Intrinsic Sliding Ferroelectricity and High-Quality Superlinear Emission in Van der Waals γ-InSe Semiconductor

High-quality single crystals are the fundamental foundation for advancing 2D van der Waals (vdW) layered semiconductors toward applications in low-consumptive nanodevices. The easily formed stacking faults (SFs) in vdW layered materials hide the intrinsic optical/electrical properties, adverse to practical application. Here, a unique low-temperature annealing strategy is adopted to eliminate the SFs in flexible γ-InSe and optimize the crystal quality. The annihilation dynamics of the SFs are revealed by using the in situ heating observation on the state-of-the-art spherical aberration-corrected transmission electron microscopy. The annealed γ-InSe not only exhibits intrinsic out-of-plane sliding ferroelectricity in a wide thickness range from a single unit cell (trilayer) to tens of nanometers, but also demonstrates efficient superlinear photoluminescence with higher beam quality and lower thresholds. This should expedite the applications of intrinsic γ-InSe in near infrared emitter and lasing, in-memory computing, and ferroelectric photovoltaics or detectors.