Borophene is a crystalline atomic monolayer of boron, i.e., it is a two-dimensionalallotrope of boron and also known as boron sheet. First predicted by theory in the mid-1990s, different borophene structures were experimentally confirmed in 2015.
Properties
Experimentally various atomically-thin, crystalline and metallic borophenes were synthesized on clean metal surfaces under ultrahigh-vacuum conditions. Their 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. Borophenes exhibit in-plane elasticity and ideal strength. They can be stronger than graphene, and more flexible, in some configurations. For example, boron nanotubes have a higher 2D Young's modulus than any other known carbon and noncarbon nanostructures. Borophenes undergo novel structural phase transition under in-plane tensile loading due to the fluxional nature of their multi-center in-plane bonding. Borophene has potential as an anode material for batteries due to high theoretical specific capacities, electronic conductivity and ion transportproperties. 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.
History
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. This led to the discovery of a so-called Aufbau principle that predicts the possibility of borophene, boron fullerenes and boron nanotubes. Additional studies showed that extended, triangular borophene is metallic and adopts a non-planar, buckled geometry. Further computational studies, initiated by the prediction of a stable B80 boron fullerene, suggested that extended borophene sheets with honeycomb structure and with partially filled hexagonal holes are stable. These borophene structures were predicted to be metallic. The so-called γ sheet is shown in Figure 1. The planarity of boron clusters was first experimentally confirmed by the research team of L.-S. Wang. Later they showed that the structure of 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. After the synthesis of silicene, multiple groups predicted that borophene could potentially be realized with the support of a metal surface. In particular, the lattice structure of borophene was shown to depend on the metal surface, displaying a disconnect from that in a freestanding state. In 2015 two research teams succeeded in synthesizing different borophene phases on silver surfaces under ultrahigh-vacuum conditions. Among the three borophene phases synthesized, the v1/6 sheet, or β12, was shown by an earlier theory to be the ground state on the Ag surface, while the χ3 borophene was previously predicted by Zeng team in 2012. So far, borophenes exist only on substrates; how to transfer them onto a device-compatible substrate is necessary, but remains a challenge. 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.
