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, different borophene structures were experimentally confirmed in 2015.
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. In terms of mechanical properties, borophenes exhibit in-plane elasticity and ideal strength that are comparable to those of graphene, but they are extremely flexible against bending; moreover, borophenes undergo novel structural phase transition under in-plane tensile loading due to the fluxional nature of their multi-center in-plane bonding.
Computational studies by I. Boustani and A. Quandt showed that small boron clusters do not adopt icosahedral geometries likes boranes, instead they turn out to be quasi-planar (see Figure 2). This led to the discovery of a so-called Aufbau principle which predicts the existence of borophene (boron sheets), boron fullerenes (borospherene) and boron nanotubes.
Additional studies showed that extended, triangular borophene (Figure 1(c)) 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 also predicted to be metallic. The so-called γ sheet (a.k.a. β12 borophene or υ1/6 sheet) is shown in Figure 1(a).
The planarity of boron clusters was first experimentally confirmed by the research team of L.-S. Wang.
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 it can be viewed 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 is shown to depend on the metal surface, displaying a disconnect from that in freestanding state. Finally, 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 v1/6 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. So far, borophenes exist only on substrates; how to transfer them onto a device-compatible substrate is necessary for further fundamental and applied study 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, such as predicted by theory before and shown in Figure 1. Scanning tunneling spectroscopy indicates that the borophenes are indeed metallic. This is in contrast to bulk boron allotropes, which are semiconducting and marked by an atomic structure based on B12 icosahedra.
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