Ferroelectric 2D Antimony Oxides with Wide Bandgaps

The first two-dimensional (2D) polymorphs of antimony dioxide, namely, $\gamma$-Sb$_2$O$_4$ and $\delta$-Sb$_2$O$_4$, are predicted using the evolutionary algorithm combined with first-principles density functional theory (DFT) calculations. Out-of-plane ferroelectricity is found in $\gamma$-Sb$_2$O$_4$, while in-plane ferroelectricity is found in $\delta$-Sb$_2$O$_4$. The predicted dipole moments of $\gamma$-Sb$_2$O$_4$ and $\delta$-Sb$_2$O$_4$ phases are 36.63 and 14.96 e\r{A}, respectively, implying that they can be good candidates for making ferroelectric devices. The calculations show that doping with other group V elements or applying strain can lower the switching energy barriers and thus facilitate switching. Results from GW calculations show indirect band gaps of 5.51 and 3.39 eV for $\gamma$-Sb$_2$O$_4$ and $\delta$-Sb$_2$O$_4$ in their monolayers, respectively. Raman spectra are calculated to facilitate the experimental investigation of the predicted structures. The existence of both in-plane and out-of-plane 2D ferroelectricity and the large band gaps make this material system particularly interesting for potential applications.