Intercalation of Si between MoS₂ layers

We report a combined experimental and theoretical study of the growth of sub-monolayer amounts of silicon (Si) on molybdenum disulfide (MoS₂). At room temperature and low deposition rates we have found compelling evidence that the deposited Si atoms intercalate between the MoS₂ layers. Our evidence relies on several experimental observations: (1) Upon the deposition of Si on pristine MoS₂ the morphology of the surface transforms from a smooth surface to a hill-and-valley surface. The lattice constant of the hill-and-valley structure amounts to 3.16 Å, which is exactly the lattice constant of pristine MoS₂. (2) The transitions from hills to valleys are not abrupt, as one would expect for epitaxial islands growing on-top of a substrate, but very gradual. (3) I(V) scanning tunneling spectroscopy spectra recorded at the hills and valleys reveal no noteworthy differences. (4) Spatial maps of dI/dz reveal that the surface exhibits a uniform work function and a lattice constant of 3.16 Å. (5) X-ray photo-electron spectroscopy measurements reveal that sputtering of the MoS₂/Si substrate does not lead to a decrease, but an increase of the relative Si signal. Based on these experimental observations we have to conclude that deposited Si atoms do not reside on the MoS₂ surface, but rather intercalate between the MoS₂ layers. Our conclusion that Si intercalates upon the deposition on MoS₂ is at variance with the interpretation by Chiappe et al. (Adv. Mater. 2014, 26, 2096–2101) that silicon forms a highly strained epitaxial layer on MoS₂. Finally, density functional theory calculations indicate that silicene clusters encapsulated by MoS₂ are stable.