Revealing unique energy level alignment at graphene/MoS ₂ 2-dimensional layered junction using in situ ambient pressure x-ray photoelectron spectroscopy

Abstract The chemical/electronic structures of a 2-dimensional (2D) molybdenum disulfide (MoS ₂ ) monolayer were characterized using ambient pressure x-ray photoelectron spectroscopy (APXPS) under various gas environments. APXPS results captured the binding energy shifts for the MoS ₂ chemical states that represent the electronic-structure of the 2D MoS ₂ monolayer in a real gas environment. The effect of the presence of a 2D graphene (Gr) layer on the properties of the 2D MoS ₂ layer was simultaneously characterized. When coating a Gr layer on the MoS ₂ layer, electron injection from MoS ₂ to Gr occurs due to the Schottky barrier at the interface. As a result, the Gr layer and MoS ₂ layer attain relatively more n-type and p-type characteristics, respectively, compared to when they exist separately. The hole-injection barrier, formed between the MoS ₂ layer and the SiO ₂ substrate, is lower by about 0.3 eV in the Gr/MoS ₂ /SiO ₂ structure compared to that in the MoS ₂ /SiO ₂ structure. This gap in the energy level alignments is stable so that there is no change in Ar, N ₂ , O ₂ , and CO ₂ environments. Taken together, our results not only present an appropriate methodology for fabricating 2D material-based device components, but also suggest the need for consideration of the charge transfer at the MoS ₂ /Gr junction when applied to a practical device.