First-principles prediction of ferroelectric Janus Si 2 XY (X/Y = S/Se/Te, X ≠ Y) monolayers with negative Poisson's ratios.

Nowadays, two-dimensional (2D) materials with Janus structures evoke much attention due to their unique mechanical and electronic properties. In this work, Janus Pma2-Si 2 XY (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 Si 2 STe and Si 2 SeTe 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 Si 2 STe and 1.149 × 10 -10 C m -1 and 196.6 meV for Si 2 SeTe. The ab initio molecular dynamics (AIMD) simulations reveal high Curie temperatures ( T c ) for Si 2 STe and Si 2 SeTe, ranging between 1300 K and 1400 K. Additionally, Si 2 XY monolayers exhibit high anisotropic carrier mobility (∼10 3 cm 2 V -1 s -1 ) and an extraordinary light absorption coefficient (∼10 5 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.