All-boron planar ferromagnetic structures: from clusters to monolayers

Magnetic semiconducting borophenes and their derivatives

Two semiconducting borophenes with layer-dependent magnetism are predicted based on spin-polarized density functional theory. Both monolayer borophenes are ferromagnetic. One is composed of B3 and B15 triangular motifs, exhibiting bipolar spin polarization and a magnetic moment of 1.00 μB per primitive cell. The other consists of B15 triangular motifs, possessing a Curie temperature of about 437 K and a magnetic moment of 3.00 μB per primitive cell. B atoms located between the triangular motifs are essential for inducing ferromagnetism in monolayer borophenes. However, bilayer borophenes with high-symmetry stacking orders are nonmagnetic. Furthermore, magnetic boron nanotubes and fullerenes could be made of monolayer borophenes. Finally, we propose to fabricate these magnetic semiconducting borophenes from the buckled triangular structure of borophenes via selective electron beam ionization of B atoms by scanning transmission electron microscopy.

Generalized Octet Rule with Fractional Occupancies for Boron

The octet rule is a fundamental theory in the chemical bonding of main-group elements, which achieve stable configurations by gaining, losing, or sharing electrons. However, the conventional octet rule, as depicted through Lewis structures, is inadequate for describing the electron delocalization in boron allotropes and boron-rich compounds due to the electron deficiency of boron. To address this, we introduce the concept of fractional electron occupancies, which more accurately reflect the electron delocalization in boron systems. Based on this, we propose a generalized octet rule that provides a more comprehensive understanding of the complex bonding configurations in boron allotropes and boron-rich compounds. Importantly, our predictions for α-B12 are validated by both first-principles calculations and existing experimental data. Beyond boron, this generalized octet rule is also applicable to systems with multiple resonance structures.

A new type of stable borophene with flat-band-induced magnetism

Based on first-principles calculations, we propose a new type of thermally and dynamically stable magnetic borophene (B11) with a tetragonal lattice. The magnetism is found coming from spin polarization of one bonding flat band located at the Fermi level. Despite of the 'anti-molecular' behavior in the monolayer, the interactions between thepzorbitals of the B atoms in the double-octahedron structural unit lead to the formation of the flat bands with localization behaviors. One tight binding model is built to comprehend the magnetic mechanism, which can guide us to tune other nonmagnetic borophene becoming magnetic. Biaxial tensile strain (>2.1%) is found triggering a phase transition from a semimetal to a semiconductor in the B11monolayer. The mechanism is analyzed based on the orbital-resolved crystal field effect. Our work provides a new route for designing and achieving two-dimensional magnetic materials with light elements.

Prediction of 2D IV-VI semiconductors: auxetic materials with direct bandgap and strong optical absorption

Auxetic materials are highly desirable for advanced applications because of their negative Poisson's ratios, which are rather scarce in two-dimensional materials. Motivated by the elemental mutation method, we predict a new class of monolayer IV-VI semiconductors, namely, δ-IV-VI monolayers (GeS, GeSe, SiS and SiSe). Distinctly different from the previously predicted IV-VI monolayers, the newly predicted δ-MX (X = Ge and Si; M = S and Se) monolayers exhibit a puckered unit cell with a space group of Pca2₁. Their stabilities were confirmed by first-principles lattice dynamics and molecular dynamics calculations. In particular, all these MX monolayers possess a large bandgap in the range of 2.08-2.65 eV and pronounced anisotropic mechanical properties, which are demonstrated by direction-dependent in-plane Young's moduli and Poisson's ratios. Furthermore, all these 2D MX monolayers possess negative Poisson's ratios (even up to about -0.3 for SiSe). Strong optical absorption is observed in these δ-IV-VI monolayers. These interesting physical properties will stimulate the development of 2D flexible devices based on IV-VI semiconductor monolayers.

Relative Stability of Boron Planar Clusters in Diatomic Molecular Model

In the recently introduced phenomenological diatomic molecular model imagining the clusters as certain constructions of pair interatomic chemical bonds, there are estimated specific (per atom) binding energies of small all-boron planar clusters B_n, n = 1–15, in neutral single-anionic and single-cationic charge states. The theoretically obtained hierarchy of their relative stability/formation probability correlates not only with results of previous calculations, but also with available experimental mass-spectra of boron planar clusters generated in process of evaporation/ablation of boron-rich materials. Some overestimation in binding energies that are characteristic of the diatomic approach could be related to differences in approximations made during previous calculations, as well as measurement errors of these energies. According to the diatomic molecular model, equilibrium binding energies per B atom and B–B bond lengths are expected within ranges 0.37–6.26 eV and 1.58–1.65 Å, respectively.

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B₂₀. In the ground state, the presently focused AB₂₀/EB₂₀ (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A⁺B₂₀^-/E²⁺B₂₀^2- frameworks. The excited electronic states of AB₂₀ and EB₂₀⁺ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB₂₀⁺ and EB₂₀²⁺. In several excited states, the radical electron is predominantly localized on the B₂₀ frames, which are counterparts of the low-lying states of bare B₂₀^-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/_A,E d-aug-cc-pVDZ/_B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB₂₀ and EB₂₀⁺ and will serve as a guide for future studies on similar metal-doped boron systems.