# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Electron Irradiation
Raman scattering indicated that both regions (with and without irradiation) exhibited the \(A^{1}_{g}\) and \(E^{1}_{2g}\) peaks of MoS\({}_{2}\). However, the two peaks were blue-shifted in the exposed region, as shown in Figure 19b. It was suggested that the electron-beam exposure introduced a local compressive strain in the MoS\({}_{2}\) crystal. The induced  
Figure 16: TEM images of (**a**) un-irradiated, (**b**) 5 keV electron-irradiated, and (**c**) 200 keV electron-irradiated MoS\({}_{2}\) 20-layers [75]. The strong reflection spots of the inset SAED come from the MoS\({}_{0.6}\) layers while the weak ones from metallic Mo. Reproduced with permission under the Creative Commons license. Copyright © 2016 IOP Publishing. strain would further change the band structure. The PL peak of the exposed left region, \(\sim\)1.85 eV, was blue-shifted 40-50 meV compared with that of unexposed right region (1.81 eV). Figure 19c shows the PL mapping of PL intensity. It was stated that the averaged PL spectrum of the unexposed region of the MoS\({}_{2}\) mono-layer could be characterized by a broad peak at 1.81 eV while the exposed region of the same MoS\({}_{2}\) sample showed a narrower feature at 1.85 eV, corresponding to a blue shift of 40-50 meV. [PERSON] et al. [81] prepared a MoS\({}_{2}\) mono-layer by micromechanical cleavage, as shown in Figure 20a. The well-crystalline MoS\({}_{2}\) mono-layer was then exposed in a vacuum under a 80 keV electron beam with an intensity of 40 A/cm\({}^{2}\). Initial defects and holes were induced in the MoS\({}_{2}\) sheet once exposed to the electron beam (Figure 20b). After a 81 second exposure under the electron irradiation, the small holes spread rapidly (Figure 20c) and extended into big holes with diameters of 3-6 nm (Figure 20d) after a 103 s irradiation. The MoS\({}_{2}\) mono-layer converted to Mo\({}_{5}\)S\({}_{4}\) nanoribbons after long-term electron irradiation. [PERSON] et al. [78] prepared freestanding single-layer MoS\({}_{2}\) samples by mechanical exfoliation of natural MoS\({}_{2}\) bulk crystals, and observed a MoS\({}_{2}\) sheet under an 80 keV electron beam in a vacuum on an aberration-corrected HRTEM. It was experimentally observed that the top and bottom sulfur atoms were removed under the 80 keV electron beam and vacancies were generated in the MoS\({}_{2}\) mono-layers under the electron irradiation. The sulfur vacancies agglomerated into line defects due to migration of the defects [123]. Figure 17: (**a**) Plan-view HRTEM images of the as-deposited sample, 1 min EBI-treated sample, and 10 min EBI-treated sample [82]. Insets show fast Fourier transformation (FFT) patterns of the areas marked as dashed squares. (**b**) Raman spectra of as-deposited and EBI-treated MoS\({}_{2}\) samples for an irradiation time of 1, 5, and 10 min [82]. (**c**) Mo-3\(d\) XPS spectrum (left panel) and S-2\(p\) XPS spectrum (right panel) from as-deposited and 1 min electron-irradiated samples [82]. Reproduced with permission under the Creative Commons Attribution 4.0 International license. Figure 19: (**a**) Optical image of a mono-layer MoS\({}_{2}\) crystal. The electron-beam exposure was performed on only the left side of the crystal while the right side is covered to avoid electron exposure. (**b**) Raman mapping of the MoS\({}_{2}\) crystal shown in (**a**), indicating the Raman shift of the \(E^{1}_{2g}\) peak as a function of position. (**c**) PL mapping of the MoS\({}_{2}\) crystal at a wavelength of 670 nm. Reprinted with permission from Reference [80]. Copyright © 2016 American Chemical Society. Figure 18: Micro-Raman spectra of 10-layered MoS\({}_{2}\) sheets before (black curves) and after (red curves) irradiation [77]. [PERSON] et al. [79] prepared MoS\({}_{2}\) single-crystalline mono-layers by CVD method and irradiated them with 200 keV electrons in an aberration-corrected TEM. Most generated defects were single sulfur vacancies. The irradiation-induced defects were quantitatively related to the electron dose with the actual defect concentration.