We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS 2 channel when the channel part is exposed to Cl 2 gas. The gas exposure and the electrical measurement of the MoS 2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS 2 FET can provide and understand the surface properties. With the Cl 2 gas exposure to the channel, the plot of the drain current versus the gate voltage (I d −V g curve) shifts monotonically toward the positive direction of V g , suggesting that the adsorbate acts as an electron acceptor. The I d −V g shifts are numerically estimated by measuring the onset of I d (threshold voltage, V th ) and the mobility as a function of the dosing amounts of the Cl 2 gas. The behaviors of both the V th shift and the mobility with the Cl 2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS 2 surface up to the monolayer coverage. The charge transfer makes the V th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS 2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl 2 molecule. The monotonic shift of V th with the coverage suggests that the MoS 2 device sensor has a good sensitivity to detect 10 −3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

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