Why Does a Transition\nMetal Dichalcogenide Nanoribbon\nNarrow into a Nanowire under Electron Irradiation?

Transition metal dichalcogenide (TMDC) nanowires have\npractical\napplications in 1D electron channels, spintronics, optoelectronics,\nand catalysis due to their authentic subnanometer width (<1 nm)\nand intrinsic metallicity. Although narrowing of a TMDC nanoribbon\ninto a nanowire under electron irradiation has been frequently observed\nin the synthesis of TMDC nanowires, the mechanism underlying this\nunexpected structural transformation remains a mystery. Here, to reveal\nthe underlying mechanism, we combine first-principles calculations\nwith a global structure search of 1D nanowires and show that a nanoribbon\nof 1H-phase MoS<sub>2</sub> with a width narrower than 6 rings is\nenergetically unfavorable compared with its nanowire counterpart due\nto the edge–edge interaction. The bending effect induced by\nS defects under electron irradiation is the major driving force for\nthe transition of MoS<sub>2</sub> nanoribbon into a nanowire. We predict\nthat the precursor of the Mo<sub>6</sub>S<sub>6</sub> nanowire is\na well-defined Mo<sub>11</sub>S<sub>11</sub>-i nanowire with an unexpected\nstoichiometry. The intrinsic local compressive strain triggers a phase\ntransition from Mo<sub>11</sub>S<sub>11</sub>-i to its slightly modified\nsister nanowire, Mo<sub>11</sub>S<sub>11</sub>-ii, which is characterized\nby the configuration (Mo<sub>1</sub>S<sub>1</sub>)<sub>5</sub>&Mo<sub>6</sub>S<sub>6</sub>. Triggered by electron irradiation, the nanoribbon\nundergoes a step-by-step narrowing process with sequential peeling\nof a Mo<sub>1</sub>S<sub>1</sub> fragment in each step to form a robust\nMo<sub>6</sub>S<sub>6</sub> nanowire. This unique narrowing mechanism\nis universal for the nanoribbon-to-nanowire transformation of other\nTMDCs under electron irradiation. Our study highlights a hitherto\nunexplored mechanism for creating individual M<sub>6</sub>X<sub>6</sub> nanowires and contributes to an in-depth understanding of the narrowing\nof TMDC nanoribbons under electron irradiation.