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

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 fewlayer 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 field-effect transistor devices fabricated on ultrathin SnS films, 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.

Purely in-plane ferroelectricity in monolayer SnS at room temperature

“…The unusual growth mode is probably due to the substrate effect, such as lattice strain and electrostatic surface charges. In the previous work on bulk SnS (~15 nm) 32 , an I D -V D hysteresis loop similar to that in the present work was observed for in-plane two-terminal Au contact devices, where the ferroelectricity in bulk SnS was achieved by extrinsically breaking the inversion symmetry through the perpendicular electric field from the back gate. It should be emphasized that the intrinsic ferroelectricity is observed for monolayer SnS in this study.…”

“…Therefore, the in situ observation of SnS growth has confirmed a very high growth rate in the perpendicular direction 38 . Thus, monolayer SnS has been realized by molecular beam epitaxy growth, although only with a limited crystal size of several tens of nanometers 32 . Otherwise, the minimum thickness was 5.5 nm via PVD growth 34 .…”

“…Finally, at T source = 470°C and T sub = 410°C, monolayer-thick SnS was realized with micrometer size of up to~5 µm, which is a reasonable size for the device fabrication. Note that monolayer-thick SnS has an atomically flat surface without wedding cake morphology owing to the Stranski-Krastanov growth mode 32 or spiral growth assisted by a screw dislocation 38 . The rounded shape could be caused by the SnS desorption during the growth and an insufficient growth time to reach the thermodynamic equilibrium state for thin SnS 35,38 .…”

Recent Advances in 2D Metal Monochalcogenides

“…These 2D materials possess intriguing optical, electronic, mechanical, and optoelectronic properties, and are being explored for exhibit orthorhombic crystal structure (Pnma space group) with low crystal symmetry C 2 , in which the inversion symmetry is further broken, which makes them possible to observe new order parameters (spin-orbital coupling) and polarization properties. Many exotic phenomena have been predicted on a monolayer MMCs, including valley physics, [37] spontaneous polarization and bulk photovoltaic effect, [38][39][40][41] piezo-phototronic, [42] giant piezoelectricity, [43] ferroelectricity, [44][45][46][47] multiferroics of ferroelectricity, [48] and ferroelasticity. [49] However, valley selective dichroism, [50] and more than 90% room temperature valley polarization (VP) degree [51] in SnS provide a completely novel platform for valleytronics.…”

“…In addition, the giant optical second harmonic generation (SHG) in MMCs is also promising in nonlinear optoelectronic applications. [46,52] In this article, the latest advances in the field of the emerging 2D group IV A -VI metal monochalcogenide materials (MMCs) are reviewed. The intriguing physical (crystal and electronic structure) and optical properties are initially highlighted and discussed.…”