Enhanced vertical polarization and ultra-low polarization switching barriers of two-dimensional SnS/SnSSe ferroelectric heterostructures

Abstract

Two-dimensional materials with ferroelectricity are verified theoretically and experimentally. However, applications of these materials in nanoelectronics are limited by weak vertical spontaneous polarization and high polarization switching barriers. Herein, we reported a new stable SnS/SnSSe heterostructure with excellent ferroelectricity by first-principles calculations. The noncentrosymmetric nature enables this heterostructure to possess enhanced vertical polarization. The largest e₃₁ in state II is 0.182 × 10⁻¹⁰ C m⁻¹, which is twice that of the SnSSe monolayer. Importantly, the direction of vertical polarization is determined by different couplings of neighboring layers, helping to achieve polarization switching via sliding the top SnS monolayer with a very low energy barrier of 0.106 eV. The strong coupling of neighboring layers is evidenced by the transfer of more electrons from the SnS layer to the SnSSe layer in the heterostructure, which leads to a reduced bandgap of the SnS layer after contact. Besides, the heterostructure inversion method can also realize the polarization switching of SnS/SnSSe. In the ground state, the direction of vertical polarization can be switched by the heterostructure inversion from SnS/SnSSe to SnSSe/SnS by overcoming a remarkable lower energy barrier of 0.043 eV even lower than that obtained using the former method. The opposite electrostatic potential distribution demonstrates the opposite vertical polarization in SnS/SnSSe and SnSSe/SnS, revealing the possibility of switching polarization via heterostructure inversion. Switchable strong vertical polarization and ultra-low switching barriers endow SnS/SnSSe with the ability to transfer mechanical vibration energy into electromagnetic waves, and with great application potential in nanoelectronics, such as vibration detectors, energy storage devices and nonvolatile memory devices.