Tunable electronic structures of Janus In2Ge2X3Y3 (X, Y = S, Se and Te) monolayers under external fields

Abstract

Two dimensional (2D) materials with ultrathin atomic thicknesses and intriguing properties have aroused tremendous attention. Inspired by emerging research studies on 2D InGeX₃ (X = S, Se and Te), the novel properties of Janus In₂Ge₂X₃Y₃ monolayers are of interest. Here, we explore their geometric structures, stabilities and electronic properties using first-principles calculations. The In₂Ge₂X₃Y₃ monolayers are dynamically and thermally stable, and the In–X/Y and Ge–X/Y bonds exhibit ionic bonding characteristics. Our calculations show that In₂Ge₂X₃Y₃ monolayers possess direct band gaps of 0 to 1.205 eV at the PBE level (0.638 to 2.323 eV at the HSE level). The sharp band dispersion of electrons induces smaller effective masses than that of holes, giving rise to mobilities as high as 59–86 cm² V⁻¹ s⁻¹. Additionally, upon application of external strain or electric fields, In₂Ge₂X₃Y₃ monolayers exhibit remarkably tunable electronic structures. Especially, the monolayer In₂Ge₂S₃Se₃ exhibits a larger band gap, higher carrier transport capability, and a wider range of tunable electronic structures compared to In₂Ge₂S₃Te₃ and In₂Ge₂Se₃Te₃ monolayers. The findings in this work establish a fundamental physical understanding of 2D Janus In₂Ge₂X₃Y₃ monolayers, which can promote the future synthesis, characterization, and application of these 2D materials.