  This work presents a detailed experimental investigation of the interaction
between molecular hydrogen (H$_2$) and monolayer MoS$_2$ field effect
transistors (MoS$_2$ FET), aiming for sensing application. The MoS$_2$ FET
exhibits a response to H$_2$ that covers a broad range of concentration (0.1 -
90%) at a relatively low operating temperature range (300-473 K). Most
important, H$_2$ sensors based on MoS$_2$ FETs show desirable properties such
as full reversibility and absence of catalytic metal dopants (Pt or Pd). The
experimental results indicate that the conductivity of MoS$_2$ monotonically
increases as a function of the H$_2$ concentration due to a reversible charge
transferring process. It is proposed that such process involves dissociative
H$_2$ adsorption driven by interaction with sulfur vacancies in the MoS$_2$
surface (VS). This description is in agreement with related density functional
theory studies about H$_2$ adsorption on MoS$_2$. Finally, measurements on
partially defect-passivated MoS$_2$ FETs using atomic layer deposited aluminum
oxide consist of an experimental indication that the VS plays an important role
in the H$_2$ interaction with the MoS$_2$. These findings provide insights for
futures applications in catalytic process between monolayer MoS$_2$ and H$_2$
and also introduce MoS$_2$ FETs as promising H$_2$ sensors.
