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 into three distinct phases: α-, β′-, and β^*-In₂Se₃, each predominant 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 driving their phase transitions, remains actively debated. Here, we investigate the evolution of atomic and electronic structure in molecular beam epitaxy (MBE)-grown ultrathin In₂Se₃ films through correlated microstructural and macroscopic physical properties analysis. By employing scanning tunneling microscopy/spectroscopy (STM/STS), temperature-dependent Raman spectroscopy, and piezoresponse force microscopy (PFM), 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 of as-grown films and achieve an electric-field-driven transition from the β^* to β' phase. Our findings not only deepen the understanding of phase transitions and polarization evolution in two-dimensional semiconductors, but also open new avenues for developing tunable non-volatile ferroelectric memory devices.