Multifarious interfaces, band alignments, and formation asymmetry of WSe 2 -MoSe 2 heterojunction grown by molecular-beam epitaxy.

Monolayer transition-metal dichalcogenides (TMDs) continue to attract research attention and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe and WSe, may host many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demands. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe-MoSe heterojunction. The corresponding electronic bands show type-II band alignment for both ML (monolayer)-ML and ML-BL (bilayer) lateral junctions irrespective of the presence or not of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe deposition precedes MoSe. Reversing the deposition order leads to alloying of the two materials without noticeable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.