Grain Boundaries and Tilt-Angle-Dependent Transport Properties of a 2D MoC Superconductor.

The grain boundaries (GBs) of graphene and molybdenum disulfide have been extensively demonstrated to have a strong influence on electronic, thermal, optical, and mechanical properties. 2D transition-metal carbides (TMCs), known as MXenes, are a rapidly growing new family of 2D materials with many fascinating properties and promising applications. However, the GB structure of 2D TMCs and the influence of GB on their properties remain unknown. Here, we used aberration-corrected scanning transmission electron microscopy combined with electrical measurements to study the GB characteristic of highly crystalline 2D MoC superconductor, a newly emerging member of the 2D TMC family. The 2D MoC superconductor shows a unique tilt-angle-dependent GB structure and electronic transport properties. Different from the reported 2D materials, the GB of 2D MoC shows a peculiar dislocation configuration or sawtooth pattern depending on the tilt angle. More importantly, we found two new periodic GBs with different periodic structures and crystallographic orientations. Electrical measurements on individual GBs show that GB structure strongly affects the transport properties. In the normal state, an increasingly stronger electron localization behavior is observed at the GB region with increasing tilt angle. In the superconducting state, the magnitude of the critical current across the GBs is dramatically reduced, associated with local suppression of superconductivity at GBs. These findings provide new understandings on the GB structure of 2D TMCs and the influence of GB on 2D superconductivity, which would be helpful for tailoring the properties of 2D TMCs through GB engineering.