Unexpected reconstruction of the α-boron (111) surface

Machine-learning-accelerated simulations to enable automatic surface reconstruction

Understanding material surfaces and interfaces is vital in applications such as catalysis or electronics. By combining energies from electronic structure with statistical mechanics, ab initio simulations can, in principle, predict the structure of material surfaces as a function of thermodynamic variables. However, accurate energy simulations are prohibitive when coupled to the vast phase space that must be statistically sampled. Here we present a bi-faceted computational loop to predict surface phase diagrams of multicomponent materials that accelerates both the energy scoring and statistical sampling methods. Fast, scalable and data-efficient machine learning interatomic potentials are trained on high-throughput density-functional-theory calculations through closed-loop active learning. Markov chain Monte Carlo sampling in the semigrand canonical ensemble is enabled by using virtual surface sites. The predicted surfaces for GaN(0001), Si(111) and SrTiO3(001) are in agreement with past work and indicate that the proposed strategy can model complex material surfaces and discover previously unreported surface terminations.

Surface Magnetism in Pristine α Rhombohedral Boron and Intersurface Exchange Coupling Mechanism of Boron Icosahedra

We report intrinsic surface magnetism in pristine α rhombohedral boron (α-boron) using first-principles calculations. Semiconducting α-boron has been cleaved along the (001), (102̅), and (101) planes to produce icosahedral-based non-van der Waals face-boron, t-face-boron, and edge-boron structures, respectively. Face-boron is found to be metallic, while t-face-boron and edge-boron show semiconducting features. In particular, edge-boron exhibits layer-dependent magnetism with a transition from an overall antiferromagnetic (AFM) state with AFM surfaces to either an AFM or a ferromagnetic (FM) state with FM surfaces as the number of layers increases. The magnetism in edge-boron arises from the spin polarization of boron atoms with unsaturated bonds at the edge sites in the upper and lower surfaces, and magnetic exchange coupling can be mediated via adjacent boron icosahedra by up to a maximum of 8.4 Å. These findings deepen our understanding of icosahedral-based boron and boron-rich materials, which may be useful in potential spintronics applications.

Electronic Structure of Boron Flat Holeless Sheet

The electronic band structure, namely energy band surfaces and densities-of-states (DoS), of a hypothetical flat and ideally perfect, i.e., without any type of holes, boron sheet with a triangular network is calculated within a quasi-classical approach. It is shown to have metallic properties as is expected for most of the possible structural modifications of boron sheets. The Fermi curve of the boron flat sheet is found to be consisted of 6 parts of 3 closed curves, which can be approximated by ellipses representing the quadric energy-dispersion of the conduction electrons. The effective mass of electrons at the Fermi level in a boron flat sheet is found to be too small compared with the free electron mass m 0 and to be highly anisotropic. Its values distinctly differ in directions Γ–K and Γ–M: m Γ – K / m 0 ≈ 0.480 and m Γ – M / m 0 ≈ 0.052 , respectively. The low effective mass of conduction electrons, m σ / m 0 ≈ 0.094 , indicates their high mobility and, hence, high conductivity of the boron sheet. The effects of buckling/puckering and the presence of hexagonal or other type of holes expected in real boron sheets can be considered as perturbations of the obtained electronic structure and theoretically taken into account as effects of higher order.

Effect of Net Charge on the Relative Stability of 2D Boron Allotropes

We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our ab initio calculations for the neutral system reveal previously unknown stable 2D ϵ-B and ω-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from ϵ-B to a buckled honeycomb structure. Additional electron doping, mimicking a transformation of boron to carbon, causes a gradual decrease in the degree of buckling of the honeycomb lattice that can be interpreted as piezoelectric response. Net electron doping can be achieved by placing borophene in direct contact with layered electrides such as Ca₂N. We find that electron doping can be doubled by changing from the B/Ca₂N bilayer to the Ca₂N/B/Ca₂N sandwich geometry.

Computational discovery and characterization of new B2O phases

We present computational discoveries of new structural phases of the B2O compound exhibiting novel bonding networks and electronic states at ambient and elevated pressures. Our advanced crystal structure searches in conjunction with density functional theory calculations have identified an orthorhombic phase of B2O that is energetically stable at ambient pressure and contains an intriguing bonding network of icosahedral B12 clusters bridged by oxygen atoms. As pressure increases above 1.9 GPa, a structural transformation takes the orthorhombic B2O into a pseudo-layered trigonal phase. We have performed extensive studies to investigate the evolution of chemical bonds and electronic states associated with the B12 icosahedral unit in the orthorhombic phase and the covalent B-O bonds in the trigonal phase. We have also examined the nature of the charge carriers and their coupling to the lattice vibrations in the newly identified B2O crystals. Interestingly, our results indicate that both B2O phases become superconducting at low temperatures, with transition temperatures of 6.4 K and 5.9 K, respectively, in the ambient and high-pressure phase. The present findings establish new B2O phases and characterize their structural and electronic properties, which offer insights and guidance for exploration toward further fundamental understanding and potential synthesis and application.

Superoctahedral two-dimensional metallic boron with peculiar magnetic properties

A novel two-dimensional ferromagnetic stable boron material has been predicted and exhaustively studied with DFT methods. Its magnetism can be described by the presence of two unpaired delocalized bonding elements inside each distorted octahedron.

Shear-Induced Brittle Failure along Grain Boundaries in Boron Carbide

The role that grain boundaries (GBs) can play on mechanical properties has been studied extensively for metals and alloys. However, for covalent solids such as boron carbide (B₄C), the role of GB on the inelastic response to applied stresses is not well established. We consider here the unusual ceramic, boron carbide (B₄C), which is very hard and lightweight but exhibits brittle impact behavior. We used quantum mechanics (QM) simulations to examine the mechanical response in atomistic structures that model GBs in B₄C under pure shear and also with biaxial shear deformation that mimics indentation stress conditions. We carried out these studies for two simple GB models including also the effect of adding Fe atoms (possible sintering aid and/or impurity) to the GB. We found that the critical shear stresses of these GB models are much lower than that for crystalline and twinned B₄C. The two GB models lead to different interfacial energies. The higher interfacial energy at the GB only slightly decreases the critical shear stress but dramatically increases the critical failure strain. Doping the GB with Fe decreases the critical shear stress of at the boundary by 14% under pure shear deformation. In all GBs studied here, failure arises from deconstructing the icosahedra within the GB region under shear deformation. We find that Fe dopant interacts with icosahedra at the GB to facilitate this deconstruction of icosahedra. These results provide significant insight into designing polycrystalline B₄C with improved strength and ductility.

Recent progress in boron nanomaterials

Various types of zero, one, and two-dimensional boron nanomaterials such as nanoclusters, nanowires, nanotubes, nanobelts, nanoribbons, nanosheets, and monolayer crystalline sheets named borophene have been experimentally synthesized and identified in the last 20 years. Owing to their low dimensionality, boron nanomaterials have different bonding configurations from those of three-dimensional bulk boron crystals composed of icosahedra or icosahedral fragments. The resulting intriguing physical and chemical properties of boron nanomaterials are fascinating from the viewpoint of material science. Moreover, the wide variety of boron nanomaterials themselves could be the building blocks for combining with other existing nanomaterials, molecules, atoms, and/or ions to design and create materials with new functionalities and properties. Here, the progress of the boron nanomaterials is reviewed and perspectives and future directions are described.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

Boron based two-dimensional crystals: theoretical design, realization proposal and applications

The successful realization of free-standing graphene and the various applications of its exotic properties have spurred tremendous research interest for two-dimensional (2D) layered materials. Besides graphene, many other 2D materials have been successfully produced by experiment, such as silicene, monolayer MoS2, few-layer black phosphorus and so on. As a neighbor of carbon in the periodic table, element boron is interesting and many researchers have contributed their efforts to realize boron related 2D structures. These structures may be significant both in fundamental science and future technical applications in nanoelectronics and nanodevices. In this review, we summarize the recent developments of 2D boron based materials. The theoretical design, possible experimental realization strategies and their potential technical applications are presented and discussed. Also, the current challenges and prospects of this area are discussed.