Prediction of freestanding semiconducting bilayer borophenes

Supported bilayerα-borophene(BL-αborophene)on Ag(111)substrate has been synthesized in recent experiments.Based on the experimentally observed quasi-planar C_(6v)B_(36)(1),its monolayer assemblyα^(+)-borophene B_(11)(P6/mmm)(2),and extensive global minimum searches augmented with density functional theory calculations,we predict herein freestanding BL-α^(+)borophenes B_(22)(Cmmm)(3)and B_(22)(C2/m)(4)which,as the most stable BL borophenes reported to date,are composed of interwoven boron triple chains as boron analogs of monolayer graphene(5)consisting of interwoven carbon single chains.The nearly degenerate eclipsed B_(22)(3)and staggered B_(22)(4)with the hexagonal hole density ofη=1/12 and interlayer bonding density of u=1/4 appear to be two-dimensional semiconductors with the indirect band gaps of 0.952 and 1.144 eV,respectively.Detailed bonding analyses reveal one delocalized 12c-2eπbond over each hexagonal hole in both the B_(22)(3)and B_(22)(4),similar to the situation in monolayer graphene which contains one delocalized 6c-2eπbond over each C_(6)hexagon.Furthermore,these BL-α^(+)borophenes appear to remain highly stable on Ag(111)substrate,presenting the possibility to form supported BL-α+borophenes.

Review of thermal transport and electronic properties of borophene

In recent years, two-dimensional boron sheets （borophene） have been experimentally synthesized and theoretically proposed as a promising conductor or transistor with novel thermal and electronic properties. We first give a general survey of some notable electronic properties of borophene, including the superconductivity and topological characters. We then mainly review the basic approaches, thermal transport, as well as the mechanical properties of borophene with different configurations. This review gives a general understanding of some of the crucial thermal transport and electronic properties of borophene, and also calls for further experimental investigations and applications on certain scientific community.

Lattice thermal conductivity of β12 and χ3 borophene

Borophene allotropes have many unique physical properties due to their polymorphism and similarity between boron and carbon.In this work,based on the density functional theory and phonon Boltzmann transport equation,we investigate the lattice thermal conductivityκof bothβ12 andχ3 borophene.Interestingly,these two allotropes with similar lattice structures have completely different thermal transport properties.β12 borophene has almost isotropicκaround 90 W/(m·K)at 300 K,whileκofχ3 borophene is much larger and highly anisotropic.The room temperatureκofχ3 borophene along the armchair direction is 512 W/(m·K),which is comparable to that of hexagonal boron nitride but much higher than most of the two-dimensional materials.The physical mechanisms responsible for such distinct thermal transport behavior are discussed based on the spectral phonon analysis.More interestingly,we uncover a unique one-dimensional transport feature of transverse acoustic phonon inχ3 borophene along the armchair direction,which results in a boost of phonon relaxation time and thus leads to the significant anisotropy and ultrahigh thermal conductivity inχ3 borophene.Our study suggests thatχ3 borophene may have promising application in heat dissipation,and also provides novel insights for enhancing the thermal transport in two-dimensional systems.

Edge-Dependent Electronic and Magnetic Characteristics of Freestanding β_(12)-Borophene Nanoribbons

This work presents an investigation of nanoribbons cut from β_(12)-borophene sheets by applying the density functional theory. In particular, the electronic and magnetic properties of borophene nanoribbons(BNR) are studied. It is found that all the ribbons considered in this work behave as metals, which is in good agreement with the recent experimental results. β_(12)-BNR has significant diversity due to the existence of five boron atoms in a unit cell of the sheet. The magnetic properties of the ribbons are strongly dependent on the cutting direction and edge profile. It is interesting that a ribbon with a specific width can behave as a normal or a ferromagnetic metal with magnetization at just one edge or two edges. Spin anisotropy is observed in some ribbons, and the magnetic moment is not found to be the same in both edges in an antiferromagnetic configuration. This effect stems from the edge asymmetry of the ribbons and results in the breaking of spin degeneracy in the band structure. Our findings show that β_(12) BNRs are potential candidates for next-generation spintronic devices.

Band engineering of B_2H_2 nanoribbons

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B_2H_2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B_2H_2 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-■ ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B_2H_2 nanoribbon increases its possibility of potential applications in nanodevices.

Anisotropic plasmon dispersion and damping in multilayer 8-Pmmn borophene structures

We investigate the collective plasma oscillations theoretically in multilayer 8-Pmmn borophene structures,where the tilted Dirac electrons in spatially separated layers are coupled via the Coulomb interaction.We calculate the energy dispersions and Landau dampings of the multilayer plasmon excitations as a function of the total number of layers,the interlayer separation,and the different orientations.Like multilayer graphene,the plasmon spectrum in multilayer borophene consists of one in-phase optical mode and N-1 out-of-phase acoustical modes.We show that the plasmon modes possess kinks at the boundary of the interband single-particle continuum and the apparent anisotropic behavior.All the plasmon modes approach the same dispersion at a sufficiently large interlayer spacing in the short-wavelength limit.Especially along specific orientations,the optical mode could touch an energy maximum in the nondamping region,which shows non-monotonous behavior.Our work provides an understanding of the multilayer borophene plasmon and may pave the way for multilayer borophene-based plasmonic devices.

Electric field manipulation of multiple nonequivalent Dirac cones in the electronic structures of hexagonal CrB_4 sheet

Two-dimensional materials with Dirac cones have significant applications in photoelectric technology. The origin and manipulation of multiple Dirac cones need to be better understood. By first-principle calculations, we study the influence of external fields on the electronic structure of the hexagonal CrB4 sheet with double nonequivalent Dirac cones. Our results show that the two cones are not sensitive to tensile strain and out-of-plane electric field, but present obviously different behaviors under the in-plane external electric field（along the B-B direction）, i.e., one cone holds while the other vanishes with a gap opening. More interestingly, a new nonequivalent cone emerges under a proper in-plane electric field. We also discuss the origin of the cones in CrB4 sheet. Our study provides a new method on how to obtain Dirac cones by the external field manipulation, which may motivate potential applications in nanoelectronics.

Surface-regulated triangular borophene as Dirac-like materials from density functional calculation investigation

By applying the first principles calculations combined with density functional theory (DFT), this study explored the optical properties, electronic structure, and structure stability of triangular borophene decorated chemically, B3X (X=F, Cl) in a systematical manner. As revealed from the results of formation energy, phonon dispersion, and molecular dynamics simulation study, all the borophene decorated chemically were superior and able to be fabricated. In the present study, triangular borophene was reported to be converted into Dirac-like materials when functionalized by F and Cl exhibiting narrow direct band gaps as 0.19 eV and 0.17 eV, separately. Significant light absorption was assessed in the visible light and ultraviolet region. According the mentioned findings, these two-dimensional (2D) materials show large and wide promising applications for future nanoelectronics and optoelectronics.

Molecular Dynamics Simulations for Anisotropic Thermal Conductivity of Borophene

The present work carries out molecular dynamics simulations to compute the thermal conductivity of the borophene nanoribbon and the borophene nanotube using the Müller-Plathe approach.We investigate the thermal conductivity of the armchair and zigzag borophenes,and show the strong anisotropic thermal conductivity property of borophene.We compare results of the borophene nanoribbon and the borophene nanotube,and find the thermal conductivity of the borophene is orientation dependent.The thermal conductivity of the borophene does not vary as changing the width of the borophene nanoribbon and the perimeter of the borophene nanotube.In addition,the thermal conductivity of the borophene is not sensitive to the applied strains and the background temperatures.

Epitaxial growth of borophene on graphene surface towards efficient and broadband photodetector

In-situ integration of multiple materials with well-defined interfaces as heterostructures is of great interest due to their unique properties and potential for new device functionality.Because of its polymorphism and diverse bonding geometries,borophene is a promising candidate for two-dimensional heterostructures,but suitable synthesis conditions have limited its potential applications.Toward this end,we demonstrate the vertical borophene and graphene heterostructures which form by epitaxial growth of borophene onto multilayer graphene on Cu substrates via chemical vapor deposition,where hydrogen and NaBH_(4) are respectively used as the carrier gas and the boron source.The lattice structure of the as-synthesized borophene well coincides with the predictedα′-boron sheet.The borophene-based photodetector shows an excellent broadband photoresponse from the ultraviolet(255 nm)to the infrared(940 nm)wavelengths,with enhanced responsivity compared to pristine borophene or graphene photodetectors.This work informs emerging efforts to integrate borophene into nanoelectronic applications for both fundamental investigations and technological applications.

Bilayer borophene:an efficient catalyst for hydrogen evolution reaction

The electrocatalytic hydrogen evolution reaction is a crucial technique for green hydrogen production.However,finding affordable,stable,and efficient catalyst materials to replace noble metal catalysts remains a significant challenge.Recent experimental breakthroughs in the synthesis of two-dimensional bilayer borophene provide a theoretical framework for exploring their physical and chemical properties.In this study,we systematically considered nine types of bilayer borophenes as potential electrocatalysts for the hydrogen evolution reaction.Our first-principles calculations revealed that bilayer borophenes exhibit high stability and excellent conductivity,possessing a relatively large specific surface area with abundant active sites.Both surface boron atoms and the bridge sites between two boron atoms can serve as active sites,displaying high activity for the hydrogen evolution reaction.Notably,the Gibbs free energy change associated with adsorption for these bilayer borophenes can reach as low as−0.002 eV,and the Tafel reaction energy barriers are lower(0.70 eV)than those on Pt.Moreover,the hydrogen evolution reaction activity of these two-dimensional bilayer borophenes can be described by engineering their work function.Additionally,we considered the effect of pH on hydrogen evolution reaction activity,with significant activity observed in an acidic environment.These theoretical results reveal the excellent catalytic performance of two-dimensional bilayer borophenes and provide crucial guidance for the experimental exploration of multilayer boron for various energy applications.

Experimental realization of honeycomb borophene

We report the successful preparation of a purely honeycomb,graphene-like borophene,by using an Al(11 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 2 D 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.

Ab initio prediction of borophene as an extraordinary anode material exhibiting ultrafast directional sodium diffusion for sodium-based batteries

Density functional theory calculations and ab initio molecular dynamics simulations are performed to study the feasibility of using borophene, a newly synthesized two-dimensional sheet of boron, as an anode material for sodium-ion and sodium-oxygen batteries. The theo- retical capacity of borophene is found to be as high as 1,218 mAh g-1 （Nao.sB）. More importantly, it is demonstrated that the sodium diffusion energy barrier along the valley direction is as low as 0.0019 eV, which corresponds to a diffusivity of more than a thousand times higher than that of conventional anode materials such as Na2Ti307 and Na3Sb. Hence, the use of borophene will revolutionize the rate capability of sodium-based batteries. Moreover, it is predicted that, during the sodiation process, the average open-circuit voltage is 0.53 V, which can effectively sup- press the formation of dendrites while maximizing the energy density. The metallic feature and structural integrity of borophene can be well preserved at different sodium concentrations, demonstrating good electronic conductivity and stable cyclability.

Borophene–graphene heterostructure: Preparation and ultrasensitive humidity sensing

Heterostructure has triggered a surge of interest due to its synergistic effects between two different layers,which contributes to desirable physical properties for extensive potential applications.Structurally stable borophene is becoming a promising candidate for constructing two-dimensional(2D)heterostructures,but it is rarely prepared by suitable synthesis conditions experimentally.Here,we demonstrate that a novel heterostructure composed of hydrogenated borophene and graphene can be prepared by heating the mixture of sodium borohydride and few-layered graphene followed by stepwise and in situ thermal decomposition of sodium borohydride under high-purity hydrogen as the carrier gas.The fabricated borophene–graphene heterostructure humidity sensor shows ultrahigh sensitivity,fast response,and long-time stability.The sensitivity of the fabricated borophene-based sensor is near 700 times higher than that of pristine graphene one at the relative humidity of 85%RH.The sensitivity of the sensor is highest among all the reported chemiresistive sensors based on 2D materials.Besides,the performance of the borophene–graphene flexible sensor maintains good stability after bending,which shows that the borophene-based heterostructures can be applied in wearable electronics.The observed high performance can be ascribed to the well-established charge transfer mechanism upon H2O molecule adsorption.This study further promotes the fundamental studies of interfacial effects and interactions between boron-based 2D heterostructures and chemical species.

Highly sensitive tuning of lattice thermal conductivity of graphenelike borophene by fluorination and chlorination

Boron-based 2D materials are of current interest.However,graphene-like geometry is unstable for B due to the electron deficiency,which can be stabilized by introducing H,F and Cl.Here,using density functional theory combined with phonon Boltzmann transport equation,we perform systematic studies on how the functionalization changes the lattice thermal conductivity(LTC).We find that when going from hydrogenation to fluorination and chlorination,the LTC along zigzag direction changes from 367.6 to 211.3 and 43.0 W/(rrvK),while the corresponding values in armchair direction are 279.6,198.9,and 41.6 W/(m·K),respectively.These huge differences imply the sensitivity of LTC to functionalization,which can be attributed to the enhanced anharmonicity as revealed by analyzing group velocity,Gruneisen parameter,anharmonic scattering rates,and three-phonon scattering space.

Borophene gas sensor

High-performance gas sensing devices have been extensively studied in industrial production,clinical medicine and environmental monitoring.Among the materials used to fabricate gas sensors,two-dimensional(2D)materials are viewed as favorable candidate sensing materials because of their high surface-to-volume ratios,abundant surface activity,defect sites.However,gas sensors based on the previously reported 2D materials have some disadvantages such as poor air-stability and slow dynamic response.Recently,borophene,as a unique 2D material,has been theoretically predicted to have excellent gas sensing characteristic,especially for nitrogen dioxide(NO_(2)).However,the gas sensing property of borophene has not been still reported experimentally.Here,we report that a chemiresistive sensor device based on borophene shows high sensitivity,fast response,high selectivity,good flexibility and long-time stability.It is found that the sensor has a low experimental detection limit of around 200 ppb,a large detection range from 200 ppb to 100 ppm,and fast response time of 30 s and recovery time of 200 s at room temperature,which are remarkably superior to those of reported 2D materials.The underlying NO_(2) sensing mechanism of borophene is revealed by first-principles calculations.In line with theoretical predication,it has also been confirmed experimentally that the borophene-based sensor has a unique selectivity to NO_(2) compared with other common gases.Furthermore,the sensor also displays superior flexibility and stability under different bending angles.This study shows excellent electronic and sensing characteristic of borophene,which indicates that it has great potential application value in high-performance sensing and detection in the future.

Ultra-low lattice thermal conductivity and promising thermoelectric figure of merit in borophene via chlorination

Monolayer boron-based materials are of current interests due to its polymorphism.Herein,motivated by the recent experimental synthesis of semiconducting hydrogenatedαʹ-borophene and the regulation of the physical properties in layered materials by surface functionalization,we study the thermal and electronic properties ofαʹ-borophene with three different types of gas functional groups(H,F,and Cl)based on first-principles and Boltzmann transport theory.It is found thatαʹ-borophene can be well stabilized by fluorination and chlorination and maintain the semiconductor nature.More interestingly,when hydrogen is replaced with fluorine or chlorine,the lattice thermal conductivity changes from 24.3 to 5.2 or 0.73 W/(m·K)along armchair direction at 300 K,exhibiting a huge reduction by two orders of magnitude.The main reason is the decrease of both phonon group velocities and acoustic phonon relaxation time resulting from the strong phonon mode softening due to the weaken B-B bond strength and heavier atomic mass of fluorine and chlorine.Consequently,the chlorinatedαʹ-borophene exhibits a high thermoelectric figure of merit~2 at 300 K along armchair direction.Our study illustrates the importance of the modulation of transport properties by gas functional groups,which may promote the thermoelectric application of boron-based materials.

Review of borophene and its potential applications

Since two-dimensional boron sheet (borophene) synthesized on Ag substrates in 2015, research on borophene has grown fast in the fields of condensed matter physics, chemistry, material science, and nanotechnology. Due to the unique physical and chemical properties, borophene has various potential applications. In this review, we summarize the progress on borophene with a particular emphasis on the recent advances. First, we introduce the phases of borophene by experimental synthesis and theoretical predictions. Then, the physical and chemical properties, such as mechanical, thermal, electronic, optical and superconducting properties are summarized. We also discuss in detail the utilization of the borophene for wide ranges of potential application among the alkali metal ion batteries, Li-S batteries, hydrogen storage, supercapacitor, sensor and catalytic in hydrogen evolution, oxygen reduction, oxygen evolution, and CO2 electroreduction reaction. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

Insight into the magnetic behavior and magnetocaloric effect of a borophene monolayer

The successful discovery of borophene has opened a new door for the development of 2D materials.Due to its excellent chemical,electronic and thermal properties,borophene has shown considerable potential in supercapacitors,hydrogen storage and batteries.In this paper,the thermodynamic characteristics and magnetocaloric effect of borophene are specifically studied using the Monte Carlo method.We find that there is an opposite impact between the spin quantum number and the crystal field on the magnetization,magnetic susceptibility,specific heat and magnetic entropy of the system.Moreover,increasing the spin quantum number or decreasing the absolute value of the crystal field can improve the relative cooling power,the coercivity(h_(c)),and the remanence(M_(R))and the area of the loop.

Density functional theory investigation on selective adsorption of VOCs on borophene

In the field of volatile organic compounds(VOCs)pollution control,adsorption is one of the major control methods,and effective adsorbents are desired in this technology.In this work,the density functional theory(DFT)calculations are employed to investigate the adsorption of typical VOCs molecules on the two-dimensional material borophenes.The results demonstrate that both structure ofχBorophene;2D material;Volatile organic compounds(VOCs);Selective adsorption;Electronic structure andβ12 borophene can chemically adsorb ethylene and formaldehyde with forming chemical bonds and releasing large energy.However,other VOCs,including ethane,methanol,formic acid,methyl chloride,benzene and toluene,are physically adsorbed with weak interaction.The analysis of density of states(DOS)reveals that the chemical adsorption changes the conductivity of borophenes,while the physical adsorption has no distinct effect on the conductivity.Therefore,bothχ^(3)andβ_(12) borophene are appropriate adsorbents for selective adsorption of ethylene and formaldehyde,and they also have potential in gas sensor applications due to the obvious conductivity change during the adsorption.