  Intercalation of alkali atoms within the lamellar transition metal
dichalcogenides is a possible route toward a new generation of batteries. It is
also a way to induce structural phase transitions authorizing the realization
of optical and electrical switches in this class of materials. The process of
intercalation has been mostly studied in three-dimensional dichalcogenide
films. Here, we address the case of a single-layer of molybdenum disulfide
(MoS$_2$), deposited on a gold substrate, and intercalated with cesium (Cs) in
ultra-clean conditions (ultrahigh vacuum). We show that intercalation decouples
MoS$_2$ from its substrate. We reveal electron transfer from Cs to MoS$_2$,
relative changes in the energy of the valence band maxima, and electronic
disorder induced by structural disorder in the intercalated Cs layer. Besides,
we find an abnormal lattice expansion of MoS$_2$, which we relate to immediate
vicinity of Cs. Intercalation is thermally activated, and so is the reverse
process of de-intercalation. Our work opens the route to a microscopic
understanding of a process of relevance in several possible future
technologies, and shows a way to manipulate the properties of two-dimensional
dichalcogenides by "under-cover" functionalization.
