Predicted septuple-atomic-layer Janus MSiGeN4 (M = Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities
2021
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.
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