# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 6 Irradiation-Induced Properties
### Electric Properties  
The density-functional method and the [PERSON]'s function approach indicated that structural defects and grain boundaries were principal contributors to electric transport properties of MoS\({}_{2}\) mono-layers [173]. Atomic vacancies could significantly reduce the conductance of the 2D layers. In experiments, MoS\({}_{2}\) 2D materials were deposited on various substrates and connected to metal electrodes to fabricate FETs to detect electric properties under irradiation. [PERSON] et al. [174] studied electrical transport properties of CVD-grown MoS\({}_{2}\) mono-layers on SiO\({}_{2}\)/Si substrates. The carrier density was significantly increased, and the mobility was reduced under a low-energy electron irradiation (5.0 keV) in an UHV environment. It was believed that the electron-irradiation generated defects in MoS\({}_{2}\) layers and caused Coulomb potentials at the MoS\({}_{2}\)/SiO\({}_{2}\) interfaces \begin{table}
\begin{tabular}{c c c} \hline \hline
**Comfiguration** & **Bulk (eV)** & **Mono-layer (eV)** \\ \hline single S-vacancy & 14.4 & 5.7 \\ single Mo-vacancy & 24.8 & 18.8 \\ single-line S-vacancy & 39.2 & 30.5 \\ double-line S-vacancy & 66.2 & 62.3 \\ DIV & 28.8 & 23.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of formation thresholds in eV for various vacancy configurations of MoS\({}_{2}\) bulks and mono-layers [77]. [PERSON] et al. [175] grew MoS\({}_{2}\) single-crystalline few-layers on SiO\({}_{2}\)/Si substrates at 700 \({}^{\circ}\)C through CVD method, and then transferred them to pre-patterned SiO\({}_{2}\)/Si substrates with Au electrodes by the poly(methyl methacrylate)-assisted method. The fabricated MoS\({}_{2}\) devices were then electron-irradiated in a scanning electron microscope with a 30 keV electron beam. The electron dose was 100-1800 \(\upmu\)C/cm\({}^{2}\). It was found that the threshold voltage shifted to the negative side and the mobility increased upon the increasing electron doses. [PERSON] et al. [79] deposited single-crystalline MoS\({}_{2}\) mono-layers onto SiN\({}_{x}\) membranes, contacted them to Au electrodes. The fabricated device was then irradiated under a TEM electron beam with 200 keV. It was found that the electrical resistance increased with the electron irradiation dose. [PERSON] et al. [86] prepared few-layer MoS\({}_{2}\) FETs, and investigated the effect of irradiation under 10 MeV proton exposures. The electrical properties of the devices were nearly unchanged in response to low fluence proton irradiation (10\({}^{12}\) protons/cm\({}^{2}\)). The current and conductance of the devices significantly decreased after high fluence irradiation (10\({}^{13}\)-10\({}^{14}\) protons/cm\({}^{2}\)). The electrical changes contributed to proton-irradiation-induced traps in the SiO\({}_{2}\) substrate layers and at the interfaces between the MoS\({}_{2}\) layers and the SiO\({}_{2}\) layers. [PERSON] et al. [128] measured electrical properties of MoS\({}_{2}\) mono-layers exposed under oxygen plasma. The plasma was generated from a gas mixture of oxygen (20%) and argon (80%) under a pressure of 250-350 mTorr at 50 kHz. Figure 33a shows the \(I_{D}\)-\(V_{DS}\) curves of the device at a back-gate voltage \(V_{G}=40\) V for various plasma exposure time. The \(I_{D}\)-\(V_{DS}\) curves were linear around the zero bias, showing Ohmic behaviors. Figure 33b demonstrates the exposure-time dependence of electric resistance. The resistance increased exponentially with increasing plasma exposure time. The drain current and mobility decreased exponentially with plasma exposure time, dropping more than four orders of magnitude after only a total of 6 s plasma exposure. It was claimed that the significant degradation of electronic properties was caused by the creation of insulating MoO\({}_{3}\)-rich disordered domains in the MoS\({}_{2}\) sheet upon oxygen plasma exposure.