## 2 Materials and Methods  
All of the devices of this work were fabricated on P+ silicon bottom gate with 300 nm-thick thermally grown SiO\({}_{2}\) that was used as the gate dielectric. Monolayer MoS\({}_{2}\) was grown on the substrate using a CVD process explained elsewhere [10]. An optical images of CVD MoS\({}_{2}\) crystal grown on 300 nm SiO\({}_{2}\) is shown in Figure 1a. The monolayer structure of the grown CVD films were confirmed by Raman and photoluminescence (PL) spectroscopy shown in Figure 2. The \(E_{2g}^{1}\) and \(A_{1g}\) peaks are at 389.3 cm\({}^{-1}\) and 409.1 cm\({}^{-1}\), respectively. This corresponds to a peak separation of 19.8 cm\({}^{-1}\), which is characteristic of single-layer MoS\({}_{2}\)[24]. The PL spectrum of the CVD MoS\({}_{2}\) also shows a strong peak at around 685 nm which corresponds to monolayer MoS\({}_{2}\)[25].  
Figure 1: (**a**) Optical microscope (Olympus, Tokyo, Japan) image of chemical vapor deposited (CVD) MoS\({}_{2}\) crystals; (**b**) An image of the MoS\({}_{2}\) field effect transistors (FET) with channel dimensions of 3 \(\upmu\)m \(\times\) 9 \(\upmu\)m.  
Figure 2: (**a**) Raman spectrum of MoS\({}_{2}\) before (red) and after (blue) applying Teflon-AF encapsulation layer; (**b**) photoluminescence (PL) spectrum of monolayer MoS\({}_{2}\) before (red) and after (blue) applying the cap.  
A conventional photolithography process was used to define the channels and contacts. The channels were isolated using Cl\({}_{2}\) plasma followed by patterning and deposition of source/drain contacts (20 nm Ag/ 20 nm Pd). The fabricated two-probe devices have a constant width of 3 \(\upmu\)m and a variable length of 3 \(\upmu\)m, 6 \(\upmu\)m and 9 \(\upmu\)m. An optical image of the MoS\({}_{2}\) FET with channel dimensions of 3 \(\upmu\)m \(\times\) 9 \(\upmu\)m, is shown in Figure 1b.  
The initial electrical characteristics of about 60 randomly-chosen devices distributed on a 5 mm \(\times\) 5 mm surface area were tested under room ambient conditions. The field-effect modulation of CVD MoS\({}_{2}\) FETs were studied by sweeping the back-gate bias in [\(-\)100, 100] V range. Afterwards, the sample was held in N\({}_{2}\) glovebox for half an hour, before being coated with Teflon-AF (Dupont Co., Wilmington, NC, USA). Vacuum and N\({}_{2}\) environment were previously reported to be equally efficient in removing the adsorbates from unpassivated back-gated MoS\({}_{2}\) devices [26]. In the next step, 140 nm Teflon-AF encapsulation cap was spin-coated on the sample with as-supplied Teflon-AF solution at 2000 rpm for 1 min. The sample was then placed on the hotplate and cured at 250 \({}^{\circ}\)C for 30 mins in N\({}_{2}\) ambient. After the cure step, the hotplate temperature was reduced to room temperature with a ramp rate of 20 \({}^{\circ}\)C/min. Teflon-AF is a soluble fluoropolymer in selected solvents, based on copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE) [27]. Teflon-AF is known to have a high-polarity amorphous structure, which is originated from a number of C-F bonds in it, with a dipole moment of 1.13D [28]. Raman and photoluminescence characterization were used to check any structural and material changes in MoS\({}_{2}\) monolayer after applying the Teflon-AF cap. Figure 2a,b compares the Raman and PL scattering of MoS\({}_{2}\) before and after applying the cap. The attenuated intensity of signal in both cases is likely due to the thickness of the cap.