Thickness dependence of surface energy and contact angle of water droplets on ultrathin MoS₂ films

We have performed a systematic density functional study of surface energy of MoS₂ films as a function of thickness from one to twelve layers with the consideration of van der Waals (vdW) interactions using the vdW-DF and DFT-D2 methods. Both vdW schemes show that the surface energy will increase with the increase of the number of atomic layers and converge to a constant value at about six layers. Based on the calculated surface energies, we further analyze the surface contact angle of water droplets on the MoS₂ film surface using Young's equation as a function of thickness in comparison with experiments, from which the water-MoS₂ interfacial energy is derived to be independent of MoS₂ thickness. Our calculations indicate that the vdW interactions between the MoS₂ layers play an important role in determining surface energy, and results in the thickness dependence of the contact angle of water droplets on the MoS₂ film surface. Our results explain well the recent wetting experiment [Nano Lett., 2014, 14(8), 4314], and will be useful for future studies of physical and chemical properties of ultrathin MoS₂ films.