The transition metal dichalcogenides (TMDCs) are one of the most promising class of two dimensional (2D) materials to be included into new-generation electronic and optoelectronic devices  as the semiconducting analogue to graphene . Molybdenum disulfide (MoS2) is the most widely explored member of TMDCs, due to its uncommon size dependent properties. MoS2 displays a shift from indirect to direct band gap when going from its bulk to monolayer form , with a more than a 104-fold enhancement of the photoluminescence quantum yield . Currently, one of the greatest challenges in materials science is to grow stable 2D layers of MoS2 via different synthetic approaches and to achieve structure-property correlations of monolayer, bilayer, and multilayer sheets . Among the various approaches attempted to grow MoS2 monolayer, the one consisting in the deposition of Mo atoms on Au(111) at room temperature, followed by annealing, both performed in H2S atmosphere, led to the growth of high quality single layer nano-islands, allowing the measurement of the band structure of monolayer MoS2 [5-7].
Here we present a high energy resolution X-ray photoelectron spectroscopy study (HR-XPS), carried out at the SuperESCA beamline of Elettra, the Italian synchrotron radiation facility, on the growth of MoS2 on Au(111) performed with the aforementioned growth method. The XPS technique, used also in its fast modality to follow transient processes (Fast-XPS), allowed us to detect the different species present on the surface and to follow their evolution during annealing, at the transition from sulfided Mo clusters to MoS2 monolayer. The HR-XPS spectra of the Au 4f core level showed the appearance of a new component upon MoS2 growth, caused by the MoS2-substrate interaction. This interaction is also detected in the S 2p core level region, where the component related to the bottom sulfur layer, which is in contact with gold, is shifted with respect to the top sulfur layer of the 'S-Mo-S' sandwich structure.
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