Charge Redistribution Mechanisms in SnSe Surfaces Exposed to Oxidative and Humid Environments and Their Related Influence on Chemical Sensing.

Tin diselenide (SnSe) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO-SnSe heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe surface increases by 0.23 and 0.40 eV upon exposure to O and air, respectively, with a charge transfer reaching 0.56 e/SnO between the underlying SnSe and the SnO skin. Remarkably, both pristine SnSe and defective SnSe display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO-SnSe interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules.