In₂F₂ monolayer: a new class of two-dimensional materials with negative Poisson's ratio and topological phase

Two-dimensional (2D) materials have garnered significant attention for their exceptional potential in electronic, optical, and flexible nanodevices. In this study, we introduce a novel 2D In₂F₂ monolayer, revealed through first-principles calculations, and demonstrate its thermal, dynamic, and mechanical stability. Our findings show that the In₂F₂ monolayer exhibits notable anisotropic mechanical behavior, including auxetic properties characterized by a negative Poisson's ratio. Electronic band structure calculations, using both PBE-GGA and HSE06 functionals, indicate that this monolayer is a semiconductor with a small, nontrivial topological bandgap of approximately 1.58 meV. The observed s-p band inversion and calculated invariant, confirm the presence of a nontrivial topological phase in this material. Furthermore, the optical absorption spectrum reveals strong anisotropy, with significant absorption in the visible to near-infrared range along the y-axis, suggesting potential applications in polarized photodetectors and anisotropic optoelectronic devices. The relatively low work function (3.86 eV) further increases its suitability for electron-emission applications, such as thermionic devices. These mechanical, electronic, and optical properties position the In₂F₂ monolayer as a promising candidate for next-generation electronics, flexible electronics, and anisotropic optoelectronics.