  Two-dimensional (2D) black phosphorus (i.e., phosphorene) has become a rising
star in electronics. Recently, 2D phosphorus oxides with higher stability have
been synthesized. In this work, we systematically explore the structures and
properties of 2D phosphorus oxides on the basis of global optimization approach
and first-principles calculations. We find that the structural features of 2D
phosphorus oxides PxOy vary with the oxygen concentration. When the oxygen
content is poor, the most stable 2D PxOy can be obtained by adsorbing O atoms
on phosphorene. However, when the oxygen concentration becomes rich, stable
structures are no longer based on phosphorene and will contain P-O-P motifs.
For the 2D P4O4, we find that it has a direct band gap (about 2.24 eV), good
optical absorption, and high stability in water, suggesting that it may be good
candidate for photochemical water splitting application. Interestingly, 2D P2O3
adopt two possible stable ferroelectric structures (P2O3-I and P2O3-II) as the
lowest energy configurations within a given layer thickness. The electric
polarizations of P2O3-I and P2O3-II are perpendicular and parallel to the
lateral plane, respectively. We propose that 2D P2O3 could be used in a novel
nanoscale multiple-state memory device. Our work suggests that 2D phosphorus
oxides may be excellent functional materials.

Materials Science;Mesoscale and Nanoscale Physics