In-depth study of the chemical/electronic structures of two-dimensional molybdenum disulfide materials with sub-micrometer-resolution scanning photoelectron microscopy

Abstract The limitation of x-ray photoelectron spectroscopy (XPS) carried out with an in-lab source can be overcome by synchrotron-based scanning photoelectron microscopy (SPEM) that integrates the XPS technique with scanning microscopy, enabling the analysis of the chemical/electronic structure of a material with sub-micrometer spatial resolution. Using this analytical method, the changes in the structural properties of two-dimensional (2D) molybdenum disulfide (MoS ₂ ) materials according to the synthetic method used were studied. In addition to the monolayer form of a 2D MoS ₂ material (MoS ₂ monolayer), multilayered 2D MoS ₂ materials and MoS ₂ monolayers doped with the widely used p-type dopants, gold chloride (AuCl ₃ ) and bis(trifluoromethane)sulfonamide (TFSI), were prepared by a controlled synthetic process. Through comparative analysis of the SPEM data with those obtained by other techniques such as Raman spectroscopy, auger electron spectroscopy (AES), and atomic force microscopy (AFM), the variation in the chemical/electronic structures of MoS ₂ 2D materials depending on the synthetic process was clarified. From SPEM data acquired from locations where AuCl ₃ or TFSI dopant molecules were present, all the MoS ₂ chemical states were confirmed to have relatively lower binding energies than those of an as-grown MoS ₂ monolayer. These differences are related to the effects of p-type doping on the MoS ₂ 2D material. Separately, the increase in the MoS ₂ layer number is manifested in the form of brightness difference in the SPEM data. Meanwhile, all the binding energies of the MoS ₂ chemical states including the onset of the valence band maximum are slightly lower in the bright regions than in the dark regions, and these binding energy differences are presumably due to the change in the bandgap change based on the MoS ₂ layer number.