Single-Layer Zirconium Dihalides ZrX₂ (X = Cl, Br, and I) with Abnormal Ferroelastic Behavior and Strong Anisotropic...

Recently, two-dimensional (2D) metal halides have brought out an intensive interest for their unique mechanical, electronic, magnetic, and topological properties. Here, we theoretically report the existence of the single-layer (SL) zirconium dihalide materials ZrX₂ (X = Cl, Br, and I) using first-principles calculations. SL ZrX₂, which can be obtained from its bulk phase through simple mechanical exfoliation, shows the dynamic, thermodynamic, and mechanical stability. Halogen atoms can effectively tune the electronic structure, dipole moment transition, band alignment, and light absorption. Specifically, ZrX₂ monolayers intrinsically exhibit a ferroelasticity with an abnormal 120° orientation rotation, possessing a moderate switching barrier of 24-39 meV/atom. Importantly, we observe superior anisotropic light absorption responses on SL ZrX₂ in the visible region. Besides, a series of ZrX₂-based excitonic solar cells have been proposed, which hold a large power conversion efficiency limit of 12.4-18.7%.