Low-dimensional boron: searching for Dirac materials

Two-dimensional (2D) boron has been predicted to exhibit various structural polymorphs and interesting physics, including superconductivity, ideal strength, negative Possion's ratio, and a higher thermal conductivity than graphene, etc. However, the difficulty in experimental synthesis of 2D boron has further reduced its exploration and applications. In this work we have used a crystal structure search method and first-principles calculations. We have proposed a scheme of ``synthesize under high pressure and peel off at ambient conditions'' to first predict a bulk phase of $\mathrm{K}{\mathrm{B}}_{4}$, and then peel off free standing 2D boron from it. Based on electron-phonon coupling calculations, we estimated that 2D boron is an intrinsic superconductor with ${T}_{c}=17.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition we found a strong anisotropy of the electron-phonon coupling strength in 2D boron, thus the superconductivity in 2D boron possesses a quasi-1D feature. Our proposed scheme for the synthesis of free standing 2D boron will stimulate further studies of superconductivity in low dimensional materials, both theoretically and experimentally.

Monte Carlo study of magnetic and thermodynamic properties of a 2D boron clusters in a magnetic field

Transfer-free synthesis of large-area 2D boron films on non-wetting liquid gallium substrate

The hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) have been envisaged on a two-dimensional (2D) boron sheet through electronic structure calculations based on a density functional theory framework. To date, boron sheets are the lightest 2D material and, therefore, exploring the catalytic activity of such a monolayer system would be quite intuitive both from fundamental and application perspectives. We have functionalized the boron sheet (BS) with different elemental dopants like carbon, nitrogen, phosphorous, sulphur, and lithium and determined the adsorption energy for each case while hydrogen and oxygen are on top of the doping site of the boron sheet. The free energy calculated from the individual adsorption energy for each functionalized BS subsequently guides us to predict which case of functionalization serves better for the HER or the OER.

The successful experimental syntheses of two-dimensional (2D) boron allotropes with intriguing properties have stimulated great interest in searching for novel low-dimensional boron. By using high-throughput first-principles calculations, we proposed a new stable 2D boron with a bilayer structure (P6̅-boron), composed of the building blocks of buckled B12 cluster. We showed the possibility of experimental syntheses of P6̅-boron on metal substrates and proposed a strategy to pursue for P6̅-boron by the B12 clusters self-assembly. Specifically, P6̅-boron possesses a topologically nontrivial Dirac nodal line, which is protected by the mirror-reflection symmetry. Furthermore, we employed a low-energy effective k·p model to prove the existence of the nodal line solution. In addition, the topological analysis of bonding suggests that the chemical bonds of P6̅-boron are all covalent rather than ionic bonds found in bilayer P6/mmm boron. We expect that our findings can favor the low-dissipation high-speed nanoelectronic devices based on 2D boron sheets.

Boron, as a unique element nearest to carbon in the periodic table, has been predicted to form many distinctive two‐dimensional (2D) structures that significantly differ from other well‐studied 2D materials, owning to its exceptional ability to form strong covalent two‐center‐two‐electron bonds as well as stable electron‐deficient multi‐center‐two‐electron bonds. Until recently, the successful syntheses of atomically thin crystalline 2D boron sheets (i.e., borophenes) provoked growing passion in 2D boron crystals. In this feature article, we present a survey of the latest achievements on 2D boron structures, starting from a concise introduction of the structures and properties of the bulk allotropes of boron, boron clusters, and especially potential building blocks for 2D boron crystals. Then we review important achievements and the current status of research on single‐layered metallic borophene, and discuss 2D few‐layered boron sheets, from their possible structures to tunable properties and potential applications in electronics, spintronics, and photoelectronics. We also systematically investigate the stability and functionalization of 2D icosahedral boron sheets in comparison with borophenes through first‐principles studies. Finally, we present an outlook on the advance in fabrications of 2D boron sheets, and the challenges and prospects in the realm of 2D boron crystals.

Recently, two-dimensional (2D) boron nanosheets have been predicted to exhibit exceptional physical and chemical properties, which is expected to be widely used in advanced electronics, optoelectronic, energy storage and conversion devices. However, the experimental application of 2D boron nanosheets in hydrogen evolution reactiuon (HER) has not been reported. Here, we have grown ultrathin boron nanosheets on tungsten foils via chemical vapor deposition. The prepared triclinic boron nanosheets are highly crystalline, which perfectly match the structure in the previous theoretical calculations. Notably, the boron nanosheets show excellent HER performance. The Tafel slope is only 64 mV dec−1 and the nanosheets can maintain good stability under long-time cycle in acidic solution. The improvement of performance is mainly due to the metal properties and a large number of exposed active sites on the boron nanosheets, which is confirmed by first-principle calculations.

We suggest the possibility to build graphene analogue with the planar hexacoordinate wheel-type Fe@B6H6 cluster as the building block through studying theoretically the geometry, stability, and electron structure of its dimer and trimer as well as the dimerization of the two trimers. Employing the dehydrogenation route to polymerization, we can obtain the hexagonal boron sheet that are partly and uniformly filled by Fe atoms in the center of the holes, achieving uniform chemical doping and a very large hexagonal-hole density. Thus, we may offer a novel cluster-assembled material for experimental chemists to construct graphene analogue.

Two-dimensional (2D) boron monolayers have been successfully synthesized on a silver substrate very recently. Their potential application is thus of great significance. In this work, we explore the possibility of boron monolayers (BMs) as electrocatalysts for the hydrogen evolution reaction (HER) by first-principles methods. Our calculations show that BMs are active catalysts for HER with nearly zero free energy of hydrogen adsorption, metallic conductivity and plenty of active sites in the basal plane. The effect of the substrate on HER activity is further assessed. It is found that the substrate has a positive effect on the HER performance caused by the competitive effect of mismatch strain and charge transfer. The in-depth understanding of the structure dependent HER activity is also provided.

According to the crystal structure of MgB2 and bandstructure calculations quasi–two-dimensional (2D) boron planes areresponsible for the superconductivity. We report on critical fieldsand resistance measurements of 30 nm thick MgB2 filmsgrown on MgO single crystalline substrate. A lineartemperature dependence of the parallel and perpendicular uppercritical fields indicates a 3D-like penetration of magnetic fieldinto the sample. Resistivity measurements, in contrast, yield atemperature dependence of fluctuation conductivity aboveTc which agrees with the Aslamazov-Larkin theory offluctuations in 2D superconductors. We consider this finding as anexperimental evidence of two-dimensional nucleation ofsuperconductivity in MgB2.

10Boron-doped carbon nanoparticles as delivery platforms for boron neutron capture therapy and photothermal therapy.

Borophene is a family of two-dimensional (2D) boron materials containing many isomers with different hole concentrations and distributions in a triangular lattice. Although it has been widely studied theoretically and some have been synthesized experimentally, their thermodynamic properties are still unexplored. Based on density functional theory (DFT), we developed an accurate potential for the kinetic Monte Carlo (kMC) simulations of borophene. Through extensive kMC simulations, new phases were discovered, such as the glass state of borophene, liquid borophene and borophene with large holes. A phase diagram of borophene is constructed to guide future experiments on borophene materials at high temperature.

Using the Full Potential Linear Augmented Plane Wave and the pseudo-potential method based on the Density Functional Theory, we investigate the physical properties of two-dimensional (2D) boron nitride, carbon nitride, and boron carbide as well as their ternary system boron carbon nitride (BCN). The structural and optoelectronic properties are determined and discussed in detail with available theoretical and experimental results. We show that the studied physical properties are influenced and tunable by atom concentration. A high concentration of nitrogen (> 50%) disturbs the honeycomb structure of binary and ternary alloys. Additionally, the optoelectronic properties are very sensitive to the amount of boron and nitrogen atoms. The zero bandgap is only conserved for B3C12N3 and B6C6N6 ternary systems. A large bandgap was observed for B9N9 (∼3.9 eV) and a moderate one for B6N12 and B3N15 (∼2 eV). The coexistence of boron, carbon, and nitrogen atoms with different concentrations has important optical properties as they can absorb light in all spectra. However, they have more active absorption in the ultraviolet than visible regions. It is more interesting to use ternary BCN than binary or pristine alloys with tunable optoelectric properties, by varying the nitrogen content in nanodevices.

Most three-dimensional (3D) and two-dimensional (2D) boron nitride (BN) structures are wide-band-gap insulators. Here, we propose two BN monolayers having Dirac points and flat bands, respectively. One monolayer is named as 5–7 BN that consists of five- and seven-membered rings. The other is a Kagome BN made of triangular boron rings and nitrogen dimers. The two structures show not only good dynamic and thermodynamic stabilities but also novel electronic properties. The 5–7 BN has Dirac points on the Fermi level, indicating that the structure is a typical Dirac material. The Kagome BN has double flat bands just below the Fermi level, and thus there are heavy fermions in the structure. The flat-band-induced ferromagnetism is also revealed. We analyze the origination of the band structures by partial density of states and projection of orbitals. In addition, a possible route to experimentally grow the two structures on some suitable substrates such as the PbO2 (111) surface and the CdO (111) surface is also discussed, respectively. Our research not only extends understanding on the electronic properties of BN structures, but also may expand the applications of BN materials in 2D electronic devices.

Since the first realization of borophene on Ag(111), two-dimensional (2D) boron nanomaterials have attracted substantial interest because of their polymorphic diversity and potential for hosting solid-state quantum phenomena. Here, we use atomic-resolution scanning tunneling microscopy (STM) and field-emission resonance (FER) spectroscopy to elucidate the structure and properties of atomically thin boron phases grown on Cu(111). Specifically, FER spectroscopy reveals charge transfer and electronic states that strongly differ from the decoupled borophene phases observed on silver, suggesting that the deposition of boron on copper results in strong covalent bonding characteristic of a 2D copper boride. This conclusion is reinforced by detailed STM characterization of line defects that are consistent with density functional theory calculations for atomically thin Cu8B14. This evidence for 2D copper boride is likely to motivate future synthetic efforts aimed at expanding the relatively unexplored family of atomically thin metal boride materials.