# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Swift-Heavy Ion Irradiation
#### 2.1.3 Xenon Ion Irradiation  
[PERSON]\(\beta\) et al. [100] fabricated ultra-thin mechanically exfoliated MoS\({}_{2}\) sheets on SiO\({}_{2}\) substrates and irradiated them using 91 MeV Xe ions. Figure 6a shows Atomic force microscopy (AFM) images of the track morphology in MoS\({}_{2}\) with various thicknesses. In the case of bulk-like MoS\({}_{2}\), chains of nanosized hillocks were generated and protruded from the surface. For few-layer MoS\({}_{2}\) sheets (thickness under 10 nm), individual hillocks were created and mixed with incisions. In even thinner MoS\({}_{2}\) sheets, i.e., tri-layered, bi-layered, and mono-layered MoS\({}_{2}\) sheets, the major part of the surface track consisted of a continuous incision. The length of the incision varied among tri-layered (3L), bi-layered (2L), and single-layered (1L) sheets. Compared with the surface track length of mono-layers, the length of the incisions was reduced by \(\sim\)25% for a bi-layer and by \(\sim\)50% for a tri-layer. Additionally, the surface tracks in tri-layered and bi-layered MoS\({}_{2}\) were accompanied by protrusions before and after the central incision. Mono-layered MoS\({}_{2}\) surface tracks generally consisted only of incisions and no protrusions. Figure 6b shows lattice information around an incision (length 100 nm) created by a projectile. The extended incisions were oriented along the direction of the incoming ion beam and extremely narrow (less than 10 nm). The edges of the incision were relatively straight at an atomic scale. The surrounding lattice remained undisturbed while the violent atomic displacements took place inside the ion track core. Several mechanisms were proposed to explain the ion irradiation damages, such as electrostatic repulsion between the atoms in the ionized region, exciton self-trapping causing a local lattice distortion, non-thermal melting caused by significant changes of the interatomic potentials, and phase transitions such as melting due to a thermal spike. It was assumed that the hillock chains in bulk-like MoS\({}_{2}\) sheets consisted of nonstoichiometric MoS\({}_{2}\) which re-solidified after a phase transition caused by the thermal spike. The nanoscaled incisions should have originated from  
Figure 5: HAADF images of the pristine and the 30 keV Ga\({}^{+}\) ion-irradiated MoS\({}_{2}\) mono-layers with different ion doses [99]. Reprinted with permission from Reference [99]. Copyright © 2018 American Chemical Society.  
the material evaporation. It was reported that the threshold for grazing incidence of Xe ions was 2.0 keV/nm.  
[PERSON] et al. [101] deposited MoS\({}_{2}\) mono-layers on KBr(100) substrates and irradiated them with highly charged Xe ions (Xe\({}^{35+}\) with kinetic energy of 25.4 keV and Xe\({}^{40+}\) with kinetic energy of 38.5 keV) in a vacuum at room temperature. The fluences ranged from \(5\times 10^{9}\)-\(2\times 10^{10}\) ions/cm\({}^{2}\). Hillocks were observed on the MoS\({}_{2}\) mono-layer surfaces. The MoS\({}_{2}\) mono-layers were exfoliated from a single crystal under ambient condition while no details were provided, so it is hard to compare their results with other groups.