2D Monoelements. Группа авторов
(2D) materials are different from traditional three-dimensional (3D) bulk materials because the movement of charges is confined to the atomic layer, resulting in many novel physical phenomena. As the most typical 2D material, monolayer graphene was successfully isolated in 2004 [1], which paves the way to discover and study more families of 2D materials. However, the valence band intersects the conduction band at the Dirac point [2], making graphene a semi-metallic material with no band gap, which limits its applications in electronic and optoelectronic devices.
Afterwards, many researchers hope to find novel 2D materials with a certain band gap. Transition metal dichalcogenides (TMDs) are also a typical class of 2D materials, these materials have tunable band gap ranging from 1.5 to 2.5 eV and some of which are semiconductors with direct band gap [3]. However, TMDs are still not very suitable for optoelectronic applications. Therefore, other groups of 2D materials have aroused interest in research. Among them, the earliest concerned and widely explored 2D material is black phosphorus (BP), which is the most stable allotrope of phosphorus. The band gap of BP can be adjusted in a large range by the number of layers to achieve light absorption from the near infrared to the visible region. BP also has a high carrier mobility of up to 103 cm2 V−1s−1, and a large on-off current ratio of 105 [4]. Therefore, it is a very promising electronic and optoelectronic materials. In addition, unlike other 2D materials, BP possesses crystal orientation-dependent carrier mobility, light absorption, and other properties as well, due to its anisotropic nature [5, 6]. Unfortunately, a fatal disadvantage of BP is its easy degradation, due to the joint influence of oxygen, water, and light [7].
In view of this, the researchers turned their attention to the other group 15 elements. One of the most representatives is antimonene with strong spin-orbit coupling. In 2015, the novel 2D antimonene was first proposed theoretically [8]. The theoretical studies show that when the antimonene is thinned from bulk to monolayer, the band structure will be abrupt, resulting in its conversion from semi-metal to semiconductor, and the 2D form is still extremely stable. Monolayer antimonene is predicted that having a band gap of 2.28 eV [8]. This makes antimonene very interesting for fabrication in field-effect-transistors (FETs) and optoelectronic devices [9, 10]. At the same time, theoretical predictions have found that Group 15 monoelements have higher carrier mobility [11]. Apart from this, 2D antimonene has also achieved many significant results in experiments. Here, we discuss the research on 2D antimonene in recent years, including four parts as below. Section 2.1 gives the predicted structure and electronic band of antimonene based on the theoretically calculations. Section 2.2 summarizes the experimental synthesis of antimonene with different thicknesses and shapes, including mechanical exfoliation, liquid phase exfoliation, epitaxial growth, and some novel technologies [12]. In Section 2.3, we discuss various potential applications of antimonene, such as nonlinear optics, electrocatalysis, energy storage, biomedicine, and magneto-optic storage. Moreover, a perspective of the future 2D antimonene research is provided in Section 2.4.
2.2 Fundamental Characteristics
2.2.1 Structure
By means of density functional theory (DFT) calculations, one can see that honeycomb-structure antimonene monolayers are composed of different allotropic forms, which are shown in Figure 2.1a [11]. In these allotropes, the theoretically and experimentally studied antimonene structures are α and β phases. β-antimonene with a buckled structure is the lowest-energy configuration, while α-antimonene has a puckered structure (Figure 2.1b). From the calculated phonon spectra of these two phases, it is observed that there are no appreciable imaginary phonon modes, indicating that both α- and β-phase antimonene structures are thermodynamically stable (Figure 2.1c). Furthermore, each Sb atom is bonded with three adjacent Sb atoms in one atomic layer, endowing them with octet stability [8]. The buckled honeycomb structure of β-antimonene stabilizes further the layered structure, which turns β phase to be the most stable structure. Due to the weak interlayer interactions, α- and β-antimonene monolayers can be easily exfoliated from their bulk crystals.
Figure 2.1 (a) Top views of the relaxed antimonene monolayer allotropic forms with five typical honeycomb structures (α, β, γ, δ, ε). (b) Calculated average binding energies of antimonene allotropes with different phases (α, β, γ, δ, ε, ζ, η, θ, ι). (c) Phonon band dispersions of α and β phases of antimonene monolayer. Reproduced with permission [11]. Copyright 2016, Wiley-VCH.
2.2.2 Electronic Band Structure
Based on first-principles calculations, bulk antimony is a typical semimetal material, while it is interesting that it will be transformed into a semiconductor when thinned to be a monolayer antimonene. Zhang et al. reported the electronic band structure of β-antimonene, which was calculated by the hybrid functional theory (HSE06) method [8]. As illustrated in Figure 2.2, trilayer and bilayer antimonene are still semi-metallic, where both valence-band tops and conduction-band bottoms cross the Fermi level at several points, causing a band gap of 0 eV in the Brillouin zone. However, for monolayer antimonene, the valence band and conduction band shift respectively down and up, resulting in the formation of a wide band gap of 2.28 eV. The valence band maximum (VBM) locates at K point, while the conduction maximum (CBM) is at Γ point, showing that monolayer antimonene is an indirect semiconductor. Due to the wide and indirect band gap, monolayer antimonene-based optoelectronic devices prefer to respond to the blue and ultraviolet light. After applying a small biaxial tensile strain, antimonene will experience an indirect-to-direct band-gap transition, making it more suitable for the applications of optoelectronic devices. The calculated electron effective masses of monolayer antimonene are
2.3 Experimental Preparation
2.3.1 Mechanical Exfoliation
Mechanical exfoliation is the most common method for preparing monolayer or few-layer 2D materials. In 2004, Novoselov and Geim obtained the first monolayer graphene via this method in human history [1]. Thereafter, this method is widely applied to the preparation of other layered materials. The van der Waals bond between the layers of the layered material is weak, and the binding energy is only 40–70 meV, so the layers and layers are more easily isolated by the external force [13]. The mechanical exfoliation is very suitable for scientific research because of its simple method, and the obtained sample is free from contamination and has a higher crystal quality.
Figure 2.2 HSE06 calculated electronic band structures of trilayer, bilayer, and monolayer antimonene. Dots: Fermi levels.