2D Boron Sheets Research Articles

Structure and exfoliation mechanism of two-dimensional boron nanosheets

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets—by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials.

Borophene: Synthesis, Chemistry, and Electronic Properties.

As a neighbor of carbon in the periodic table, boron exhibits versatile structural and electronic configurations, with its allotropes predicted to possess intriguing structures and properties. Since the experimental realization of two-dimensional (2D) boron sheets (borophene) on Ag(111) substrates in 2015, the experimental study of the realization and characteristics of borophene has drawn increasing interest. In this review, we summarize the synthesis and properties of borophene, which are mainly based on experimental results. First, the synthesis of borophene on different substrates, as well as borophane and bilayer borophene, featuring unique phases and properties, are discussed. Next, the chemistry of borophene, such as oxidation, hydrogenation, and its integration into heterostructures with other materials, is summarized. We also mention a few works focused on the physical properties of borophene, specifically its electronic properties. Lastly, the brief outlook addresses challenges toward practical applications of borophene and possible solutions.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity.

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be2M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be2Al and Be2Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (Tc) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their Tc. The high thermochemical and kinetic stability of Be2M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be2M on the electride Ca2N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be2M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

THE RISE OF BOROPHENE AND BOROPHENE NANORIBBONS: A POTENTIAL QUANTUM ELECTRONICS MATERIAL

Borophene, a boron analogue of graphene, has been considered as an innovative two-dimensional (2D) material for the scientific and research community. First 2D boron sheet was reported in 2015 by growing it on Ag substrate. Attributable to its exceptional structural, electronic, and spin properties, it has earned enormous exploration interests in the multifaceted fields of material science, nanotechnology and computational physics. Borophene is the newest rising 2D material, which demonstrates various diversified configurations that can be marked by anisotropic and polymorphism. These configurations enabled enhanced tailoring properties of borophene for various applications. This review provides brief discussion on various experimentally synthesized borophene and borophene nanoribbon (BNR), their structural, electronic, and magnetic properties variations along with the width effect. Based upon the type of edges, BNRs are categorized as line edge-BNR (LE-BNR) and zigzag-BNR (ZZ-BNR). Electronic properties of LE-BNR with vacancy defect show semiconductor behavior and ZZ-BNR indicates metallic property. It covers all the structural allotropes, their synthesis, stability and other crucial intrinsic properties, characterization and tailoring of physical properties. Hence, this comprehensive review reported here incorporates the crucial recent experimental and theoretical advances on borophene and BNRs till date. This work also covers recent modifications reported in borophene and BNR to alter their properties as per the demands of various application areas such as nanoelectronic devices, interconnects, biosensors, metal detectors and gas sensors.

Designing and optimizing β1-borophene organic gas sensor: A theoretical study

Borophene, a two-dimensional (2D) boron sheet, has received widespread attention recently. β1-borophene is one of the allotropes and has the representative structure. Using density functional theory and non-equilibrium Green's function method, here we investigated the adsorption behaviors of small organic molecules CH4, CH3OH, C6H6, C2H2, C2H4, HCHO and HCOOH on β1-borophene, and the corresponding sensing applications. We found that CH4, CH3OH and C6H6 are physically adsorbed on β1-borophene while C2H2, C2H4, HCHO and HCOOH are chemically adsorbed on β1-borophene. For the cases of chemisorption, the chemical bonds of the C atoms in the four organic molecules all change to sp3 hybridization, owing to the strong interactions with β1-borophene. The transport properties of β1-borophene are sensitive to the adsorption of C2H2, C2H4, HCHO and HCOOH, thus β1-borophene can be used to construct gas sensors for the organic molecules. Especially, β1-borophene is very sensitive to C2H4 and the sensing performance can be improved by applying proper in-plane strain. In addition, the influence of the substrates has also been considered. We found that β1-borophene can form van der Waals heterostructures with 2D semiconductors MoS2, MoSe2, WS2 or WSe2, and WS2 can be chosen as the substrate since it enhances the thermal stability of β1-borophene and significantly improves the sensing performance. This suggests a possible way to design borophene-based organic gas sensors.

Structural Evolution of Boron Clusters on Ag(111) Surfaces - From Atomic Chains to Triangular Sheets with Hexagonal Holes.

Unlike graphene and other 2D materials, borophene is 2D polymorphic with diverse hexagonal holes (HHs)-triangles ratios and the concentrations of HHs are highly substrate dependent. Here, we systematically explored the evolution of boron cluster on Ag(111) surface, BN @Ag(111) (N=1∼36), to understand the nucleation of 2D boron sheet on metal surface. Our calculation showed that, with the size increasing, the structures of most stable BN clusters undergo an evolution from compact triangular lattice, such as double-chains or triple-chains, to the ones with mixed triangular-hexagonal lattices. The first single-HH appears at N=12 and the first double-HH appears at N=27. The stability of large BN clusters with mixed structures is derived from the charge transfer between triangular lattice and the HHs, as well as between the substrates and the BN clusters. Our results provide a deep understanding on the formation of small boron clusters in the initial nucleation stage of borophene growth.

Chemical Vapor Deposition of Two-Dimensional Boron Sheets by Thermal Decomposition of Diborane.

Two-dimensional (2D) boron sheets (borophenes) are promising materials for the next generation of electronic devices because of their metallic conductivity. Molecular beam epitaxy has remained the main approach for the growth of borophene, which considerably restricts large-scale production of 2D boron sheets. The high melting point of boron and the growth of borophenes at moderate temperatures posed a significant challenge for the synthesis of borophenes. Employing diborane (B2H6) pyrolysis as a pure boron source, we report, for the first time, the growth of atomic-thickness borophene sheets by chemical vapor deposition (CVD). A methodical study on the effect of temperature, deposition rate, and pressure on the growth of 2D boron sheets is provided and detailed analyses about the morphology and crystalline phase of borophene sheets are presented. The CVD-borophene layers display an average thickness of 4.2 Å, χ3 crystalline structure, and metallic conductivity. We also present experimental evidence supporting the formation of stacked bilayer and trilayer borophene sheets. Our method paves the way for empirical investigations on borophenes.

The thermal transport characterization of borophene: A molecular dynamics study

Over recent years, two-dimensional (2D) boron sheets (borophene) have been reported to have unique properties, but studies on the thermal properties of borophene using molecular dynamics simulation are still few and far between. In this research, phase transition temperature, heat capacity (Cp), thermal expansion coefficients, and the thermal conductivity of borophene were investigated using the molecular dynamics (MD) simulation method via the reactive force field (ReaxFF). The present work is the first to report the exact phase transition temperature for the borophene sheets. The effects of temperature, point defects, and chirality angle on the thermal conductivity of these sheets were further studied. In addition, an approximate structure parameter on the relaxed system was obtained. Although a theoretical study predicted the structure parameter of borophene, it can be inferred that the ReaxFF was incapable of accurately predicting the borophene lattice parameters. Our simulation results were well in line with the available values reported in other studies where the thermal conductivity of borophene decreased with the increase in temperature. Moreover, the chirality angle effect showed that the thermal conductivity of borophene was anisotropic, and the thermal conductivity in the armchair direction was noticeably larger than that in the zigzag direction. Furthermore, it was confirmed that the thermal conductivity was extremely dependent on the vacancy defects. Understanding the thermal properties of borophene may be crucial for expanding its application in novel nanosystems.

Borophene: Current Status, Challenges and Opportunities

Borophenes (2D boron sheets) have triggered a surge of interest both theoretically and experimentally because of its distinct structural, optical and electronic properties for extensive potential applications. Although theoretical efforts have guided the research directions of borophene, only few synthetic borophene sheets have been demonstrated experimentally. Borophene sheets have been successfully synthesized experimentally on metal substrates until 2015. Afterwards, more efforts were put on the controlled synthesis of crystalline and semiconducting borophene sheets as well as on the investigation of their novel and fascinating physical properties. This report provides a brief review on theoretical and experimental progress in borophene research. Some typical structures and properties of borophenes have been reviewed. The focus is laid on summarizing the experimental synthesis of borophene in recent years, and on showing some ultrastable and semiconducting borophenes which have been applied in electronic information devices. Finally, the future challenges and opportunities regarding experimental realization and practical applications of borophenes are presented.

Borophene: A Metal‐free and Metallic Electrocatalyst for Efficient Converting CO2 into CH4

AbstractExploring novel and metal‐free electrocatalysts for CO2 reduction is of great significance not only for reducing of CO2 emission but also for energy storage. However, searching metal‐free and highly efficient electrocatalysts for CO2 reduction is still challenging. Here, we report that a metal‐free and metallic borophene can be used as an efficient electrocatalyst for CO2 reduction. Through a comprehensive DFT investigation of CO2 activation and conversion on newly fabricated metallic 2D boron sheets (borophene, β12 or χ3 boron sheets), our calculational results indicate that electron deficient borophene can provide electrons to CO2 and effectively activate the inserted CO2 by breaking π bond of CO2. Moreover, the study demonstrates that the most feasible product of CO2 reduction on the borophene is CH4 with a limiting potential of −0.27 V and the largest activation barrier of 0.98 eV along the minimum energy pathway. The very small values of the limiting potential and activation barrier indicate that CO2 reduction reaction on borophene is both thermodynamically and kinetically feasible. Overall, our work provides insight that the metal free and metallic borophene can be used as an excellent electrocatalyst for CO2 reduction.

Polymorphism of low dimensional boron nanomaterials driven by electrostatic gating: a computational discovery.

The successful synthesis of two-dimensional (2D) boron sheets typically relies on the utilization of a silver surface, which acts as a gated substrate compensating for the electron-deficiency of boron. However, how the structures of one-dimensional (1D) boron are affected by the gating effect remains unclear. By means of an unbiased global minimum structure search and density functional theory (DFT) computations, we discovered the coexistence of 2D boron sheets and 1D ribbons triggered by electrostatic gating. Specifically, at a low excess charge density level (<0.1 e per atom), 2D boron sheets dominate the low energy configurations. As the charge density increases (>0.3 e per atom), more 1D boron ribbons emerge, while the number of 2D layers is reduced. Additionally, a number of low-lying 1D boron ribbons were discovered, among which a flat borophene-like ribbon (FBR) was predicted to be stable and possess high mechanical strength. Moreover, the electride Ca2N was identified as an ideal substrate for the fabrication of the FBR because of its ability to supply a strong electrostatic field. This work bridges the gap between 2D and 1D boron structures, reveals the polymorphism of 1D boron ribbons under the electrostatic gating effect, and in general provides broad implications for future synthesis and applications of low-dimensional boron materials.

2D Boron Sheets: Structure, Growth, and Electronic and Thermal Transport Properties

AbstractThe structures of boron clusters, such as flat clusters and fullerenes, resemble those of carbon. Various two‐dimensional (2D) borophenes have been proposed since the production of graphene. The recent successful fabrication of borophene sheets has prompted extensive researches, and some unique properties are revealed. In this review, the recent theoretical and experimental progress on the structure, growth, and electronic and thermal transport properties of borophene sheets is summarized. The history of prediction of boron sheet structures is introduced. Existing with a mixture of triangle lattice and hexagonal lattice, the structures of boron sheets have peculiar characteristics of polymorphism and show significant dependence on the substrate. Due to the unique structure and complex BB bonds, borophene sheets have many interesting electronic and thermal transport properties, such as strong nonlinear effect, strong thermal transport anisotropy, high thermal conductance in the ballistic transport and low thermal conductivity in the diffusive transport. The growth mechanism and synthesis of borophene sheets on different metal substrates are also presented. The successful prediction and synthesis will shed light on the exploration of new novel materials. Besides, the outstanding and peculiar properties of borophene make them tempting platform for exploring novel physical phenomena and extensive applications.

Borophene Is a Promising 2D Allotropic Material for Biomedical Devices

Allotropic 2D materials are the new frontier of materials science, due to their unique strategic properties and application within several sciences. Allotropic 2D materials have shown tunable physical, chemical, biochemical, and optical characteristics, and among the allotropic materials, graphene has been widely investigated for its interesting properties, which are highly required in biomedical applications. Recently, the synthesis of thin 2D boron sheets, developed on Ag(111) substrates, was able to create a 2D triangular structure called borophene (BO). Borophene has consistently shown anisotropic behavior similar to graphene. In this topical review, we will describe the main properties and latest applications of borophene. This review will critically describe the most interesting uses of borophene as part of electronic and optical circuits. Moreover, we will report how borophene can be an innovative component of sensors within biomedical devices, and we will discuss its use in nanotechnologies and theranostic applications. The conclusions will provide insight into the latest frontiers of translational medicine involving this novel and strategic 2D allotropic material.

Adsorption of DNA Bases on Two‐Dimensional Boron Sheets

AbstractUtilizing two‐dimensional (2D) sheets for DNA fingerprinting applications is an active field of research, with extensive research being carried out on graphene, h‐BN and transition metal dichalcogenides. In this theoretical study we propose the newly discovered 2D boron sheets (α, α1 and β1 sheets) as a promising adsorbate material for DNA fingerprinting applications. The relatively high binding energies of the DNA bases Adenine (A), thymine (T), guanine (G) and cytosine (C) and the DNA base pairs A:T and G:C on the sheets suggest stabilization of the DNA constructs on these sheets. The stabilization energies were found to be higher in comparison with graphene sheets. The stabilization of the adsorbed nucleobases were found to be mediated by Van‐der Waals interaction, with no direct chemical bond being formed between the DNA bases and the sheets, similar to the scenario in graphene and h‐BN sheets. Physisorption of the DNA bases was also reflected in the relatively small changes in the work function after adsorption.

Two-Dimensional Boron Sheets as Metal-Free Catalysts for Hydrogen Evolution Reaction

Current metal-free catalysts for hydrogen evolution reaction (HER) are mainly carbon-based. In this work, HER catalytic activity on two-dimensional (2D) boron sheets (α and β12) are explored using periodic density functional theory. Using the binding free energy of H (ΔGH*) as the descriptor, we found that both α and β12 sheets present superior activity, with ΔGH* being close to zero. It is expected that 2D boron sheets would be a promising metal-free catalyst in the electrolysis of water and may offer novel thoughts for the design of new catalysts for other reactions.

Stretch‐Driven Increase in Ultrahigh Thermal Conductance of Hydrogenated Borophene and Dimensionality Crossover in Phonon Transmission

AbstractInspirited by the wide range of applications of graphene and the similarity between boron and carbon, 2D boron sheets have gained extensive research interest. In this work, using first‐principles combined with a nonequilibrium Green's function method, thermal conductance of fully hydrogenated borophene, named borophane, is studied. Interestingly and in contrast to widely perceived sense, at 300 K, it is found that the thermal conductance of borophane in the armchair direction is remarkably larger than that of graphene. More interesting, a dimensionality crossover is observed in phonon transmission where low‐frequency phonons exhibit 2D characteristic, while high‐frequency phonons behave like a 1D system, oriented along armchair direction, which results in the ultrahigh thermal conductance. An anomalous increase of thermal conductance with uniaxial tensile strain is observed, which is well explained by the unique puckered structure and chemical bonding in borophane. The excellent in‐plane stiffness and flexibility together with the high thermal conductance suggest that borophane is promising for soft thermal channel. Moreover, this unique dimensionality crossover in phonon transmission offers a perfect platform for studying the effect of phonon population in mode space, which is of primary importance for thermal transport in low‐dimensional systems.

Stabilization and Metallic to Semiconducting Transition in 2D Boron Sheet

Stabilization and Metallic to Semiconducting Transition in 2D Boron Sheet

Spindle nodal chain in three-dimensional α' boron.

Topological metals/semimetals (TMs) have emerged as a new frontier in the field of quantum materials. A few two-dimensional (2D) boron sheets have been suggested as Dirac materials, however, to date TMs made of three-dimensional (3D) boron structures have not been found. Herein, by means of systematic first principles computations, we discovered that a rather stable 3D boron allotrope, namely 3D-α' boron, is a nodal-chain semimetal. In momentum space, six nodal lines and rings contact each other and form a novel spindle nodal chain. This 3D-α' boron can be formed by stacking 2D wiggle α' boron sheets, which are also nodal-ring semimetals. In addition, our chemical bond analysis revealed that the topological properties of the 3D and 2D boron structures are related to the π bonds between boron atoms, however, the bonding characteristics are different from those in the 2D and 3D carbon structures.

Design of Two-Dimensional Graphene-like Dirac Materials β12-XBeB5 (X = H, F, Cl) from Non-graphene-like β12-Borophene.

Two-dimensional (2D) Dirac materials and boron sheets have attracted intensive interest recently. However, 2D Dirac materials remain rare and difficult to be realized experimentally, and 2D boron sheets generally have high dynamical instability. Stimulated by the experimental observation of Dirac cones in nongraphene-like β12 boron sheets and based on the understanding of boron sheet electronic organization, we theoretically design new 2D Dirac materials β12-XBeB5 (X = H, F, Cl) with high stability. We confirm β12-HBeB5 as the global energy minimum among its 2D allotropes based on global structure search methods, a strong indication of its experimental feasibility. Our designed β12-HBeB5 has not only a high Fermi velocity, but also a Dirac state very robust against extraordinary large tensile strains, an advantage for flexible electronics applications. Our work opens a new avenue to designing feasible 2D Dirac materials and stabilizing borophene sheets.

Semimetallic Two-Dimensional TiB12: Improved Stability and Electronic Properties Tunable by Biaxial Strain

Recently, the successful synthesis of two-dimensional (2D) boron on metallic surfaces has motivated great interest in improving the stability of 2D boron to allow the realization of promising materials. Through the use of an ab initio evolutionary search algorithm, we have discovered a series of TiBx (2 ≤ x ≤ 16) structures that consist of earth-abundant titanium and boron atoms in 2D arrangements. These structures are greatly stabilized by electron transfer from Ti to B, therefore, leading to much better stability than the 2D boron sheets proposed so far. In particular, TiB12 has a low enough energy to make it competitive to a mixture of the well-known TiB2 compound and a 2D α-boron sheet and exhibits a quasi-Dirac point with a 0.02 eV gap. Interestingly, the work function and conductivity of this 2D TiB12 material are calculated to be tunable through the application of biaxial strain. The possibility of synthesis and novel electronic properties expected for 2D TiB12 render it a promising new 2D material...