Phase transformations in single-layer MoTe₂ stimulated by electron irradiation and annealing

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), MoTe₂ is predestined for phase-engineering applications due to the small difference in free energy between the semiconducting H-phase and metallic 1T′-phase. At the same time, the complete picture of the phase evolution originating from point defects in single-layer of semiconducting H-MoTe₂ via Mo₆Te₆ nanowires to cubic molybdenum has not yet been reported so far, and it is the topic of the present study. The occurring phase transformations in single-layer H-MoTe₂ were initiated by 40-80 kV electrons in the spherical and chromatic aberration-corrected high-resolution transmission electron microscope and/or when subjected to high temperatures. We analyse the damage cross-section at voltages between 40 kV and 80 kV and relate the results to previously published values for other TMDs. Then we demonstrate that electron beam irradiation offers a route to locally transform freestanding single-layer H-MoTe₂ into one-dimensional (1D) Mo₆Te₆ nanowires. Combining the experimental data with the results of first-principles calculations, we explain the transformations in MoTe₂ single-layers and Mo₆Te₆ nanowires by an interplay of electron-beam-induced energy transfer, atom ejection, and oxygen absorption. Further, the effects emerging from electron irradiation are compared with those produced by in situ annealing in a vacuum until pure molybdenum crystals are obtained at temperatures of about 1000 °C. A detailed understanding of high-temperature solid-to-solid phase transformation in the 2D limit can provide insights into the applicability of this material for future device fabrication.