Oxygen-Assisted Direct Synthesis of Two-Dimensional Ferroelectric Bi2SeO5

Abstract Two-dimensional (2D) ferroelectrics are promising for nonvolatile memory, nanoelectronics, and neuromorphic devices. Among them, bismuth selenite (Bi2SeO5) has recently emerged as a high-performance van der Waals (vdW) ferroelectric oxide. However, its current synthesis mainly relies on the post-oxidation of Bi2O2Se, which complicates the fabrication process and limits its integration potential. Here, we report the oxygen-assisted direct synthesis of 2D ferroelectric Bi2SeO5 nanosheets by regulating the oxygen chemical potential and the resulting relative thermodynamic stability of competing Bi–Se–O phases. The presence of O2 increases the oxygen chemical potential, rendering Bi2SeO5 thermodynamically more stable than Bi2O2Se and enabling its direct growth. The as-grown Bi2SeO5 nanosheets exhibit high crystallinity, uniform composition, well-defined chemical states, and robust in-plane (IP) and out-of-plane (OOP) ferroelectricity, while temperature-dependent second-harmonic generation (SHG) measurements reveal that the non-centrosymmetric phase persists up to approximately 690 K. Lateral and vertical devices further demonstrate stable polarization-controlled resistive switching. This work establishes a direct synthesis route to 2D ferroelectric Bi2SeO5 and provides a foundation for ferroelectric memory and nanoelectronic devices.