Structural and Phase Engineering in Two-Dimensional Polymorphic In₂Se₃

The two-dimensional van der Waals layered semiconductor In₂Se₃ has emerged as a promising candidate for non-volatile ferroelectric memory, optoelectronic devices, and polymorphic phase engineering. Polymorphic In₂Se₃ typically stabilizes in three distinct phases: α-, β′-, and β^*-In₂Se₃, each dominant within specific temperature ranges. Although the crystal structures and ferroelectric properties of these phases have been widely studied, the unambiguous assignment of their in-plane and out-of-plane ferroelectric behaviors, as well as the mechanisms governing their phase transitions, remains a subject of active debate. In this study, we investigate the evolution of atomic and electronic structures in molecular beam epitaxy-grown ultrathin In₂Se₃ films through correlated microstructural and macroscopic physical property analysis. By employing scanning tunneling microscopy/spectroscopy, temperature-dependent Raman spectroscopy, and piezoresponse force microscopy, we demonstrate a reversible temperature-induced phase transition between the in-plane ferroelectric β^* and antiferroelectric β′ phases. Furthermore, we confirm robust out-of-plane ferroelectric polarization in the as-grown films and achieve an electric-field-driven transition from the β^* to β′ phase. Our findings not only advance the fundamental understanding of phase transitions and polarization evolution in two-dimensional semiconductors but also open new avenues for the design of tunable, non-volatile ferroelectric memory devices.