Na transport in bilayer MoS2 and MoS2-WS2 heterojunction with S vacancy defect: First-principles study

Based on first-principles calculations, the diffusion properties of Na+ in bilayer MoS2 and the MoS2-WS2 heterojunction were investigated. Owing to the similar structures, the diffusion properties of Na are almost identical in both systems. On perfect surfaces, the diffusion energy barrier is smaller than 0.1 eV. The diffusion characteristics of Na+ ions between layers are related to the size of the system. For a small size (<3 nm2), the Na+ diffusion energy barrier is around 0.40 eV because of sliding between layers. With the size of the system increasing, the energy barrier for Na+ diffusion between layers increases to 0.90 eV. S vacancy defects can trap Na+ ions and hinder Na+ diffusion. The diffusion energy barrier on the surface with S vacancy defects will increase to around 0.5 eV. When Na ions are transported between layers, they are tightly bound by S vacancy defects. The energy barrier exceeds 2.0 eV.