Ultrahigh Out-of-Plane Piezoelectricity Meets Giant Rashba Effect in 2D Janus Monolayers and Bilayers of Group IV Transition-Metal Trichalcogenides

The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX2Y (X not equal Y = S, Se), thereby inducing a large out-of-plane piezoelectric constant d(33) (similar to 68 pm/V) in them. d(33) can be further enhanced to a super high value of similar to 1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d(33) in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (similar to 268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin-orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, alpha(R) similar to 1.1 eV angstrom. It can be raised to 1.41 eV angstrom via compressive strain. Most of the 2D materials reported to date mainly show in-plane electric polarization, which severely limits their prospects in piezotronic devices. In this present work, the piezoelectricity shown by the Janus monolayers of group IV TMTCs and their bilayers is significantly higher than the ones generally utilized in the form of three-dimensional bulk piezoelectric solids, for example, alpha-quartz (d(11) = 2.3 pm/V), wurtzite-GaN (d(33) = 3.1 pm/V), and wurtzite-AlN (d(33) = 5.6 pm/V). It is exceedingly higher than that in Janus multilayer/bulk structures of Mo- and W-based transition-metal dichalcogenides, for example, MoSTe (d(33) similar to 10 pm/V). The 2D Janus TMTC monolayers and their bilayers reported herewith straddle giant Rashba spin-splitting and ultrahigh piezoelectricity, thereby making them immensely promising candidates in the next-generation electronics, piezotronics, spintronics, flexible electronics, and piezoelectric devices.