Revealing unique energy level alignment at graphene/MoS₂ 2-dimensional layered junction using in situ ambient pressure x-ray photoelectron spectroscopy

The chemical/electronic structures of a 2-dimensional (2D) molybdenum disulfide (MoS₂) monolayer were characterized using ambient pressure x-ray photoelectron spectroscopy (APXPS) under various gas environments. APXPS results captured the binding energy shifts for the MoS₂ chemical states that represent the electronic-structure of the 2D MoS₂ monolayer in a real gas environment. The effect of the presence of a 2D graphene (Gr) layer on the properties of the 2D MoS₂ layer was simultaneously characterized. When coating a Gr layer on the MoS₂ layer, electron injection from MoS₂ to Gr occurs due to the Schottky barrier at the interface. As a result, the Gr layer and MoS₂ layer attain relatively more n-type and p-type characteristics, respectively, compared to when they exist separately. The hole-injection barrier, formed between the MoS₂ layer and the SiO₂ substrate, is lower by about 0.3 eV in the Gr/MoS₂/SiO₂ structure compared to that in the MoS₂/SiO₂ structure. This gap in the energy level alignments is stable so that there is no change in Ar, N₂, O₂, and CO₂ 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 MoS₂/Gr junction when applied to a practical device. (paper)