Ion-Pair Design Rules for Defect Passivation in 2D WS2

Two-dimensional (2D) semiconductors hold great promise for next-generation optoelectronics; however, device performance is frequently limited by intrinsic defects. In monolayer WS2, sulfur vacancies strongly quench optical emission, while existing chemical passivation strategies remain largely empirical. Here, we establish ion-pair interactions as a predictive design principle for lithium-based chemical passivation. Systematic comparison of lithium salts reveals that weaker Li+-anion interactions increase the effective availability of Li+ for vacancy binding and enhance photoluminescence (PL), which is further supported by low-temperature PL and femtosecond transient absorption (fs-TA) measurements. Density functional theory (DFT) calculations reveal that strong ion-pair formation counteracts passivation, directly linking ion-pair interaction energies to the experimentally observed PL trends. Guided by this framework, we predict and experimentally validate lithium bis(pentafluoroethylsulfonyl)imide (Li-BETI) as a new passivation agent that matches or exceeds the performance of Li-TFSI. Together, these findings establish molecular-level chemical design rules for defect passivation in 2D semiconductors.