PEEM and micro-UPS studies of cleaved and exfoliated molybdenum disulfide surfaces

Thin films of molybdenum disulfide (MoS2) are generally exfoliated from a mineral crystal. Impurities and contaminants of mineral origin may spoil the expected functions of MoS2 thin-film devices because they have a strong influence on the properties of the exfoliated films. The effects of surface contaminants on cleaved or exfoliated MoS2 surfaces were investigated using photoemission electron microscopy and selected-area ultraviolet photoelec-tron spectroscopy under several exfoliation conditions. The chemical composition and crystallinity of surfaces were also investigated using Auger electron spectroscopy and electron diffraction. The bulk crystal included a limited number of contaminated interfaces, at which the crystal was easily cleaved. Therefore, repeated exfoliations are required to expose a clean surface for exfoliated films. Annealing at ca. 400◦C in a vacuum was effective to reduce contamination of the cleaved MoS2 surface.