Two-dimensional ferroelectricity in all-inorganic halide perovskite monolayer Cs2PbX4 ( X=F , Cl, Br, I)

Ferroelectricity in lead halide perovskites has attracted tremendous interest due to its promising applications in high-performance optoelectronic devices. At present, ferroelectricity has been reported in some inorganic lead halide perovskite bulks, but whether it can exist in their ultra-thin films or even in the two-dimensional (2D) limit remains an unresolved issue. Here, we investigate ferroelectricity in lead halide perovskite monolayer films ${\mathrm{Cs}}_{2}\mathrm{Pb}{X}_{4}$ ($X$ = F, Cl, Br, I) using first-principles calculations, and demonstrate that the intrinsic ferroelectricity driven by the lone-pair electron effect of ${\mathrm{Pb}}^{2+}$ ions can exist in both freestanding and epitaxial strained ${\mathrm{Cs}}_{2}{\mathrm{PbF}}_{4}$ monolayer. Epitaxial strain maintains the polar ground-state phase established by octahedral rotation plus ferroelectric distortion in the ${\mathrm{Cs}}_{2}{\mathrm{PbF}}_{4}$ monolayer, whereas it does not induce ferroelectricity in the other monolayers. All the monolayers behave as semiconductors with a direct band gap that decrease with increasing anion radius. This work develops 2D ferroelectric semiconductors based on all-inorganic halide perovskites.