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Two-dimensional planar heterojunctions composed of single-layer transition metal dichalcogenides have great potential applications in low-power, high-performance, and flexible optoelectronic devices. The localized atomic structure and crystal defects at interface govern the electronic, magnetic, optical, catalytic, and topological quantum properties. However, accurate characterization of interface atomic structure is still a challenge, so far. To determine the accurate atomic position, a spherical aberration-corrected electron microscope with segmented detector is employed, and the calculation is performed by integrated differential phase contrast (iDPC) imaging algorithm. By using the iDPC method, the atomic structure of WS2-MoSe2 monolayer heterojunction interface is characterized, and the W, Se, Mo, and S atoms are imaged simultaneously. Statistics show that the angles between the lattices on both sides of the WS2-MoSe2 planar heterojunction are distributed around 29° and 35°. Additionally, it is found that the lattice near the boundary experiences the strains of approximately 4‰ and 2% in the two lattice vector directions, with significant distortion occurring only at the interface. In this work, several typical atomic configurations, including merge type, quadrilateral type, and pentagonal type are found. The interface atomic configuration can help to release stress at the lateral interface. This study provides a useful method for accurately characterizing the structures for planar heterojunctions of monolayer transition metal dichalcogenide. It is of great significance for in-depth research on the structure-property relationship at single-atom resolution in various interface structures.
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Keywords:
- integrated differential phase contrast /
- 2-dimensional material /
- heterojunction /
- interface atomic structure
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图 1 DPC-STEM示意图 (a) 电子束受样品内电场作用发生偏转; (b) 四分区DPC探头
Figure 1. Schematic diagram of DPC-STEM: (a) Electron beam deflected by electric field in specimen; (b) 4 segment DPC detector.
图 2 分区探头示意图以及iDPC成像算法简要示意 (a) 八分区探头的示意图; (b) 每一个分区在STEM扫描模式下得到的图像信号; (c) 分区信号经由成像算法计算得到样品iDPC图像
Figure 2. Schematic diagram of the segmented probe and a brief illustration of the iDPC imaging algorithm: (a) Schematic diagram of the eight-segment probe; (b) signals obtained from each segment in STEM scanning mode; (c) the segmented signals processed through the imaging algorithm to obtain the sample's iDPC image.
图 3 平面异质结的电镜成像与原子模型 (a) WS2-MoSe2的低倍电镜像; (b) 平面异质结的HAADF-STEM像; (c) WS2区域的放大图(图(b)中红框部分); (d) 放大区域的DPC图像; (e) 放大区域的iDPC信号; (f) 放大区域对应的原子模型(红色标记: W原子, 蓝色标记: S原子)
Figure 3. Electron microscopy imaging and atomic model of the planar heterojunction: (a) Low magnification STEM image of WS2-MoSe2; (b) HAADF-STEM image of the planar heterojunction; (c) magnified display of the WS2 region (red box in Figure (b)); (d) DPC image of the magnified region, scale bar: 3 Å; (e) iDPC signal of the magnified region; (f) corresponding atomic model of the magnified region (red: W atoms, blue: S atoms).
图 4 HAADF与iDPC成像的对比 (a) WS2区域的HAADF图像以及框选区域的线扫描强度分布, 线扫描横坐标单位为pixel; (b)同一区域的iDPC图像以及同区域的线扫描强度分布; (c)平面异质结界面处的HAADF图像与原子模型; (d) 同一区域的iDPC图像与原子模型
Figure 4. Comparison of HAADF and iDPC imaging: (a) HAADF image of the WS2 region and Line profile of the selected area, line profile exhibit in pixels; (b) iDPC image of the same region and the line profile of the same area; (c) HAADF image at the interface of the planar heterojunction and the corresponding atomic model; (d) iDPC image and atomic model at the same region.
图 5 平面异质结界面典型原子构型 (a) 平面异质结WS2的界面过渡; (b) 融合型界面; (c) 四边形过渡构型; (d) 五边形过渡构型; (e) 四边性、五边形界面构型示意图
Figure 5. Typical configurations of the planar heterojunction interface: (a) Illustration of the transition at the WS2 interface of the planar heterojunction; (b) merged interface; (c) quadrilateral transition configuration; (d) pentagon transition configuration; (e) Individual display of quadrilateral and pentagon interface configurations.
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