## 3 Results and Discussion
The pressure was raised gradually by controlling the air flow via a high precision vacuum valve. We observe that the I\({}_{\text{ds}}\)-V\({}_{\text{gs}}\) transfer curve shifts towards a positive bias that means a change from \(n\)-type to \(p\)-type conduction in the investigated gate bias range. By keeping the device exposed to air (atmospheric pressure) for about 8 h, the \(p\)-type behavior is further enhanced (Figure 2e), with no indication of ambipolarity. Successively, by reducing the pressure back to 10\({}^{-6}\) Torr, we demonstrate (in Figure 2f) that the process can be reversed by restoring the ambipolar conduction centered almost at V\({}_{\text{gs}}\) = 0 V. The reversibility of this phenomenon provides an opportunity to use the pressure as a controller to tune the conduction type from \(n\) to \(p\) or vice versa in PdSe\({}_{2}\) devices. However, it has been reported that long air exposure (>15 days) can result in the definitive suppression of \(n\)-type conduction [26]. The observed reversible conduction type can be due to the activated chemisorption of molecular O\({}_{2}\), probably in the Se vacancies, and desorption by vacuum annealing [7; 26]. A similar characterization of the effect of pressure on the transport properties of 2D field-effect transistors was performed on monolayer MoS\({}_{2}\)-based devices. In Figure 3a, we report the I\({}_{\text{ds}}\)-V\({}_{\text{ds}}\) output characteristics, measured for different values of the gate voltage. The experimental data evidence a clear modulation of the current up to five orders of magnitude and an asymmetric behavior. Usually, the Fermi level at the metal-MoS\({}_{2}\) interface is pinned above the midgap of MoS\({}_{2}\) by either a metal work function modification due to interface dipole formation or by the production of gap states owing to the intralayer S-Mo bonding being weakened by metal [27]. As a consequence of the Fermi level pinning, Schottky barriers are formed at the contacts, causing the small rectification observed in the output curves as well as a dominant \(n\)-type behavior [9; 28; 29]. In Figure 3b we show a \(\mathrm{I_{ds}}\) -V\({}_{\mathrm{gs}}\) transfer curve, measured in a high vacuum (10\({}^{-6}\) Torr) and at room temperature, by applying a bias \(\mathrm{V_{ds}}\) = 100 mV. We note that the device has the expected \(n\)-type conduction and clear hysteresis. In addition, we estimate a field-effect mobility of about 1 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\) and an on-off ratio of about six orders of magnitude. The \(\mathrm{I_{ds}}\) -V\({}_{\mathrm{gs}}\) transfer curves at atmospheric pressure (Figure 3c,d) were then measured by filling the SEM chamber with selected gases. We observe that the exposure to gas provokes significant modifications of the curves, in particular causing a reduction in the conductance and/or a widening of the hysteresis. More precisely, we stress that when monolayer MoS\({}_{2}\) FET is exposed to argon or hydrogen atmospheric pressure, the main effect is the enlargement of the hysteresis, keeping the same conductance values at the maximum (positive and negative) gate bias (Figure 3c). On the other hand, when oxygen, nitrogen, and methane are employed, a reduction in the conductance is also observed (Figure 3d). In all cases, the device remains in the \(n\)-type conduction due to the intrinsic doping of MoS\({}_{2}\) by S vacancies and to the pinning effect [27; 30]. Moreover, S vacancies and other defects like unreacted MoO\({}_{3}\) play a role as centers for gas adsorption [8], favoring charge trapping and an increase in the hysteresis. From the curves of Figure 3c,d, we can obtain the relation between the hysteresis width, \(\mathrm{H_{w}}\) (here defined as the V\({}_{\mathrm{gs}}\) difference at 10 pA current) and the gas adsorption energy \(\mathrm{E_{ads}}\)[31; 32; 33]. The results are summarized in Table 1. \(\mathrm{E_{ads}}\) are ordered for increasing absolute value to evidence the monotonic dependence of \(\mathrm{H_{w}}\) on \(\mathrm{E_{ads}}\).