Honeycomb Borophene Research Articles

Highly efficient catalytic properties of Sc and Fe single atoms stabilized on a honeycomb borophene/Al(111) heterostructure via a dual charge transfer effect.

Theoretical design and experimental fabrication of highly efficient single-atom catalysts (SACs) containing isolated metal atoms monodispersed on appropriate substrates have surged to the forefront of heterogeneous catalysis in recent years. Nevertheless, the instability of SACs, i.e., preferential clustering in chemical reaction processes, dramatically hinders their practical applications. In this paper, using first-principles calculations, we predict that a honeycomb borophene/Al(111) heterostructure can be an ideal candidate to stabilize and enhance the catalysis of many transition metal (TM) SACs via a dual charge transfer mechanism. The Al(111) substrate donates electrons to the pre-covered two-dimensional honeycomb borophene (h-B) to stabilize the latter, and the deposited TM atoms further provide electrons to the h-B, enhancing the covalent binding between the h-B and the Al(111) substrate. Intriguingly, during CO oxidation, the h-B/Al(111) heterostructure can in turn serve as an efficient electron reservoir to accept electrons from or donate electrons to the deposited TM-SACs and the reactants. Such a flexible dual charge transfer mechanism not only facilitates stabilizing the TM-SACs rather than clustering, but also effectively reduces the reaction barriers. Particularly, in contrast to expensive noble metal atoms such as Pd and Pt, low-cost Sc- and Fe-SACs are found to be the most promising SAC candidates that can be stabilized on h-B/Al(111) for O2 activation and CO oxidation, with fairly low reaction barriers (around 0.50-0.65 eV). The present findings may provide important theoretical guidance for the experimental fabrication of highly stable, efficient, and economic SACs stabilized on various heterostructure substrates.

Double‐sided surface functionalization: An effective approach to stabilize and modulate the electronic structure of graphene‐like borophene

AbstractGraphene‐like borophene was theoretically proposed and recently synthesized on Al (111) surface, however, how to conquer its structural instability is still an open question. By means of density functional theory computations, we theoretically predicted that honeycomb borophene can be well stabilized by double‐sided surface passivation with monovalent functional groups (X = F, Cl, Br, I, OH, and NH2) due to the electron redistributions. The system undergoes the transition from metallic to semiconducting upon functionalization, while the energy gap depends on the choice of functional groups. Under external strain, the gap values can be manipulated over a broad range. Our further calculations indicated that the functionalized borophene possesses moderate and anisotropic carrier mobility, which is comparable to or even higher than some 2D materials such as MoS2 and phosphorene. Our work provides a feasible strategy to effectively stabilize the graphene‐like borophene and tune the electronic properties with great potentials for electronic applications.Double‐sized functionalization by monovalent functional groups (F, Cl, Br, I, OH, and NH2) is predicted to be an effective and feasible way to stabilize the graphene‐like borophene and modulate the electronic properties. The functionalized borophenes are semiconductors with a wide range of gaps, which can be further tuned by external strains, thus are promising for various nanoelectronics applications.image

Theoretical dissection of superconductivity in two-dimensional honeycomb borophene oxide B2O crystal with a high stability

Atomically thin borophene has recently been synthesized experimentally, significantly enriching the boron chemistry and broadening the family of two-dimensional (2D) materials. Recently, oxides of 2D materials have been widely investigated for next-generation electronic devices. Based on the first-principles calculations, we predict the existence of the superconductivity in honeycomb borophene oxide (B2O), which possesses a high stability and could be potentially prepared by intrinsically incorporating oxygen into the recently synthesized borophene. The mechanical, electronic, phonon properties, as well as electron–phonon coupling of metallic B2O monolayer, have been systematically scrutinized. Within the framework of the Bardeen–Cooper–Schrieffer theory framework, the B2O monolayer exhibits an intrinsic superconducting feature with a superconducting transition temperature (Tc) of ~10.3 K, higher than many 2D borides (0.2–7.8 K). Further, strain can be utilized to tune the superconductivity with the optimal Tc of 14.7 K under a tensile strain of 1%. The superconducting trait mainly originates from the out-of-plane soft-mode vibrations of the system, which are significantly enhanced via the light O atoms’ incorporation compared to other 2D metal-boride superconductors. This strategy would open a door to design 2D superconducting structures via the participation of light elements. We believe our findings greatly bloom the 2D superconducting family and pave the way for future nanoelectronics.

Experimental evidence of monolayer AlB2 with symmetry-protected Dirac cones

Monolayer AlB$_2$ is composed of two atomic layers: honeycomb borophene and triangular aluminum. In contrast with the bulk phase, monolayer AlB$_2$ is predicted to be a superconductor with a high critical temperature. Here, we demonstrate that monolayer AlB$_2$ can be synthesized on Al(111) via molecular beam epitaxy. Our theoretical calculations revealed that the monolayer AlB$_2$ hosts several Dirac cones along the $\Gamma$--M and $\Gamma$--K directions; these Dirac cones are protected by crystal symmetries and are thus resistant to external perturbations. The extraordinary electronic structure of the monolayer AlB$_2$ was confirmed via angle-resolved photoemission spectroscopy measurements. These results are likely to stimulate further research interest to explore the exotic properties arising from the interplay of Dirac fermions and superconductivity in two-dimensional materials.

Theoretical analysis of borophene for lithium ion electrode

Borophene, a 2-dimensional material with efficacious properties (electrical, thermal and mechanical), is a very promising material that is expected to contribute, significantly, in the revolutionization of electrochemical energy storage, for improved energy and power density. In order to prove the potency of borophene as a potential anode electrode in lithium-ion batteries, density function theory simulation is performed. The study considered the interaction of stable striped, (β12), and honeycomb (Hb) borophene, with lithium ions. The main interest is to investigate the adsorption energy of the materials, and their specific capacities. The calculated average adsorption energies for the two promising electrodes are: −1.710 eV, and −0.517 eV. Moreover, the estimated specific capacities of the batteries, are: 929 mAh/g and 584 mAh/g; these results evidently proved that borophene may deliver the total energy demand of the globe, if more attention is paid into it, through dedicated research and development.

Theory of Epitaxial Growth of Borophene on Layered Electride: Thermodynamic Stability and Kinetic Pathway

Based on first-principles calculations, we propose that a layered electride Mg2O can serve as an effective substrate, in place of metal substrate, to grow honeycomb borophene (h-borophene). We first confirm the thermodynamic stability of h-B@Mg2O heterostructure by energetics analysis and dynamic stability by absence of imaginary frequency in phonon spectra. Then, kinetically, we identify the atomistic pathways for a preferred 2D growth mode over 3D growth, and reveal a growth transition from 2D compact islands to h-borophene at ~13-atom cluster size, indicating the feasibility of epitaxial growth of h-borophene on Mg2O. The h-borophene is found to be stabilized by nearly one electron transfer from Mg2O to h-borophene based on the Bader charge analysis. Moreover, the intrinsic band structure of the free-standing h-borophene is only weakly perturbed by the Mg2O substrate, a significant advantage over metal substrate. We envision that layered electrides provide an attractive family of substrates for epitaxial growth of a range of 2D materials that can otherwise only be grown on undesirable metal substrates.

Honeycomb Borophene Fragment Stabilized in Polyanionic Sandwich Lithium Salt: A New Type of Two-Dimensional Material with Superconductivity

For seeking new types of two-dimensional (2D) materials based on honeycomb borophene, honeycomb borophene fragments stabilized by Li atoms in sandwich molecules are found theoretically. This is bas...

Monolayer Honeycomb Borophene: A Promising Anode Material with a Record Capacity for Lithium-Ion and Sodium-Ion Batteries

Two-dimensional (2D) materials are a promising candidate for the anode material of lithium-ion battery (LIB) and sodium-ion battery (NIB) for their unique physical and chemical properties. Recently, a honeycomb borophene (h-borophene) has been fabricated by molecular beam epitaxy (MBE) growth in ultra high vacuum. Here, we adopt the first-principles density functional theory calculations to study the performance of monolayer (ML) h-borophene as an anode material for the LIB and NIB. The binding energies of the ML h-borophene-Li/Na systems are all negative, indicating a steady adsorption process. The diffusion barriers of the Li and Na ions in h-borophene are 0.53 and 0.17 eV, respectively, and the anode overall open-circuit voltages for the LIB and NIB are 0.747 and 0.355 V, respectively. The maximum theoretical storage capacity of h-borophene is 1860 mAh·g−1 for NIB and up to 5268 mAh·g−1 for LIB. The latter is more than 14 times higher than that of commercially used graphite (372 mAh·g−1) and is also the highest theoretical capacity among all the 2D materials for the LIB discovered to date. Our study suggests that h-borophene is a promising anode material for high capacity LIBs and NIBs.

2D honeycomb borophene oxide: a promising anode material offering super high capacity for Li/Na-ion batteries

Rational design of novel two-dimensional (2D) electrode materials with high capacity is crucial for the further development of Li-ion and Na-ion batteries. Herein, based on first-principles calculations, we systemically investigate Li and Na storage behaviors in the recently discovered 2D topological nodal-loop metal—the honeycomb borophene oxide (h-B2O). We show that h-B2O is an almost ideal anode material. It has good conductivity before and after Li/Na adsorption, fast ion diffusion with diffusion barrier less than 0.5 eV, low open-circuit voltage (<1 V), and small lattice change (<6.2%) during intercalation. Most remarkably, its theoretical storage capacity is extremely high, reaching up to 2137 mAh · g−1 for Li and 1425 mAh · g−1 for Na. Its Li storage capacity is more than six times higher than graphite (~372 mAh · g−1), and is almost the highest among all 2D materials discovered to date. Our results strongly suggest that 2D h-B2O is an exceedingly promising anode material for both Li- and Na-ion batteries with super high capacity.

Electron-phonon coupling in a honeycomb borophene grown on Al(111) surface

Recently, a honeycomb borophene was reported to grow successfully on Al(111) surface. Since the metallic $\sigma$-bonding bands of honeycomb boron sheet play a crucial role in the 39 K superconductivity of MgB$_2$, it is physically interesting to examine whether similar property exists in this material. We have calculated the electronic structures and the electron-phonon coupling for honeycomb borophene by explicitly considering the substrate effect using first-principles density functional theory in conjunction with the Wannier interpolation technique. We find that the $sp^2$-hybridized $\sigma$-bonding bands of honeycomb borophene are metallized due to moderate charge transfer from the Al substrate, similar as in MgB$_2$. However, the electron-phonon coupling in honeycomb borophene is much weaker than in MgB$_2$ due to the hardening of the bond-stretching boron phonon modes and the reduction of phonon density of states. Nevertheless, the interlayer coupling between Al-associated phonons and electrons in borophene is strong. Based on this observation, we predict that a 6.5 K superconducting transition can be observed in a free-standing borophene decorated by a single Al layer, namely monolayer AlB$_2$. Accordingly, similar superconducting transition temperature could be expected in honeycomb borophene on Al(111).

How is Honeycomb Borophene Stabilized on Al(111)?

Because of the electron deficiency, freestanding honeycomb borophene (HB) is predicted to be kinetically unstable. Very recently, the honeycomb phase of two-dimensional boron, nevertheless, has bee...

Band engineering of B2H2 nanoribbons**Project supported by the National Natural Science Foundation of China (Grant Nos. 61888102, 61390501, and 51872284), the CAS Pioneer Hundred Talents Program, the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB30000000 and XDB28000000), the Beijing Nova Program, China (Grant No. Z181100006218023), and the University of Chinese Academy of Sciences.

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B2H2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B2H2 nanoribbons. Based on first-principles calculations, we have found that all narrow armchair nanoribbons with and without mirror symmetry (ANR-s and ANR-as, respectively) are semiconducting. The energy gap has a relation with the width of the ribbon. When the ribbon is getting wider, the gap disappears. The zigzag ribbons without mirror symmetry (ZNR-as) have the same trend. But the zigzag ribbons with mirror symmetry (ZNR-s) are always metallic. We have also found that the metallic ANR-as and ZNR-s can be switched to semiconducting by applying a tensile strain along the nanoribbon. A gap of 1.10 eV is opened under 16% strain for the 11.0-Å ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B2H2 nanoribbon increases its possibility of potential applications in nanodevices.

Two-dimensional honeycomb borophene oxide: strong anisotropy and nodal loop transformation.

The search for topological semimetals is mainly focused on heavy-element compounds by following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features. Here, using first-principles calculations, we propose a new two-dimensional light-element material-the honeycomb borophene oxide (h-B2O), which has nontrivial topological properties. The proposed structure is based on the recently synthesized honeycomb borophene on an Al (111) substrate [W. Li, L. Kong, C. Chen, J. Gou, S. Sheng, W. Zhang, H. Li, L. Chen, P. Cheng and K. Wu, Sci. Bull., 2018, 63, 282-286]. The h-B2O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B2O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B2O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different [Doublestruck Z] × [Doublestruck Z] topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.

Stabilizing honeycomb borophene by metal decoration: a computational study

Adsorption of light metal atoms like Be on a borophene nanosheet can stabilize the sheet structurally and electronically, leading to a graphene like planar honeycomb structure.

Optimized tight binding parameters for single layer honeycomb borophene

Motivated by experimental realization of honeycomb borophene (HB) on an Al(111) surface, we propose a non-orthogonal Slate-Koster tight binding (TB) model for single layer HB, of which parameters are optimized via a combination of the genetic algorithm and the simplex method using electronic structures obtained from density functional theory (DFT) as targets. Our proposed Slate-Koster parameters can well produce both energies and orbital compositions of electronic bands of a single layer HB. The direct calculation of band structure and atomic orbital resolved density of states show excellent agreement with DFT results. The highly accurate TB model would facilitate the future large-scale atomistic modeling on electronic, optical, and transport properties of HB based nano-materials.

Corrigendum: Gate-Voltage Control of Borophene Structure Formation.

In this Communication, the minimum thickness of the jellium slab (to allow for high doping) was set at d=4 Å, which is incorrect. In fact, d=3 Å is the correct thickness limit: it is computationally convergent, is reasonably larger than the covalent diameter of boron (1.74 Å), and the calculations with d=3 Å still avoid spurious occupancy of the vacuum states even at doping as high as q=0.5–1 e/atom. Importantly, for q ≥0.5 e/atom, the v1/3 honeycomb sheet emerges as the ground state (Figure 1). Furthermore, at q=1 e/atom the honeycomb borophene becomes isoelectronic to graphene and Dirac cones appear at the Fermi level (Figure 1 c, inset). This honeycomb structure is the same as that of the boron lattice responsible for superconductivity in MgB2. a) Top: Total energy per atom of all symmetry-inequivalent B sheets calculated by CE fits at a doping level of q=0.03 e/atom as a function of v in the B1−v[…]v systems. Diamonds show the data expanded from a rhombus cell while squares show those expanded from a rectangular cell, as shown in the inset. Hexagonal lattice sites, filled B sites, and hollow hexagons are indicated. The solid line shows the ground states from precise DFT calculations. Bottom: Top three stable sheets, with the stability decreasing as the circle fades from dark to light gray. b) and c) are the same as (a) but for q=0.2 e/atom, and q=0.5–1.0 e/atom, respectively. Bottom panel of (c) shows the the ground state structures for all q=0.5–1.0 e/atom, and inset shows the band structure of the ground state v=1/3 for a doping of q=1.0 e/atom (black solid line) and q=0 e/atom (red dashed lines). Note that the Dirac cone shifts to the Fermi level in this doped configuration.

Experimental realization of honeycomb borophene

We report the successful preparation of a purely honeycomb, graphene-like borophene, by using an Al(1 1 1) surface as the substrate and molecular beam epitaxy (MBE) growth in ultrahigh vacuum. Scanning tunneling microscopy (STM) images reveal perfect monolayer borophene with planar, non-buckled honeycomb lattice similar as graphene. Theoretical calculations show that the honeycomb borophene on Al(1 1 1) is energetically stable. Remarkably, nearly one electron charge is transferred to each boron atom from the Al(1 1 1) substrate and stabilizes the honeycomb borophene structure, in contrast to the negligible charge transfer in case of borophene/Ag(1 1 1). The existence of honeycomb 2D allotrope is important to the basic understanding of boron chemistry, and it also provides an ideal platform for fabricating boron-based materials with intriguing electronic properties such as Dirac states.