  Atomically thin transition metal dichalcogenides (TMDs) are ideal candidates
for ultrathin optoelectronics that is flexible and semitransparent.
Photodetectors based on TMDs show remarkable performance, with responsivity and
detectivity higher than 10^3 A/W and 10^12 Jones, respectively, but they are
plagued by response times as slow as several tens of seconds. Although it is
well established that gas adsorbates such as water and oxygen create charge
traps and significantly increase both the responsivity and the response time,
the underlying mechanism is still unclear. Here we study the influence of
adsorbates on MoS2 photodetectors under ambient conditions, vacuum and
illumination at different wavelengths. We show that, for wavelengths
sufficiently short to excite electron-hole pairs in the MoS2, light
illumination causes desorption of water and oxygen molecules. The change in the
molecular gating provided by the physisorbed molecules is the dominant
contribution to the device photoresponse in ambient conditions.
