We studied the effects of thermal annealing (in air and in ultra-high vacuum from room temperature up to 300 C) of mechanically exfoliated mono-layer and few layer MoS 2 onto 270 nm SiO 2/Si (1 0 0). The experiments were performed with optical microscopy, atomic force microscopy, non resonant Raman spectroscopy, and photoluminescence (PL) spectroscopy on the mono-layer flakes. We demonstrate the presence of a nano-confined water layer at the interface with the silicon substrate. The thickness of this water layer can be increased by immersing the exfoliated samples in water for one hour, or decreased by post exfoliation annealing. Then, we directly demonstrate the sublimation with annealing of the bottom layer at the interface with SiO 2. PL experiments performed on the mono-layers in the 250 C–300 C annealing range, together with previous x-ray photoemission experiments, demonstrate the direct …