Using density functional theory based first-principles calculations, we predict the dynamically stable 1H-phase of a Janus single-layer composed of S-Mo-O atomic layers. It is an indirect band gap semiconductor exhibiting strong polarization arising from the charge difference on the two surfaces. In contrast to 1H-phases of MoS${}_2$ and MoO${}_2$, Janus MoSO is found to possess four Raman active phonon modes and a large out-of-plane piezoelectric coefficient which is absent in fully-symmetric single-layers of MoS${}_2$ and MoO${}_2$. We investigate the electronic and phononic properties under applied biaxial strain and found an electronic phase transition with tensile strain while the conduction band edge displays a shift when under compressive strain. Furthermore, single-layer MoSO exhibits phononic stability up to 5% of compressive and 11% of tensile strain with significant phonon shifts. The phonon instability is shown to arise from the soft in-plane and out-of-plane acoustic modes at finite wave vector. The large strain tolerance of Janus MoSO is important for nanoelastic applications. In view of the dynamical stability even under moderate strain, we expect that Janus MoSO can be fabricated in the common 1H-phase with a strong out-of-plane piezoelectric coefficient.

中文翻译：

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Janus SMoO的稳定单层：强大的面外压电性

使用基于密度泛函理论的第一性原理计算，我们预测了由S-Mo-O原子层组成的Janus单层的动态稳定1H相。它是一种间接带隙半导体，由于两个表面上的电荷差异而显示出强烈的极化。与MoS的1H相相反${}_2$ 和MoO${}_2$，发现Janus MoSO具有四个拉曼有源声子模式和大的面外压电系数，这在MoS的完全对称单层中是不存在的${}_2$ 和MoO${}_2$。我们研究了施加的双轴应变下的电子和声子性质，并发现了在拉伸应变下电子相变，而导带边缘在压缩应变下显示出位移。此外，单层MoSO表现出高达5％的压缩力和11％的拉伸应变的声子稳定性，并具有明显的声子位移。声子不稳定性被证明是由有限波矢量处的软面内和面外声模引起的。Janus MoSO的大应变容限对于纳米弹性应用非常重要。考虑到即使在中等应变下的动态稳定性，我们希望Janus MoSO可以在具有强平面外压电系数的普通1H相中制造。