Borophene is a proposed crystalline allotrope of boron. One unit consists of 36 atoms arranged in a 2-dimensional sheet with a hexagonal hole in the middle. Another form made in 2015 is a buckled two dimensional sheet on silver.
Computational studies suggested that extended borophene sheets with partially filled hexagonal holes are stable. Global minimum searches for B−
36 lead to a quasiplanar structure with a central hexagonal hole. Borophene is predicted to be fully metallic.
Borophene is analogous to graphene in that it is expected to form extended sheets. The latter is a semi-metal, implying that borophene may be a better conductor. The boron-boron bond is also nearly as strong as graphene’s carbon-carbon bond. At the atomic-cluster scale, pure boron forms simple planar molecules and cage-like fullerenes.
Boron is adjacent to carbon in the periodic table and has similar valence orbitals. Unlike carbon, boron cannot form a honeycomb hexagonal framework (like graphene) because of its electron deficiency.
In 2014 a research team at Brown University, led by Lai-Sheng Wang, showed that the structure of B
36 was not only possible but highly stable. Photoelectron spectroscopy revealed a relatively simple spectrum, suggesting a symmetric cluster. Neutral B
36 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.
In 2015 a research team synthesized borophene on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal. Notably, the atomic structure of borophene on Ag(111) was revealed to be the same as the one predicted by an earlier theory on the same substrate.
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