borophene

Experimentally various atomically thin, crystalline and metallic borophenes were synthesized on clean metal surfaces under ultrahigh-vacuum conditions. Its atomic structure consists of mixed triangular and hexagonal motifs, such as shown in Figure 1. The atomic structure is a consequence of an interplay between two-center and multi-center in-plane bonding, which is typical for electron deficient elements like boron.{{cite book |last1=Pauling |first1=Linus |title=The nature of the chemical bond |url=https://archive.org/details/natureofchemical00paul |url-access=registration |publisher=Cornell University Press |isbn=0-8014-0333-2 |edition=3rd|year=1960 }}

Borophenes exhibit in-plane elasticity and ideal strength. It can be stronger than graphene, and more flexible, in some configurations.{{Cite web|url=https://www.technologyreview.com/s/613267/borophene-the-new-2d-material-taking-chemistry-by-storm/|title=Sorry, graphene—borophene is the new wonder material that's got everyone excited|last=arXiv|first=Emerging Technology from the|website=MIT Technology Review|language=en-US|access-date=2019-08-02}} Boron nanotubes are also stiffer than graphene, with a higher 2D Young's modulus than any other known carbon and noncarbon nanostructures.{{cite journal |last1=Kochaev |first1=A. |title=Elastic properties of noncarbon nanotubes as compared to carbon nanotubes |journal=Physical Review B |date=11 October 2017 |volume=96 |issue=15 |pages=155428 |doi=10.1103/PhysRevB.96.155428|bibcode=2017PhRvB..96o5428K }}

Since borophene is theoretically predicted to have metallic electronic structures and boron is lighter than most elements, borophene is expected to be the lightest experimentally realizable 2D metal.{{cite journal |last1=Mannix |first1=A.J. |last2=Zhang |first2=Z. |last3=Guisinger |first3=N.P. |last4=Yakobson |first4=B.I. |last5=Hersam |first5=M.C.|title=Borophene as a prototype for synthetic 2D materials development |journal=Nature Nanotechnology |date=6 June 2018 |volume=13 |issue=6 |pages=444–450 |doi=10.1038/s41565-018-0157-4|pmid=29875501 |bibcode=2018NatNa..13..444M |osti=1482112 |s2cid=205567992 }} As with most 2D materials, borophene is expected to have anisotropic properties. In terms of mechanical properties, v_1/6 (where the fraction denotes the hollow hexagon density) borophene is theoretically predicted to have an in-plane modulus of up to 210 N/m, Poisson's ratio of up to 0.17.{{cite journal |last1=Zhang |first1=Z.|last2=Yang |first2=Yang. |last3=Penev |first3=E.S. |last4=Yakobson |first4=B.I. |title=Elasticity, Flexibility, and Ideal Strength of Borophenes |journal=Advanced Functional Materials |date=11 January 2017 |volume=27 |issue=9 |pages=1605059 |doi=10.1002/adfm.201605059|arxiv=1609.07533 |s2cid=119199830}} Furthermore, the modulus is predicted to be relatively invariant to phase as v varies from 1/5 to 1/9. This is a particularly salient point because borophenes undergo novel structural phase transition under in-plane tensile loading instead of fracturing due to the fluxional nature of their multi-center in-plane bonding. v_1/6 borophene is also predicted to have an out-of-plane bending stiffness of 0.39 eV, smaller than any reported 2D material. The ratio of the modulus to the stiffness (a.k.a. the Foppl–von Karman number per unit area) which effectively characterizes a material's flexibility is around 570 nm⁻² for the v_1/6 phase.

These predicted properties are partially supported by experimental work, where v_1/6 borophene was synthesized on a surface reconstructed Ag(111) substrate.{{cite journal |last1=Zhang |first1=Z. |last2=Mannix |first2=A.J. |last3=Hu|first3=Z. |last4=Kiraly |first4=B. |last5=Hersam |first5=M.C.|last6=Yakobson |first6=B.I.|title=Substrate-Induced Nanoscale Undulations of Borophene on Silver |journal=Nano Letters |date=22 September 2016 |volume=16 |issue=10 |pages=6622–6627 |doi=10.1021/acs.nanolett.6b03349|pmid=27657852 |bibcode=2016NanoL..16.6622Z }} Instead of growing as flat, planar borophene sheets as expected for flat Ag(111) substrates, the borophene took on an undulating configuration closely following the protruding rows arising from the surface reconstructed Ag(111) substrate. Ideal flexible electronics require the ability to be stressed, compressed, and even twisted into a wide array of geometries; however, most 2D materials reported to date are unable to meet all of these criteria since they are stiff against in-plane deformation. Undulated borophene is a promising material for flexible electronics as undulated 2D materials adhered to elastomeric substrates should remain easy to bend and afford large in-plane deformations. The undulated borophene's mechanical properties were studied using first principles calculations and were found to have similar values for the aforementioned mechanical properties. Comparing these values to graphene, the prototypical 2D material, the modulus and bending stiffness of borophene is lower while the Poisson's ratio is similar. Notably, the Foppl-von Karman number for the v_1/6 phase is more than twice that of graphene, indicating that borophenes are flexible atomic layers. Thus, borophenes may have applications such as reinforcing elements for composites and in flexible electronic interconnects, electrodes, and displays.

Borophene also has potential as an anode material for batteries due to high theoretical specific capacities, electronic conductivity, and ion transport properties. Hydrogen easily adsorbs to borophene, offers potential for hydrogen storage – over 15% of its weight. Borophene can catalyze the breakdown of molecular hydrogen into hydrogen ions, and reduce water.

File:Borophene.png

Computational studies by I. Boustani and A. Quandt showed that small boron clusters do not adopt icosahedral geometries like boranes, instead they turn out to be quasi-planar (see Figure 2). This led to the discovery of a so-called Aufbau principle{{cite journal |last1=Boustani |first1=Ihsan |title=Systematic ab initio investigation of bare boron clusters: Determination of the geometry and electronic structures of B_n (n=2–14)|journal=Physical Review B |date=15 June 1997 |volume=55 |issue=24 |pages=16426–16438 |doi=10.1103/PhysRevB.55.16426|bibcode=1997PhRvB..5516426B }} that predicts the possibility of borophene (boron sheets), boron fullerenes (borospherene){{cite journal |last1=Boustani |first1=Ihsan |title=New Convex and Spherical Structures of Bare Boron Clusters |journal=Journal of Solid State Chemistry |date=October 1997 |volume=133 |issue=1 |pages=182–189 |doi=10.1006/jssc.1997.7424|bibcode=1997JSSCh.133..182B }} and boron nanotubes.{{cite journal |last1=Boustani |first1=I |last2=Quandt |first2=A |title=Nanotubules of bare boron clusters: Ab initio and density functional study |journal=Europhysics Letters |date=1 September 1997 |volume=39 |issue=5 |pages=527–532 |doi=10.1209/epl/i1997-00388-9|bibcode=1997EL.....39..527B |s2cid=250905974 }}{{cite journal |last1=Gindulytė |first1=Asta |last2=Lipscomb |first2=William N. |last3=Massa |first3=Lou |title=Proposed Boron Nanotubes |journal=Inorganic Chemistry |date=December 1998 |volume=37 |issue=25 |pages=6544–6545 |doi=10.1021/ic980559o|pmid=11670779 }}{{cite journal |last1=Quandt |first1=Alexander |last2=Boustani |first2=Ihsan |title=Boron Nanotubes |journal=ChemPhysChem |date=14 October 2005 |volume=6 |issue=10 |pages=2001–2008 |doi=10.1002/cphc.200500205|pmid=16208735 }}

Additional studies showed that extended, triangular borophene (Figure 1(c)) is metallic and adopts a non-planar, buckled geometry.{{cite journal |last1=Boustani |first1=Ihsan |last2=Quandt |first2=Alexander |last3=Hernández |first3=Eduardo |last4=Rubio |first4=Angel |title=New boron based nanostructured materials |journal=The Journal of Chemical Physics |date=8 February 1999 |volume=110 |issue=6 |pages=3176–3185 |doi=10.1063/1.477976|bibcode=1999JChPh.110.3176B |doi-access=free }}{{cite journal |last1=Kunstmann |first1=Jens |last2=Quandt |first2=Alexander |title=Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties |journal=Physical Review B |date=12 July 2006 |volume=74 |issue=3 |pages=035413 |doi=10.1103/PhysRevB.74.035413|arxiv=cond-mat/0509455 |bibcode=2006PhRvB..74c5413K |s2cid=73631941 }} Further computational studies, initiated by the prediction of a stable B₈₀ boron fullerene,{{cite journal |last1=Gonzalez Szwacki |first1=Nevill |last2=Sadrzadeh |first2=Arta |last3=Yakobson |first3=Boris I. |title= B80 Fullerene: An Ab Initio Prediction of Geometry, Stability, and Electronic Structure |journal=Physical Review Letters |date=20 April 2007 |volume=98 |issue=16 |doi=10.1103/PhysRevLett.98.166804 |pmid=17501448 |page=166804 |bibcode=2007PhRvL..98p6804G}} suggested that extended borophene sheets with honeycomb structure and with partially filled hexagonal holes are stable.{{Cite journal | doi = 10.1103/PhysRevLett.99.115501 | title=Novel Precursors for Boron Nanotubes: The Competition of Two-Center and Three-Center Bonding in Boron Sheets | journal= Physical Review Letters | year=2007 |author1=Tang, Hui |author2=Ismail-Beigi, Sohrab |name-list-style=amp |arxiv = 0710.0593 |bibcode = 2007PhRvL..99k5501T | volume=99 | issue=11 | pmid=17930448 | page=115501| s2cid=30421181 }}{{cite journal |last1=Özdoğan |first1=C. |last2=Mukhopadhyay |first2=S. |last3=Hayami |first3=W. |last4=Güvenç |first4=Z. B. |last5=Pandey |first5=R. |last6=Boustani |first6=I. |title=The Unusually Stable B100 Fullerene, Structural Transitions in Boron Nanostructures, and a Comparative Study of α- and γ-Boron and Sheets|journal=The Journal of Physical Chemistry C |date=18 March 2010 |volume=114 |issue=10 |pages=4362–4375 |doi=10.1021/jp911641u}} These borophene structures were predicted to be metallic. The so-called γ sheet (a.k.a. β₁₂ borophene or υ_1/6 sheet) is shown in Figure 1(a).

The planarity of boron clusters was first experimentally confirmed by the research team of L.-S. Wang.{{cite journal |last1=Zhai |first1=Hua-Jin |last2=Kiran |first2=Boggavarapu |last3=Li |first3=Jun |last4=Wang |first4=Lai-Sheng |title=Hydrocarbon analogues of boron clusters — planarity, aromaticity and antiaromaticity |journal=Nature Materials |date=9 November 2003 |volume=2 |issue=12 |pages=827–833 |doi=10.1038/nmat1012|pmid=14608377 |bibcode=2003NatMa...2..827Z |s2cid=23746395 }} Later they showed that the structure of {{chem|B|36}} (see Figure 2) is the smallest boron cluster to have sixfold symmetry and a perfect hexagonal vacancy, and that it can serve as a potential basis for extended two-dimensional boron sheets.{{Cite journal | doi = 10.1038/ncomms4113|pmid=24445427|bibcode=2014NatCo...5.3113P| title = Planar hexagonal B₃₆ as a potential basis for extended single-atom layer boron sheets| journal = Nature Communications| volume = 5| year = 2014| last1 = Piazza | first1 = Z. A. | last2 = Hu | first2 = H. S. | last3 = Li | first3 = W. L. | last4 = Zhao | first4 = Y. F. | last5 = Li | first5 = J. | last6 = Wang | first6 = L. S. | pages=3113| doi-access = free }}

After the synthesis of silicene, multiple groups predicted that borophene could potentially be realized with the support of a metal surface.{{cite journal |last1=Zhang |first1=L. Z. |last2=Yan |first2=Q. B. |last3=Du |first3=S. X. |last4=Su |first4=G. |last5=Gao |first5=H.-J. |title=Boron Sheet Adsorbed on Metal Surfaces: Structures and Electronic Properties |journal=The Journal of Physical Chemistry C |date=15 August 2012 |volume=116 |issue=34 |pages=18202–18206 |doi=10.1021/jp303616d}}{{cite journal |last1=Liu |first1=Yuanyue |last2=Penev |first2=Evgeni S. |last3=Yakobson |first3=Boris I. |title=Probing the Synthesis of Two-Dimensional Boron by First-Principles Computations |journal=Angewandte Chemie International Edition |date=11 March 2013 |volume=52 |issue=11 |pages=3156–3159 |doi=10.1002/anie.201207972|pmid=23355180 |arxiv=1312.0656 |s2cid=44779429 }}{{cite journal |last1=Liu |first1=Hongsheng |last2=Gao |first2=Junfeng |last3=Zhao |first3=Jijun |title=From Boron Cluster to Two-Dimensional Boron Sheet on Cu(111) Surface: Growth Mechanism and Hole Formation |journal=Scientific Reports |date=18 November 2013 |volume=3 |issue=1 |pages=3238 |doi=10.1038/srep03238|pmid=24241341 |pmc=3831238 |bibcode=2013NatSR...3E3238L }} In particular, the lattice structure of borophene was shown to depend on the metal surface, displaying a disconnect from that in a freestanding state.{{cite journal |last1=Zhang |first1=Z. |last2=Yang |first2=Y. |last3=Gao |first3=G. |last4=Yakobson |first4=B.I. |title=Two-Dimensional Boron Monolayers Mediated by Metal Substrates |journal=Angewandte Chemie International Edition |date=2 September 2015 |volume=54 |issue=44 |pages=13022–13026 |doi=10.1002/anie.201505425 |pmid=26331848|doi-access=free }}

In 2015 two research teams succeeded in synthesizing different borophene phases on silver (111) surfaces under ultrahigh-vacuum conditions. Among the three borophene phases synthesized (see Figure 1), the v_1/6 sheet, or β₁₂, was shown by an earlier theory to be the ground state on the Ag(111) surface, while the χ₃ borophene was previously predicted by Zeng team in 2012.{{cite journal |last1=Wu |first1=Xiaojun |last2=Dai |first2=Jun |last3=Zhao |first3=Yu |last4=Zhu |first4=Zhiwen |last5=Yang |first5=Jinlong |last6=Zeng |first6=Xiao Cheng |title=Two-Dimensional Boron Monolayer Sheets |journal=ACS Nano |date=20 July 2012 |volume=6 |issue=8 |pages=7443–7453 |doi=10.1021/nn302696v|pmid=22816319 }} So far, borophenes exist only on substrates; how to transfer them onto a device-compatible substrate is necessary, but remains a challenge.{{cite journal |last1=Zhang |first1=Z. |last2=Penev |first2=E.S.|last3=Yakobson |first3=B.I. |title=Two-dimensional boron: structures, properties and applications |journal=Chemical Society Reviews |date=31 October 2017 |volume=46 |issue=22 |pages=6746–6763 |doi=10.1039/c7cs00261k|pmid=29085946 }}

Molecular beam epitaxy is the main approach for the growth of high-quality borophene. The high melting point of boron and the growth of borophenes at moderate temperatures posed a significant challenge for the synthesis of borophenes. Utilizing diborane (B2H6) pyrolysis as a pure boron source, a group of researchers reported the growth of atomic-thickness borophene sheets via chemical vapor deposition (CVD) for the first time.{{Cite journal |last1=Mazaheri |first1=Ali |last2=Javadi |first2=Mohammad |last3=Abdi |first3=Yaser |date=2021-02-24 |title=Chemical Vapor Deposition of Two-Dimensional Boron Sheets by Thermal Decomposition of Diborane |url=https://pubs.acs.org/doi/10.1021/acsami.0c22580 |journal=ACS Applied Materials & Interfaces |language=en |volume=13 |issue=7 |pages=8844–8850 |doi=10.1021/acsami.0c22580 |pmid=33565849 |s2cid=231862792 |issn=1944-8244}} The CVD-borophene layers display an average thickness of 4.2 Å, χ3 crystalline structure, and metallic conductivity.

Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters consisting of mixed triangular and hexagonal motives, as previously predicted by theory and shown in Figure 1. Scanning tunneling spectroscopy confirmed that the borophenes are metallic. This is in contrast to bulk boron allotropes, which are semiconducting and marked by an atomic structure based on B₁₂ icosahedra.{{Citation needed|date=August 2019}}

In 2021 researchers announced hydrogenated borophene on a silver substrate, dubbed borophane. The new material was claimed to be far more stable than its component.{{Cite web|last=Lavars|first=Nick|date=2021-04-06|title=2D "borophane" offers new building block for advanced electronics|url=https://newatlas.com/materials/2d-borophane-material-advanced-electronics/|url-status=live|access-date=2021-04-09|website=New Atlas|language=en-US|archive-url=https://web.archive.org/web/20210406050133/https://newatlas.com/materials/2d-borophane-material-advanced-electronics/ |archive-date=April 6, 2021 }} Hydrogenation reduces oxidation rates by more than two orders of magnitude after ambient exposure.{{Cite journal|last1=Li|first1=Qiucheng|last2=Kolluru|first2=Venkata Surya Chaitanya|last3=Rahn|first3=Matthew S.|last4=Schwenker|first4=Eric|last5=Li|first5=Shaowei|last6=Hennig|first6=Richard G.|last7=Darancet|first7=Pierre|last8=Chan|first8=Maria K. Y.|author8-link=Maria Chan|last9=Hersam|first9=Mark C.|date=2021-03-12|title=Synthesis of borophane polymorphs through hydrogenation of borophene|url=https://www.science.org/doi/10.1126/science.abg1874|journal=Science|language=en|volume=371|issue=6534|pages=1143–1148|doi=10.1126/science.abg1874|issn=0036-8075|pmid=33707261|bibcode=2021Sci...371.1143L|s2cid=232199843}}

Experimental evidence supporting the formation of stacked bilayer and trilayer borophene sheets was first observed in CVD-grown borophene layers. Soon after that, the creation of two-layer borophene was announced in August 2021.Liu, Xiaolong; Li, Qiucheng; Ruan, Qiyuan; Rahn, Matthew S.; Yakobson, Boris I.; Hersam, Mark C. "Borophene synthesis beyond the single-atomic-layer limit." Nature Materials (26 August 2021). https://doi.org/10.1038/s41563-021-01084-2

• Allotropes of boron
• Borospherene
• Boron nitride nanosheet

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Category:Allotropes of boron

Category:Monolayers

Category:Substances discovered in the 2010s