Nanoscale Friction on Confined Water Layers Intercalated between MoS₂ Flakes and Silica

Frictional energy dissipation at the interfaces of two-dimensional (2D) materials through the excitation and transfer processes of kinetic energy into the bulk can be easily influenced by an intercalated water film. An enhancement of friction on water-intercalated graphene has been observed. Is this frictional enhancement by confined water a general phenomenon? We address this issue by investigating the frictional behavior of confined water layers intercalated between single-layer molybdenum disulfide (MoS2), synthesized using chemical vapor deposition, and a silica substrate. The icelike water was intercalated by exposure to high-humidity air. We found that the intercalated water molecules morphologically deform the 2D MoS2 sheet, forming distinct subdomains after the exposure to high humidity. We found that the adsorption of the icelike water layer between the MoS2 and the silica leads to friction enhancement, compared with a pristine MoS2/silica sample, which is associated with additional phononic friction energy dissipation at the solid-liquid interface, as indicated by the phonon distribution analysis from the empirical force-field calculations. Moreover, the atomic stick-slip behavior shows that the lattice orientation of the hydrophilic MoS2 affects water molecule diffusion at the interface of the MoS2/silica substrate. Chemical mapping of the water-intercalated MoS2 on silica using scanning photoelectron microscopy and vacuum annealing processes shows water intercalation without changing the intrinsic composition of the MoS2 on silica.