Bonnet Noémie, Baaboura Jassem, Castioni Florian, Woo Steffi Y, Ho Ching-Hwa, Watanabe Kenji, Taniguchi Takashi, Tizei Luiz H G, Coenen Toon
Delmic B.V., Kanaalweg 4, 2628 EB Delft, The Netherlands.
Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, F-91405, Orsay, France.
Nanotechnology. 2024 Jul 18;35(40). doi: 10.1088/1361-6528/ad3d62.
Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSeflakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiOmake this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.
利用电子显微镜，通过阴极发光（CL）和电子能量损失谱（EELS）可以探索过渡金属二硫属化物（TMD）单层中光学性质的纳米级变化。为了提高TMD单层的CL发射强度，将MoSe2薄片封装在六方氮化硼（hBN）中，形成范德华（VdW）异质结构。到目前为止，研究一直分别专门集中在扫描透射电子显微镜（STEM-CL）或扫描电子显微镜（SEM-CL）上。在这里，我们展示了在同一样品上使用这两种技术的结果，从而探索了较大的加速电压范围。我们将CL测量结果与用不同能量色散获取的STEM-EELS测量结果相关联，以同时获取超高光谱分辨率下的低损耗区域和芯损区域。这提供了有关各种吸收现象权重的信息，包括直接TMD吸收、hBN带间跃迁、hBN体等离子体以及异质结构中存在的原子的芯损。来自TMD单层的S(T)EM-CL测量仅显示A激子的发射。结合STEM-EELS和S(T)EM-CL测量，我们可以重建导致A激子CL发射的不同衰减途径。与SEM-CL的比较表明，这也是一种用于TMD异质结构表征的良好技术，其对样品制备的要求较低很有吸引力。为了展示SEM-CL成像的能力，我们还在SiO/Si衬底上进行了测量，这在二维材料的样品制备中是典型的，它是电子不透明的，只能在SEM-CL中测量。SiO的CL发射缺陷使得该衬底难以使用，但我们证明可以通过使用较低的电子能量来抑制这种背景。
