Higher photoluminescence yield from the boundaries as compared to the interiors in monolayer (1L) islands of transition metal dichalcogenides grown by chemical vapor deposition (CVD) has been frequently documented in the literature. However, the detailed understanding of this phenomenon is still lacking. Here, we investigate the effect observed in CVD grown 1L-MoS2 islands on c-sapphire substrates. The study reveals a blue shift of the A-excitonic feature from the interiors to the edges of the monolayers, suggesting the release of the tensile strain, which is resulting in the interiors due to lattice and/or thermal expansion coefficient mismatch between the layer and the substrate, toward the boundaries. The degree of valley polarization is also found to increase at the edges. However, when the as-grown monolayers are transferred on a SiO2 surface, the intensity, position, and valley polarization of the A-excitonic peak do not show any inhomogeneity over the surface. The study attributes the decrease in PL intensity and the valley polarization in the interiors as compared to the edges of these as-grown islands to the reduction of the energy gap between the Γ- and K-valley valence band maxima with the increase in the tensile strain in the layer. First principles density functional theory based calculations for the geometry optimization are performed on a 1L-MoS2 flake residing on a (0001) sapphire surface, which indeed shows the relaxation of tensile strain toward the edges.

中文翻译：

---

解决化学气相沉积生长的 MoS2 单层边缘光致发光产率较高的难题


文献中经常记录了通过化学气相沉积 （CVD） 生长的过渡金属二硫化物的单层 （1L） 岛的内部，边界的光致发光产率更高。然而，对这一现象的详细了解仍然缺乏。在这里，我们研究了在 CVD 生长的 1L-MoS2 岛对 c 蓝宝石衬底上观察到的影响。该研究揭示了 A 激子特征从单层内部到边缘的蓝移，表明拉伸应变的释放，由于层和衬底之间的晶格和/或热膨胀系数不匹配，导致内部朝向边界。还发现谷极化的程度在边缘处增加。然而，当生长的单层转移到 SiO2 表面上时，A 激子峰的强度、位置和谷极化在表面上没有表现出任何不均匀性。该研究将与这些生长岛的边缘相比，内部 PL 强度和谷极化的降低归因于 Γ 和 K 谷价带最大值之间的能量间隙的减少，以及层中拉伸应变的增加。在位于 （0001） 蓝宝石表面的 1L-MoS2 薄片上进行基于密度泛函理论的第一性原理几何优化计算，这确实显示了拉伸应变向边缘的松弛。