Predicted septuple-atomic-layer Janus MSiGeN4 (M = Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities

Abstract

Janus two-dimensional (2D) materials have attracted much attention as they possess unique properties caused by their out-of-plane asymmetry, and have been achieved in many 2D families. In this work, Janus monolayers are predicted for a new 2D MA₂Z₄ family by means of first-principles calculations, of which MoSi₂N₄ and WSi₂N₄ have been synthesized experimentally (Science, 2020, 369, 670–674). The predicted MSiGeN₄ (M = Mo and W) monolayers exhibit dynamical, thermodynamic and mechanical stability, and they are indirect band-gap semiconductors. The inclusion of spin–orbit coupling (SOC) gives rise to Rashba-type spin splitting, which is observed in the valence bands, which are different from the common conduction bands. The calculated results show valley polarization due to SOC, together with inversion symmetry breaking. It is found that MSiGeN₄ (M = Mo and W) monolayers have much higher electron mobilities with respect to the hole mobilities. Both in-plane and much weaker out-of-plane piezoelectric polarizations can be observed when a uniaxial strain is applied in the basal plane. The values of the piezoelectric strain coefficient d₁₁ of the Janus MSiGeN₄ (M = Mo and W) monolayers fall in between those of the MSi₂N₄ (M = Mo and W) and MGe₂N₄ (M = Mo and W) monolayers, as expected. It is proved that strain can tune the positions of the valence band maximum (VBM) and the conduction band minimum (CBM), and can enhance the strength of the conduction band convergence caused by compressive strain. It is also found that tensile biaxial strain can enhance the d₁₁ of the MSiGeN₄ (M = Mo and W) monolayers, and the compressive strain can improve the d₃₁ (absolute values). Our predicted MSiGeN₄ (M = Mo and W) monolayers, as derivatives of the 2D MA₂Z₄ family, enrich the field of Janus 2D materials, and these results can motivate related experimental work.