Van der Waals Gap-Confined Molecular Engineering of MoS₂ for Selectivity-Enhanced and Humidity-Tolerant Gas Sensing

Controlling interactions between sensor surfaces and target molecules is central to achieving high selectivity and stability in chemical sensing. However, in two-dimensional (2D) material-based sensors, unintended adsorption, particularly of water molecules, often disrupts these interactions, degrading selectivity and long-term stability. Here, we exploit molecular confinement within van der Waals (vdW) gaps of MoS₂ to engineer sensor-analyte interactions. Ethanol (EtOH) confinement induces electron doping and hydrophobicity, whereas water confinement produces negligible doping and hydrophilicity. This combined modulation suppresses H₂O adsorption while promoting selective NO₂ adsorption, enabling the EtOH-confined sensor to detect NO₂ from 200 ppb to 5 ppm, with a maximum response of 48% at 60% relative humidity and a detection limit of 36 ppb. Under highly humid conditions, it exhibits a sevenfold higher NO₂ response and retains 95% of its initial response after 5 weeks, compared with 44% for H₂O-confined films. First-principles calculations reveal that molecular confinement regulates sulfur-vacancy (V_s) configurations: EtOH favors isolated and lined V_s, which promote NO₂ adsorption while suppressing H₂O binding, whereas H₂O stabilizes clustered V_s, which increase H₂O affinity. This vdW-gap confinement provides a design strategy to tune surface chemistry, electronic structure, and defect configurations for selective, humidity-tolerant, and reliable 2D chemical sensors.