2D Organometallic Chalcogenide for Sub‐ppt Ratiometric Sensing

ABSTRACT Semiconductor‐based sensors suffer from baseline drift that compromises measurement accuracy. Ratiometric sensing offers a solution by introducing a photoresponse internal reference, yet conventional materials face an inherent trade‐off between gas sensitivity and photoelectric stability. Here, we report a 2D organometallic chalcogenide, PbBD, featuring a multi‐quantum well structure composed of a continuous inorganic {PbS} n layer partially covered by compact and flexible organic ligands. This unique configuration exposes active sites on the inorganic layer, allowing gas molecules to directly interact with the inorganic semiconductor layer, bypassing the indirect electron transfer pathway typically imposed by functional groups. This direct interaction facilitates electron transfer efficiency and significantly enhances the sensitivity in gas sensing applications. The PbBD nanosheet‐based sensor achieves an ultrahigh response of 52900% to 10 ppm NO 2 and a ratiometric value of 0.84, which is 3.67 times higher than that of AgBDT. PbBD delivers both enhanced ratiometric sensitivity and an exceptionally low detection limit of 0.245 ppt, marking the lowest value ever reported for room‐temperature chemiresistive NO 2 sensing. Furthermore, the ratiometric mode reduces the coefficient of variation from 21.8% to 5.5% against temperature fluctuations. This work provides a strategy for co‐optimizing photoelectric and gas‐sensing properties toward high‐precision sensing applications.