Ultrathin Copper\nMonosulfide Films for an Optically\nSemitransparent,\nHighly Selective Ammonia Chemosensor

Transition-metal sulfides are emerging as promising materials\nfor\nchemiresistive gas sensorsa field still dominated by semiconducting\nmetal oxides. Despite the availability of materials with tunable electronic,\noptical, physical, and chemical properties, few studies have moved\nbeyond synthesis to provide strategies for enhancing gas sensing performance\nthrough material modification. Here, we present a simple, scalable\nsynthetic strategy for developing an optically semitransparent, flexible\nNH<sub>3</sub> gas sensor with a highly uniform, ultrathin CuS (covellite)\nactive sensing layer. The optical and chemical properties of the CuS\nwere precisely controlled near the percolation threshold of thin-film\nformation by varying key experimental parameters such as the Cu film\nthickness (<10 nm) and the sulfurization time (∼90 s) under\nambient conditions. Experimental and computational studies of CuS\nand its NH<sub>3</sub> sensing characteristics identify key physicochemical\nproperties. The controlled surface chemistry and morphology of the\nultrathin CuS layer demonstrate its effectiveness in functional NH<sub>3</sub> sensing devices, which achieve a calculated detection limit\nof 1.38 ppm for NH<sub>3</sub> gas at 150 °C, along with exceptional\nmechanical robustness and optical semitransparency in the visible-light\nspectrum.