Borophene is a crystalline atomic monolayer of boron, i.e., it is a two-dimensional allotrope of boron and also known as boron sheet. First predicted by theory in the mid-1990s, [1] different borophene structures were experimentally confirmed in 2015. [2] [3]
Experimentally various atomically thin, crystalline and metallic borophenes were synthesized on clean metal surfaces under ultrahigh-vacuum conditions. [2] [3] 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. [4]
Borophenes exhibit in-plane elasticity and ideal strength. It can be stronger than graphene, and more flexible, in some configurations. [5] Boron nanotubes are also stiffer than graphene, with a higher 2D Young's modulus than any other known carbon and noncarbon nanostructures. [6] 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. [7] As with most 2D materials, borophene is expected to have anisotropic properties. In terms of mechanical properties, v1/6 (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. [8] Furthermore, the modulus is predicted to be relatively invariant to phase as v varies from 1/5 to 1/9. [8] 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. [8] v1/6 borophene is also predicted to have an out-of-plane bending stiffness of 0.39 eV, smaller than any reported 2D material. [8] 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 v1/6 phase. These predicted properties are partially supported by experimental work, where v1/6 borophene was synthesized on a surface reconstructed Ag(111) substrate. [9] 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. [9] 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. [9] 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. [9] The undulated borophene's mechanical properties were studied using first principles calculations and were found to have similar values for the aforementioned mechanical properties. [9] 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. [8] Notably, the Foppl-von Karman number for the v1/6 phase is more than twice that of graphene, indicating that borophenes are flexible atomic layers. [8] Thus, borophenes may have applications such as reinforcing elements for composites and in flexible electronic interconnects, electrodes, and displays. [7] [8]
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. [5]
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). [1] This led to the discovery of a so-called Aufbau principle [10] that predicts the possibility of borophene (boron sheets), [1] boron fullerenes (borospherene) [11] and boron nanotubes. [12] [13] [14]
Additional studies showed that extended, triangular borophene (Figure 1(c)) is metallic and adopts a non-planar, buckled geometry. [15] [16] Further computational studies, initiated by the prediction of a stable B80 boron fullerene, [17] suggested that extended borophene sheets with honeycomb structure and with partially filled hexagonal holes are stable. [18] [19] These borophene structures were predicted to be metallic. The so-called γ sheet (a.k.a. β12 borophene or υ1/6 sheet) is shown in Figure 1(a). [19]
The planarity of boron clusters was first experimentally confirmed by the research team of L.-S. Wang. [20] Later they showed that the structure of 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. [21]
After the synthesis of silicene, multiple groups predicted that borophene could potentially be realized with the support of a metal surface. [22] [23] [24] In particular, the lattice structure of borophene was shown to depend on the metal surface, displaying a disconnect from that in a freestanding state. [25]
In 2015 two research teams succeeded in synthesizing different borophene phases on silver (111) surfaces under ultrahigh-vacuum conditions. [2] [3] Among the three borophene phases synthesized (see Figure 1), the v1/6 sheet, or β12, was shown by an earlier theory to be the ground state on the Ag(111) surface, [25] while the χ3 borophene was previously predicted by Zeng team in 2012. [26] So far, borophenes exist only on substrates; how to transfer them onto a device-compatible substrate is necessary, but remains a challenge. [27]
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. [28] The CVD-borophene layers display an average thickness of 4.2 Å, χ3 crystalline structure, and metallic conductivity. [28]
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 ]
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. [29] Hydrogenation reduces oxidation rates by more than two orders of magnitude after ambient exposure. [30]
Experimental evidence supporting the formation of stacked bilayer and trilayer borophene sheets was first observed in CVD-grown borophene layers. [28] Soon after that, the creation of two-layer borophene was announced in August 2021. [31]
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form.
A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometer range (nanoscale). They are one of the allotropes of carbon.
Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. The name is derived from "graphite" and the suffix -ene, reflecting the fact that the graphite allotrope of carbon contains numerous double bonds.
Phaedon Avouris is a Greek chemical physicist and materials scientist. He is an IBM Fellow and was formerly the group leader for Nanometer Scale Science and Technology at the Thomas J. Watson Research Center in Yorktown Heights, New York.
Marvin Lou Cohen is an American–Canadian theoretical physicist. He is a physics professor at the University of California, Berkeley. Cohen is a leading expert in the field of condensed matter physics. He is widely known for his seminal work on the electronic structure of solids.
Graphene nanoribbons are strips of graphene with width less than 100 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.
Alex K. Zettl is an American experimental physicist, educator, and inventor.
Rodney S. "Rod" Ruoff is an American physical chemist and nanoscience researcher. He is one of the world experts on carbon materials including carbon nanostructures such as fullerenes, nanotubes, graphene, diamond, and has had pioneering discoveries on such materials and others. Ruoff received his B.S. in chemistry from the University of Texas at Austin (1981) and his Ph.D. in chemical physics at the University of Illinois-Urbana (1988). After a Fulbright Fellowship at the MPI fuer Stroemungsforschung in Goettingen, Germany (1989) and postdoctoral work at the IBM T. J. Watson Research Center (1990–91), Ruoff became a staff scientist in the Molecular Physics Laboratory at SRI International (1991–1996). He is currently UNIST Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST), and the director of the Center for Multidimensional Carbon Materials, an Institute for Basic Science Center located at UNIST.
Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene. Contrary to graphene, silicene is not flat, but has a periodically buckled topology; the coupling between layers in silicene is much stronger than in multilayered graphene; and the oxidized form of silicene, 2D silica, has a very different chemical structure from graphene oxide.
Lai-Sheng Wang is an experimental physical chemist currently serving as the Chair of the Chemistry Department at Brown University. Wang is known for his work on atomic gold pyramids and planar boron clusters.
Potential graphene applications include lightweight, thin, and flexible electric/photonics circuits, solar cells, and various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene. Single-layer materials that are compounds of two or more elements have -ane or -ide suffixes. 2D materials can generally be categorized as either 2D allotropes of various elements or as compounds.
A two-dimensional semiconductor is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms. This coupling could enable applications. One research focus is on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor.
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Two-dimensional silica (2D silica) is a layered polymorph of silicon dioxide. Two varieties of 2D silica, both of hexagonal crystal symmetry, have been grown so far on various metal substrates. One is based on SiO4 tetrahedra, which are covalently bonded to the substrate. The second comprises graphene-like fully saturated sheets, which interact with the substrate via weak van der Waals bonds. One sheet of the second 2D silica variety is also called hexagonal bilayer silica (HBS); it can have either ordered or disordered (amorphous) structure.
Graphene-Boron Nitride nanohybrid materials are a class of compounds created from graphene and boron nitride nanosheets. Graphene and boron nitride both contain intrinsic thermally conductive and electrically insulative properties. The combination of these two compounds may be useful to advance the development and understanding of electronics.
Boron nitride nanosheet is a two-dimensional crystalline form of the hexagonal boron nitride (h-BN), which has a thickness of one to few atomic layers. It is similar in geometry as well as physical and thermal properties to its all-carbon analog graphene, but has very different chemical and electronic properties – contrary to the black and highly conducting graphene, BN nanosheets are electrical insulators with a band gap of ~5.9 eV, and therefore appear white in color.
A graphene morphology is any of the structures related to, and formed from, single sheets of graphene. 'Graphene' is typically used to refer to the crystalline monolayer of the naturally occurring material graphite. Due to quantum confinement of electrons within the material at these low dimensions, small differences in graphene morphology can greatly impact the physical and chemical properties of these materials. Commonly studied graphene morphologies include the monolayer sheets, bilayer sheets, graphene nanoribbons and other 3D structures formed from stacking of the monolayer sheets.
Two dimensional hexagonal boron nitride is a material of comparable structure to graphene with potential applications in e.g. photonics., fuel cells and as a substrate for two-dimensional heterostructures. 2D h-BN is isostructural to graphene, but where graphene is conductive, 2D h-BN is a wide-gap insulator.
David Tománek (born July 1954) is a U.S.-Swiss physicist of Czech origin and researcher in nanoscience and nanotechnology. He is Emeritus Professor of Physics at Michigan State University. He is known for predicting the structure and calculating properties of surfaces, atomic clusters including the C60 buckminsterfullerene, nanotubes, nanowires and nanohelices, graphene, and two-dimensional materials including phosphorene.