## 3 Results and Discussion
Figure 3a,b show the detailed binding energy profiles of Mo and S for the CHF\({}_{3}\)-treated and untreated MoS\({}_{2}\) samples, respectively. Two peaks at 229.8 and 232.9 eV are shown in Figure 3a. They were attributed to the doublet Mo 3d\({}_{5/2}\) and Mo 3d\({}_{3/2}\) of the untreated MoS\({}_{2}\), respectively, while the peaks of the S 2p\({}_{3/2}\) and S 2p\({}_{1/2}\) orbitals of the divalent sulfide ions were observed at 162.7 and 163.8 eV, respectively. These results are consistent with the reported values for untreated MoS\({}_{2}\) crystals [21; 22]. In comparison, all relevant peaks of the CHF\({}_{3}\)-treated sample were broader, and red-shifted by 0.57 eV. This was because of the shift in Fermi levels of the CHF\({}_{3}\)-treated samples towards the valence band edge [23]. This red-shift of peaks indicates the proper \(p\)-type doping of MoS\({}_{2}\) films. The specific types of dopants introduced by the plasma processes were confirmed by the XPS spectra in Figure 3c. A prominent binding energy peak associated with F was observed  
Figure 2: (**a**) Atomic force microscopy (AFM) height image of the as-grown MoS\({}_{2}\) films. (**b**) Line scan of the as-grown MoS\({}_{2}\)-substrate interface. (**c**) AFM height image and (**d**) line scan across the CHF\({}_{3}\)-treated and untreated interface of the MoS\({}_{2}\) film. in CHF\({}_{3}\)-treated samples, while the F peak was absent for untreated samples. The excess electrons were preferentially transferred from the MoS\({}_{2}\) layers onto F atoms with strong electronegativity when incorporating F dopants into MoS\({}_{2}\) layers. This surface charge transfer process suggests that F atoms are the critical dopants responsible for the \(p\)-type doping in MoS\({}_{2}\) film [24]. In order to measure the electrical characteristics of the CHF\({}_{3}\)-treated multilayer MoS\({}_{2}\), a back-gated FET device was fabricated as shown in the schematic illustrated in Figure 4a. Photolithography was carried out to define the exposed area of MoS\({}_{2}\) films for plasma treatment. After plasma treatment, 80 nm of Au was deposited as the source and as drain contacts for the FET by sputtering and was followed by lift-off. Due to the screening of the electric field in MoS\({}_{2}\), only the top few layers of the MoS\({}_{2}\) encountered a surface charge transfer process. Therefore, a thinner \(n\)-type MoS\({}_{2}\) channel at the untreated bottom layers could result in effective gate modulation and higher on/off ratios for FET applications. Figure 4b shows the output characteristics, drain current (I\({}_{\text{ds}}\))-drain voltage (V\({}_{\text{ds}}\)), of the MoS\({}_{2}\) FET under varying gate voltages (V\({}_{\text{g}}\), from 0 to \(-\)10 V). Id\({}_{\text{s}}\) decreased with the increasing V\({}_{\text{g}}\) values, indicating typical \(p\)-type behavior. The Id\({}_{\text{s}}\)-V\({}_{\text{ds}}\) characteristic of the device exhibited a low onset voltage and a linear increase of Id\({}_{\text{s}}\) versus V\({}_{\text{ds}}\) up to a value of V\({}_{\text{ds}}\) approximately equal to 0.6 V, where a kink in the Id\({}_{\text{s}}\)-V\({}_{\text{ds}}\) curves was observed. This was followed by saturation of the Id\({}_{\text{s}}\)-V\({}_{\text{ds}}\) curve for V\({}_{\text{ds}}\) greater than 0.6 V. Figure 4c represents the transfer characteristic of the back-gated MoS\({}_{2}\) FET at V\({}_{\text{ds}}\) of 4 V. The back-gated MoS\({}_{2}\) FET showed excellent gating control capability. The field-effect mobility, \(\mu_{h}\), of this MoS\({}_{2}\) device can be estimated based on the following equation:  
\[\mu_{h}=\frac{L}{W\times C_{g}\times V_{ds}}\times\frac{dI_{ds}}{dV_{g}}\]  
Figure 3: X-ray photoelectron spectroscopy (XPS) spectra for (**a**) Mo and (**b**) S signals before and after plasma treatment, showing a downshift of 0.57 eV in binding energy for all peaks. (**c**) XPS spectra for MoS\({}_{2}\) exhibit the presence of the F 1s peak in CHF\({}_{3}\) plasma-treated MoS\({}_{2}\).