Ultrahigh mechanical flexibility induced superior piezoelectricity of InSeBr-type 2D Janus materials

Spin-orbit splitting and piezoelectric properties of Janus Ge2XY (X ≠ Y = P, As, Sb and Bi)

The coexistence of spin-orbit coupling and piezoelectricity in a single material may have potential application in multifunctional devices, including spintronics, nanorobotics and piezotronics. Spin-orbit coupling provides a new means to manipulate electron's spin without an additional external magnetic field, while piezoelectricity refers to the interplay between mechanical stresses and electric polarization. Using first-principles calculations, the structural, electronic, optical, spin, and piezoelectric properties of the Janus Ge₂XY (X ≠ Y = P, As, Sb, and Bi) monolayers were systematically investigated. All the Ge₂XY are energetically and dynamically stable in the α phase. At the GW level, Ge₂AsSb, Ge₂AsBi, and Ge₂SbBi have direct fundamental band gaps of 0.65, 0.64, and 0.91 eV. At the GW + BSE level, their optical gaps are 0.42, 0.45, and 0.63 eV, and the optical absorption coefficients can reach about 10^-5 cm^-1 in the infrared light region, which reveals that they have potential for application in infrared photodetectors. For Ge₂PBi, Ge₂AsBi, and Ge₂SbBi containing the heavy Bi element, the lowermost conduction band and uppermost valence band have large spin splitting along the M-K and K-Γ lines, and the bands near the Fermi level possess Rashba spin splitting at the Γ point. Ge₂PBi and Ge₂SbBi have both large in-plane piezoelectric coefficients d₁₁ (-0.75 and -3.18 pm V^-1) and out-of-plane piezoelectric coefficients d₃₁ (0.37 and 0.30 pm V^-1). Our findings are helpful to understand the mechanism of the spin-orbit physics and piezoelectricity of Janus Ge₂XY monolayers and guide experiments in exploring novel multifunctional materials.

W4PCl11 monolayer: an unexplored 2D material with moderate direct bandgap and strong visible-light absorption for highly efficient solar cells

The discovery of novel two-dimensional (2D) materials with excellent electronic and optoelectronic properties have attracted much scientific attention. Based on the first-principles calculations, we predict an unexplored 2D W₄PCl₁₁ monolayer, which is potentially strippable from its bulk counterpart with the exfoliation energy of only 0.16 J m^-2. The dynamical, thermal, and mechanical stabilities have also been confirmed. Remarkably, W₄PCl₁₁ monolayer is direct semiconductor with a bandgap of 1.25 eV, which endows the monolayer with very strong visible-light absorption in the magnitude of 10⁵ cm^-1. Meanwhile, the calculated carrier mobilities of W₄PCl₁₁ monolayer can reach to 10³ cm² V^-1 s^-1. Considering the moderate direct bandgap and high carrier mobility, W₄PCl₁₁ monolayer should be a superior candidate for the donor material of excitonic solar cells. The estimated power conversion efficiency of the fabricated W₄PCl₁₁/Bi₂WO₆ heterojunction reaches as high as 21.64%, which much superior to those of most recently reported 2D heterojunction. All these outstanding properties accompanied with its experimental feasibility endows W₄PCl₁₁ monolayer with promising photovoltaic applications.