In Situ STEM Observation of Phase Transition Triggered by Mo Migration in MoTe₂

The heterostructure of transition metal dichalcogenides (TMDs), such as one-dimensional (1D) nanowires embedded in two-dimensional (2D) nanosheets, has drawn much research attention due to its unique electronic, spintronic, magnetic, and catalytic properties. The general approach for preparing such a heterostructure is through electron beam lithography or annealing on the 2D template, triggering direct formation of the 1D component within the 2D matrix. However, the thermodynamic mechanism behind the transition from 2D to 1D is still not well clarified. Here, by in situ scanning transmission electron microscopy (STEM), we present a direct observation of two metastable phases, M-Mo_1+<i>x</i>Te₂ and M-Mo₆Te₆, which enable a smooth transition from 2D 2H-MoTe₂ nanosheets to 1D Mo₆Te₆ nanowires (NWs). As a result, an atomically sharp "NW-MoTe₂-NW" heterojunction is formed, with a coherent interface between 2D 2H-MoTe₂ and 1D Mo₆Te₆ NWs. The study provides a deep understanding of the growth of 1D Mo₆Te₆ NWs from 2D MoTe₂ and a pathway for predictive and controlled atomic-level manipulation for the directed synthesis of the 1D/2D heterostructure.