Thickness characterization of atomically-thin WSe2 on epitaxial graphene by low-energy electron reflectivity oscillations

In this work, low-energy electron microscopy is employed to probe structural as well as electronic information in few-layer WSe2 on epitaxial graphene on SiC. The emergence of unoccupied states in the WSe2–graphene heterostructures are studied using spectroscopic low-energy electron reflectivity. Reflectivity minima corresponding to specific WSe2 states that are localized between the monolayers of each vertical heterostructure are shown to reveal the number of layers for each point on the surface. A theory for the origin of these states is developed and utilized to explain the experimentally observed featured in the WSe2 electron reflectivity. This method allows for unambiguous counting of WSe2 layers, and furthermore may be applied to other 2D transition metal dichalcogenide materials.