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.