# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Argon Ion Irradiation
Copyright © 2017 by the American Physical Society. [PERSON] et al. [95] deposited MoS\({}_{2}\) layers (roughly 200 nm) on steel and Si substrates via ion-beam assisted deposition. The produced MoS\({}_{2}\) layers were fully dense with basal-oriented microstructures. The deposited MoS\({}_{2}\) layers were then irradiated with 180 keV Ar\({}^{2+}\) ions at doses of \(1\times 10^{15}\) ions/cm\({}^{2}\), \(1\times 10^{16}\) ions/cm\({}^{2}\), and \(5\times 10^{16}\) ions/cm\({}^{2}\) at room temperature in a vacuum. Figure 9a shows X-ray diffraction (XRD) patterns of the as-deposited and ion-irradiated coatings. The intensity of the (002) main peak at 13.5\({}^{\circ}\) decreased with increasing irradiation doses. The basal plane peak (\(2\theta=11^{\circ}\)) disappeared after the irradiation. Figure 9b shows Raman spectra of the samples. The main peaks of the Raman spectra were kept after the irradiation, while their intensity decreased with increasing doses. To explore the irradiation effects, the microstructure of the MoS\({}_{2}\) layers was characterized by high-resolution transmission electron microscopy (HRTEM) as shown in Figure 9c. The as-deposited MoS\({}_{2}\) consisted of basal-oriented and nanocrystalline MoS\({}_{2}\) with horizontal (002) fringes. After an irradiation dose of \(1\times 10^{15}\) ions/cm\({}^{2}\), some regions became amorphous, and the surviving crystalline regions (still with the basal orientation) were embedded in the amorphous matrix. After a high irradiation dose of \(5\times 10^{16}\) ions/cm\({}^{2}\), the MoS\({}_{2}\) regions were nearly amorphous, although few crystalline regions existed. The surviving crystalline regions oriented randomly after the high dose irradiation. The irradiation-reduced microstructural change degraded tribological behaviors of MoS\({}_{2}\) coatings, such as accelerated wear and eliminated lubrication. [PERSON] et al. [116] irradiated mechanically exfoliated MoS\({}_{2}\) few-layers (5-15 layers) under low-energy Ar (200 eV and 1 \(\times 10^{13}\)-\(10^{15}\) ions /cm\({}^{2}\)). The electric resistance increased 100 times after a high dose of irradiation because of induced defects. [PERSON] et al. [84] also prepared MoS\({}_{2}\) bi-layers by chemical exfoliation from MoS\({}_{2}\) micron powders in a hexane solution of butylllithium. The prepared bi-layers were then irradiated by 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\({}^{2}\). Figure 10 shows Raman spectra of the MoS\({}_{2}\) bi-layers before and after Ar\({}^{+}\) irradiation. The \(A_{1g}\) and \(E_{2g}^{1}\) modes were broadened. It was claimed that a new broad peak emerged at approximately 373 cm\({}^{-1}\), which was 10 cm\({}^{-1}\) away from the \(E_{2g}^{1}\) peak. There was a broad feature at around 435 cm\({}^{-1}\). The mode was relatively enhanced after the irradiation. The new vibration mode was identified as a unique signature of the sulfur vacancies. Figure 9: (**a**) XRD patterns, (**b**) Raman spectra, and (**c**) cross-sectional HRTEM images of MoS\({}_{2}\) coatings on Si substrates as-deposited and irradiated with 180 keV Ar\({}^{2+}\)-ions [95]. Insets: selected-area electron diffraction (SAED) patterns. Reprinted from Reference [95]. Copyright © 2000, with permission from Elsevier. [PERSON] et al. [84] claimed that MoS\({}_{2}\) mono-layers were destroyed by Ar\({}^{+}\) irradiation with 500 eV. [PERSON] et al. [117] sputtered MoS\({}_{2}\) mono-layers with Ar\({}^{+}\) ions (500 eV and emission current of 20 mA), defect peaks of Raman scattering were observed and its hydrogen evolution performance was enhanced. However, no details were reported in the literature. [PERSON] et al. [97] treated CVD-grown MoS\({}_{2}\) mono-layers under radio-frequency (RF) argon plasma with 13.56 MHz at room temperature for 40 s. The very weak Ar plasma had enough kinetic energy to trench the S-Mo bond while no enough energy to etch molybdenum and sulfur atoms.