A family of superconducting boron crystals made of stacked bilayer borophenes

Monolayer borophenes tend to be easily oxidized, while thicker borophenes have stronger antioxidation properties. Herein, we proposed four novel metallic boron crystals by stacking the experimentally synthesized borophenes, and one of the crystals has been reported in our previous experiments. Bilayer units tend to act as blocks for crystals as determined by bonding analyses. Their kinetic, thermodynamic and mechanical stabilities are confirmed by our calculated phonon spectra, molecular dynamics and elastic constants. Our proposed allotropes are more stable than the boron α-Ga phase below 1000 K at ambient pressure. Some of them become more stable than the α-rh or γ-B₂₈ phases at appropriate external pressure. More importantly, our calculations show that three of the proposed crystals are phonon-mediated superconductors with critical temperatures of about 5-10 K, higher than those of most superconducting elemental solids, in contrast to typical boron crystals with significant band gaps. Our study indicates a novel preparation method for metallic and superconducting boron crystals dispensing with high pressure.

An ideal two-dimensional nodal-ring semimetal in tetragonal borophene oxide

Free-standing stable two-dimensional (2D) boron monolayers, i.e., borophenes, usually settle into triangular lattices with different ratios of monoatomic vacancies. However, a stable polymorph can be drastically distinct from a free-standing one upon charge doping or on a substrate, as evidenced by the free-standing unstable hexagonal borophene that was prepared on the Al(111) substrate [Sci. Bull., 2018, 63, 282]. Moreover, 2D borophenes prefer to be oxidized to form more stable borophene oxides under ambient conditions. In this work, with the help of first-principles calculations, we propose a stable borophene oxide (t-B₂O) through oxidizing the free-standing unstable T-borophene. More interestingly, t-B₂O is a topological nodal-ring semimetal protected by in-plane mirror symmetry and characterized by a topological index. The energy fluctuation of the nodal ring is small and no extraneous bands are entangled with the nodal ring around the Fermi level. Two tight-binding models are developed to elucidate the orbital interactions and the formation of the nodal ring. Our work not only discovers a new ideal 2D topological nodal-ring semimetal, but the method used here also provides a fresh view in the search for 2D materials.

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.

Enhanced field-emission properties of buckled α-borophene by means of Li decoration: a first-principles investigation

In this study, the structures and field-emission properties of Li-decorated buckled α-borophene (BBP) were investigated by first-principles density functional theory at the PW91 level. Using the computed binding energies, Hirshfeld- and electrostatic potential-derived charges, induced dipole moments, densities of states, and ionization potentials, we evaluated the influence of an applied electric field on the structural stability, work function, and field-emission current of the Li-decorated BBP nanostructures. Furthermore, we also explored the quantitative dependence of the emission current on the electric field, Li concentration, and molecular orbitals. The computed results indicated that increasing the electric field and Li concentration has a considerably positive effect on the field-emission performance of the Li-decorated BBPs. Besides advantages including small work functions and low ionization potentials, most remarkably, the field-emission current can be as high as 48.81 μA in Li4/BBP (supercell with 36 atoms only) under a rather small applied electric field of 0.05 V Å-1, which rivals the highest value of the graphene-BN nanocomposite among all the theoretical nanostructures presented to date. Our results highly support the fact that Li-decorated BBPs can be appealing field-emission cathode materials with an extremely high emission current.

Two-dimensional boron: structures, properties and applications

Situated between metals and non-metals in the periodic table, boron is one of the most chemically versatile elements, forming at least sixteen bulk polymorphs composed of interlinked boron polyhedra. In low-dimensionality, boron chemistry remains or becomes even more intriguing since boron clusters with several to tens of atoms favor planar or cage-like structures, which are similar to their carbon counterparts in terms of conformation and electronic structure. The similarity between boron and carbon has raised a question of whether there exists stable two-dimensional (2D) boron, as a conceptual precursor, from which other boron nanostructures may be built. Here, we review the current theoretical and experimental progress in realizing boron atomic layers. Starting by describing a decade-long effort towards understanding the size-dependent structures of boron clusters, we present how theory plays a role in extrapolating boron clusters into 2D form, from a freestanding state to that on substrates, as well as in exploring practical routes for their synthesis that recently culminated in experimental realization. While 2D boron has been revealed to have unusual mechanical, electronic and chemical properties, materializing its potential in practical applications remains largely impeded by lack of routes towards transfer from substrates and controlled synthesis of quality samples.