Selective Growth of Ferroelectric Ultrathin Nonlayered γ-In₂Se₃

Indium selenide (In₂Se₃) has received increasing interest due to its diverse polytypes and associated polytype-dependent ferroelectric properties, making it one of the promising platforms guiding the development of ultrathin ferroelectric nanodevices. To date, α-, β-, and β'-phase In₂Se₃ have been well studied, but nonlayered γ-In₂Se₃ remains underexplored because of sophisticated formation paths and minor energy differences with other phases. Therefore, understanding the growth mechanisms and electronic structures of γ-In₂Se₃ is crucial to present the full roadmap of the In_xSe_y family. Herein, a precursor-guided chemical vapor deposition (CVD) method is proposed to selectively grow ultrathin γ-In₂Se₃ crystals with a nonlayered, vacancy-ordered screw form (VOSF) structure. The sublimation temperature of precursors plays a critical role in selectively synthesizing α-, β-, and γ-phase In₂Se₃ using different precursors (In₂Se₃, In₂O₃, InCl₃). Ex situ scanning transmission electron microscopy (STEM) reveals a γ-In₂Se₃-to-InSe phase transition at 600 °C, consecutively triggered by interlayer bonds broken, vacancy reorganization and removal of Se atoms. Away from existing literature, only out-of-plane ferroelectricity in γ-In₂Se₃, driven by vertical off-center displacements of Se atoms, is detected. Therefore, a full spectrum of In₂Se₃'s polytypes, particularly nonlayered ferroelectric γ-In₂Se₃, is selectively obtained, presenting great potential for next-generation In₂Se₃-based polytypes-dependent nanoelectromechanical and memory devices.