Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences , Shenyang 110016, P. R. China.
Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University , Beijing 100871, P. R. China.
Nano Lett. 2016 Jul 13;16(7):4243-50. doi: 10.1021/acs.nanolett.6b01265. Epub 2016 Jun 23.
The properties of two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides are strongly influenced by domain boundaries. Ultrathin transition metal carbides are a class of newly emerging 2D materials that are superconducting and have many potential applications such as in electrochemical energy storage, catalysis, and thermoelectric energy conversion. However, little is known about their domain structure and the influence of domain boundaries on their properties. Here we use atomic-resolution scanning transmission electron microscopy combined with large-scale diffraction-filtered imaging to study the microstructure of chemical vapor deposited high-quality 2D α-Mo2C superconducting crystals of different regular shapes including triangles, rectangles, hexagons, octagons, nonagons, and dodecagons. The Mo atom sublattice in all these crystals has a uniform hexagonal closely packed arrangement without any boundaries. However, except for rectangular and octagonal crystals, the C atom sublattices are composed of three or six domains with rotational-symmetry and well-defined line-shaped domain boundaries because of the presence of three equivalent off-center directions of interstitial carbon atoms in Mo octahedra. We found that there is very small lattice shear strain across the domain boundary. In contrast to the single sharp transition observed in single-domain crystals, transport studies across domain boundaries show a broad resistive superconducting transition with two distinct transition processes due to the formation of localized phase slip events within the boundaries, indicating a significant influence of the boundary on 2D superconductivity. These findings provide new understandings on not only the microstructure of 2D transition metal carbides but also the intrinsic influence of domain boundaries on 2D superconductivity.
二维（2D）材料如石墨烯和单层过渡金属二硫属化物的性质强烈受到畴界的影响。超薄过渡金属碳化物是一类新兴的 2D 材料，具有超导性，并在电化学储能、催化和热电能量转换等方面具有许多潜在应用。然而，人们对它们的畴结构以及畴界对其性质的影响知之甚少。在这里，我们使用原子分辨扫描透射电子显微镜结合大规模衍射过滤成像技术研究了不同规则形状的化学气相沉积高质量 2D α-Mo2C 超导晶体的微观结构，这些晶体包括三角形、矩形、六边形、八边形、九边形和十二边形。所有这些晶体中的 Mo 原子亚晶格都具有均匀的六方紧密堆积排列，没有任何边界。然而，除了矩形和八边形晶体之外，C 原子亚晶格由三个或六个具有旋转对称性和明确定义的线状畴界组成，这是由于 Mo 八面体中存在三个等效的中心外碳原子间隙位置。我们发现畴界处的晶格剪切应变非常小。与单畴晶体中观察到的单一尖锐转变相反，穿过畴界的输运研究显示出一个宽的电阻超导转变，具有两个明显的转变过程，这是由于边界内形成局部相滑移事件所致，这表明边界对 2D 超导性有显著影响。这些发现不仅提供了对二维过渡金属碳化物的微观结构的新认识，也提供了对二维超导性中畴界内在影响的新认识。
