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

Abstract

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 In 2 S 3 . 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 In 2 S 3 . 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 NO 2 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 NO 2 gas at room temperature. • Free-standing Janus indium oxysulfide is formed from In 2 S 3 with oxygen incorporation. • Surface crystal transfer is due to the replacement of sulfur by diffused oxygen. • Deteriorated exciton binding energy and recombination rate; prolonged PL lifetime. • Visible-light-driven gas sensor is ultrasensitive to NO 2 gas at room temperature. • The limit of detection (LOD) is in parts-per-trillion (ppt) range.