# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 2 Charged Particle Irradiation
### Plasma Irradiation
#### 2.7.2 Active Oxygen O\({}_{2}^{*}\)  
Oxygen plasma species include ionized oxygen atoms O\({}^{+}\), excited oxygen atoms O\({}^{*}\), ionized oxygen molecules O\({}_{2}^{+}\), metastable excited oxygen molecules O\({}_{2}^{*}\), ozone O\({}_{3}\), ionized ozone O\({}_{3}^{+}\), excited ozone O\({}_{3}^{3}\), and free electrons. Therefore, oxygen plasma can effectively clean and etch MoS\({}_{2}\) 2D materials, introducing defects and doping oxygen to MoS\({}_{2}\) 2D layers. [PERSON] et al. [125] treated MoS\({}_{2}\) layers under oxygen plasma irradiation with 13.56 MHz and 5 W under 5 Pa. As shown in Figure 23 and the inset, the PL intensity increased with increasing plasma irradiation time. The PL enhancement could be increased as high as 100 times over. Considered the unchanged Raman scattering and XPS information, it was concluded that oxygen plasma introduced defects and oxygen bonding in MoS\({}_{2}\), enhancing PL intensity. [PERSON] et al. [126] treated MoS\({}_{2}\) multi-layers under oxygen plasma in a reactive ion etcher. Initial \(n\)-type MoS\({}_{2}\) layers were selected-area doped to \(p\)-type, forming \(p\)-\(n\) junctions in MoS\({}_{2}\). The fabricated  
Figure 23: PL spectra of mono-layer MoS\({}_{2}\) after oxygen plasma irradiation with different durations [125]. The change of PL intensities with the plasma irradiation time is shown in the inset. Reprinted with permission from Reference [125]. Copyright © 2014 American Chemical Society. Figure 22: Raman spectraof pristine and N\({}_{2}^{*}\)-irradiated MoS\({}_{2}\) mono-layers [98]. Reprinted from Reference [98], with the permission of AIP Publishing. highly rectifying diodes exhibited high forward/reverse current ratios and a superior long-term stability at ambient conditions. [PERSON] et al. [127] treated MoS\({}_{2}\) mono-layers with oxygen plasma. The photoluminescence evolved from a higher intense to completely quenched with increasing plasma exposure time because of a direct-to-indirect band-gap transition. The MoS\({}_{2}\) lattice was distorted after oxygen bombardment and MoO\({}_{3}\) disordered regions were generated in the MoS\({}_{2}\) flakes. [PERSON] et al. [128] deposited MoS\({}_{2}\) mono-layers (mechanically exfoliated from crystals) on Si/SiO\({}_{2}\) wafers and exposed them under oxygen plasma for several seconds. The plasma treatments were carried out at a power of 100 W operating at 50 kHz, using a gas mixture of oxygen (20%) and argon (80%) with a pressure of 250-350 mTorr. Raman spectroscopy and XPS spectroscopy indicated that MoS\({}_{2}\) mono-layers were oxidized and MoO\({}_{3}\) was produced in the reaction, 2 MoS\({}_{2}+7\)O\({}_{2}\to 2\)MoO\({}_{3}+4\)SO\({}_{2}\). The resulting MoO\({}_{3}\)-rich domains significantly decreased the mobility and conductivity of the fabricated MoS\({}_{2}\) mono-layer devices. [PERSON] et al. [129] exposed CVD-grown MoS\({}_{2}\) mono-layers under oxygen plasma at a pressure of 10 Torr with oxygen gas. Figure 24 shows the morphologies of MoS\({}_{2}\) exposed to oxygen plasma for 10 s, 20 s, and 30 s. After 10 s of oxygen plasma treatments, short and isolated cracks were observed on the continuous basal plane. Angles of those connected cracks were around 120\({}^{\circ}\). After 20 s of oxygen plasma treatments, cracks became longer and were connected to each other, forming a continuous network. When treated for 30 s, the widths of the cracks were further enlarged, most angles between the interconnected cracks were 120\({}^{\circ}\), and the MoS\({}_{2}\) was decomposed into even smaller fragments with a greater number of exposed edges. The structural change significantly decreased the intensity of A\({}_{1g}\) and E\({}_{2g}^{1}\) Raman modes of the MoS\({}_{2}\) mono-layers, and shifted the A\({}_{1g}\) mode to shorter wave-numbers and the E\({}_{2g}^{1}\) mode to longer wave-numbers. The exposure also decreased the PL intensity.