Anisotropy in colossal piezoelectricity, giant Rashba effect and ultrahigh carrier mobility in Janus structures of quintuple Bi<sub>2</sub>X<sub>3</sub> (X = S, Se) monolayers
Nilakantha Tripathy, Abir De Sarkar
Abstract
Abstract 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mi>R</mml:mi> <mml:mrow> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Γ</mml:mi> </mml:mrow> </mml:mrow> <mml:mo>−</mml:mo> <mml:mrow> <mml:mtext> K</mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:math> , is 3.30 (2.27) eV Å, whereas along Γ–M, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mi>R</mml:mi> <mml:mrow> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Γ</mml:mi> </mml:mrow> </mml:mrow> <mml:mo>−</mml:mo> <mml:mrow> <mml:mtext> M</mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:math> is 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.