Abstract

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS₂, MoS₂ and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (∼ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.

中文翻译：

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原子薄的二硫族化合物和异质结构中的直接光学结构相关性

摘要

原子稀薄的过渡金属二硫化碳（TMD）具有独特的光电特性，包括增强的发光特性和高开关电流比，可以通过制造更复杂的TMD异质结构来进一步调节。但是，常规光学方法的分辨率极限不允许在这些材料中进行直接的纳米级光学结构相关性测量，尤其是TMD异质结构中的掩埋界面。在这里，我们首次在扫描透射电子显微镜（CL-STEM）中使用电子束诱导的阴极发光来测量单层TMD（WS ₂，MoS ₂和WSSe合金）封装在hBN层之间。我们观察到由局部（约100 nm）不完善的界面和单层折叠导致的暗区，这表明与该应用相关的异质结构中各层之间的紧密接触对于正确的层间耦合是必需的。我们还实现了一种合适的成像方法，该方法可最大程度地减少电子束引起的变化并提供内在特性的测量。为了克服TMD单层中电子相互作用体积小的限制（从而降低光子产量），我们发现用hBN封装TMD单层并随后进行退火非常重要。CL-STEM提供了一种强大的方法，可以直接测量横向或垂直异质结构和合金中的结构-光学对应关系。