First-principles prediction of ferroelectric Janus Si2XY (X/Y = S/Se/Te, X ≠ Y) monolayers with negative Poisson's ratios

Abstract

Nowadays, two-dimensional (2D) materials with Janus structures evoke much attention due to their unique mechanical and electronic properties. In this work, Janus Pma2-Si₂XY (X/Y = S/Se/Te, X ≠ Y) ferroelectric monolayers are firstly proposed and systematically investigated by first-principles calculations. These monolayers exhibit remarkable mechanical properties, including small Young's modulus values, negative Poisson's ratios (NPRs) and large critical strains, reflecting their exceptional flexibility and stretchability. More strikingly, the novel structures of Si₂STe and Si₂SeTe also endow them with in-plane spontaneous polarization (P_s) and low energy barrier for phase transition, with P_s and energy barrier values being 1.632 × 10⁻¹⁰ C m⁻¹ and 159 meV for Si₂STe and 1.149 × 10⁻¹⁰ C m⁻¹ and 196.6 meV for Si₂SeTe. The ab initio molecular dynamics (AIMD) simulations reveal high Curie temperatures (T_c) for Si₂STe and Si₂SeTe, ranging between 1300 K and 1400 K. Additionally, Si₂XY monolayers exhibit high anisotropic carrier mobility (∼10³ cm² V⁻¹ s⁻¹) and an extraordinary light absorption coefficient (∼10⁵ cm⁻¹). Our research not only broadens the family of 2D Janus ferroelectric materials, but also demonstrates their potential applications in nanomechanical, nanoelectronic and optoelectronic devices.