# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 3 Electromagnetic Irradiation
### Visible Light Irradiation  
[PERSON] et al. [140] tested photocatalytic performance of N-doped MoS\({}_{2}\) nanoflowers under visible light. It was reported that the N-doped MoS\({}_{2}\) nanoflowers showed excellent photocatalytic activities and durability on the elimination of the organic pollutants under visible light irradiation. Laser has been widely employed in Raman scattering of MoS\({}_{2}\) few-layers. Laser has also been used to thin multilayered MoS\({}_{2}\) down to a single-layer two-dimensional crystal [141; 142] (power density up to 80-140 mW/\(\upmu\)m\({}^{2}\)) and generate ripples of MoS\({}_{2}\)[143; 142]. It is reported that the upper atoms of MoS\({}_{2}\) layers can be removed by high-power lasers because of laser ablation [141; 112]. Early investigation indicated that natural MoS\({}_{2}\) crystals do not undergo any kind of significant laser-assisted oxidation when exposed to high laser power (up to 32,000 W/cm\({}^{2}\)) (wavelength: 632.8 nm) [144] while their Raman mode intensity changes slightly with laser power. For microcrystalline MoS\({}_{2}\) powders, Raman scattering indicated that MoS\({}_{2}\) oxidized to MoO\({}_{2}\) under a high-power laser. Raman mode intensity and position were also significantly affected by the laser power. [PERSON] et al. [113] mechanically exfoliated MoS\({}_{2}\) few-layers from a bulk natural crystal and subsequently deposited them on Si/SiO\({}_{2}\) wafers, irradiated the few-layers under a pulsed laser with 800 nm wavelength and 1 kHz repetition rate. No modification of MoS\({}_{2}\) mono-layers was observed up to a certain single-pulse fluence of 50 mJ/cm\({}^{2}\) (2.5 mW) while damage occurred beyond the fluence via the material ablation. Further work indicated that MoS\({}_{2}\) mono-layers were practically unaffected by a low-power (600 \(\upmu\)W) pulsed laser irradiation when exposed to \(10^{3}\) pulses at a fluence of 20 mJ/cm\({}^{2}\) (lower than the damage threshold). However, the MoS\({}_{2}\) mono-layers were damaged upon \(10^{5}\) pulses at 20 mJ/cm\({}^{2}\). The \(A_{1g}\) and \(E_{2g}\) intensities were almost constant with pulse times, then rapidly decreased at a critical exposure time. The abrupt decrease possibly came from ablation and eventual sublimation of the MoS\({}_{2}\) atoms. [PERSON] et al. [111] in situ studied Raman scattering of MoS\({}_{2}\) layers during the laser thinning of MoS\({}_{2}\). Due to the high surface-area-to-volume ratio, thinner MoS\({}_{2}\) layers are less stable and easier to decompose under high-power laser irradiation because of laser-induced thermal effects and sublimation. [PERSON] et al. [145] irradiated MoS\({}_{2}\) layers under laser power of 1 mW, 5 mW, and 10 mW for an exposure time of 60 s. No significant changes were observed in topographic images of the irradiated regions under the 1 mW laser. However, significant amounts of particles or adsorbates were formed on the MoS\({}_{2}\) surface after irradiated under 5 mW and 10 mW lasers. To minimize thermal effects on atomically thin MoS\({}_{2}\) during Raman spectral measurements, low laser powers (0.14 mW to 2 mW) are usually employed [145] to avoid potential laser-induced local surface temperatures. [PERSON] et al. [112] employed a focused laser beam to directly pattern MoS\({}_{2}\) mono-layers and few-layers. Focused laser beam irradiation modified and thinned MoS\({}_{2}\) layers to create well-defined structures and controllable thickness. [PERSON] et al. [146] used a 532 nm laser to irradiate mechanically exfoliated MoS\({}_{2}\) few-layers (less than 5 layers) on SiO\({}_{2}\)/Si substrates. The power ranged between 0.93 and 8.3 mW and the treatment time was 0.01-180 s. In ambient conditions, MoS\({}_{2}\) nanosheets were etched and amorphous MoS\({}_{2}\) redeposited on the nanosheets at low laser powers while the few-layers were oxidized and MoS\({}_{2}\) nanoparticles formed at high laser powers. The nanoparticle formation and oxidation were dependent on the number of layers and laser exposure time.