Monolayer Boron Research Articles

From planar boron clusters to borophenes and metalloborophenes

Elemental boron and its compounds exhibit unusual structures and chemical bonding owing to the electron deficiency of boron. Joint photoelectron spectroscopy and theoretical studies over the past decade have revealed that boron clusters possess planar or quasi-planar (2D) structures up to relatively large sizes, laying the foundations for the discovery of boron-based nanostructures. The observation of the 2D B36 cluster provided the first experimental evidence that extended boron monolayers with hexagonal vacancies were potentially viable and led to the proposition of ‘borophenes’ — boron analogues of 2D carbon structures such as graphene. Metal-doping can expand the range of potential nanostructures based on boron. Recent studies have shown that the CoB18− and RhB18− clusters possess unprecedented 2D structures, in which the dopant metal atom is part of the 2D boron network. These doped 2D clusters suggest the possibilities of creating metal-doped borophenes with potentially tunable electronic, optical and magnetic properties. Here, we discuss the recent experimental and theoretical advances in 2D boron and doped boron clusters, as well as their implications for metalloborophenes. The unusual electronic characteristics of boron atoms lead boron clusters to adopt a wide variety of structural arrangements, most of which are 2D. This Perspective discusses the possibility of expanding the range of boron-based 2D structures by metal doping, as well as the use of the resulting clusters for conceptualizing metalloborophenes.

Semiconducting phase in borophene: role of defect and strain

Boron is an interesting element due to its chemical and structural complexity. Recent synthesis of borophene led scientists to study boron monolayer-based materials for various applications. Using density functional theory calculations, nineteen different phases of boron monolayer (with hexagonal hole densities from 1/32–8/32) are studied to understand their origin of buckling, stability, and planarity. Projected densities of states of various phases of borophene-based systems with defect are plotted into in-plane (s + px + py) and out-of-plane (pz) orbitals to understand the role of the σ and π-bands towards their geometry and stability. Interestingly, the λ5-sheet shows semiconducting properties under uniaxial/biaxial tensile/compressive strains and it shows excellent dynamical, thermal, and mechanical properties and is thus a promising semiconducting phase for electronic devices.

Selective gas adsorption and I–V response of monolayer boron phosphide introduced by dopants: A first-principle study

Abstract Two-dimensional (2D) materials have gained tremendous research interests for gas sensing applications because of their ultrahigh theoretical specific surface areas and unique electronic properties. Here, we investigate the adsorption of CO, SO2, NH3, O2, NO and NO2 gas molecules on pure and doped boron phosphide (BP) systems using first-principles calculations to exploit their potential in gas sensing. Our results predict that all six gas molecules show stronger adsorption interactions on impurities-doped BP over the pristine monolayer BP. Al-doped BP shows the highest sensitivity to all gas molecules, but N-doped BP is more suitable as a sensing material for SO2, NO and NO2 due to the feasibility of desorption. We further calculated the current–voltage (I–V) relation by mean of nonequilibrium Green’s function (NEGF) formalism. The I–V curves indicate that the electronic properties of the doping systems change significantly with gas adsorption by studying the nonparamagnetic molecules NH3 and the paramagnetic molecules NO, which can be more likely to be measured experimentally compared to graphene and phosphorene. This work explores the possibility of BP as a superior sensor through introducing the appropriate dopants.

Structure and stability of bilayer borophene: The roles of hexagonal holes and interlayer bonding

Two-dimensional (2D) boron monolayers with diversity of structures and extraordinary physical properties have been extensively investigated using first-principles calculations. A series of boron bilayer sheets with pillars and hexagonal holes have been constructed. Many of them have lower formation energy than α-sheet boron monolayer. The structural stability and chemical bonding character of these boron bilayers are analyzed by charge density, electron localization function and Bader charge, indicating that the optimal proportions of pillars and hexagonal holes can be obtained by balancing the surplus electrons. Meanwhile, the distribution and arrangement of hexagonal holes can cause ignorable effect on the stability of structures. These findings pave the way for the structural diversity of 2D boron materials.

Nanosheet Supported Single-Metal Atom Bifunctional Catalyst for Overall Water Splitting

Nanosheet supported single-atom catalysts (SACs) can make full use of metal atoms and yet entail high selectivity and activity, and bifunctional catalysts can enable higher performance while lowering the cost than two separate unifunctional catalysts. Supported single-atom bifunctional catalysts are therefore of great economic interest and scientific importance. Here, on the basis of first-principles computations, we report a design of the first single-atom bifunctional eletrocatalyst, namely, isolated nickel atom supported on β12 boron monolayer (Ni1/β12-BM), to achieve overall water splitting. This nanosheet supported SAC exhibits remarkable electrocatalytic performance with the computed overpotential for oxygen/hydrogen evolution reaction being just 0.40/0.06 V. The ab initio molecular dynamics simulation shows that the SAC can survive up to 800 K elevated temperature, while enacting a high energy barrier of 1.68 eV to prevent isolated Ni atoms from clustering. A viable experimental route for the synthesis of Ni1/β12-BM SAC is demonstrated from computer simulation. The desired nanosheet supported single-atom bifunctional catalysts not only show great potential for achieving overall water splitting but also offer cost-effective opportunities for advancing clean energy technology.

Dirac Nodal Lines and Tilted Semi-Dirac Cones Coexisting in a Striped Boron Sheet.

The enchanting Dirac fermions in graphene stimulated us to seek other 2D Dirac materials, and boron monolayers may be a good candidate. So far, a number of monolayer boron sheets have been theoretically predicted, and three have been experimentally prepared. However, none of intrinsic sheets possess Dirac electrons near the Fermi level. Herein, by means of density functional theory computations, we identified a new boron monolayer, namely, hr-sB, with two types of Dirac fermions coexisting in the sheet: One type is related to Dirac nodal lines traversing Brillouin zone (BZ) with velocities approaching 106 m/s, and the other is related to tilted semi-Dirac cones with strong anisotropy. This newly predicted boron monolayer consists of hexagon and rhombus stripes. With an exceptional stability comparable to the experimentally achieved boron sheets, it is rather optimistic to grow hr-sB on some suitable substrates such as the Ag (111) surface.

Dirac cones in a boron monolayer.

2D Materials When pure boron is evaporated onto a silver substrate, a structure made up of a single layer of boron atoms can form. Several different structures have been predicted and fabricated, but it is unclear whether the electronic properties of this so-called borophene can rival those of its better known cousin graphene. Feng et al. made a sheet of boron atoms on a silver substrate that had a characteristic striped and flat structure. Although the atoms were not arranged in a hexagonal lattice, like carbon atoms are in graphene, the electronic structure of borophene showed similar linearly dispersing electronic states, called Dirac cones. The discovery may lead to applications of monolayer boron in electronic devices. Phys. Rev. Lett. 118 , 096401 (2017).

Dirac Fermions in Borophene.

Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β_{12} sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β_{12} sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

Suppressed superconductivity in substrate-supported β12 borophene by tensile strain and electron doping

Planar borophene, the truly 2D monolayer boron, has been independently successfully grown on Ag(1 1 1) by two groups (2016 Nat. Chem. 8 563 and 2015 Science 350 1513), which has received widespreading attentions. The superconducting property has not been unambiguously observed, which is unexpected because light element boron should have strong electron–phonon coupling. To resolve this puzzle, we show that the superconducting transition temperature Tc of β12 borophene is effectively suppressed by the substrate-induced tensile strain and electron doping via first principles calculations. The biaxial tensile strain of 2% induced by Ag(1 1 1) significantly reduces Tc from 14 K to 2.95 K; electron doping of 0.1 e− per boron atom further shrinks Tc to 0.09 K. We also predict that the superconducting transition temperature in β12 can be enhanced to 22.82 K with proper compressive strain (−1%) and 18.97 K with hole doping (0.1 h+ per boron). Further studies indicate that the variation of Tc is closely related to the density of states of σ bands near the Fermi surface. Our results help to explain the challenges to experimentally probe superconductivity in substrate-supported borophene.

The electronic properties, electronic heat capacity and magnetic susceptibility of monolayer boron nitride graphene-like structure in the presence of electron-phonon coupling

In this work, we have studied the influences of electron-phonon (e-ph) coupling and chemical potential on the boron nitride graphene-like sheet. In particular, by starting the Green's function technique and Holstein model, the electronic density of states (DOS), electronic heat capacity (EHC) and magnetic susceptibility (MS) of this system have been investigated in the context of self-consistent second order perturbation theory which has been implemented to find the electronic self-energy. Our findings show that the band gap size decreases (increases) with e-ph coupling (chemical potential) parameters. The Schottky anomaly (crossover) decreases in EHC (MS) as soon as e-ph coupling increases. Also, the corresponding temperature with Schottky anomaly is considerably affected by e-ph coupling.

High intrinsic catalytic activity of two-dimensional boron monolayers for the hydrogen evolution reaction.

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.

Metal-free spin and spin-gapless semiconducting heterobilayers: monolayer boron carbonitrides on hexagonal boron nitride.

The interfaces between monolayer boron carbonitrides and hexagonal boron nitride (h-BN) play an important role in their practical applications. Herein, we respectively investigate the structural and electronic properties of two metal-free heterobilayers constructed by vertically stacking two-dimensional (2D) spintronic materials (B4CN3 and B3CN4) on a h-BN monolayer from the viewpoints of lattice match and lattice mismatch models using density functional calculations. It is found that both B4CN3 and B3CN4 monolayers can be stably adsorbed on the h-BN monolayer due to the van der Waals interactions. Intriguingly, we demonstrate that the bipolar magnetic semiconductor (BMS) behavior of the B4CN3 layer and the spin gapless semiconductor (SGS) property of the B3CN4 layer can be well preserved in the B4CN3/BN and B3CN4/BN heterobilayers, respectively. The magnetic moments and spintronic properties of the two systems originate mainly from the 2pz electrons of the carbon atoms in the B4CN3 and B3CN4 layers. Furthermore, the BMS behavior of the B4CN3/BN bilayer is very robust while the electronic property of the B3CN4/BN bilayer is sensitive to interlayer couplings. These theoretical results are helpful both in understanding the interlayer coupling between B4CN3 or B3CN4 and h-BN monolayers and in providing a possibility of fabricating 2D composite B4CN3/BN and B3CN4/BN metal-free spintronic materials theoretically.

A first-principles study on alkaline earth metal atom substituted monolayer boron nitride (BN)

This paper presents first-principles density functional theory (DFT) calculations for the structural, electronic, magnetic and optical properties of monolayer boron nitride (BN) doped with different alkaline earth metal (AEM) atoms.

Mechanics of Materials Creation: Nanotubes, Graphene, Carbyne, Borophenes

In this article, we provide brief overview of how mechanics and computations play a role in understanding materials growth, creating new low-dimensional materials and exploring structural defects. First, we introduce a concept of screw dislocation for describing carbon nanotube growth and derive a kinetic relationship between growth rate and chiral angle. Deeper analysis of the subtle balance between the kinetic and thermodynamic views reveals sharply peaked distribution of near-armchair nanotubes, explaining puzzling (n, n-1) types observed experimentally. A combination of ab initio calculations and Monte Carlo models further explains the low symmetry shapes of graphene on substrates. Being monoatomic chains of carbon, carbynes are shown to be strong yet flexible, and undergo metal-semiconductor transition under tension, offering promising innovations for future nanotechnology. We then reveal how metal substrates could facilitate the formation of boron monolayers whose bulk counterparts are non-layered and lower in energy. Further remarks are given to High Burger's vector graphene defects called D-loops and interfaces in hybrid graphene-BN materials, both with significant out-of plane distortion and impact on the mechanical properties. All of these computationally modeled systems have significant implications for the future use of these nanomaterials.

Atomistic simulation of free transverse vibration of graphene, hexagonal SiC, and BN nanosheets

Free transverse vibration of monolayer graphene, boron nitride (BN), and silicon carbide (SiC) sheets is investigated by using molecular dynamics finite element method. Eigenfrequencies and eigenmodes of these three sheets in rectangular shape are studied with different aspect ratios with respect to various boundary conditions. It is found that aspect ratios and boundary conditions affect in a similar way on natural frequencies of graphene, BN, and SiC sheets. Natural frequencies in all modes decrease with an increase of the sheet’s size. Graphene exhibits the highest natural frequencies, and SiC sheet possesses the lowest ones. Missing atoms have minor effects on natural frequencies in this study.

First-Principles Prediction of the Electronic Structure and Carrier Mobility in Hexagonal Boron Phosphide Sheet and Nanoribbons

Using density functional theory coupled to the Boltzmann transport equation with relaxation time approximation, we study the electronic structure and carrier mobility of graphene-like hexagonal boron phosphide (h-BP) monolayer and H-terminated armchair boron phosphide nanoribbons (ABPNRs). Our results show that the carrier mobility can reach over 104 cm2 V–1 s–1 for electron and 5 × 103 cm2 V–1 s–1 for hole in monolayer sheet. The carrier mobility in the ABPNRs is in the range of 103 to 104 cm2 V–1 s–1, and we find that the width of nanoribbon plays an important role in tuning the polarity of the carrier transport, which exhibits a distinct 3p (p is a positive integer) alternating behavior. The staggering oscillating behavior of mobility should be attributed to different bond characteristics of the edge states in the ABPNRs. Moreover, the H-terminated zigzag boron phosphide nanoribbons (ZBPNRs) have the characteristics of p-type semiconductors in electrical conduction, and the carrier mobility is increase...

Computational prediction of the diversity of monolayer boron phosphide allotropes

We propose previously unrecognized allotropes of monolayer boron phosphorus (BP) based on ab initio density functional calculations. In addition to the hexagonal structure of h-BP, four types of boron phosphide compounds were predicted to be stable as monolayers. They can form sp2 hybridized planar structures composed of 6-membered rings, and buckled geometries including 4–8 or 3–9 membered rings with sp3 like bonding for P atoms. The calculated Bader charges illustrate their ionic characters with the charge transfers from B to P atoms. The competing between the electrostatic energy and the bonding energy of sp2 and sp3 hybridizations reflected in P atoms results in multiple structures of BP. These 2D BP structures can be semiconducting or metallic depending on their geometric structures. Our findings significantly broaden the diversity of monolayer BP allotropes and provide valuable guidance to other 2D group-III-V allotropes.

Direct evidence of metallic bands in a monolayer boron sheet

The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the \b{eta}12-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the S point and a pair of hole pockets near the X point, which is supported by the first-principles calculations. The metallic boron allotrope in \b{eta}12 sheet opens the way to novel physics and chemistry in material science.

Phonon-mediated superconductivity in borophenes

We use first-principles calculations to systematically investigate electronic, vibrational, and superconducting properties in borophenes (boron monolayer sheets). Remarkably, superconducting transition temperature Tc is a V-like function of hexagon hole density and has a similar tendency to the variations of the total energy and density of states at the Fermi level, which shows that the larger density of states at the Fermi level corresponds to the higher Tc. In consideration of substrate, the Ag(111) surfaces weaken the superconductivity in borophenes, which results in Tcμ*=0.1 of about 5.2 K in the buckled triangular sheet. As synthesis of borophenes was reported, superconducting boron sheets are feasible.

Anomalous thermal conductivity of monolayer boron nitride

In this paper, we use nonequilibrium molecular dynamics modeling to investigate the thermal properties of monolayer hexagonal boron nitride nanoribbons under uniaxial strain along their longitudinal axis. Our simulations predict that hexagonal boron nitride shows an anomalous thermal response to the applied uniaxial strain. Contrary to three dimensional materials, under uniaxial stretching, the thermal conductivity of boron nitride nanoribbons first increases rather than decreasing until it reaches its peak value and then starts decreasing. Under compressive strain, the thermal conductivity of monolayer boron nitride ribbons monolithically reduces rather than increasing. We use phonon spectrum and dispersion curves to investigate the mechanism responsible for the unexpected behavior. Our molecular dynamics modeling and density functional theory results show that application of longitudinal tensile strain leads to the reduction of the group velocities of longitudinal and transverse acoustic modes. Such a phonon softening mechanism acts to reduce the thermal conductivity of the nanoribbons. On the other hand, a significant increase in the group velocity (stiffening) of the flexural acoustic modes is observed, which counteracts the phonon softening effects of the longitudinal and transverse modes. The total thermal conductivity of the ribbons is a result of competition between these two mechanisms. At low tensile strain, the stiffening mechanism overcomes the softening mechanism which leads to an increase in the thermal conductivity. At higher tensile strain, the softening mechanism supersedes the stiffening and the thermal conductivity slightly reduces. Our simulations show that the decrease in the thermal conductivity under compressive strain is attributed to the formation of buckling defects which reduces the phonon mean free path.