Structural symmetry-breaking is a key strategy to modify the physical and chemical properties of two-dimensional transition metal dichalcogenides. However, little is known about defect formation during this process. Here, with atomic-scale microscopy, we investigate the evolution of defect formation in monolayer MoS2 exposed indirectly to hydrogen plasma. At the beginning of the treatment only top-layer sulfur atoms are removed, while vacancies and the molybdenum atomic layer are maintained. As processing continues, hexagonal-shaped nanocracks are generated along the zigzag edge during relaxation of defect-induced strain. As defect density increases, both photoluminescence and conductivity of MoS2 gradually decreases. Furthermore, MoS2 showed increased friction by 50% due to defect-induced contact stiffness. Our study reveals the details of defect formation during the desulfurization of MoS2 and helps to design the symmetry-breaking transition metal dichalcogenides, which is of relevance for applications including photocatalyst for water splitting, and Janus heterostructures.

the confined 2D layer due to the strongly enhanced Coulomb interaction of electron-hole (e-h) pairs and the reduced dielectric screening. [2,3] Recent advances in achieving high intrinsic quantum yield of TMDs close to unity increases the possibility for practical use in light emitting devices. [4,5] In this regard, efforts have been devoted to the development of various 2D light emitting devices with different device structures, such as the Schottky-junction, split-gated lateral p-n junctions, vertical p-n junctions, ionic gel-gated ambipolar transistors, quantumwell-based tunnel junction, and AC-driven transistors. [6-14] Nevertheless, there are still several issues, such as low efficiency and less tunability, that should be addressed. For efficient light emission, both electrons and holes should be injected efficiently into the light emitting layer. However, it is very difficult to achieve ambipolar transport in monolayer TMDs with large direct bandgap and electrically modulate a broad range of carrier densities due to a limit of the dielectric breakdown. [15] Even though an ionic liquid gating has been proposed as an alternative to conventional gate 2D semiconductors have shown great potential for application to electrically tunable optoelectronics. Despite the strong excitonic photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDs), their efficient electroluminescence (EL) has not been achieved due to the low efficiency of charge injection and electron-hole recombination. Here, multioperationmode light-emitting field-effect transistors (LEFETs) consisting of a monolayer WSe 2 channel and graphene contacts coupled with two top gates for selective and balanced injection of charge carriers are demonstrated. Visibly observable EL is achieved with the high external quantum efficiency of ≈6% at room temperature due to efficient recombination of injected electrons and holes in a confined 2D channel. Further, electrical tunability of both the channel and contacts enables multioperation modes, such as antiambipolar, depletion, and unipolar regions, which can be utilized for polarity-tunable field-effect transistors and photodetectors. The work exhibits great potential for use in 2D semiconductor LEFETs for novel optoelectronics capable of high efficiency, multifunctions, and heterointegration.