumulated difference between two sets of EPFs can be taken as the distance between configurations. Exemplified with the cases of carbon allotropes and boron sheets, our EPF approach shows exceptional rationality and efficiency to distinguish the atomic structures, which is crucial in structure recognition, comparison, and analysis.

High-coverage stable structures of 3d transition metal intercalated bilayer graphene.

Alkali-metal intercalated graphite and graphene have been intensively studied for decades, where alkali-metal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. Using first-principles calculations, we have predicted various stable structures of high-coverage 3d transition metal (TM) intercalated bilayer graphene (BLG) stabilized by the strain. Specifically, with reference to the bulk metal, Sc and Ti can form stable TM-intercalated BLG without strain, while the stabilization of Fe, Co, and Ni intercalated BLG requires the biaxial strain of over 7%. Under the biaxial strain ranging from 0% to 10%, there are four ordered sandwich structures for Sc with the coverage of 0.25, 0.571, 0.684, and 0.75, in which the Sc atoms are all distributed homogenously instead of locating at the hollow sites. According to the phase diagram, a homogenous configuration of C8Ti3C8 with the coverage of 0.75 and another inhomogeneous structure with the coverage of 0.692 were found. The electronic and magnetic properties as a function of strain were also analyzed to indicate that the strain was important for the stabilities of the high-coverage TM-intercalated BLG.

Understanding the stable boron clusters: A bond model and first-principles calculations based on high-throughput screening.

The unique electronic property induced diversified structure of boron (B) cluster has attracted much interest from experimentalists and theorists. B30-40 were reported to be planar fragments of triangular lattice with proper concentrations of vacancies recently. Here, we have performed high-throughput screening for possible B clusters through the first-principles calculations, including various shapes and distributions of vacancies. As a result, we have determined the structures of Bn clusters with n = 30-51 and found a stable planar cluster of B49 with a double-hexagon vacancy. Considering the 8-electron rule and the electron delocalization, a concise model for the distribution of the 2c-2e and 3c-2e bonds has been proposed to explain the stability of B planar clusters, as well as the reported B cages.

Waiting endurance time estimation of electric two-wheelers at signalized intersections.

The paper proposed a model for estimating waiting endurance times of electric two-wheelers at signalized intersections using survival analysis method. Waiting duration times were collected by video cameras and they were assigned as censored and uncensored data to distinguish between normal crossing and red-light running behavior. A Cox proportional hazard model was introduced, and variables revealing personal characteristics and traffic conditions were defined as covariates to describe the effects of internal and external factors. Empirical results show that riders do not want to wait too long to cross intersections. As signal waiting time increases, electric two-wheelers get impatient and violate the traffic signal. There are 12.8% of electric two-wheelers with negligible wait time. 25.0% of electric two-wheelers are generally nonrisk takers who can obey the traffic rules after waiting for 100 seconds. Half of electric two-wheelers cannot endure 49.0 seconds or longer at red-light phase. Red phase time, motor vehicle volume, and conformity behavior have important effects on riders' waiting times. Waiting endurance times would decrease with the longer red-phase time, the lower traffic volume, or the bigger number of other riders who run against the red light. The proposed model may be applicable in the design, management and control of signalized intersections in other developing cities.

The stability of CsPb(BrxCl1-x)3 all-inorganic mixed halide perovskites.

The use of mixed halide perovskites in the preparation of blue light-emitting diodes (LEDs) is considered to be the most effective and direct approach. However, the introduction of chlorine (Cl) element might raise stability issues in the system and lead to low efficiency, thereby impeding the development of deep blue light-emitting diodes with high efficiency and stability. Determining the alloy concentration and the atomic distribution of bromine-chlorine (Br-Cl) mixed systems is essential for further application of deep blue light-emitting diodes. In this work, we have systematically investigated the stability of bromine-chlorine (Br-Cl) mixed alloy systems in various substitution configurations using high-throughput theoretical calculations. Based on this, we have examined the relationship between configuration stability and three aspects: the type of octahedra, the orientation of the octahedra and the Pb-X-Pb distortion angle in the configuration.

Theoretical studies of the passivants' effect on the Si(x)Ge(1-x) nanowires: composition profiles, diameter, shape, and electronic properties.

Theoretically, we have performed a systematic investigation on the passivants' effect on the geometrical and electronic properties of Si(x)Ge(1-x) nanowires. First-principles calculations revealed that, in the nanowires passivated by fluorine (F)∕chlorine (Cl)∕hydrogen (H) atoms, Si atoms preferred to segregate towards the surface due to the stronger Si-X bonds than that of Ge-X bonds (X = F, Cl, H). The energy barriers of X atoms' desorption is higher than that of the Si∕Ge atoms' exchanging, inducing a feasible and strong surface segregation of Si atoms at proper temperature. Considering the Si∕Ge interactions and mixing entropy, the composition profiles of Si∕Ge distributions are obtained by minimizing the Gibbs free energy, which indicates the outmost layer of surface should be mostly occupied by Si. With total Si surface segregation, the diameter and shape of most stable Si(x)Ge(1-x) nanowires are found to be determined by the composition x and the passivants' chemical potential. In addition, charge distribution of near-gap levels can be modulated through the surface passivants. Our finding provides a practical avenue to tune the electronic properties of Si(x)Ge(1-x) nanowires, by modulating the morphologies of nanowires with the composition control of Si∕Ge and the chemical potential of passivants.

Exact results for the residual entropy of ice hexagonal monolayer.

Since the problem of the residual entropy of square ice was exactly solved, exact solutions for two-dimensional realistic ice models have been of interest. In this work, we study the exact residual entropy of ice hexagonal monolayer in two cases. In the case that the external electric field along the z-axis exists, we map the hydrogen configurations into the spin configurations of the Ising model on the kagome lattice. By taking the low temperature limit of the Ising model, we derive the exact residual entropy, which agrees with the result determined previously from the dimer model on the honeycomb lattice. In another case that the ice hexagonal monolayer is under the periodic boundary conditions in the cubic ice lattice, the residual entropy has not been studied exactly. For this case, we employ the six-vertex model on the square lattice to represent the hydrogen configurations obeying the ice rules. The exact residual entropy is obtained from the solution of the equivalent six-vertex model. Our work provides more examples of the exactly soluble two-dimensional statistical models.

High Concentration Intrinsic Defects in MnSb2Te4.

MnSb2Te4 has a similar structure to an emerging material, MnBi2Te4. According to earlier theoretical studies, the formation energy of Mn antisite defects in MnSb2Te4 is negative, suggesting its inherent instability. This is clearly in contrast to the successful synthesis of experimental samples of MnSb2Te4. Here, the growth environment of MnSb2Te4 and the intrinsic defects are correspondingly investigated. We find that the Mn antisite defect is the most stable defect in the system, and a Mn-rich growth environment favors its formation. The thermodynamic equilibrium concentrations of the Mn antisite defects could be as high as 15% under Mn-poor conditions and 31% under Mn-rich conditions. It is also found that Mn antisite defects prefer a uniform distribution. In addition, the Mn antisite defects can modulate the interlayer magnetic coupling in MnSb2Te4, leading to a transition from the ideal antiferromagnetic ground state to a ferromagnetic state. The ferromagnetic coupling effect can be further enhanced by controlling the defect concentration.

Multiple Magnetic Topological Phases in the van der Waals Crystal Mn(Bi,Sb)4Se7.

Magnetic topological materials have drawn markedly attention recently due to the strong coupling of their novel topological properties and magnetic configurations. In particular, the MnBi2Te4/(Bi2Te3)n family highlights the researches of multiple magnetic topological materials. Via first-principles calculations, we predict that Mn(Bi, Sb)4Se7, the close relatives of MnBi2Te4/(Bi2Te3)n family, are topological nontrivival in both antiferromagnetic and ferromagnetic configurations. In the antiferromagnetic ground state, Mn(Bi, Sb)4Se7 are simultaneously topological insulators and axion insulators. Massless Dirac surface states emerge on the surfaces parallel to the z axis. In ferromagnetic phases, they are axion insulators. Particularly, when the magnetization direction is along the x axis, they are also topological crystalline insulators. Mirror-symmetry-protected gapless surface states exist on the mirror-invariant surfaces. Hence, the behaviors of surface states are strongly dependent on the magnetization directions and surface orientations. Our work provides more opportunities for the study of magnetic topological physics.

Surface structure and phase transition of K adsorption on Au(111): by ab initio atomistic thermodynamics.

We studied the interactions between atomic potassium (K) and Au(111) at a range of coverage (i.e., Θ(K) = 0.11-0.5 monolayer (ML)) by ab initio atomic thermodynamics. For K on-surface adsorption, we found that K energetically favors the three-fold hollow sites (fcc or hcp), while the most significant surface rumpling was obtained at the atop sites. The incorporation of gold atoms in the adsorbate layer gradually becomes energetically favorable with increasing K coverage. We proposed a possible model with a stoichiometry of K(2)Au for the (2 × 2)-0.5 ML phase observed in lower energy electron diffraction (LEED): one K at atop site and the other K as well as one Au adatom at the second-nearest fcc/hcp and hcp/fcc, respectively. Clear theoretical evidences were given for the ionic interaction of K on Au surface. Additionally, phase transitions were predicted based on chemical potential equilibrium of K, largely in line with the earlier reported LEED observations: the clean surface → (√3 × âˆš3)R30° → (2 × 2), and (2 × 2) → (√3 × âˆš3)R30° reversely at an elevated temperature.

Five-fold symmetry in Au-Si metallic glass.

The first metallic glass of Au-Si alloy for over half a century has been discovered, but its atomic structure is still puzzling. Herein, Au8Si dodecahedrons with local five-fold symmetry are revealed as building blocks in Au-Si metallic glass, and the interconnection modes of Au8Si dodecahedrons determine the medium-range order. With dimensionality reduction, the surface ordering is attributed to the motif transformation of Au8Si dodecahedrons into planar Au5Si pyramids with five-fold symmetry, and thus the self-assembly of Au5Si pyramids leads to the formation of the ordered Au2Si monolayer with the lowest energy. Furthermore, structural similarity analysis is performed to unveil the physical origin of the structural characteristics in different dimensions. The amorphism of Au-Si is due to the smooth energy landscape around the global minimum, while the ordered surface structure occurs due to the steep energy landscape.