# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Proton Irradiation  
[PERSON] et al. [85] prepared MoS\({}_{2}\) flakes with a thickness of 200 \(\upmu\)m and irradiated them at room temperature using a 3.5 MeV proton ion beam. The Raman spectra of the pristine and irradiated samples at a fluence of \(5\times 10^{18}\) ions/cm\({}^{2}\) are shown in Figure 13. A new peak at 483 cm\({}^{-1}\) was clearly visible in the irradiated samples. The appearances of the mode at 483 cm\({}^{-1}\) along with the broadening of the mode at 452 cm\({}^{-1}\) indicated the presence of lattice defects due to proton irradiation of the samples. The FWHM of the E\({}_{2g}^{1}\) and A\({}_{1g}\) modes did not increase in the irradiated MoS\({}_{2}\), indicating that the lattice structure was preserved in the near-surface region of the irradiated samples. The intensity ratio of A\({}_{1g}\)/E\({}_{2g}\) enhanced 16% after the proton irradiation, showing the enhanced interaction of electrons with A\({}_{1g}\) phonons. \(\chi\)-ray photoelectron spectroscopy indicated there were zone-edge phonons in the irradiated samples and the molybdenum valence of irradiated samples was higher than \(+\)4. The irradiation-induced changes of structures and chemical states affected their magnetic moments. [PERSON] et al. [86] micromechanically exfoliated MoS\({}_{2}\) few-layers from a bulk MoS\({}_{2}\) crystal and fabricated them into MoS\({}_{2}\) field-effect transistor (FET) devices on highly doped Si substrates coated with SiO\({}_{2}\). Source and drain electrodes were made by depositing Au/Ti electrodes. The FET devices were irradiated with a 10 MeV proton beam under fluences of \(10^{12}\), \(10^{13}\), and \(10^{14}\) ions/cm\({}^{2}\). Sufficiently high irradiation fluences decreased the electrical current and conductance of the devices while low dose did not change them significantly. The threshold voltage was shifted towards the positive gate voltage direction under proton irradiation. However, these changes were recovered over a time scale of days. It was believed that proton irradiation changed the SiO\({}_{2}\)/MoS\({}_{2}\) interface states, and the interface trap states at the SiO\({}_{2}\)/MoS\({}_{2}\) interfaces affected the electrical behaviors of the FET devices. [PERSON] et al. [87] exfoliated MoS\({}_{2}\) bi-layers, transferred them to silicon nitride membranes, and irradiated them at 100 keV protons (H\({}^{+}\)) with a fluence of \(6\times 10^{14}\) particles/cm\({}^{2}\). Figure 14a-b shows the PL spectra of the bi-layers. Both the suspended and substrate-supported bi-layers showed almost  
Figure 13: Raman spectra of (a) irradiated and (b) pristine MoS\({}_{2}\) nanosheets under a proton fluence of \(5\times 10^{18}\) ions/cm\({}^{2}\)[85]. Reprinted from Reference [85], with the permission of AIP Publishing. complete suppression of the indirect emission (1.55-1.60 eV) after the irradiation and the emergence of a defect-induced sideband peak at around 1.70 eV. The direct band emission at 1.83 eV increased 1.6\(\times\) after irradiation while the substrate-supported direct band emission increased by a factor of 2.7. After being annealed at 300 \({}^{\circ}\)C for 1 h in argon, the defect-induced sideband peak disappeared for both kinds of bi-layers. The indirect band emission was suppressed after annealing, indicating that the bi-layers underwent an irreversible indirect-to-direct band-gap transition. [PERSON] et al. [87] also examined MoS\({}_{2}\) tri-layers and four-layers. These multi-layers showed similar behaviors as bi-layers. [PERSON] et al. [87] also exfoliated MoS\({}_{2}\) mono-layers and transferred them to silicon nitride membranes with holes. The freestanding MoS\({}_{2}\) mono-layers were then irradiated at 100 keV protons (H\({}^{+}\)) with fluences ranging from \(2\times 10^{12}\) particles/cm\({}^{2}\) to \(6\times 10^{14}\) particles/cm\({}^{2}\).