Global Structure Search Research Articles

Why Does a Transition Metal Dichalcogenide Nanoribbon Narrow into a Nanowire under Electron Irradiation?

Transition metal dichalcogenide (TMDC) nanowires have practical applications in 1D electron channels, spintronics, optoelectronics, and catalysis due to their authentic subnanometer width (<1 nm) and intrinsic metallicity. Although narrowing of a TMDC nanoribbon into a nanowire under electron irradiation has been frequently observed in the synthesis of TMDC nanowires, the mechanism underlying this unexpected structural transformation remains a mystery. Here, to reveal the underlying mechanism, we combine first-principles calculations with a global structure search of 1D nanowires and show that a nanoribbon of 1H-phase MoS2 with a width narrower than 6 rings is energetically unfavorable compared with its nanowire counterpart due to the edge-edge interaction. The bending effect induced by S defects under electron irradiation is the major driving force for the transition of MoS2 nanoribbon into a nanowire. We predict that the precursor of the Mo6S6 nanowire is a well-defined Mo11S11-i nanowire with an unexpected stoichiometry. The intrinsic local compressive strain triggers a phase transition from Mo11S11-i to its slightly modified sister nanowire, Mo11S11-ii, which is characterized by the configuration (Mo1S1)5&Mo6S6. Triggered by electron irradiation, the nanoribbon undergoes a step-by-step narrowing process with sequential peeling of a Mo1S1 fragment in each step to form a robust Mo6S6 nanowire. This unique narrowing mechanism is universal for the nanoribbon-to-nanowire transformation of other TMDCs under electron irradiation. Our study highlights a hitherto unexplored mechanism for creating individual M6X6 nanowires and contributes to an in-depth understanding of the narrowing of TMDC nanoribbons under electron irradiation.

Theoretical exploration of novel compounds in Sr(B,C)x (x = 7,8) at high pressure

Boron-carbon-based compounds have attracted considerable attention due to their exceptional properties and increasing applications in industry. In this study, we conducted a systematic structural prediction of the SrBxC7-x and SrBxC8-x systems under ambient and high pressures up to 20 GPa using an unbiased global structure search. Through the calculations of formation enthalpy and phonon dispersion curves, we identified three new compounds, namely SrB5C2, SrB6C and SrB5C3, as thermodynamically and dynamically stable under ambient pressure and high pressures up to 20 GPa. Calculations of mechanical properties reveal that these three compounds have significant mechanical strength, with estimated theoretical Vickers hardness values ranging from 23.2 to 39.8 GPa. Electronic structure calculations suggest that both SrB5C2 and SrB5C3 are hole conductors, with the bands crossing the Fermi level, and SrB6C is a semiconductor with an indirect bandgap of 2.4 eV. The electron-phonon coupling calculations using the Migdal-Eliashberg theory indicate that SrB5C2 exhibits superconductivity at 10.5 K under ambient pressure, while SrB5C3 shows superconductivity at a much lower temperature of 0.4 K. This finding adds to our knowledge of boron-carbide materials and provides valuable insights for the development of similar materials with good mechanical properties.

Hund’s three rules in actinide-containing superatoms with spin-orbit coupling calculations

The intriguing and challenge issue in magnetic superatoms is searching for the suitable candidates to validate the Hund’s rules. Here, early actinide elements (An: Ac, Th, Pa, U, Np, Pu, Am) whose 5f electrons may crossover the localization and delocalization characteristics have been chosen to alloy with Al atoms in designing magnetic An@Al12 superatoms. By doing the global minimum structure search and the spin-orbital coupling density functional theory calculations, we provide an original idea to give theoretical argument that Hund’s three rules are still applicable in superatoms, which can be related to the fillings of highly localized An-5f orbitals into large exchange-splitting 2 F superatom orbitals. Specifically, selective 5f sub-orbitals of several An dopants can exhibit a dual nature in superatomic bonding, i.e. partial 5f electrons of Pa, U and Pu are reactive whereas all 5f electrons of Np and Am are highly localized. The molecular orbital analyses, combined with the qualitative interpretation of the phenomenological superatom sub-shell model, address the intricate interplays between the structure symmetry, electronic structure, spin and orbital magnetic moments. These findings have important implications for understanding the bonding and magnetic behaviors of An-containing superatoms and pave the way for designing novel magnetic superatoms.

Theoretical prediction of a novel hexagonal narrow-gap silicon allotrope under high pressures

Silicon material plays a vital role in contemporary technology-related fields, including electronics and the photovoltaics. There is a growing demand for exploring new silicon structures with potential applications, and numerous metastable structures have been reported. In this study, we present the prediction of a novel stable sp 3 hybridized silicon allotrope using particle swarm optimization global structure search. The predicted Si allotrope is a semiconductor with an indirect band gap of approximately 0.21 eV. It possesses three Si basis atoms in the unit cell, and we named it Si3. Interestingly, when subjected to strain, it undergoes a transition from a semiconductive state to a metallic state. Furthermore, moderate tensile strain enhances the interactions between silicon and lithium atoms, suggesting its potential for Li-ion batteries. Additionally, Si3 exhibits exceptional sunlight absorption across a wide range of wavelengths, with a significantly higher light absorption intensity than cubic diamond silicon. These findings have important implications for photovoltaic applications.

Role of Edge Reconstruction in the Synthesis of Few-Layer Black Phosphorene.

Recent advancements in preparing few-layer black phosphorene (BP) are hindered by edge reconstruction challenges. Our previous studies have revealed the factors contributing to the difficulty of growing few-layer BP. In this study, we have successfully identified three reconstructed edges in bi- and multilayer BP through a combination of the crystal structure analysis by particle swarm optimization (CALYPSO) global structure search and density functional theory (DFT). Notably, the reconstruction between adjacent layers proves more beneficial than self-passivation or maintaining pristine edges. Among the reconstructed edges, the reconstructed ZZ edge is the most stable, regardless of the number of layers. Calculated electronic band structures reveal a significant transition in the electronic properties of black phosphorus nanoribbons (BPNRs), changing from metallic to semiconducting. This insight not only enhances the understanding of the fundamental properties of BP but also provides valuable theoretical guidance for the experimental growth of BPNRs or black phosphorus nanowires (BPNWs).

Penta-MN8 family: First realization of type-5 pentagonal tessellation in 2D hexagonal crystals with intriguing properties

Tessellation of the Euclidean plane by congruent pentagons has long been of great interest. However, periodic pentagonal patterns realized in crystallography reported so far are limited to type-2 or type-4 tiling in typical square lattices. Based on the recently synthesized N18 macrocycles together with Hückel aromaticity and 18-electron rules, we design a family of pentagonal metal polynitrides, penta-MN8 (M = Mg, Ca, Sr, Ba; Sn, Pb; Cd; Mo, W), the first materials realization of type-5 mathematical pattern with novel pentagonal tiles in 2D hexagonal lattices. The penta-MN8 family exhibits rich stable phases (γ, α/α′, and β) and diverse Poisson's ratios ranging from 0.299 to -0.488. Among these nitrogen-richest monolayer nitrides hitherto, penta-MgN8 possesses firm stability up to 700 K and under low pressure originating from unique aromaticity over the polymeric nitrogen network, which could be synthesized under ∼50 GPa as confirmed by global structure search. Moreover, penta-MgN8 exhibits a fully flat band near Fermi level over the whole 2D Brillouin zone, deriving from the combination of pentagonal pattern and hexagonal lattice as verified with tight-binding model analysis. Such electronic structures fill in the blank of clear kagome flat bands in known kagome nitrides and pentagon-based materials, resulting in very heavy fermions with huge effective mass (55.7 m0) that are favorable to achieve quantum zero-field Wigner crystals. Our work presents a new pentagonal-materials class with unprecedented coordination, lattice symmetry, and physical properties compared to existing pentagon-based systems, which makes a breakthrough in exploring the materials realization of the mathematical models for pentagonal tessellation.

Crystal system and space group prediction of two-dimensional materials from chemical formula via deep neural networks

Predictive modeling of undiscovered two-dimensional (2D) materials plays an important role in 2D materials research. In the initial stage of modeling, predicting the crystal system and space group, two fundamental symmetry parameters of a crystal, from their chemical formula is essential for global structure search and discovery of new 2D crystalline materials. Here, we develop a deep neural network (DNN) model containing 290 descriptors to predict the crystal system and space group of 2D materials in AA stacking based on their chemical formulas. Our model achieves an accuracy of 74.56% in predicting the crystal system and 73.14% in predicting the space group with significant generality. In addition, the probability of obtaining the correct space group from the top five candidates reaches 89.58%. Our results also demonstrate that approximate predictions can be generated with the top 80 descriptors with the predicted results close to those obtained with all 290 descriptors, indicating the relative feature importance difference of descriptors. This study provides an effective DNN model for the quick determination of crystal systems and space groups for the global structure search of 2D materials.

Off-Stoichiometric Restructuring and Sliding Dynamics of Hexagonal Boron Nitride Edges in Conditions of Oxidative Dehydrogenation of Propane.

Boron-containing materials, such as hexagonal boron nitride (h-BN), recently shown to be active and selective catalysts for the oxidative dehydrogenation of propane (ODHP), have been shown to undergo significant surface oxyfunctionalization and restructuring. Although experimental ex situ studies have probed the change in chemical environment on the surface, the structural evolution of it under varying reaction conditions has not been established. Herein, we perform global optimization structure search with a grand canonical genetic algorithm to explore the chemical space of off-stoichiometric restructuring of the h-BN surface under ambient as well as ODHP-relevant conditions. A grand canonical ensemble representation of the surface is established, and the predicted 11B solid-state NMR spectra are consistent with previous experimental reports. In addition, we investigated the relative sliding of h-BN sheets and how it influences the surface chemistry with ab initio molecular dynamics simulations. The B-O linkages on the edges are found to be significantly strained during the sliding, causing the metastable sliding configurations to have higher reactivity toward the activation of propane and water.

Data-driven prediction of complex crystal structures of dense lithium

Lithium (Li) is a prototypical simple metal at ambient conditions, but exhibits remarkable changes in structural and electronic properties under compression. There has been intense debate about the structure of dense Li, and recent experiments offered fresh evidence for yet undetermined crystalline phases near the enigmatic melting minimum region in the pressure-temperature phase diagram of Li. Here, we report on an extensive exploration of the energy landscape of Li using an advanced crystal structure search method combined with a machine-learning approach, which greatly expands the scale of structure search, leading to the prediction of four complex Li crystal structures containing up to 192 atoms in the unit cell that are energetically competitive with known Li structures. These findings provide a viable solution to the observed yet unidentified crystalline phases of Li, and showcase the predictive power of the global structure search method for discovering complex crystal structures in conjunction with accurate machine learning potentials.

Scanning Intrinsic Superconductivity and the Spin Hall Effect in Niobium Borides

Although niobium borides have been extensively studied on their mechanical properties, a systematic investigation on their potential metastable phases, superconducting excitations, and spin Hall effects (SHE) is still lacking. Herein, we carry out a global structure search in the Nb-B system based on the evolutionary algorithm and density functional theory. Thermodynamical, dynamical, mechanical, and quasiharmonic approximation investigations unveil four new phases, Nb3B5, Nb2B5, Nb3B7, and Nb5B4, which are promising candidates for experimental preparations. Moreover, the superconducting transitions of all Nb-B compounds are performed from electron–phonon calculations. In addition, all Nb-B compounds are predicted to be topologically nontrivial. More intriguingly, Nb2B5 is unveiled as a promising noncentrosymmetric superconductor to explore topological superconducting excitation. Furthermore, nonzero spin Hall conductivity (SHC) tensor elements of all Nb-B compounds are predicted, and NbB is predicted to own a maximal SHC value of 320 (ℏ/e) (S/cm) among all Nb-B compounds. Our theoretical results fill in the gap on superconducting and SHC parameters of Nb-B binaries, demonstrating that Nb-B binaries are a potential choice to explore superconducting excitations and topological states and to investigate the SHE.

Structural and Composition Evolution of Palladium Catalyst for CO Oxidation under Steady-State Reaction Conditions

Understanding how the composition and structure of heterogeneous catalysts change during in situ reaction is of critical importance but greatly challenging because of the complexity of multiple time–space scale. Herein, we propose a self-adaptive simulation strategy driven by the first-principles microkinetic modeling and genetic-algorithm-based global structural search accelerated by machine learning that allows the interplay of surface reaction and catalyst evolution, and uncover the structural/compositional evolution of a Pd(111) single-crystal catalyst under the reaction conditions for CO oxidation, which is a classic open problem lasting for decades. The possible active phases at the kinetically steady state are identified and their common nature is unraveled. We show that, in the presence of CO/O2 reaction mixtures, a dynamically stable partially oxidized nonstoichiometric palladium oxide (PdO0.44 layer) grown on Pd(111) is formed with the O adatoms inserted into the Pd(111) sublayer that drives the formation of the surface oxide. Remarkably, the first-principles microkinetic analyses demonstrate that this self-evolved PdO0.44 surface at the steady state exhibits higher catalytic activity for CO oxidation as compared with Pd(111) and overoxidized PdO catalyst, which may explain the long-standing puzzle of the “PdOx” active phase for Pd catalyst during CO oxidation.

Boron-based Pd3B26 alloy cluster as a nanoscale antifriction bearing system: tubular core-shell structure, double π/σ aromaticity, and dynamic structural fluxionality.

Boron-based nanoclusters show unique geometric structures, nonclassical chemical bonding, and dynamic structural fluxionality. We report here on the theoretical prediction of a binary Pd3B26 cluster, which is composed of a triangular Pd3 core and a tubular double-ring B26 unit in a coaxial fashion, as identified through global structural searches and electronic structure calculations. Molecular dynamics simulations indicate that in the core-shell alloy cluster, the B26 double-ring unit can rotate freely around its Pd3 core at room temperature and beyond. The intramolecular rotation is virtually barrier free, thus giving rise to an antifriction bearing system (or ball bearing) at the nanoscale. The dimension of the dynamic system is only 0.66 nm. Chemical bonding analysis reveals that Pd3B26 cluster possesses double 14π/14σ aromaticity, following the (4n + 2) Hückel rule. Among 54 pairs of valence electrons in the cluster, the overwhelming majority are spatially isolated from each other and situated on either the B26 tube or the Pd3 core. Only one pair of electrons are primarily responsible for chemical bonding between the tube and the core, which greatly weaken the bonding within the Pd3 core and offers structural flexibility. This is a key mechanism that effectively diminishes the intramolecular rotation barrier and facilitates dynamic structural fluxionality of the system. The current work enriches the field of nanorotors and nanomachines.

Chemical bonding and dynamic structural fluxionality of a boron-based Al2B8 binary cluster: the robustness of a doubly 6π/6σ aromatic [B8]2- molecular wheel.

Despite the isovalency between Al and B elements, Al-doping in boron clusters can deviate substantially from an isoelectronic substitution process. We report herein on a unique sandwich di-Al-doped boron cluster, Al2B8, using global structural searches and quantum chemical calculations. The cluster features a perfectly planar B8 molecular wheel, with two isolated Al atoms symmetrically floating above and below it. The two Al atoms are offset from the center of the molecular wheel, resulting in a C 2v symmetry for the cluster. The Al2B8 cluster is shown to be dynamically fluxional even at far below room temperature (100 K), in which a vertical Al2 rod slides or rotates freely within a circular rail on the B8 plate, although there is no direct Al-Al interaction. The energy barrier for intramolecular rotation is only 0.01 kcal mol-1 at the single-point CCSD(T) level. Chemical bonding analysis shows that the cluster is a charge-transfer complex and can be formulated as [Al]+[B8]2-[Al]+. The [B8]2- molecular wheel in sandwich cluster has magic 6π/6σ double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al]+, [B8]2-, and [Al]+ layers.

Machine learning search for stable binary Sn alloys with Na, Ca, Cu, Pd, and Ag.

We present our findings of a large-scale screening for new synthesizable materials in five M-Sn binaries, M = Na, Ca, Cu, Pd, and Ag. The focus on these systems was motivated by the known richness of M-Sn properties with potential applications in energy storage, electronics packaging, and superconductivity. For the systematic exploration of the large configuration space, we relied on our recently developed MAISE-NET framework that constructs accurate neural network interatomic potentials and utilizes them to accelerate ab initio global structure searches. The scan of over two million candidate phases at a fraction of the typical ab initio calculation cost has uncovered 29 possible intermetallics thermodynamically stable at different temperatures and pressures (1 bar and 20 GPa). Notable predictions of ambient-pressure materials include a simple hP6-NaSn2 phase, fcc-based Pd-rich alloys, tI36-PdSn2 with a new prototype, and several high-temperature Sn-rich ground states in the Na-Sn, Cu-Sn, and Ag-Sn systems. Our modeling work also involved ab initio (re)examination of previously observed M-Sn compounds that helped explain the entropy-driven stabilization of known Cu-Sn phases. The study demonstrates the benefits of guiding structure searches with machine learning potentials and significantly expands the number of predicted thermodynamically stable crystalline intermetallics achieved with this strategy so far.

Scanning the latent phases and superconductivity in the Nb-Pb system at high pressure.

So far, few literature studies have been reported on niobium-lead binary intermetallic compounds, which are expected to have very different properties compared to existing niobium-carbon binary compounds, due to the distinct electronic properties of lead when compared to other carbon-group elements. Herein, we carry out a global structure search for the Nb-Pb system based on the evolutionary algorithm and density functional theory. Based on the dynamical and mechanical stability analyses, we unveiled five new phases, P4/m-Nb9Pb, Cmcm-Nb3Pb, I4/mmm-Nb2Pb, Pmm2-Nb5Pb3, and I4/mmm-NbPb2, that are promising candidates for experimental synthesis. Moreover, the superconducting transitions of all Nb-Pb binary intermetallic compounds are performed with electron-phonon calculations. As Nb9Pb exhibited the maximum Tc in the Nb-Pb intermetallics, greater than 3.0 K at 20 GPa, the phonon band structures, partial phonon density of states (PHDOS), the corresponding Eliashberg spectral functions α2F(ω), and integral electron-phonon coupling (EPC) parameters λ as a function of frequency of Nb9Pb were also studied. This work filled the gap in the pressure-tuned Nb-Pb phase transitions from a systematic first principles study for the first time.

Design of Lanthanide Single-Chain Magnets Based on Tubular Segment Clusters

One-dimensional (1D) atomic chains that exhibit large magnetic anisotropy are highly relevant to spintronic applications. Such 1D magnetic systems have usually been produced by atom/molecular assembling on solid surfaces or by coordination polymers. Here, we demonstrate an alternative approach for the formation of a confined ferromagnetic holmium (Ho) atom chain, which is composed of repeating units of the tubular Ho doped boron (B) cluster HoB20. The tubular HoB20 is discovered by the global structural search of a series of small-sized boron clusters doped by a single Ho atom (HoBn, n = 12, 14, 16, 18, 20) using ab initio calculations. Electronic analysis reveals that the individual HoB20 cluster is stabilized by the double σ + π aromaticity. Extending the tubular structure results in a single Ho atom chain being confined inside a boron nanotube, which shows ferromagnetic behaviors and a large magnetic anisotropy of more than 40 meV/atom. Our findings indicate that the magnetic lanthanide chain is a promising candidate for high-density magnetic information storage.

Atomistic structure search using local surrogate model.

We describe a local surrogate model for use in conjunction with global structure search methods. The model follows the Gaussian approximation potential formalism and is based on the smooth overlap of atomic positions descriptor with sparsification in terms of a reduced number of local environments using mini-batch k-means. The model is implemented in the Atomistic Global Optimization X framework and used as a partial replacement of the local relaxations in basin hopping structure search. The approach is shown to be robust for a wide range of atomistic systems, including molecules, nanoparticles, surface supported clusters, and surface thin films. The benefits in a structure search context of a local surrogate model are demonstrated. This includes the ability to benefit from transfer learning from smaller systems as well as the possibility to perform concurrent multi-stoichiometry searches.

Structural Evolution of C60 Molecular Crystal Predicted by Neural Network Potential

AbstractThe preparation of fullerene and a wide range of derivatives has attracted much attention in past decades. Understanding the structural evolution starting from fullerene is critical to guide the experimental exploration but has been paid less attention yet. Using ab initio molecular dynamics or global structural search algorithm accompanied with density functional theory calculations can give a glance of the potential energy surface (PES), but suffers high cost and low efficiency. Herein, by using neural network potentials and stochastic surface walking global structural search method, an accelerated energy calculation, and the higher efficient PES mapping starting from C60 molecular crystal are reported. The structural evolution is found to follow an order of molecular crystal → polymers → opened caged ordered structures → 3D) curved carbon and graphite under zero pressure. Under high pressures, e.g., 50 GPa, the evolution follows a pathway of fullerene → sp2 and sp3 hybrid carbon → sp3 amorphous carbon and crystal diamond. The electronic property calculation of the obtained structures in the evolution pathway shows a band gap depending on the order parameter of the generated structures, suggesting that more novel carbons can be potentially prepared in experiments, starting from C60.

Global structure search for new 2D PtSSe allotropes and their potential for thermoelectirc and piezoelectric applications

Two new phases of pentagon-based 2D PtSSe , named α-penta- and β-penta-PtSSe, are identified based on global structure search method. We demonstrate that the two structures are dynamically, thermally and mechanically stable and exhibit prior thermoelectric and piezoelectric performances, suggesting their great potential for thermoelectric and piezoelectric applications. The highest ZT values of α-penta- and β-penta-PtSSe reach 0.60 and 1.13 at 800 K, respectively. Besides, β-penta-PtSSe possesses a large intrinsic piezoelectric response with d16 = 1.30 pm/V. This study expands the family of 2D TMDs, and throws new light on the explorations of new materials with high-performance thermoelectric and piezoelectric properties.

Structural diversity and unusual valence states in compressed Na-Hg system

Global structural searches performed for sodium amalgam compounds reveal six new and stable Na-Hg stoichiometries (e.g., NaHg3, NaHg4, Na2Hg, Na4Hg, Na5Hg and Na6Hg) under ambient and high-pressure conditions. With increasing Hg content in the compounds, the structure topology of Hg evolves from isolate atom (NamHgn, m/n ≥ 3, 0D), linear chains (Na2Hg, 1D), puckered honeycomb layers (Na3Hg2, 2D), diamond networks (NaHg, 3D), dodecahedron (NaHg2, 3D), to tetrakaidecahedron (NaHg3, 3D). Electronic structure analysis shows that Hg can attain higher negative oxidation states, transferring more than one electron from Na atoms to the Hg 6p orbitals. In NamHgn (m/n < 3) compounds, the covalent Hg-Hg interactions are found stemming from the sp hybridization. In Na4Hg, quasi-zero-dimensional (0-D) electride is found with the electrons located within the octahedrons of Na in the lattice. The present results establish the richness of sodium amalgam stoichiometries under ambient and high-pressure conditions.