2D Boron: Boraphene, Borophene, Boronene

For many years it was believed that boron can only form 3D allotropes due to its intrinsic electron deficiency. The last 20 years, however, have seen a true revolution in the knowledge and understanding of boron chemistry with synthesis of 2D forms of boron recently being made possible. The key works that contributed to this discovery were influenced by investigations in boron allotrope in various dimensions, including 3D (bulk), 0D (cluster), 1D (nanotube), and 2D (sheet). This chapter is a brief historical overview.

The structural chemistry of Boron has always been very challenging. The recent discovery of two-dimensional boron phases (borophenes) on the metal templates has been one of the remarkable developments in the chemistry of boron. However, borophene is bound to have different structural variations, depending upon the synthetic techniques. Though always consisting of triangular networks, the nonplanar distortions and distribution of hexagonal holes to varying hole densities lead to dramatic modifications in structural preferences and stability. In this chapter, the recent theoretical advancements, which led fundamental contributions in understanding the structural chemistry of two- dimensional boron phases are discussed. Drawing relationships to the planar boron clusters and extended boron compounds, many structural possibilities are predicted, which eventually influenced several experimental efforts to synthesize these phases. Electron counting strategies are introduced based upon benzenoid aromaticity and MgB₂, to understand their electronic structure and stability. With the help of molecular dynamics simulations on different metal surfaces and the electron density mapping, probable atomic arrangements and growth mechanisms of the recently synthesized borophenes phases are arrived at. These structural variations in borophene phases are reminiscent of the structural chemistry of 3D boron allotropes, where the presence of fractional occupancies and crystallization procedures bring in significant structural polymorphism.

The complexity of the chemical bonds in boron results in the polymorphism of borophene. To date, a variety of borophene polymorphs have been predicted. Because of its high chemical reactivity, boron reacts easily with other elements. Therefore, the most suitable method for the synthesis of borophene is molecular beam epitaxy, which is performed in an ultrahigh vacuum. In addition, some catalytic metal substrates can assist the growth process that leads to the experimental synthesis of borophene. In this chapter, we will review the synthesis processes of the different borophene polymorphs.

In the previous chapters, we have seen that borophene has been predicted to form numerous polymorphs, and some of these polymorphs have been experimentally synthesized by molecular beam epitaxy. Theoretically, borophenes are expected to host a rich variety of physical and chemical properties and a study of their electronic structures is thus highly desirable. In this chapter, we will review recent studies on the electronic structures of borophenes, with particular emphasis on experimental works. We will see that borophene hosts various exotic properties, including massless Dirac fermions, Dirac nodal line fermions, and superconductivity. These novel properties make borophene a promising material for use in future quantum devices.

Two-dimensional (2D) materials have great potential in several applications such as batteries, catalysts, and electronic devices because of their unique properties, such as large surface area and novel electronic states (Butler et al. ACS Nano. 7(4):2898–926 (2013); Osada and Sasaki. Adv Mater. 24(2):210–28 (2012); Deng et al. Nat Nanotechnol. 11(3):218–30 (2016)). Among these 2D materials, boron-related materials exhibit polymorphisms (Zhang et al. Chem Soc Rev. 46(22):6746–63 (2017); Kondo. Sci Technol Adv Mater. 18(1):780–804 (2017); Jiao et al. Angew Chemie Int Ed. 55(35):10292–5 (2016)), which are unique characteristics differentiating them from 2D materials—that is, there are a wide variety of stable 2D phases owing to the ability to form multicenter bonding configurations of boron (Oganov et al. J Superhard Mater. 31(5):285–291 (2009)). Single monoatomic 2D boron (borophene) layers have been fabricated on solid surfaces with several different stable structures (Mannix et al. Nat Rev Chem. 1:0014 (2017); Xie et al. Adv Mater. 1900392:1–13 (2019)), which is consistent with theoretical predictions regarding polymorphs of borophene (Boustani. Surf Sci. 370(2–3):355–63 (1997); Penev et al. Nano Lett. 12(5):2441–5 (2012); Wu. ACS Nano. 6(8):7443–53 (2012)). Chemically modified borophene should also exhibit polymorphisms owing to these characteristics. Several stable structures are predicted for hydrogenated borophene (borophane) (Jiao et al. Angew Chemie Int Ed. 55(35):10292–5 (2016)). Chemically modified borophene can thus be regarded as a material with potential to exhibit several intriguing functionalities, physical properties, and chemical properties in a wide variety of applications. We note that a wide variety of chemically modified borophenes could also be used as building blocks from the viewpoint of large-scale material production. Indeed, combining 2D materials through layer stacking in a controlled manner has already been focused on and is reported to produce several novel functionalities including superconductivity in the form of new three-dimensional (3D) layered materials (van der Waals heterostructures) (Geim and Grigorieva. Nature. 499(7459):419–25 (2013)). In this paper, both theoretically predicted results and experimentally realized results of chemically modified borophene are reviewed.

Bulk boron is reviewed with description of physical and chemical properties. Varieties of the reported boron crystals, such as α-rhombohedral, β-rhombohedral, α-tetragonal, and β-tetragonal, and γ-orthorhombic, are described. A new class of the boron quasicrystal is also introduced. Optical, electrical, and magnetic properties of the well-known boron solid, especially β-rhombohedral boron, are described with the data.

This chapter is composed of the English translation with the additional information from Chap. 1, Sections 1 and 2, in the Japanese book, Fundamentals and Applications of Boron, Borides and Related Materials (CMC Publishing Co., LTD. 2008), written by the same authors.