# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Alpha-Particle/Helium-Ion Irradiation  
Alpha particles consist of two protons and two neutrons which are tightly bound together bound together, which is identical to a helium ion (He\({}^{2+}\)). They are produced either from particle accelerators in the form of helium-ion beams or from the process of alpha decay of alpha-particle-emitting radionuclides. Alpha particles have been widely studied in radiopharmaceutical therapy that is a promising treatment approach under active pre-clinical and clinical investigation [119; 120]. Decayed alpha particles generally have a kinetic energy of about 5 MeV, which induce defects and even 'cut' the MoS\({}_{2}\) nanosheets. [PERSON] et al. [88] studied the effects of alpha-particle (helium nuclei) irradiation (1.66 MeV) on both bulk and liquid-phase exfoliated MoS\({}_{2}\) nanosheets using Raman spectroscopy and energy-dispersive X-ray spectroscopy. Liquid-phase exfoliated MoS\({}_{2}\) layers were often more defective than mechanically exfoliated MoS\({}_{2}\) and CVD-grown MoS\({}_{2}\) layers. Besides water, the solvent used during liquid exfoliation could be retained between the MoS\({}_{2}\) layers during ultrasonication and annealing/cleaning post-procedures. Raman spectroscopy showed a small blueshift of the E\({}_{2g}^{1}\) and \(A_{1g}\) modes in the MoS\({}_{2}\) bulk under a high total absorbed dose (\(\sim\)900 MGy), accompanied by a small broadening of both peaks. The exfoliated MoS\({}_{2}\) layers were less radiation tolerant than the bulk material. Both spectroscopies proved the presence of amorphous carbon in the exfoliated MoS\({}_{2}\) membranes, which could be formed by radiolytic amorphization of residual solvent and retained within the exfoliated nanosheets. [PERSON] et al. [89] irradiated freestanding mechanically exfoliated MoS\({}_{2}\) few-layers in a helium-ion microscope. Figure 12 shows HRTEM images of a MoS\({}_{2}\) few-layer. The hexagonal structure of the pristine MoS\({}_{2}\) was destroyed to an amorphous state. Energy-dispersive X-ray spectroscopy (EDX) indicated that preferential sputtering of sulfur occurred in the mechanically exfoliated MoS\({}_{2}\) few-layers. Its stoichiometry was modified by the preferential sputtering of sulfur at nanometer scales. It was believed that He\({}^{2+}\) ions transfer more energy to sulfur atoms than to molybdenum atoms because of the lighter mass of sulfur atoms. Electric properties of the MoS\({}_{2}\) layers were altered to be semiconducting, metallic-like, or insulating depending on doses. When the dose was above  
Figure 11: X-ray photoelectron spectra of the Mo-3d and S-2s in the MoS\({}_{2}\) mono-layers on Au substrates before and after Ar\({}^{+}\) sputtering. XPS of Au-4f electrons were shown as a standard reference. Reprinted with permission from Reference [117]. Copyright © 2018 American Chemical Society. \((2.56\pm 0.05)\times 10^{18}\) ion/cm\({}^{2}\), complete removal of material (milling) was observed. Figure 12c shows He\({}^{2+}\)-ion fabricated freestanding nanoribbons in MoS\({}_{2}\) few-layers. The edges of the milled regions may be amorphous or crystalline, depending on He\({}^{2+}\) beam sizes. MoS\({}_{2}\) layers were milled when the dose was higher than \((1.30\pm 0.03)\times 10^{18}\) ion/cm\({}^{2}\). When the dose was below \((1.00\pm 0.02)\times 10^{17}\) ion/cm\({}^{2}\), MoS\({}_{2}\) layers were extensively damaged but not completely etched away. [PERSON] et al. [89] also irradiated MBE-grown pristine MoS\({}_{2}\) 6-layers supported on an MgO substrate. With increasing He\({}^{2+}\) doses, the full width at half maximum (FWHM) of the E\({}_{2g}^{1}\) (the in-plane Mo-S vibration) increased, indicating more in-plane defects during irradiation. The FWHM of A\({}_{1g}\) peaks decreased with increasing doses, inferring material removal with high He\({}^{2+}\) doses. [PERSON] et al. [90] irradiated mechanically exfoliated MoS\({}_{2}\) mono-layers with a high-energy He\({}^{2+}\) beam (3.04 MeV) with doses up to \(8\times 10^{13}\) ions/cm\({}^{2}\).