A two-dimensional (2D) MA₂Z₄ family with and phases has been attracting tremendous interest, the MoSi₂N₄ and WSi₂N₄ of which have been successfully fabricated ( Science 369, 670 (2020)). Janus monolayers have been achieved in many 2D families, so it is interesting to construct a Janus monolayer from the MA₂Z₄ family. In this work, Janus MSiGeN₄ (M = Zr and Hf) monolayers are predicted from -MA₂Z₄, which exhibit dynamic, mechanical and thermal stabilities. It is found that they are indirect band-gap semiconductors by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). With biaxial strain from 0.90 to 1.10, the energy band gap shows a nonmonotonic behavior due to a change of conduction band minimum (CBM). A semiconductor to metal transition can be induced by both compressive and tensile strains, and the phase transformation point is about 0.96 for compressive strain and 1.10 for tensile strain. The tensile strain can change the positions of CBM and valence band maximum (VBM), and can also induce the weak Rashba-type spin splitting near CBM. For MSiGeN₄ (M = Zr and Hf) monolayers, both an in-plane and out-of-plane piezoelectric response can be produced, when a uniaxial strain in the basal plane is applied, which reveals the potential as piezoelectric 2D materials. The high absorption coefficients in the visible light region suggest that MSiGeN₄ (M = Zr and Hf) monolayers have potential photocatalytic applications. Our works provide an idea to achieve a Janus structure from the MA₂Z₄ family, and can hopefully inspire further research exploring Janus MA₂Z₄ monolayers.

中文翻译：

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Janus MSiGeN4 (M = Zr 和 Hf) 来自中心对称 β-MA2Z4 的单分子层：第一性原理研究

二维 (2D) MA ₂ Z ₄系列 和 相引起了极大的兴趣，其中MoSi ₂ N ₄和WSi ₂ N ₄已成功制造（科学 369、670 (2020)）。Janus 单分子层已在许多 2D 家族中实现，因此从 MA ₂ Z ₄家族构建 Janus 单层很有趣。在这项工作中，Janus MSiGeN ₄ (M = Zr 和 Hf) 单层预测来自 -MA ₂ Z ₄，具有动态、机械和热稳定性。通过使用广义梯度近似（GGA）加自旋轨道耦合（SOC）发现它们是间接带隙半导体。双轴应变 从 0.90 到 1.10，由于导带最小值 (CBM) 的变化，能带隙表现出非单调行为。压缩应变和拉伸应变都可以引起半导体到金属的转变，压缩应变的相变点约为 0.96，拉伸应变的相变点约为 1.10。拉伸应变可以改变CBM和价带最大值（VBM）的位置，还可以在CBM附近诱导弱Rashba型自旋分裂。对于 MSiGeN ₄ (M = Zr 和 Hf) 单层，当在基面施加单轴应变时，可以产生面内和面外压电响应，这揭示了压电二维材料的潜力。可见光区域的高吸收系数表明 MSiGeN ₄(M = Zr 和 Hf) 单层具有潜在的光催化应用。_{我们的工作提供了从 MA 2} Z ₄家族中实现 Janus 结构的想法，并有望激发探索 Janus MA ₂ Z ₄单分子层的进一步研究。