Semiconducting Ferroelectric SnS_1-Se van der Waals Alloy Flakes

Single-layer monochalcogenides are predicted to be in-plane ferroelectrics but are challenging to obtain in the 2D limit. Recent work showed that synthetic few-layer SnSe and SnS flakes also support ferroelectricity. Key properties such as the Curie temperature may become tunable via anion substitution in SnS_1-Se alloys. Hence, protocols need to be developed that produce ferroelectric few-layer alloy crystals. Here, we report SnS_1-Se alloy flakes across the entire composition range obtained by a highly reproducible growth process using mixed SnS/SnSe precursors. Characterization by electron microscopy and diffraction, X-ray dispersive spectroscopy, and Raman spectroscopy shows the flakes to be high-quality single crystals whose phonon modes and optical bandgaps interpolate between SnSe and SnS. Thin SnS_1-Se flakes across all compositions carry ubiquitous stripe domain patterns, i.e., the flakes are ferroelectric with twin domain walls. Such composition-tunable alloy flakes can support research on the fundamental mechanisms of in-plane ferroelectricity in few-layer monochalcogenide van der Waals semiconductors.