  The development of room-temperature sensing devices for detecting small
concentrations of molecular species is imperative for a wide range of low-power
sensor applications. We demonstrate a room-temperature, highly sensitive,
selective, and reversible chemical sensor based on a monolayer of the
transition metal dichalcogenide Re0.5Nb0.5S2. The sensing device exhibits
thickness dependent carrier type, and upon exposure to NO2 molecules, its
electrical resistance considerably increases or decreases depending on the
layer number. The sensor is selective to NO2 with only minimal response to
other gases such as NH3, CH2O, and CO2. In the presence of humidity, not only
are the sensing properties not deteriorated, but also the monolayer sensor
shows complete reversibility with fast recovery at room temperature. We present
a theoretical analysis of the sensing platform and identify the
atomically-sensitive transduction mechanism.
