Thickness- and strain-tunable electronic structures of two-dimensional Bi2O2Se

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Two-dimensional (2D) Bi2O2Se that exhibits high carrier mobility, moderate band gap, and good stability is a promising candidate of 2D semiconductors for next-generation computing technologies. However, the electronic properties of ultrathin Bi2O2Se films are poorly understood, especially the thickness and strain tunability of their electronic structures. In this work, we performed a systematic investigation of electronic structures of 2D Bi2O2Se using HSE06 method with high accuracy. Our results show that Bi2O2Se multi-layers display a weak quantum size effect. The origin of this unusual phenomenon is elucidated by the crystal orbital Hamilton population (COHP) analysis. Imposed with biaxial strain, band gaps of monolayer and bilayer can be modulated in a large energy range, e.g., direct band gap of monolayer from 3.08 eV to 1.17 eV which strides over the whole visible light region. Besides, the band gap of bilayer closes up at large tensile strain of ~8%, indicating a semiconductor-metal transition. Further analysis of valence band and conduction band edges shows that high tunability of band gap originates from the sensitivity of band edge with strain, especially conduction band minimum. With the strain from compressive to tensile, Bi-Se bond length gradually increase with the stretch of lattice, which reduces the Bi-Se bond strength and thus lowers the energies of valence band and conduction band edge states due to their antibonding character. Highly tunable electronic properties via changing thickness or adding strain impart Bi2O2Se more possibilities for its application in electronic and optoelectronic devices.