Experimental Realization of Two-Dimensional Boron Sheets

Abstract: Boron is the fifth element in the periodic table and possesses rich chemistry second only to carbon. A striking feature of boron is that B12 icosahedral cages occur as the building blocks in bulk boron and many boron compounds. This is in contrast to its neighboring element, carbon, which prefers 2D layered structure (graphite) in its bulk form. On the other hand, boron clusters of medium size have been predicted to be planar or quasi-planar, such as B12+ , B13+, B19-, B36, and so on. This is also in contrast to carbon clusters which exhibit various cage structures (fullerenes). Therefore, boron and carbon can be viewed as a set of complementary chemical systems in their bulk and cluster structures. Now, with the boom of graphene, an intriguing question is that whether boron can also form a monoatomic-layer 2D sheet structure? Here, we report the first successful experimental realization of 2D boron sheets. We have revealed two types of boron sheet structures, corresponding to a triangular boron lattice with different arrangements of the hexagonal holes. Moreover, our boron sheets were found to be relatively stable against oxidization, and interacts only weekly with the substrate. The realization of such a long expected 2D boron sheet could open a door toward boron electronics, in analogous to the carbon electronics based on graphene.

• The paper experimentally achieved 2D boron sheets on Ag(111) via molecular beam epitaxy, identifying two distinct phases (S1 and S2) at different annealing temperatures.
• The study uses STM, XPS, and DFT analysis to confirm the sheets’ hexagonal structure, periodicity, metallic behavior, and resistance to oxidation.
• The weak interaction with the Ag(111) substrate suggests potential for detaching freestanding boron sheets, paving the way for microelectronic applications.

Experimental Realization of Two-Dimensional Boron Sheets on Ag(111)

In the field of two-dimensional (2D) materials, the synthesis and characterization of 2D boron sheets have long posed significant challenges, remaining largely theoretical until now. This paper presents a successful experimental realization of 2D boron sheets using a molecular beam epitaxy (MBE) method on a silver (Ag(111)) surface. The authors elucidate two distinct boron sheet structures, denoted as S1 and S2 phases, offering noteworthy details about their stability, formation, and potential applications.

Structural Realization and Characteristics

The synthesis was performed under controlled conditions with boron deposited on Ag(111) substrate. The synthesis showed that upon annealing at specific temperatures, boron formed monolayer islands characterized by hexagonal arrangements with unique vacancy chains. At temperatures around 570 K, the S1 phase emerges, exhibiting a structure akin to the predicted β<sub\>12</sub>-sheet, with characteristic periodicity supported by the commensuration with the Ag(111) lattice. Transition to the S2 phase, identified with features corresponding to the χ<sub\>3</sub>-sheet, occurs upon annealing to 650 K and is stabilized by higher thermal environments up to 800 K.

Atomic arrangements in the two phases were confirmed through both scanning tunneling microscopy (STM) images and first-principles calculations. The S1 phase aligns with a periodicity of 1.5 nm along boron rows, corresponding precisely to its predicted structural model. Similarly, structural and STM analyses verified the characterization of the S2 phase. Both phases showed planar configurations and revealed inhomogeneous charge distributions upon Bader charge analysis, deriving from the precise commensurate lattice match with Ag(111).

Stability and Electronic Properties

An intriguing aspect of these 2D boron sheets is their unexpected stability against oxidation, especially in a monolayer form compared to boron clusters. Ex-situ X-ray photoelectron spectroscopy (XPS) analysis corroborated this, showing minimal oxidation and the resilience of boron atoms within the sheet structure. The metallic nature of these boron sheets is affirmed by both experimental scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. The results indicate significant density of states around the Fermi level, making these structures conductive.

Implications and Future Directions

These findings open new pathways for microelectronics based on boron, paralleling the electronic applications seen with graphene. The weak interaction between 2D boron sheets and Ag(111) suggests potential for detaching the sheets, providing a feasible approach for obtaining freestanding boron sheets in the future. This weak adhesion also permits leveraging substrates like Au(111) and Cu(111) for further exploration of 2D boron allotropes.

While the paper does not identify the boron sheet structures as the most energetically favorable configurations in isolation, it highlights that the interfacial interactions and lattice matching with Ag(111) are critical in determining the observed forms. This indicates that substrate choice significantly impacts resultant boron phases, which can foster new studies exploring boron allotropes with novel properties, potentially including electronic properties like Dirac fermions or unique reconstruction geometries.

Conclusion

This comprehensive study convincingly demonstrates the experimental realization of stable 2D boron sheets. This research not only advances our understanding of boron's allotropy and surface interactions but also unlocks new opportunities for applications in electronic devices and materials science. Future research can benefit from exploring different substrates and investigating the synthesis and properties of these boron structures, further broadening the scope of 2D materials research.