Electronic Structures of Polymorphic Layers of Borophane

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.

Prediction of a Cyclic Hydrogenated Boron Molecule as a Promising Building Block for Borophane

“…The α-HB sheet in Figure 4 a,b was predicted by topological band theory and confirmed to be thermodynamically and kinetically stable by first-principles calculations [ 15 , 17 ]. It is noteworthy that its kinetic stability was supported by the positive vibrational energy of the phonon modes.…”

“…An example is the recent synthesis of sheets of hydrogen boride (HB) or borophane [ 11 , 12 , 13 ], which show intriguing functions such as hydrogen storage [ 14 ]. Boron and hydrogen atoms create a three-center two-electron (3c-2e) bond with unique chemical characteristics that allow us to design new 2D HB structures [ 15 , 16 , 17 ].…”

“…The molecule has a 3c-2e bond at the center and two five-membered rings on the sides. The nomenclature of the molecule is based on references [ 19 , 20 ]; its atomic structure can be regarded as a building block of a topological borophane layer, discovered recently [ 12 , 13 , 15 , 17 ]. This research shows intriguing relationships between the planar molecules and atomic sheets of the hydrogenated boron compound.…”

Electronic Topological Transition of 2D Boron by the Ion Exchange Reaction

“…Such 2D Dirac materials have been reported to show exotic optical and electronic properties. − In recent times, novel monolayer materials have been pursued and atomic sheets of boron, so-called borophene, have been found as one of the family of 2D Dirac materials. − However, the borophene layers were synthesized only on crystal surfaces in ultrahigh vacuum. − Nowadays, chemical terminations, such as hydrogenations, have been examined to prepare 2D boron layers that can survive under ambient condition. A method of the ion exchange has been successful to extract sheets of hydrogen boride (HB, borophane or hydrogenated borophene) from crystals of metal borides that are composed of boron layers and metal ions. ,− Borophane is formed by processes of liquid exfoliation of layers in the mother crystal, deintercalation of metallic cation, and hydrogenation. Since there are varieties of 2D polymorphs of boron in metal borides, this chemical approach has an advantage to prepare borophane with various atomic structures.…”

Polymorph Separation by Ordered Patterning

“…As the crystalline structure has a fundamental role in determining the chemical and physical properties of a given material, distinct polymorphs can dramatically differ in their properties [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. This effect severely impacts the technological application of materials.…”