Free-standing ultra-thin Janus indium oxysulfide for ultrasensitive visible-light-driven optoelectronic chemical sensing

Atomically-thin Janus heterojunctions exhibit extraordinary electronic and optoelectronic properties, mainly due to their intrinsic built-in electric field. However, current investigations are limited within layered metal chalcogenides. Here, a free-standing ultra-thin Janus metal oxychalcogenide is realized from non-layered In2S3. In the presence of strong mechanical agitation in the liquid medium, the original tetragonal crystal is cleaved. Ambient oxygen atoms are subsequently diffused and incorporated into limited part of the crystal structure, forming the oxysulfide phase with the gradual transition into a hexagonal structure. While the integrity of the covalent bonding system is maintained, a tensile strain is generated as a result of the crystal coordination mismatching between the pure sulfide and oxysulfide phases, leading to a more than two orders enhancement on visible-light-driven exciton lifetime compared to that of pure In2S3. Such an impressive excitonic interaction provides the fundamentals to establish an ultra-sensitive and power-saving optoelectronic chemical sensing platform. As an example, dipoles generated by the surface adsorbed NO2 molecules cause significant re-distribution of photoinduced charges in the anisotropic non-layered Janus structure under visible light low-power excitation, resulting in a superior sub-ppb detection limit of NO2 gas at room temperature.