Gate-Tunable In-Plane Ferroelectricity in Few-Layer SnS

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Abstract
Ultrathin ferroelectrics hold great promise for modern miniaturized sensors, memories and optoelectronic devices. However, in most ferroelectric materials, polarization is destabilized in ultrathin films by the intrinsic depolarization field. Here, we report robust in-plane ferroelectricity in few-layer tin sulfide (SnS) 2D crystals that is coupled anisotropically to lattice strain. Specifically, the intrinsic polarization of SnS manifests as nanoripples along the armchair direction due to a converse piezoelectric effect. Most interestingly, such nanoripples show an odd-and-even effect in terms of its layer dependence, indicating that it is highly sensitive to changes in inversion symmetry. Ferroelectric switching is demonstrated in ferroelectric field effect transistor devices, in which a stronger ferroelectric response is achieved at negative gate voltages. Our work shows the promise of 2D SnS in ultrathin ferroelectric field effect transistors as well as nanoscale electromechanical systems. Keywords: 2D materials, SnS, in-plane ferroelectricity, field effect transistors, resistive switching, second-harmonic generation

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