Stacking dependent piezoelectric response of bilayer and heterobilayer group-IV monochalcogenides under applied external strain

Layered two-dimensional (2D) materials are promising materials for piezoelectric and optoelectronic devices due to the introduction of new and interesting properties not seen in the single layers alone. In particular, the group-IV monochalcogenides (MX, M = Ge/Sn and X = S/Se) are highly piezoelectric layered materials which have outstanding optical adsorption properties in the isolated monolayer form. It is possible that combinations of MX monolayers, in a bilayer or heterobilayer system, could exhibit properties that are different to their monolayer counterparts. Using density functional theory calculations, the stacking-dependent piezoelectric response of group-IV monochalcogenide bilayers and heterobilayers with and without applied strain was determined. Of the four materials, SnSe yields the largest e₂₂ value in both the monolayer (0.86 C m^-2) and bilayer (1.36 C m^-2) form. Of the different heterobilayers examined, GeSe/SnSe has the largest e₂₂ value (1.58 C m^-2). With the application of strain, the piezoelectric response can be significantly enhanced, allowing bilayer SnSe to achieve a maximum response of 6.13 C m^-2, which is a ∼450% increase compared to its unstrained form, and is 450-730% higher than reported for other layered materials, such as ZnO and MoSTe. Indirect-to-direct band gap transitions can also be achieved using differing stacking arrangements, with the GeSe/SnSe heterobilayer being determined to have a type II band gap, demonstrating there are potential applications for the heterobilayers in optoelectronic devices. Overall, the group-IV monochalcogenide bilayers have the potential to achieve extraordinary piezoelectric responses under applied strain, making them ideal for nano-based piezoelectric and optoelectronic devices.