# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Electron Irradiation
The distance between two neighboring molybdenum atoms is 3.12 Å (**a** top panel), 3.24 Å (**b** top panel), 2.94 Å (**c** top panel), 3.23 Å (**d** top panel), and 3.22 Å (**e** top panel). Defects are circled. The \(V_{S}\) vacancies in the (**a**) bottom panel come from the irradiation of the electron beam during observations. Reprinted from [74], with the permission of AIP Publishing. [PERSON] et al. [82] deposited five to seven atomic layers of MoS\({}_{2}\) on a SiO\({}_{2}\)/Si wafer at room temperature using the RF magnetron sputtering method. The thickness of the deposed amorphous MoS\({}_{2}\) layers was about 4 nm. The films were then exposed under a collimated electron beam with an energy of 1 keV for 1-10 min at room temperature without any additional heating processes. HRTEM images indicated that the as-deposited MoS\({}_{2}\) was amorphous (left panel in Figure 17a). Under the electron beam irradiation (EBI) of 1 min, the random atoms re-arranged from amorphous to crystalline structure, forming MoS\({}_{2}\) crystal domains with a size about 5 nm (middle panel in Figure 17a). However, longer irradiation time would damage crystalline domains (right panel in Figure 17a) because of the Mo-S bond breaking. The amorphous-crystalline transformation was confirmed by Raman scattering, as shown in Figure 17b. The two prominent Raman peaks of MoS\({}_{2}\), the in-plane mode \(E^{1}_{2g}\) and the out-of-plane mode \(A^{1}_{g}\), did not appear in the as-deposited film because of its amorphous nature. The peak intensities of the \(E^{1}_{2g}\) and \(A^{1}_{g}\) modes increased dramatically after the electron irradiation at room temperature. Their XPS measurements provided a stronger proof that the amorphous films crystallized under the 1 keV electron irradiation. Figure 17c shows the XPS spectra of MoS\({}_{2}\) samples characterized with an Al-K\({}_{a}\) ray with an energy of 1486 eV. The Mo-\(3d\) peak was de-convoluted into three chemical bonding states of Mo-Mo, Mo-S, and Mo-O in the as-deposited sample. After 1 min of electron irradiation, the Mo-\(3d\) spectrum only consisted of two peaks originating from the states of the Mo-S and Mo-O bonds, with dramatically increased peak intensity of the Mo-S bonds. Similarly, the S-\(2p\) peak from the as-deposited sample were de-convoluted into S-S and S-Mo bonding while only the S-Mo bond remained in the 1 min electron-irradiated sample. The authors also checked the samples under TEM at 400 keV and then measured the Raman scattering with a 532 nm laser. It was not clear if the amorphous films were changed under the TEM electron-beam irradiation and laser irradiation. [PERSON] et al. [77] investigated the electron irradiation effect of MoS\({}_{2}\) few-layers through micro-Raman scattering. The few-layered MoS\({}_{2}\) samples were prepared by mechanical exfoliation and irradiated under a vacuum of \(10\times 10^{-7}\) torr. The thickness of the few-layers was about 7 nm, which was equivalent to about ten layers of MoS\({}_{2}\). Figure 18 shows the Raman scattering before and after irradiation. In addition to the in-plane \(E^{1}_{2g}\) and out-of-plane \(A_{1g}\) modes, extra Raman modes appeared after being irradiated under 5 keV, 7 keV, and 10 keV, showing the maximum change under 10 keV irradiation. Obviously, the electron-irradiation-induced defects, resulting in the breaking of inversion symmetry. The 15 keV irradiation did not lead to any changes in the spectra because the electron beam passed the few-layers without creating any damage. [PERSON] et al. [80] irradiated triangular-shaped MoS\({}_{2}\) mono-layers grown on SiO\({}_{2}\)/Si substrates under an electron beam with an acceleration voltage of 15 kV and an areal dose of 280 uC/cm\({}^{2}\) charge. Figure 19a shows an optical image of a mono-layer MoS\({}_{2}\) crystal. The right half region was covered by polymethyl methacrylate (PMMA) and was not electron irradiated.