Global Structure Search Research Articles

Two-Dimensional AuMX2 (M = Al, Ga, In; X = S, Se) Monolayers Featuring Intracrystalline Aurophilic Interactions with Novel Electronic and Optical Properties.

Aurophilicity, known as aurophilic interaction, is a strong attractive van der Waals interaction between cationic gold(I) centers, whose strength is comparable to the hydrogen bond. Here, we show that aurophilicity can serve as an engineering approach to expand the structural dimensionality for nanomaterials design. Specifically, based on a global-structure search method and density functional theory calculations, we predict a series of stable two-dimensional (2D) AuMX2 (M = Al, Ga, In; X = S, Se) structures featuring intracrystalline aurophilic interactions. All the AuMX2 monolayers designed are semiconductors with moderate band gaps, excellent carrier mobilities, and good optical properties. The intriguing chemistry of aurophilicity coupled with novel electronic properties render AuMX2 monolayers a potentially new series of 2D materials that are of fundamental importance in gold chemistry and of technological importance for nanoelectronics.

Crystal structures of CsSi6 at high pressures

High pressure is well known to be a useful tool to explore novel crystal structures of materials with unique chemical and physical properties that are not possible to access at ambient conditions. We here report some interesting crystal structures of CsSi6 using a global structure search at a pressure range of 0–30 GPa. Remarkably, we identify a new cubic Im-3m phase of CsSi6 at ambient conditions, and that phase undergoes to an orthorhombic Cmcm phase at ∼8.2 GPa, then transforms to a monoclinic C2/m phase at ∼20.8 GPa. It is found that Si atoms have 5-fold coordination in the high-pressure C2/m phase. These newly predicted structures are all dynamically stable, and their electronic properties are well presented in this work. The present results clearly show some interesting crystal structures of CsSi6 under high pressure, providing insights for exploring other high-pressure behaviors of alkali metal silicides.

A New Anisotropic Dirac Cone Material: A B2S Honeycomb Monolayer.

Different from the isotropic Dirac cones existing in other two-dimensional (2D) materials, anisotropic Dirac cones have the merit of anisotropic carrier mobility for applications in direction-dependent quantum devices. Motivated by the recent experimental finding of an anisotropic Dirac cone in borophene, here we report a new 2D anisotropic Dirac cone material, B2S monolayer, identified by using a global structure search method and first-principles calculation combined with a tight-binding model. The B2S monolayer is found to be stable mechanically, thermally, and dynamically and exhibits an anisotropic Dirac cone exactly at the Fermi level, showing a Fermi velocity of 106 m/s in the same order of magnitude as that of graphene. Moreover, B2S monolayer is the first anisotropy Dirac cone material with a pristine honeycomb structure stabilized by S in free-standing conditions where each atom has four valence electrons on average being isoelectronic to C. This study would expand the Dirac cone material family with new features.

High-pressure-assisted design of porous topological semimetal carbon for Li-ion battery anode with high-rate performance

It has been a great challenge to develop a high-rate anode material with high-capacity, fast Li-ions diffusion and long cycling life going beyond the commercially used graphite in Li-ion battery. Here for the first time we propose a strategy combined high-pressure synthesis method with the global structure search to find a topological semimetal porous carbon as the desired anode. Our crystal-structure searching shows that we can obtain the ground state of an orthorhombic phase $\mathrm{Li}{\mathrm{C}}_{6}$ with regular pores at 30 GPa, and when the Li atoms are removed, the resulting carbon structure is the recently predicted interlocked graphene network (IGN) that is a topological semimetal with an intrinsic high electronic conductivity. Based on the state-of-the-art first-principles calculations, we further find that the Li-ion migration energy barrier in the IGN is extremely low and the estimated diffusion coefficient can reach a magnitude of ${10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ at both low and high Li concentrations, which is three orders of magnitude larger than that of graphite anode. Moreover, the volume changes during the Li insertion and deinsertion are smaller than $3.2%$, while the theoretical specific capacity is the same as that of graphite anode. Our studies not only suggest a practical way of synthesizing the topological semimetal carbon but also propose a new anode material for Li-ion battery.

Prediction of a BeP2 monolayer with a compression-induced Dirac semimetal state

We have identified a two-dimensional (2D) beryllium diphosphide (${\mathrm{BeP}}_{2}$) structure using a global structure search combined with first-principles calculations. Phonon calculations and molecular dynamics simulation confirm that the structure is dynamically and thermally stable. Electronic structure calculations show that the 2D sheet is a direct band gap semiconductor with a small band gap of 0.15 eV, and the intrinsic acoustic-phonon-limited carrier mobility of the structure can reach $\ensuremath{\sim}{10}^{4}\phantom{\rule{4pt}{0ex}}{\text{cm}}^{2}{\text{V}}^{\ensuremath{-}1}{\text{s}}^{\ensuremath{-}1}$ for both electrons and holes with anisotropic features in the $x$ and $y$ directions. More interestingly, both mechanical and chemical compression can close the band gap and the structure turns to a Dirac semimetal with the Dirac cones located exactly at the Fermi level. The emerged Dirac semimetal state is direction dependent, with a linear band dispersion in the $x$ direction and a quadratic one in the $y$ direction. Moreover, it is demonstrated that the Dirac point is symmetry protected in the absence of spin-orbit coupling (SOC). In ${\mathrm{BeP}}_{2}$, the SOC is too weak to alter the semimetal feature except for the cases at extremely low temperatures. The band gap closing mechanism is further clarified by using the tight-binding (TB) method.

Stabilization of golden cages by encapsulation of a single transition metal atom.

Golden cage-doped nanoclusters have attracted great attention in the past decade due to their remarkable electronic, optical and catalytic properties. However, the structures of large golden cage doped with Mo and Tc are still not well known because of the challenges in global structural searches. Here, we report anionic and neutral golden cage doped with a transition metal atom MAu16 (M = Mo and Tc) using Saunders ‘Kick' stochastic automation search method associated with density-functional theory (DFT) calculation (SK-DFT). The geometric structures and electronic properties of the doped clusters, MAu16q (M = Mo and Tc; q = 0 and −1), are investigated by means of DFT theoretical calculations. Our calculations confirm that the 4d transition metals Mo and Tc can be stably encapsulated in the Au16− cage, forming three different configurations, i.e. endohedral cages, planar structures and exohedral derivatives. The ground-state structures of endohedral cages C2v Mo@Au16−-(a) and C1 Tc@Au16−-(b) exhibit a marked stability, as judged by their high binding energy per atom (greater than 2.46 eV), doping energy (0.29 eV) as well as a large HOMO–LUMO gap (greater than 0.40 eV). The predicted photoelectron spectra should aid in future experimental characterization of MAu16− (M = Mo and Tc).

Stabilizing benzene-like planar N6 rings to form a single atomic honeycomb BeN3 sheet with high carrier mobility.

It is a longstanding quest to use the planar N6 ring as a structural unit to build stable atomic sheets. However, unlike C6H6, the neutral N6 ring is unstable due to the strong repulsion of the lone-pair of electrons. Using first-principles calculations and the global structure search method, we show that the N6 unit can be stabilized by the linkage of Be atoms, forming a h-BeN3 honeycomb monolayer, in which the geometry and the π-molecular orbitals of the N6 rings are well kept. This sheet is not only energetically, dynamically and thermally stable, but also can withstand high temperatures up to 1000 K. Band structure calculation combined with a group theory analysis and a tight-binding model uncover that h-BeN3 has a π-band dominated band structure with an indirect band gap of 1.67 eV. While it possesses a direct band gap of 2.07 eV at the Γ point lying in the photon energy region of visual light, its interband dipole transition is symmetrically allowed so that electrons can be excited by photons free of phonons. Based on deformation potential theory, a systematic study of the transport properties reveals that the h-BeN3 sheet possesses a high carrier mobility of ∼103 cm2 V-1 s-1, superior to the extensively studied transition metal dichalcogenide monolayers. We further demonstrate that this sheet can be rolled up into either zigzag or armchair nanotubes. These nanotubes are also dynamically stable, and are all direct band gap semiconductors with carrier mobility comparable to that of their 2D counterparts, regardless of their chirality and diameter. The robust stability and novel electronic and transport properties of the h-BeN3 sheet and its tubular derivatives endow them with great potential for applications in nanoelectronic devices.

2D framework C2N as a potential cathode for lithium–sulfur batteries: anab initiodensity functional study

Quantum chemical calculations combined with a genetic algorithm for a global structure search using a C2N nanosheet as a LSB cathode.

Two-Dimensional Co2S2 monolayer with robust ferromagnetism

Design and synthesis of two-dimensional (2D) materials with robust intrinsic ferromagnetism is highly desirable due to their potential applications in spintronics devices. In this work, we identify a new 2D cobalt sulfide (Co2S2) material by using first-principles calculations and particle swarm optimization (PSO) global structure search. We show that the 2D Co2S2 is most stable in the litharge type tetragonal structure with space group of P4/nmm. The elastic constants, phonon spectrum, and molecular dynamics simulation confirm its mechanical, dynamical and thermal stability, respectively. It is also found that Co2S2 monolayer is a ferromagnetic metal with a Curie temperature up to 404 K. In addition, we propose a feasible procedure to synthesize the Co2S2 monolayer by chemically exfoliating from bulk TlCo2S2 phase.

Origin of polymorphism of the two-dimensional group-IV monochalcogenides

Unlike other two-dimensional (2D) isovalent materials, the 2D group IV monochalcogenides, $MX$ ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{Si}$, Ge, Sn, and Pb; $X\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{S}$, Se, and Te), are found to be either in a black phosphorene-derived distorted NaCl-type ($d$-NaCl) structure or a recently predicted $Pma2$ structure. Both $M$ and $X$ atoms in the $d$-NaCl structure are threefold coordinated, whereas $M$ and $X$ in the $Pma2$ structure are fourfold and twofold coordinated, respectively. Using first-principles total energy and electronic structure calculations and a global structural search technique, we systematically investigated the mechanism underlying the polymorphism of the 2D group-IV monochalcogenides. Our analysis show that the relative stability of the two distinct crystallographic phases depends on the strength of the $M\ensuremath{-}M$ covalent bond and the electronegativity difference between the constituent elements $M$ and $X$. For small cations, the covalency plays more important role, whereas for large cations the Coulomb interaction becomes more dominant. Therefore, the $\mathrm{Si}X$ and $\mathrm{Ge}X$ compounds assume the $Pma2$ structure, whereas the $MX$ compounds with heavy cation elements ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{Sn}$ and Pb) tend to adopt the $d$-NaCl structure.

Identifying the Ground State Geometry of a MoN2 Sheet through a Global Structure Search and Its Tunable p-Electron Half-Metallicity

MoN2 and MoS2 sheets are representatives of two-dimensional transition metal dinitrides and dichalcogenides, respectively. Their similarity in atomic ratios misled people to make an incorrect assumption in previous studies that the former adopts the geometry of the latter. However, compared with S, N is smaller and has fewer valence electrons, and N is more effective in mediating magnetic couplings; therefore, a different geometry and different properties can be expected for the MoN2 sheet. Here using a global structure search, for the first time we have identified the ground state geometry of this sheet named Tetra-MoN2 that is much more stable than the H phase proposed previously. Different from the metallic nature of H-MoN2, Tetra-MoN2 is a semiconductor having an indirect band gap of 1.41 eV with a flexible strain tunability. In particular, Tetra-MoN2 can exhibit rich half-metallic behaviors mediated by the polarized p electron of N and induced by low-concentration hole doping and small strains that are readily achievable experimentally.

Design of Two-Dimensional Graphene-like Dirac Materials β12-XBeB5 (X = H, F, Cl) from Non-graphene-like β12-Borophene.

Two-dimensional (2D) Dirac materials and boron sheets have attracted intensive interest recently. However, 2D Dirac materials remain rare and difficult to be realized experimentally, and 2D boron sheets generally have high dynamical instability. Stimulated by the experimental observation of Dirac cones in nongraphene-like β12 boron sheets and based on the understanding of boron sheet electronic organization, we theoretically design new 2D Dirac materials β12-XBeB5 (X = H, F, Cl) with high stability. We confirm β12-HBeB5 as the global energy minimum among its 2D allotropes based on global structure search methods, a strong indication of its experimental feasibility. Our designed β12-HBeB5 has not only a high Fermi velocity, but also a Dirac state very robust against extraordinary large tensile strains, an advantage for flexible electronics applications. Our work opens a new avenue to designing feasible 2D Dirac materials and stabilizing borophene sheets.

Surface reconstruction of fluorites in vacuum and aqueous environment

Surfaces and interfaces of bulk materials with liquids are of importance for a wide range of chemical processes. In this work, we systematically explore reconstructions on the (100) surface of calcium fluoride $({\mathrm{CaF}}_{2})$ and other fluorites $(M{\mathrm{F}}_{2})$, $M={\text{Sr},\text{Cd},\text{Ba}}$ by sampling the configurational space with the minima hopping structure prediction method in conjunction with density functional theory calculations. We find a large variety of structures that are energetically very close to each other and are connected by very low barriers, resulting in a high mobility of the topmost surface anions. This high density of configurational states makes the ${\mathrm{CaF}}_{2}$ (100) surface a very dynamic system. The majority of the surface reconstructions found in ${\mathrm{CaF}}_{2}$ are also present in ${\mathrm{SrF}}_{2},{\mathrm{CdF}}_{2}$, and ${\mathrm{BaF}}_{2}$. Furthermore, we investigate in detail the influence of these reconstructions on the crystal growth of ${\mathrm{CaF}}_{2}$ in solvents by modeling the fluorite-water interface and its wetting properties. We perform a global structural search both by explicitly including water molecules and by employing a recently developed soft-sphere solvation model to simulate an implicit aqueous environment. The implicit approach correctly reproduces both our findings with the explicit-water model and the experimentally reported contact angles for the partial-hydrophobic (111) and hydrophilic (100) surfaces. Our simulations show that the high anion mobility and the low coordination of the (100) surface atoms strongly favors the adsorption of water molecules over the (111) surface. The aqueous environment makes terminations with low-coordination surface atoms more stable, promoting (100) growth instead of the (111).

TGMin: A global-minimum structure search program based on a constrained basin-hopping algorithm

In this article, we introduce Tsinghua Global Minimum (TGMin) as a new program for the global minimum searching of geometric structures of gas-phase or surface-supported atomic clusters, and the constrained basin-hopping (BH) algorithm implemented in this program. To improve the efficiency of the BH algorithm, several types of constraints are introduced to reduce the vast search space, including constraints on the random displacement step size, displacement of low-coordination atoms, and geometrical structure adjustment after displacement. The ultrafast shape-recognition (USR) algorithm and its variants are implemented to identify duplicate structures during the global minimum search. In addition to the Metropolis acceptance criterion, we also implemented a morphology-based constraint that confines the global minimum search to a specific type of morphology, such as planar or non-planar structures, which offers a strict divide-and-conquer strategy for the BH algorithm. These improvements are implemented in the TGMin program, which was developed over the past decade and has been used in a number of publications. We tested our TGMin program on global minimum structural searches for a number of metal and main-group clusters including C60, Au20 and B20 clusters. Over the past five years, the TGMin program has been used to determine the global minimum structures of a series of boron atomic clusters (such as [B26]–, [B28]–, [B30]–, [B35]–, [B36]–, [B39]–, [B40]–, [MnB16]–, [CoB18]–, [RhB18]–, and [TaB20]–), metal-containing clusters Li n (n = 3–20), Au9(CO)8 + and [Cr6O19]2–, and the oxide-supported metal catalyst Au7/γ-Al2O3, as well as other isolated and surface-supported atomic clusters. In this article we present the major features of TGMin program and show that it is highly efficient at searching for global-minimum structures of atomic clusters in the gas phase and on various surface supports.

Structure Prediction for Surface-Induced Phases of Organic Monolayers: Overcoming the Combinatorial Bottleneck.

Structure determination and prediction pose a major challenge to computational material science, demanding efficient global structure search techniques tailored to identify promising and relevant candidates. A major bottleneck is the fact that due to the many combinatorial possibilities, there are too many possible geometries to be sampled exhaustively. Here, an innovative computational approach to overcome this problem is presented that explores the potential energy landscape of commensurate organic/inorganic interfaces where the orientation and conformation of the molecules in the tightly packed layer is close to a favorable geometry adopted by isolated molecules on the surface. It is specifically designed to sample the energetically lowest lying structures, including the thermodynamic minimum, in order to survey the particularly rich and intricate polymorphism in such systems. The approach combines a systematic discretization of the configuration space, which leads to a huge reduction of the combinatorial possibilities with an efficient exploration of the potential energy surface inspired by the Basin-Hopping method. Interfacing the algorithm with first-principles calculations, the power and efficiency of this approach is demonstrated for the example of the organic molecule TCNE (tetracyanoethylene) on Au(111). For the pristine metal surface, the global minimum structure is found to be at variance with the geometry found by scanning tunneling microscopy. Rather, our results suggest the presence of surface adatoms or vacancies that are not imaged in the experiment.

Crystal Structures of CaB3N3 at High Pressures.

Using global structure searches, we have explored the structural stability of CaB3N3, a compound analogous to CaC6, under pressure. There are two high-pressure phases with space groups R3c and Amm2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC6. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the R3c phase is a semiconductor with a band gap of 2.21 eV and that the Amm2 phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.

A new metallic carbon allotrope with high stability and potential for lithium ion battery anode material

Metallic carbon has been extensively studied for its potential novel applications in catalysis, superconductivity, and electronic devices. Currently, the design of metallic carbon is mainly by educated intuition which could miss some more stable allotropes. Here we carry out a global structure search on the potential energy surface, and identify a new three-dimensional (3D) metallic carbon phase, termed Hex-C18, which is energetically more favorable than most of the previously identified 3D metallic carbon allotropes. Using state-of-the-art theoretical calculations, we show that Hex-C18 not only possesses a high thermodynamic stability, large heat capacity, high Debye stiffness, anisotropic elasticity, and super hardness, but also is a promising anode material for lithium ion batteries (LIBs). As compared to graphite commercially used in LIBs, Hex-C18 exhibits a lower Li diffusion energy barrier and a higher Li capacity because of its intrinsic metallicity and regular porosity. In addition, the simulated x-ray diffraction of Hex-C18 matches well with a previously unidentified low-angle diffraction peak in experimental XRD spectra of detonation soot, implying the possibility of its existence in the specimen. This study expands the family of metallic carbon, and may open a new frontier in design of high performance anode materials for LIBs as well.

New carbon allotropes with metallic conducting properties: a first-principles prediction

Combining density functional theory computation and the global minimum structural search, we have found new α-, β-, and δ-phographene carbon allotropes.

Coexistence of Superconductivity and Superhardness in Beryllium Hexaboride Driven by Inherent Multicenter Bonding.

Unique multicenter bonding in boron-rich materials leads to the formation of complex structures and intriguing properties. Here global structural searches are performed to unearth the structure of beryllium hexaboride (BeB6) synthesized decades ago. Three BeB6 phases (α, β, and γ) were predicted to be stable at ambient and high pressures. The ground state at ambient pressure, α-BeB6, consists of a strong and uniformly distributed covalent B-B network, which results in exceptional elastic properties and a hardness of 46 GPa comparable to γ-B. Even more surprisingly, α-BeB6 retains credible electron phonon coupling in the boron sublattice, and is predicted to be superconducting at 9 K. Above 4 GPa, β-BeB6 is stabilized with alternating boron slabs and triangular beryllium layers analogous to the structure of MgB2. The β-BeB6 is predicted to be superconducting at 24 K, similar to Nb3(Al,Ge). The γ-BeB6 is stable above 340 GPa. The understanding of intrinsic multicenter-bonding mechanism and related properties demonstrated in the very example of BeB6 provides new insights for the design of tunable multifunctional materials.

Structural Evolution of Core-Shell Gold Nanoclusters: Aun- (n = 42-50).

Gold nanoclusters have attracted great attention in the past decade due to their remarkable size-dependent electronic, optical, and catalytic properties. However, the structures of large gold clusters are still not well-known because of the challenges in global structural searches. Here we report a joint photoelectron spectroscopy (PES) and theoretical study of the structural evolution of negatively charged core-shell gold nanoclusters (Aun-) for n = 42-50. Photoelectron spectra of size-selected Aun- clusters are well resolved with distinct spectral features, suggesting a dominating structural type. The combined PES data and density functional calculations allow us to systematically identify the global minimum or candidates of the global minima of these relatively large gold nanoclusters, which are found to possess low-symmetry structures with gradually increasing core sizes. Remarkably, the four-atom tetrahedral core, observed first in Au33-, continues to be highly robust and is even present in clusters as large as Au42-. Starting from Au43-, a five-atom trigonal bipyramidal core appears and persists until Au47-. Au48- possesses a six-atom core, while Au49- and Au50- feature seven- and eight-atom cores, respectively. Notably, both Au46- and Au47- contain a pyramidal Au20 motif, which is stacked with another truncated pyramid by sharing a common 10-atom triangular face. The present study sheds light on our understanding of the structural evolution of the medium-sized gold nanoclusters, the shells and core as well as how the core-shell structures may start to embrace the golden pyramid (bulk-like) fragment.