Vertical ferroelectricity in two-dimensional mixed-valence tin sulfide system: Unprecedented piezoelectricity, efficient nanogenerator and facile control of morphotoropic phase transformations

Highlights: • Novel approach for inducing the current highest vertical polarization via utilizing mixed-valence states. • Prediction of mechanical switching of a high polarization, which may be utilized for nanogenerators with ultrahigh voltages. • Prediction of multiple metastable phases where the phase transformations can be facilely controlled by multiple approaches. • Prediction of unprecedented high piezoelectric coefficient via utilizing morphotoropic phases in 2D non-toxic tin sulfides. For the recently emerging two-dimensional van der Waals ferroelectrics, their promising prospects in nanoelectronic applications are hindered by their low vertical polarizations. Despite recent experimental breakthroughs and theoretical high throughput screening of material database, the obtained vertical polarizations of two-dimensional ferroelectrics are still limited. Here we propose a strategy of constructing two-dimensional mixed-valence compounds. We show first-principles evidence that Sn₂S₃ monolayer can be synthesized via epitaxial growth of SnS on SnS₂ monolayer with degenerate mixed-valence bi-states. Such a system possesses a room-temperature robust vertical polarization higher than 10 pC/m, which can be switched via interchange of oxidation states between two layers crossing an energy-saving low barrier, either by applying an electric field or mechanical bending. If such Sn₂S₃ monolayer is utilized as a nanogenerator, an unprecedented alternating voltage of ~130 V can be generated by applying an oscillating driving force that repeatedly reverses its polarization. Moreover, it possesses multiple metastable phases with distinct electronic properties. The transformations between them are not only strain-tunable, but can also be controlled via external electric field or low frequency linearly polarized light. Due to the small energy difference between polar and nonpolar states with distinct thicknesses, akin to morphotoropic phases, an ultra-high piezoelectric coefficient (> 2700 pm/V) can be obtained in the phase transformation under pressure, which can be further greatly enhanced via critical doping.