Tri-MX: New group-IV monochalcogenide monolayers with excellent piezoelectricity and special optical properties

Two-dimensional (2D) group-IV monochalcogenides (MX) have shown great value in the fields of sensing, energy harvesting and information storing, etc. To explore MX with multiple performances, we predict four new MXs with three atomic sub-layers in monolayer, namely tri-MX (i.e. trilayer-MX, M = Ge, Sn and X = Se, S) maintaining non-centrosymmetric structures by using first-principles calculations. We systematically investigate the geometry, stabilities, electronic structures, optical properties and piezoelectric coefficients of tri-MX, together with their transformation mode. Among them, the piezoelectric coefficients of tri-SnSe and tri-SnS reach 182.31 pm/V and 179.44 pm/V, surpassing α-SnS which exhibits the highest piezoelectric coefficient experimentally, which is expected to further improve the nanogenerator efficiency. Further analysis shows that the electronegativity differences between elements in tri-MX determine the differences in structure and deformation mode. Moreover, tri-MX is transmissive in the visible range and can hinder ultraviolet (UV) light. Combining their excellent piezoelectricity and special optical properties, they will sparkle in the fields of electronic screens, energy harvesting, and UV protection. Finally, their transition barriers from α-MXs to tri-MXs range in 0.105–0.208 eV/atom, indicating the synthetic feasibility of tri-MX. These results are of guiding significance for experimental synthesis of tri-MX and the development of nanodevices.