Direct imaging of single monolayer tungsten diselenide flake on bilayer graphene with photoelectron momentum microscope

A photoelectron momentum microscope (MM) [1-2] was used to investigate the electronic structures of μm-sized monolayer WSe₂ flakes epitaxially grown on bilayer graphene (BLG). Our study began with microscopically allocating the targeted WSe₂ single flake in direct space (r-space) followed by acquiring the two-dimensional (2D) constant energy contours of that flake in momentum space (k-space). Next, multiple k-space images were recorded as a function of electron kinetic energy to complete a 3D data set from which the band structure of a single WSe₂ flake was extracted. Our study concluded that the distinct flake orientations often seen in epitaxial WSe₂/BLG systems do not lead to significant variation in the electron structures of WSe₂. However, there is a spatially inhomogeneous electrostatic field on the surface of our specimen that causes the entire WSe₂ and BLG bands to shift 60 to 100 meV locally. Finally, we show that MM can measure the twisted angle between lattices of exfoliated WSe₂ flakes and the underlying BLG through a single k-space image. In conclusion, we demonstrate that MM can directly access the electronic structures of 2D materials/heterostructures from μm-sized regions with a few 10’s meV energy sensitivity.