# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Argon Ion Irradiation  
Argon ion beams (500 eV) were employed as early as 1987 [9] to irradiate MoS\({}_{2}\) crystalline films (cleaved from a single crystal) with a thickness of 0.7 \(\upmu\)m. The Ar bombardments created sulfur vacancies and reduced molybdenum of MoS\({}_{2}\) edge surfaces. [PERSON] et al. [93] studied defects generated by Ar\({}^{+}\) ion irradiation of MoS\({}_{2}\) bulk surface by scanning tunneling microscopy (STM). A clean MoS\({}_{2}\) surface was prepared by cleaving the surface layers of a MoS\({}_{2}\) bulk crystal with Scotch tape. The cleaved clean MoS\({}_{2}\) was then degassed in a UHV chamber by electrical heating at 538 K for half an hour to provide a clean and contamination-free surface. The sample was irradiated with Ar\({}^{+}\) ions at an energy of 500 eV and an irradiation density of \(75\times 10^{11}\) ions/cm\({}^{2}\). The incident angle of Ar\({}^{+}\) ion was \(\pm 2\)-3\({}^{\circ}\) from the normal to the MoS\({}_{2}\) surface. The irradiated MoS\({}_{2}\) sample was heated at 583 K for an hour by direct current heating through the substrate to remove residual ions on the sample surface after irradiation. Low-density individual defects were observed. Some surface defects were formed by removal of sulfur atoms from the top MoS\({}_{2}\) surface. Some defects may originate from the hybridized dangling bond composed of the Mo-4\(d\) orbital and removal of MoS\({}_{2}\) layer fragments. [PERSON] et al. [84] MoS\({}_{2}\) prepared slabs with a thickness of several microns from MoS\({}_{2}\) crystals using the mechanical exfoliation method and then irradiated them with Ar\({}^{+}\) ions with an energy of 500 eV in an UHV, with fluences of \(5.65\times 10^{14}\) ions/cm\({}^{2}\) and \(2.26\times 10^{15}\) ions/cm\({}^{2}\). The incident angle of Ar\({}^{+}\) was almost normal to the surface of the samples. Residual argon on sample surfaces was removed by heating at 583 K for one hour in an UHV after the irradiation process. Figure 7 shows the Raman spectra of Arirradiated samples. After Ar\({}^{+}\) irradiation, additional broad satellite peaks appeared in the lower-frequency side of both the \(E_{2g}^{1}\) and \(A_{1g}\) peaks, located at 377-381 cm\({}^{-1}\) and 404-406 cm\({}^{-1}\) respectively. The Raman satellite peaks shifted to lower wave-numbers while their intensity increased with increasing Ar\({}^{+}\) doses. First-principles calculation showed that new satellite modes were related to molybdenum vacancies, sulfur vacancies, or MoS\({}_{6}\) vacancy clusters. The lower shift of the satellites of the \(E_{2g}^{1}\) mode should have come from the large cluster vacancy. The \(A_{1g}\) and \(E_{2g}^{1}\) mode of MoS\({}_{2}\) materials remained unchanged before and after the irradiation. It is believed that the new peaks were induced by lattice defects introduced by Ar\({}^{+}\) irradiation. Additionally, the intensity of the satellite peaks increased with the increasing irradiation dose and dominated over the \(E_{2g}^{1}\) and \(A_{1g}\) modes of pristine MoS\({}_{2}\), indicating the formation of molybdenum vacancies or MoS\({}_{6}\) vacancies. [PERSON] et al. [94] irradiated MoS\({}_{2}\) powders under a 3 keV Ar\({}^{+}\) ion beam with a current density of 0.1 \(\mu\)A/cm\({}^{2}\). The Ar\({}^{+}\) ion-beam bombardment caused the preferential sputtering of sulfur and produced an amorphous MoS\({}_{x}\) phase of depth of 3.8 nm. The Mo/S ratio increased with increasing bombardment time, as shown in Figure 8.  
Figure 8: Surface composition of MoS\({}_{2}\) powders when exposed to a 3 keV Ar\({}^{+}\) ion beam [94]. The atomic percent was calculated from energy-dispersive X-ray spectroscopy. Reprinted from Reference [94]. Copyright ©1999, with permission from Elsevier. Figure 7: Raman spectra of (bottom) MoS\({}_{2}\) crystalline bulk, Ar\({}^{+}\)-irradiated MoS\({}_{2}\) micron crystals at fluences of (middle) \(5.65\times 10^{14}\) ions/cm\({}^{2}\) and (top) \(2.26\times 10^{15}\) ions/cm\({}^{2}\)[84]. Reprinted with permission from Reference [84].