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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 and , indicating that it owns a high carrier mobility.