Group-IV(A) Janus dichalcogenide monolayers and their interfaces straddle gigantic shear and in-plane piezoelectricity

Abstract

Inversion symmetry in the 1T-phase of pristine dichalcogenide monolayer MX₂ (M = Ge, Sn; X = S, Se) is broken in their Janus structures, MXY (M = Ge, Sn; X ≠ Y = S, Se), which induces an in-plane piezoelectric coefficient, d₂₂ = 4.09 (2.15) pm V⁻¹ and a shear piezoelectric coefficient, d₁₅ = 7.90 (13.68) pm V⁻¹ in the GeSSe (SnSSe) monolayer. High flexibility arising from the small Young's modulus (60–70 N m⁻¹) found in these Group-IV(A) Janus monolayers makes them suitable for large-scale strain engineering. Application of 7% uniaxial tensile strain increases d₂₂ and d₁₅ colossally to 267.07 pm V⁻¹ and 702.34 pm V⁻¹, respectively, thereby reaching the level of bulk piezoelectric perovskite materials. When the Janus GeSSe monolayers are stacked to form a van der Waals (vdW) homo-bilayer, d₂₂ lies between 19.87 and 73.26 pm V⁻¹, while d₁₅ falls into the range between 83.01 and 604.34 pm V⁻¹, depending on the stacking order. The chalcogen exchange energies and overall stabilities of the monolayers and bilayers confirm the feasibility of their experimental synthesis. Moreover, hole mobility in the GeSSe monolayer is greater than the electron mobility along its zigzag directions (μ_e = 883 cm² V⁻¹ s⁻¹ and μ_h = 1134 cm² V⁻¹ s⁻¹). Therefore, the semiconducting, flexible, and piezoelectric Janus GeSSe monolayer and bilayers are immensely promising for futuristic applications in energy harvesting, nanopiezotronic field-effect transistors, atomically thin sensors, shear/torsion actuators, transducers, self-powered circuits in nanorobotics, and electromechanical memory devices, and biomedical and other nanoelectronic applications.