Hydrophilic Character of Single-Layer MoS₂ Grown on Ag(111)

The study of MoS₂/metal interfaces is crucial for engineering efficient semiconductor-metal contacts in 2D MoS₂-based devices. Here we investigate a MoS₂/Ag heterostructure fabricated by growing a single MoS₂ layer on Ag(111) by pulsed laser deposition under ultrahigh vacuum (UHV) conditions. The surface structure is observed in situ by scanning tunneling microscopy, revealing the hexagonal moiré pattern characteristic of the clean MoS₂/Ag(111) interface. Ex situ Raman spectroscopy reveals an anomalous behavior of vibrational modes, induced by the strong MoS₂-Ag interaction. After few-hours exposure to ambient conditions the Raman response significantly changes and the formation of molybdenum oxysulfides is revealed by X-ray photoelectron spectroscopy. These effects are due to the interplay with water vapor and can be reversed by a moderate UHV annealing. A polymeric (PMMA) capping is demonstrated to hinder water-induced modifications, preserving the original interface quality for months.