# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Electron Irradiation  
[PERSON] et al. [122] irradiated MoS\({}_{2}\) single crystals with a thickness of about 50 \(\upmu\)m by using high-energy electrons in ambient conditions at room temperature. The electron dose was 300 kGy (\(6.70\times 10^{14}\) electrons/cm\({}^{2}\)) and the acceleration energy of electrons was 0.7 MeV. There was a  
Figure 14: PL spectra of bi-layer MoS\({}_{2}\) (a) suspended over holes and (b) deposited on substrates, and mono-layer MoS\({}_{2}\) (c) suspended over holes and (d) deposited on substrates taken before and after 100 keV proton irradiation with a fluence of \(6\times 10^{14}\) particles/cm\({}^{2}\), and after annealing [87]. Reprinted from [87], with the permission of AIP Publishing. negligible reduction of sulfur XPS intensity compared to the molybdenum XPS intensity of the samples after the irradiation. However, 1T-like defects were generated in the MoS\({}_{2}\) surface, inducing a weak ferromagnetic state at room temperature and improving transport properties. [PERSON] et al. [74] prepared single-crystalline MoS\({}_{2}\) lamellae with a thickness of 100 \(\upmu\)m. The lamellae were electron-irradiated in ambient conditions at room temperature. Figure 15 shows HRTEM images of the irradiated samples, showing honeycomb lattices of the MoS\({}_{2}\). The vacancies depended on the acceleration energy and dose. Without irradiation, the lattice of crystalline MoS\({}_{2}\) was a honeycomb with few defects, shown in Figure 15a. After being electron-irradiated with 150 kilogray(kGy) dose at 0.7 MeV acceleration energy, additional defects were produced. Double-sulfur vacancies (\(V_{S2}\)) were more frequently observed than the monosulfur vacancies (\(V_{S}\)) after electron irradiation. Slightly displaced molybdenum atoms were occasionally found, as marked by the red arrow in Figure 15b. Under electron irradiation with a 300 kGy dose at 0.7 MeV acceleration energy, the honeycomb lattice was heavily distorted (Figure 15c) and perfect honeycomb lattices were rarely observed. The numerous \(V_{S}\) and \(V_{S2}\) vacancies distorted the honeycomb lattice and the irradiation considerably decreased the lattice parameters. \(V_{S2}\) defects were more frequently observed under the electron irradiation with a 100 kGy dose at 2.0 MeV. With an increasing dose of 2.0 MeV electrons, the \(V_{S}\) and \(V_{S2}\) concentration increased significantly. Molybdenum vacancies were produced too. The lattice parameters were also significantly increased. Under electron irradiation at higher energy of 10 MeV, different types of defects were produced. Under all irradiation, the vacancy densities increased with the electron irradiation dose and electron energy. [PERSON] et al. [75] mechanically exfoliated MoS\({}_{2}\) 20-layers and transferred them to carbon-coated copper grids. Figure 16a shows a transmission electron microscopy (TEM) image of a typic MoS\({}_{2}\) few-layer. The multi-layers were then irradiated under a scanning electron microscopy (SEM) beam with an accelerating voltage of 5 keV and an electron-beam current of 50 nA for 15 min. Many crystalline islands were formed throughout the multi-layers after the irradiation, as shown in Figure 16b. The selected-area electron diffraction (SAED) patterns indicated that the islands were metallic molybdenum. Under high-energy (200 keV) electron irradiation, the MoS\({}_{2}\) layers were drastically modified, and their sulfur stoichiometry was changed, inducing the formation of the molybdenum nanoislands, as shown in Figure 16c. In both cases, massive surface sulfur depletion was induced together with the consequent formation of molybdenum nanoislands. Figure 15: Zoomed HRTEM images of MoS\({}_{2}\) lamellae without electron irradiation (**a**) and with electron irradiation with (**b**) 150 kGy dose at 0.7 MeV energy, (**c**) 300 kGy dose at 0.7 MeV energy, (**d**) 100 kGy dose at 2.0 MeV energy, and (**e**) 250 kGy dose at 2.0 MeV energy [74].