Interlayer Coupling Dependent Discrete H → T' Phase Transition in Lithium Intercalated Bilayer Molybdenum Disulfide

In this work, the interlayer coupling dependent lithium intercalation induced phase transition in bilayer MoS₂ (BL-MoS₂) was investigated using an atomic-resolution annual dark-field scanning transmission electron microscope (ADF-STEM). It was revealed that the lithiation induced H → T' phase transition in BL-MoS₂ strongly depended on the interlayer twist angle; i.e., the H → T' phase transition occurred in well-stacked H phase BL-MoS₂ (with a twist angle of θ_t = 0°) but not for θ_t ≠ 0° BL-MoS₂. The lithiated BL-MoS₂ appeared in homophase stacking, either T'/T' or H/H (locally, no phase transformation) stacking, without any heterophase stacking such as H/T' or T'/H observed. This finding indicated the H → T' phase transition occurred via a domain-by-domain mode rather than layer-by-layer. Up to 15 types of stacking orders were experimentally identified locally in lithiated bilayer T'-MoS₂, and the formation mechanism was attributed to the discrete interlayer translation with a unit step of (m/6a, n/6b) (m, n = 0, 1, 2, 3), where a and b were the primitive lattice vectors of T'-MoS₂. Our experimental results were further corroborated by ab initio density functional theory (DFT) calculations, where the occurrence of different stacking orders can be quantitatively correlated with the variation of intercalated lithium contents into the BL-MoS₂. The present study aids in the understanding of the phase transition mechanisms in atomically thin 2D transition metal dichalcogenides (TMDCs) and will also shed light on the precisely controlled phase engineering of 2D materials for memory applications.