Self-Assembled SnO 2 /SnSe 2 Heterostructures: A Suitable Platform for Ultrasensitive NO 2 and H 2 Sensing.

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe ₂ ) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO ₂ surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO ₂ -like layer, which is finally transformed into a SnO ₂ skin. Remarkably, the self-assembled SnO ₂ /SnSe _2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe ₂ sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO ₂ /SnSe _{2- x} at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm] ^-1 for NO ₂ and H ₂ , respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO ₂ and H ₂ , respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.