Understanding and controlling defect in two-dimensional materials is important for both linear and nonlinear optoelectronic devices, especially in terms of tuning nonlinear optical absorption. Taking advantage of an atomic defect formed easily by smaller size, molybdenum disulfide nanosheet is prepared successfully with a different size by gradient centrifugation. Interestingly, size-dependent sulfur vacancies are observed by high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The defect effect on nonlinear absorption is investigated by Z-scan measurement at the wavelength of 800 nm. The results suggest the transition from saturable absorption to reverse saturable absorption can be observed in both dispersions and films. First principle calculations suggest that sulfur vacancies act as the trap state to capture the excited electrons. Moreover, an energy-level model with the trap state is put forward to explain the role of the sulfur vacancy defect in nonlinear optical absorption. The results suggest that saturable absorption and reverse saturable absorption originate from the competition between the excited, defect state and ground state absorption. Our finding provides a way to tune the nonlinear optical performance of optoelectronic devices by defect engineering.

中文翻译：

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缺陷工程在二硫化钼分散体和薄膜中的饱和和反饱和吸收

了解和控制二维材料中的缺陷对于线性和非线性光电器件都很重要，尤其是在调谐非线性光吸收方面。利用较小尺寸容易形成的原子缺陷，通过梯度离心成功制备了不同尺寸的二硫化钼纳米片。有趣的是，通过高分辨率 X 射线光电子能谱、原子力显微镜和透射电子显微镜观察到与尺寸有关的硫空位。通过在 800 nm 波长下的 Z 扫描测量研究了缺陷对非线性吸收的影响。结果表明在分散体和薄膜中都可以观察到从饱和吸收到反饱和吸收的转变。第一原理计算表明，硫空位充当捕获激发电子的陷阱态。此外，提出了具有陷阱态的能级模型来解释硫空位缺陷在非线性光吸收中的作用。结果表明，饱和吸收和反饱和吸收源于激发态、缺陷态和基态吸收之间的竞争。我们的发现提供了一种通过缺陷工程调整光电器件非线性光学性能的方法。结果表明，饱和吸收和反饱和吸收源于激发态、缺陷态和基态吸收之间的竞争。我们的发现提供了一种通过缺陷工程调整光电器件非线性光学性能的方法。结果表明，饱和吸收和反饱和吸收源于激发态、缺陷态和基态吸收之间的竞争。我们的发现提供了一种通过缺陷工程调整光电器件非线性光学性能的方法。