Sliding potential energy surfaces of Ni-doped 3R-MoS₂ and comparison to 2H, from DFT calculations

Sliding 2D layers can modulate properties important for strain engineering and ferroelectrics. While the sliding of pristine 2D materials has been studied extensively, much less is understood about doped layers. Our previous work studied 1D [arXiv:2209.15629] and 2D sliding potential energy surfaces (PES) of Ni-doped 2H-MoS₂. Here, we extend that investigation to Ni-doped 3R-MoS₂, which differs in the number of layers, their orientation, and thus unit cell symmetry. We analyze bulk and 3R-like bilayers, considering previously established structures with Ni substituting S or Mo, or intercalated in tetrahedral coordination [Phys. Rev. Materials 5, 074006 (2021)]. Using density-functional theory and a series of constraints on structural optimization, we slide the layers and study the PES and change in structure with sliding in the presence of a dopant. We provide a comparative analysis of the interactions governing sliding in 3R- and 2H-doped structures. While 2H has inversion symmetry, 3R does not. In both 2H and 3R, the symmetry of the PES is reduced from 6-fold symmetry in pristine to 3-fold in doped structures, except for Mo-substitution which has broken symmetry due to a pseudo-Jahn-Teller distortion. Additionally, we find that the polarization of the structure can be modulated by stacking. This work improves our understanding of the interactions behind the sliding of doped 2D materials.