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 WSe 2 on epitaxial graphene on SiC. The emergence of unoccupied states in the WSe 2 –graphene heterostructures are studied using spectroscopic low-energy electron reflectivity. Reflectivity minima corresponding to specific WSe 2 states that are localized between the monolayers of each vertical het-erostructure 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 WSe 2 electron reflectivity. This method allows for unambiguous counting of WSe 2 layers, and furthermore may be applied to other 2D transition metal dichalcogenide materials.