Correlating Chemical and Electronic States from Quantitative Photoemission Electron Microscopy of Transition-Metal Dichalcogenide Heterostructures
Vertical heterostructures of MoS2 and WSe2 layers are studied by spectroscopic photoemission electron microscopy as an effective technique for correlating chemical and electronic states at the micrometer scale. Element-specific, surface-sensitive images recorded at high lateral and energy resolution from core-level photoelectrons using different laboratory excitation sources are postprocessed to obtain laterally resolved maps of elemental composition and energy shifts in the Mo3d spectra of a few hundred meV. For monolayer MoS2, the method reveals substrate-dependent charge transfer properties within the narrow energy range of 360 meV, with MoS2 becoming more n-type after transfer onto WSe2. The band structure data from momentum microscopy taken over the same areas confirm the charge transfer from WSe2 to MoS2 by the shift of the K-bands away from the Fermi level and illustrates the layer-specific contributions to the electronic band structure of the heterostructure. From work function mapping, the reconstructed energy-level diagram reveals a type II heterostructure but with a very small conduction-band offset.