Welding 2D Semiconducting Crystals by Covalent Stitching of Grain Boundaries in WS₂

The advent of transition metal dichalcogenides (TMDCs) has revolutionized the field of optoelectronics. In this context, recent advances in the large-area synthesis of monolayer WS₂ opened the door to potential optoelectronic applications because of its inherently high photoluminescence (PL) yield and superior electron mobility. However, randomly distributed point and line defects are key bottlenecks for efficient charge transport, hindering further development of system-on-chip (SoC) technologies. Herein, we report a molecular welding strategy using benzene-1,4-dithiol (BDT) to simultaneously passivate sulfur vacancies and bridge grain boundary (GB) fissures in monolayer WS₂. The GB welding was monitored on the atomic scale by high-angle annular dark-field scanning transmission electron microscopy. This treatment yields uniform PL emission, ∼200-fold enhancement in electron mobility, and a three-order-of-magnitude increase in both on-state current and I_on/I_off ratio for transistors across GBs, comparable to intragrain characteristics. Additionally, temperature-dependent PL spectroscopy was employed to identify the defect types and the activation energy of GBs. Our approach, utilizing the facile vapor deposition of ad-hoc molecules to repair line defects in 2D crystals, offers a scalable and effective solution to repair extended line defects in two-dimensional (2D) semiconductors, advancing the development of high-performance, uniform 2D optoelectronic systems.