Two‐Dimensional Boron Crystals: Structural Stability, Tunable Properties, Fabrications and Applications

Abstract

(1 of 22)allotropes of boron have been discovered up to now. Four of them are thermodynamically stable, including α-rhombohedral, [6] β-rhombohedral, [7] γ-orthorhombic, [1f,2d,8] and β-tetragonal boron crystals. [1d,9] These materials are often called boron-icosahedral cluster solids (B-ICSs), [5b,10] consisting of icosahedral closo-cluster B 12 to link with each other or with other clusters to form boron allotropes and B-rich compounds, such as various types of BN, [11] B 12 As 2 , [12] B 12 P 2 , [13] B 12 O 2 , [14] YB 66 , [15] AlB 12 , [16] and boron carbide. [17] Apart from three-dimensional (3D) boron icosahedral solids, 2D boron also exhibits many unique structures owning to its electron deficiency, different from other well-known 2D materials. Generally, 2D boron crystals can be classified into three categories: 1) graphene-like atomically monolayered boron sheets, [18] 2) 2D boron structures with thickness of a single or a few unit cells, [19] and 3) new kinds of 2D boron structures reported recently. [20] For the first category, "borophene" was coined to refer to a general class of atomically thin boron sheets, [21] unlike other structures with the suffix ene, where each name always corresponds to a certain structure. For example, graphene, as the most attractive 2D crystals, is a single monolayer of carbon atoms, while phosphorene (monolayered black phosphorus) exists with puckered layer structures in nature. Both graphene and phosphorene have corresponding bulk counterparts, allowing for facile access to 2D-layered van der Waals structures through mechanical exfoliations. Other elemental 2D materials, such as silicene, [22] germanene, [23] stanene, [24] arsenene, [25] and antimonene, [26] actually do not have layered bulk counterparts. Similarly, 2D single-layered boron could not be produced by exfoliating from its bulk materials because there is no layered bulk boron. As a result, it is suggested that 2D boron sheets can be synthesized via chemical vapor deposition, thermal evaporation deposition, or molecular beam epitaxy. In the past decade, extensive theoretical efforts have been paid to investigate possible boron sheets, many of which have been predicted to have potential applications in electronic devices, [18c,d] photoelectric devices, [27] superconductivity, [20a,28] field-emission (FE) materials, [29] hydrogen storage media, [30] and lithium-ion batteries. [31] However, the fabrication of 2D boron crystals is a great challenge. Until very recently, three types of monolayered Boron, as a unique element nearest to carbon in the periodic table, has been predicted to form many distinctive two-dimensional (2D) structures that significantly differ from other well-studied 2D materials, owning to its exceptional ability to form strong covalent two-center-two-electron bonds as well as stable electron-deficient multi-center-two-electron bonds. Until recently, the successful syntheses of atomically thin crystalline 2D boron sheets (i.e., borophenes) provoked growing passion in 2D boron crysta...

A Novel Top‐Down Synthesis of Ultrathin 2D Boron Nanosheets for Multimodal Imaging‐Guided Cancer Therapy

“…In general, these exfoliation methods require the presence of layered structure in the bulk precursors, by which the weak Van der Waal's force among layers can be broken to form NSs. Unlike bulk graphite, BP and others with naturally layered structures, B exhibits more complex and diverse bulk structures due to rich bonding configurations among B atoms, thus making it highly challenging to prepare ultrathin B NSs by direct exfoliation . On the other hand, various etching technologies (e.g., plasma etching, thermal oxidation etching, and acid etching) have been developed to produce porous monolayer structures by breaking strong covalent bond in a layer of NSs, which have recently been used as high‐performance energy‐storage and hydrogen evolution reaction materials .…”

Realizing an Epitaxial Decorated Stanene with an Insulating Bandgap

“…The great success of graphene research has stimulated tremendous effort to search for other innovative 2D materials such as silicene, germanene, stanene, phosphorene, borophene, and transition metal dichalcogenides . The prosperous and diverse 2D materials family constitutes an arsenal for exploring new‐concept electronic devices as well as fundamental mysteries of condensed matter .…”

Freestanding Borophene and Its Hybrids

“…Although borophene exhibits these exciting properties, as of now experimental realization of borophene has been achieved via only atomic layer deposition and molecular beam epitaxy (MBE) on the Ag [111] substrate under ultrahigh vacuum condition . Recent computational studies proposed that boron in its bulk is not naturally layered and borophene sheet in its pure form (without any holes or protrusions) has been deemed as metastable (higher energy than its bulk) . In this context, the synthesis of freestanding borophene has not even been thought of to date.…”