Fig. 2: A library summary of OOP piezo/ferroelectricity in nonstoichiometric 2D metal chalcogenides.
From: Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides
a Periodic table illustration of metal elements and chalcogen elements for the formation of nonstoichiometric two-dimensional (2D) metal chalcogenides. Deep green indicates already reported piezo/ferroelectricity in their metal chalcogenides compounds, while light green indicates congeners with intense green. The dark blue suggests nonstoichiometric 2D transition metal dichalcogenides (TMDs) that are experimentally realized and identified as piezo/ferroelectricity in this work. Light blue color indicates congeners with dark blue. The orange symbolizes chalcogens for preparing 2D metal chalcogenides and nonstoichiometric 2D TMDs. b–g Optical images (upper left), atomic force microscopy (AFM) images and thickness profile (lower left), out-of-plane (OOP) amplitude images driven by different alternating current (AC) voltages (upper right) and corresponding phase images (lower right) of nonstoichiometric 2D Fe1+αTe2 (b), Co1+βS2 (c), Mn1+γSe2 (d), Ni1+δSe2 (e), V1+εSe2 (f), Cu1+ζS2 (g). Each nonstoichiometric 2D TMDs exhibits a voltage-dependent amplitude, indicating a pronounced piezoelectric effect.
