Two-dimensional materials are emerging as the ideal candidates for innovative electronic and optoelectronic devices, since they intrinsically push the scaling process down to the atomic level, overcoming common technologic limits. Among the plethora of two-dimensional materials, Transition Metal Dichalcogenide (TMD) monolayers, such as the direct-bandgap semiconducting WS₂, have been intensively studied since they exhibit a unique combination of atomic-scale thickness, optoelectronic and mechanical properties, resulting suitable for a wide range of applications. Moreover, Chemical Vapour Deposition (CVD) techniques allow for a fairly easy synthesis of large flakes. However, WS₂ properties are strongly influenced by the adsorption of chemical species in the surrounding environment due to the large surface-volume ratio in the monolayers.