Mechanisms of sliding in bulk and bilayer transition-metal-doped MoS₂

MoS₂ is a useful solid lubricant owing to the ease of shearing along the weakly-bonded basal planes, and dopants such as Ni improve its lubrication properties. The atomistic mechanisms of sliding have been studied in pristine MoS₂, but not in doped MoS₂. We use density functional theory to study the structure and energetics of Ni-doped MoS₂ during sliding, in both bulk and bilayer form. We consider the four reasonable dopant sites of our previous work (J. Phys. Chem. C 125, 13401-13412 (2021)): Mo/S substitutions or octahedral/tetrahedral intercalations. The shape of the sliding potential is similar to that of undoped MoS₂, but we find overall increases in the sliding barrier, particular in intercalated MoS₂ which has interlayer covalent bonds. The energetics in bulk can be well described by pairwise interactions between layers from our bilayer calculations. We study bond breaking, symmetry breaking, and structural changes during sliding, and compute forces as a function of out-of-plane load. The increased interlayer interactions in the presence of a dopant lead to shearing preferentially between undoped layers. Our results provide an atomistic view of how sliding occurs in a doped transition-metal dichalogenide.