Abstract
The search for free-standing 2D materials has been one of the most important subjects in the field of studies on 2D materials and their applications. Recently, a free-standing monolayer of hydrogenated boron (HB) sheet has been synthesized by hydrogenation of borophene. The HB sheet is also called borophane, and its application is actively studied in many aspects. Here, we review recent studies on the electronic structures of polymorphic sheets of borophane. A hydrogenated boron sheet with a hexagonal boron frame was shown to have a semimetallic electronic structure by experimental and theoretical analyses. A tight-binding model that reproduces the electronic structure was given and it allows easy estimation of the properties of the material. Hydrogenated boron sheets with more complicated nonsymmorphic boron frames were also analyzed. Using the symmetry restrictions from the nonsymmorphic symmetry and the filling factor of hydrogenated boron sheets, the existence of a Dirac nodal line was suggested. These studies provide basic insights for research on and device applications of hydrogenated boron sheets.
Highlights
It is known that the linear dispersive electronic band of graphene, which is called Dirac electron, and the topological properties of the wave function around it play an important role in the unique physical properties of graphene
We have reviewed the electronic band structures of (5–7)-α-hydrogenated boron (HB) sheets that are obtained from TmAlB4
HB sheets have recently been synthesized by the proton ion-exchange reaction and have been attracting attention as a new 2D material in both experimental and theoretical studies
Summary
Two-dimensional (2D) materials have attracted much interest due to their unique physical properties and potential applications in a variety of quantum devices [1–5]. As an analog of graphene, honeycomb-lattice materials with other atoms (Xenes) such as silicene [7–11], phosphorene [12–16], germanene [11,17–19], arsenene [14,20,21], antimonene [14,20,22], and bismuthene [14,23,24], have been actively studied. These materials are expected to be a new platform to explore unique physical properties qualitatively different from those of graphene, taking advantage of the difference in model parameters such as the strength of spin–orbit interaction. Many nodal line materials have been proposed in 3D crystals [27–32], and Cu2 Si [5] and CuSe [33] are known as 2D Dirac nodal line materials While these 2D materials possess attractive properties, most of them are restricted to preparation on substrates. A monolayer of boron sheet has been synthesized by hydrogenation of a borophene frame in a MgB2 crystal [35,36], which is one of the Polymorphic Layers of Borophane
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