Stable freestanding two-dimensional anionic electrons in YCl with extremely weak interlayer interaction

Even van-der-Waals (vdW) materials are renowned for the easiness of exfoliation down to few-layer or monolayer, many of these materials still have non-negligible interlayer couplings which do not originate from pure dispersions force but various covalent-like quasi-bond or long-range Coulombic interaction. Exploration and understanding of vdW materials with vanishing interlayer interaction are still in their infancy. Here, based on first-principles calculations, we reveal that the two-dimensional (2D) vdW halide YCl has an extremely weak interlayer interaction. It leads to the layer-independent band structure, much lower exfoliation energy and interlayer sliding energy barrier than those in graphene and MoS2. Moreover, stable monolayer YCl can host delocalized electrons in the [YCl] blocking layer, forming a freestanding anionic electron (AE) layer. Surprisingly, these highly reactive 2D AEs exhibit strong stability against gas molecules. Further analysis indicates that the unique Cl-Y-Y-Cl sandwich-like structure of YCl and the chemical inertia of the outer layer of chloride ions with large ionic radii play a decisive role in all of the above exotic properties. More importantly, behaviors like weak interlayer interaction are universal for most vdW halides with similar sandwich-like structures. Thus, combined with various functional layers, this class of halide materials provides a novel platform to study the intrinsic properties of vdW systems and explore their various potential applications at the 2D limit.