SnSe 2 is an anisotropic binary-layered material with rich physics, which could see it used for a variety of potential applications. Here, we investigate the gas-sensing properties of SnSe 2 using first-principles calculations and verify predictions using a gas sensor made of few-layer SnSe 2 grown by chemical vapor deposition. Theoretical simulations indicate that electrons transfer from SnSe 2 to NO 2 , whereas the direction of charge transfer is the opposite for NH 3 . Notably, a flat molecular band appears around the Fermi energy after NO 2 adsorption and the induced molecular band is close to the conduction band minimum. Moreover, compared with NH 3 , NO 2 molecules adsorbed on SnSe 2 have a lower adsorption energy and a higher charge transfer value. The dynamicsensing responses of SnSe 2 sensors confirm the theoretical predictions. The good match between the theoretical prediction and experimental demonstration suggests that the underlying sensing mechanism is related to the charge transfer and induced flat band. Our results provide a guideline for designing high-performance gas sensors based on SnSe 2 . KEYWORDS: SnSe 2 , gas sensing, charge transfer, first-principles calculations, selective gas sensing, NO 2 gas sensor, NH 3 gas sensor

Graphene is a single layer of carbon atoms with a large surface-to-volume ratio, providing a large capacity gas molecule adsorption and a strong surface sensitivity. Chemical vapor deposition-grown graphene-based NO 2 gas sensors typically have detection limits from 100 parts per billion (ppb) to a few parts per million (ppm), with response times over 1000 s. Numerous methods have been proposed to enhance the NO 2 sensing ability of graphenes. Among them, surface decoration with metal particles and metal-oxide particles has demonstrated the potential to enhance the gas-sensing properties. Here, we show that the NO 2 sensing of graphene can be also enhanced via decoration with monodisperse polymer beads. In dark conditions, the detection limit is improved from 1000 to 45 ppb after the application of polystyrene (PS) beads. With laser illumination, a detection limit of 0.5 ppb is determined. The enhanced gas sensing is due to surface plasmon polaritons excited by interference and charge transfer between the PS beads. This method opens an interesting route for the application of graphene in gas sensing.

We report the interaction between gas molecules (NO2 and NH3) and the SnSe2 monolayers with vacancy and dopants (O and N) for potential applications as gas sensors. Compared with the gas molecular adsorbed on pristine SnSe2 monolayer, the Se-vacancy SnSe2 monolayer obviously enhances sensitivity to NO2 adsorption. The O-doped SnSe2 monolayer shows similar sensitivity to the pristine SnSe2 monolayer when adsorbing NO2 molecule. However, only the N-doped SnSe2 monolayer represents a visible enhancement for NO2 and NH3 adsorption. This work reveals that the selectivity and sensitivity of SnSe2-based gas sensors could be improved by introducing the vacancy or dopants.

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