Abstract

At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., 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.

Bonding between boron atoms is more complex than in carbon; for example, both two- and three-center B-B bonds can form (1). The interaction between these bonding configurations results in as many as 16 bulk allotropes of boron (1–۳), composed of icosahedral B12 units, small interstitial clusters, and fused supericosahedra. In contrast, small (n < 15) boron clusters form simple covalent, quasiplanar molecules with carbon-like aromatic or anti-aromatic electronic structure (4–۷). Recently, Zhai et al. have shown that B40 clusters form a cage-like fullerene (6), further extending the parallels between boron and carbon cluster chemistry.

To date, experimental investigations of nanostructured boron allotropes are notably sparse, partly owing to the costly and toxic precursors (e.g., diborane) typically used. However, numerous theoretical studies have examined two-dimensional (2D) boron sheets [i.e., borophene (7)]. Although these studies propose various structures, we refer to the general class of 2D boron sheets as borophene. Based upon the quasiplanar B7 cluster (Fig. 1A), Boustani proposed an Aufbau principle (8) to construct nanostructures, including puckered monolayer sheets (analogous to the relation between graphene and the aromatic ring). The stability of these sheets is enhanced by vacancy superstructures (7, 9) or out-of-plane distortions (10, 11). Typically, borophene is predicted to be metallic (7, 9–۱۲) or semimetallic (10) and is expected to exhibit weak binding (13) and anisotropic growth (14) when adsorbed on noble-metal substrates. Early reports of multiwall boron nanotubes suggested a layered structure (15), but their atomic-scale structure remains unresolved. It is therefore unknown whether borophene is experimentally stable and whether the borophene structure would reflect the simplicity of planar boron clusters or the complexity of bulk boron phases.