## 3 Results  
Figure 3a,b compares the electrical transfer characteristics of 15 representative devices before and after treating the surface with Teflon-AF. No drain saturation current is observed in both cases due to the thick back gate dielectric. A large variability of the device performance is observed in Figure 3a before applying the Teflon cap. This variability, which may be caused by the charges trapped between the channel and gate dielectric or the moisture and oxygen molecules adsorbed on the surface of channel, is reduced in Figure 3b after applying the cap. These changes can be explained by first, the hydrophobic nature of Teflon-AF that significantly reduces the charges adsorbed on the channel surface. Second, neutralization of charge traps, within the channel and at the interface of channel and gate dielectric, by the local electric field induced in Teflon-AF. This local electric field could screen the scattering impact of trapped charges between the channel and gate dielectric. The impact of the reduction of adsorbates on the surface of the channel shows up in the form of reduced gate hysteresis, observed in all tested devices and presented in Figure 4a. For the specific device presented here, the hysteresis is reduced from \(\sim\)25 V to \(\sim\)10 V. The reduction of the gate hysteresis has been reported previously for devices tested under vacuum and those tested after long-term vacuum annealing steps. Note that in none of these cases the hysteresis is reduced to zero. This is believed to be due to the remaining oxygen to be strongly bonded to the defects and edges of the MoS\({}_{2}\) channel [18]. Improvement in the off-current states upon coating with Teflon-AF is possibly due to the polar molecules of the fluoropolymer which will partially neutralize charged dopants [22; 29]. Figure 4b compares the transfer characteristics of the same device in the linear scale before and after applying the cap. The threshold voltage (\(V_{th}\)) is calculated from the x-intercept of the linear fit in the strong inversion region (dotted lines in Figure 4b). The positive shift in \(V_{th}\), observed in all measured devices after applying the cap, is attributed to the net impact of the local dipole electric field and the back gate electric field. Since MoS\({}_{2}\) works like an n-type FET, this leads to a lower density of carriers in the device channel. The inset in Figure 4b shows the drain current (\(I_{d}\)) for the same overdrive gate bias. The reduction of \(I_{d}\) at the same overdrive gate bias after applying the cap, is likely due to the local electric field caused by the alignment of the dipoles of the cap. As a result of this local electric field the density of the charge carriers and the drain current decreases. In order to study the impact of Teflon-AF, approximately 60 devices were tested under ambient conditions before and after applying the cap. The distribution of threshold voltage (\(V_{th}\)), gate hysteresis, subthreshold swing and current on-off ratio of the tested devices are shown in Figures 5a,b and 6a,b. The average value of the \(V_{th}\) shifts from 15.8 V to 58 V after applying the cap. This leads to about two orders of magnitude reduction of the drain current at \(V_{g}\) = 0 V and effectively turning off the MoS\({}_{2}\) FET at zero back gate bias. Figure 4: **(a)** Transfer curves shows a reduction in the gate hysteresis and the desirable enhancement mode character of the transistor after applying the polymer cap; **(b)** Transfer curves plotted in the linear scale shows a positive \(V_{th}\) shift after applying the polymer cap that could be explained either by the trapped charges between the cap and channel or hole doping of the channels. The inset shows reduction of off-state current and slight improvement of on-state current at high overdrive gate bias after applying the cap. Figure 5: Histogram distribution of **(a)** threshold voltage; **(b)** gate hysteresis of 60 measured devices before and after applying Teflon-AF cap.