Mechanisms of Semiconducting 2H to Metallic 1T Phase Transition in Two-dimensional MoS2 Nanosheets

In the present work, phase transition mechanisms from semiconducting 2H phase to metallic IT phase in MoS2 nanosheets were studied using density functional theory (DFT) method. Various 2H -> IT phase transition mechanisms that consist of nucleation and propagation steps, which were simulated by collective rotational and rotational/translational movements, single atom translational movement, and the gliding movement of one row for sulfur (5) atoms, on both the basal plane and Mo- and S-edges with different S coverages were investigated. On the perfect basal plane, the IT phase nucleation is unlikely due to the extremely high barrier of 2.25 eV/atom, whereas the presence of defective S vacancies on the basal plane dramatically facilitate the IT phase nucleation and propagation around the defective sites by the collective rotational movement of three S atoms. On the 2H phase basal plane with two S vacancies, the kinetic barriers for the T phase nucleation are as low as of 0.66-0.77 eV/atom. Like the promoting effect of S vacancies on the phase transition over the basal plane, DFT results suggest that the S coverage on the Mo- and S-edges will affect the 1T phase nucleation and propagation. The 1T phase nucleation starting with the translational movement of single S atom on the bare Mo-edge and the gliding movement of an entire row of S atoms on the S-edge with 50% S coverage are kinetically favorable. While the 1T phase formation at the Mo-edge with 50% S coverage and the S-edge with 100% S coverage are unlikely. The present work not only confirms the important role of S vacancies/coverages in the 2H-1T phase transition but also provides new insight into how and where the 2H-1T phase transition occurs at the atomic level, which also sheds light on the general phase transition mechanism for two-dimensional transition metal dichalcogenide materials.