# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 4 Other Irradiation
### Thermal Irradiation  
MoS\({}_{2}\) bulks sublimate at 450 \({}^{\circ}\)C and melt at 2375 \({}^{\circ}\)C. MoS\({}_{2}\) crystals are not stable in the presence of oxygen, resulting in MoO\({}_{3}\). It was reported that the material oxidized at 315-375 \({}^{\circ}\)C in air and converts to MoO\({}_{3}\) at 400 \({}^{\circ}\)C [144]. MoS\({}_{2}\) mono-layers are not very stable at ambient conditions either. [PERSON] et al. [159] examined atomic-resolution STM images of the basal plane of mechanically exfoliated MoS\({}_{2}\) mono-layers after one month and one year of ambient exposure (in air, at room temperature and ambient light). STM measurements clearly revealed modifications in the atomic structure of the MoS\({}_{2}\) basal plane during the ambient exposure. New point-defects were formed after one month of the ambient exposure and the defect concentration increased 20-30 times. The point-defects increased 100-500 times after one year of exposure. The oxidation of MoS\({}_{2}\) basal planes yielded 2D MoS\({}_{2-x}\)O\({}_{x}\) layers. [PERSON] et al. [117] annealed the liquid-exfoliated MoS\({}_{2}\) mono-layers on various substrates (SiO\({}_{2}\), Au, graphene, BN, and CeO\({}_{2}\)) in a vacuum. Thermal annealing did not introduce any noticeable defects into MoS\({}_{2}\) layers up to 450 \({}^{\circ}\)C and MoS\({}_{2}\) remained stable, as shown in Figure 11. Other group reported that thermal annealing in a vacuum caused S vacancies in MoS\({}_{2}\) at 500 \({}^{\circ}\)C [90]. [PERSON] et al. [160] annealed MoS\({}_{2}\) crystals in UHV. Single sulfur vacancies were generated in bulks while the S/Mo ratio in the bulk did not change with the annealing temperature up to 400 \({}^{\circ}\)C. They also examined the liquid-exfoliated MoS\({}_{2}\) few-layers. The S/Mo ratio of MoS\({}_{2}\) few-layers gradually decreased once the annealing temperature increased up to 400 \({}^{\circ}\)C. Two kinds of sulfur vacancies (single and double) were introduced in the molybden layers. The threshold for the formation of double vacancies typically occurred upon thermal annealing at 200 \({}^{\circ}\)C.  
[PERSON] et al. [125] mechanically exfoliated MoS\({}_{2}\) mono-layers from bulk crystals and transferred them to Si wafers with SiO\({}_{2}\) capping layer. The mono-layers were then annealed at temperature of 350 \({}^{\circ}\)C or 500 \({}^{\circ}\)C for 1 h in a vacuum (0.1 Pa). Figure 28a shows PL intensity image of an as-prepared MoS\({}_{2}\) mono-layer, which was uniform across the whole sample. After being annealed for 1 h at 350 \({}^{\circ}\)C in a vacuum, the PL intensity was enhanced by 6-fold (Figure 28b). At the same time, the PL peak blue-shifted after the annealing. Figures 28c-d show another MoS\({}_{2}\) mono-layer before and after annealing at 500 \({}^{\circ}\)C for 1 h in a vacuum. The PL image becomes highly inhomogeneous after the annealing. More detailed investigation indicated that the PL enhancement and inhomogeneity should come from the reaction of oxygen (due to not very high vacuum condition) with MoS\({}_{2}\) at high temperature and followed chemical bonding of oxygen molecules to MoS\({}_{2}\). [PERSON] et al. [161] mechanically exfoliated MoS\({}_{2}\) single-layers and few-layers from MoS\({}_{2}\) bulk crystals, deposited them onto 300 nm thick SiO\({}_{2}\)/Si substrates. The MoS\({}_{2}\) layers were then exposed to an Ar/O\({}_{2}\) mixture at temperatures ranging from 27 \({}^{\circ}\)C to 400 \({}^{\circ}\)C. The flow rates of Ar and O\({}_{2}\) were 1.0 L/min and 0.7 L/min, respectively. Raman spectroscopy measurements indicated that the oxygen treatments led to triangular etch pits on the surfaces of the atomically thin MoS\({}_{2}\). MoS\({}_{2}\) was etched preferentially along the crystallographic directions of the zigzag edges with a preferential termination, resulting in uniform orientations of the pits, as shown in Figure 29a-e. The pit size increased with increasing oxidation exposure (Figure 29f), and the growth rate was larger at higher temperature.