Two-Dimensional interfaces may not be flat*

The interfaces between two-dimensional (2D) materials and 2D-3D materials host rich and diverse phenomena and applications. However, most studies only show a rather local picture of these interfaces. The understanding of the interface interaction including interface flatness on a larger scale remains limited. Here we present the flatness of 2D-2D and 2D-3D interfaces in arrays of field-effect transistors with a total length of over 16 µm. This approach shows a more complete picture of the 2D interfaces from multiple metal contacts and transistor channels, with and without hexagonal boron nitride (hBN) encapsulation. By using cross-sectional transmission electron microscopy (TEM), we observe that transferring hBN onto contacted 2D materials can dramatically alter the interface structure. Intriguingly, the interface between 3D metal and 2D materials is also not flat. We observe highly non-uniform adhesion between evaporated metal on 2D materials. Finally, we correlate the interface characterization with electrical device measurements, highlighting that interface dynamics can profoundly impact device operation in transistors and beyond. Our results reveal the intricacy and complexity of 2D interfaces and challenge the conventional assumption that 2D interfaces are always flat and uniform.