A room temperature all-optical sensor based on two-dimensional SnS for highly sensitive and reversible NO sensing.

Fiber-optic gas sensors have been considered a low-cost, effective, and robust approach for monitoring nitrogen dioxide (NO) gas which is a major toxic gaseous pollutant. The integration of functional nanoscale materials provides additional dimensions for realizing ultra-sensitive and selective NO detection, however, the trade-off is the need for sophisticated photonic structures or external non-optical peripherals (e.g. electrical heaters). In this work, we demonstrate the development of a room temperature, all-optical, and high-performance NO sensor based on a simple D-shaped optical fiber incorporated with ultra-thin two-dimensional (2D) tin disulfide (SnS). A visible light source at 473 nm is used to power the optical fiber, and at the same time excite the 2D SnS layer via the evanescent field, to generate extra charge carriers. Upon exposure to NO at room temperature, the physisorbed gas molecules induce charge exchange with the 2D SnS. This significantly re-distributes the photo-excited charge carriers in the ultra-thin material, therefore manipulating the corresponding optical absorption and scattering. As a result, the optical output power intensity varies as the sensor output through the evanescent field coupling. This all-optical sensor demonstrates an optical power variation of up to 7 µW upon the exposure of NO gas at a low concentration of 50 ppb. This response is fully reversible with an extremely low limit of detection (LOD) of 0.464 ppb. We consider that this work provides a feasible and simple solution to realize high-performance optical gas sensors without the integration of external non-optical peripherals for effective monitoring of environmentally hazardous gases.