Anisotropy in colossal piezoelectricity, giant Rashba effect and ultrahigh carrier mobility in Janus structures of quintuple Bi 2 X 3 (X = S, Se) monolayers.

Due to the asymmetric structures, two-dimensional Janus materials have gained significant attention in research for their intriguing piezoelectric and spintronic properties. In the present work, quintuple Bi 2 X 3 (X = S, Se) monolayers (MLs) have been modified to create stable Janus Bi 2 X 2 Y (X ≠ Y = S, Se) MLs that display piezoelectricity in both the planes along with Rashba effect. The out-of-plane piezoelectric constant ( d 33 ) is 41.18 (-173.14) pm V -1 , while the in-plane piezoelectric constant ( d 22 ) is 5.23 (6.21) pm V -1 for Janus Bi 2 S 2 Se (Bi 2 Se 2 S) ML. Including spin-orbit coupling in the Janus MLs results in anisotropic giant Rashba spin splitting (RSS) at the Γ point in the valence band, with RSS proportional to d 33 . The Rashba constant along the Γ-K path,αRΓ- K, is 3.30 (2.27) eV Å, whereas along Γ-M,αRΓ- Mis 3.58 (3.60) eV Å for Janus Bi 2 S 2 Se (Bi 2 Se 2 S) ML. The MLs exhibit ultrahigh electron mobility (∼5442 cm 2 V -1 s -1 ) and have electron to hole mobility ratio of more than 2 due to their tiny electron-effective masses. The flexibility of the MLs allows for a signification alteration in its properties, like band gap, piezoelectric coefficient, and Rashba constant, via mechanical (biaxial) strain. For the MLs, band gap and d 33 value are enhanced with compressive strain. The d 33 value of Janus Bi 2 Se 2 S reaches 4886.51 pm V -1 under compressive strain. The coexistence of anisotropic colossal out-of-plane piezoelectricity, giant RSS, and ultrahigh carrier mobilities in Janus Bi 2 S 2 Se and Bi 2 Se 2 S MLs showcase their tremendous prospects in nanoelectronic, piezotronics, and spintronics devices.