On the role of nano-confined water at the 2D/SiO2 interface in layer number engineering of exfoliated MoS2 via thermal annealing

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 MoS2 onto 270 nm SiO2/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 SiO2. 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 correlation of the PL integrated spectral intensity with the concentration of sulfur vacancies that are passivated by oxygen after exposure to air. Namely, the vacancy formation is triggered in the 250 °C-300 °C range independently of the annealing environment, and correspondingly in the same temperature range the PL spectral intensity proportionally increases. With respect to pristine exfoliated MoS2 we estimate via PL an increase by a factor three of the vacancy concentration upon annealing to 300 °C. This, considering the typical vacancy concentration values of mechanical exfoliated MoS2, leads to sulfur vacancy concentration values close to 1014 cm-2. Considering such high value, the sublimation of the bottom layer is then explained via oxidation of the sulfur vacancies upon dissociation of water preferentially supplied by the film present at the interface with the SiO2 substrate.