# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 1 Introduction
### MoS\({}_{2}\) 2D Materials  
MoS\({}_{2}\) has two main types of natural phases: hexagonal structure (2H type) with the space group of P6\({}_{3}\)/mmc (D\({}^{4}_{6h}\), No. 194) and the trigonal structure (3R type) with the space group of R3m (C\({}^{5}_{3v}\), No. 160). The 2H phase contains two layers per unit cell, \(a=3.15\) A and \(c=12.30\) A. Similar to graphite, the 2H type crystals can be easily cleaved to form (0001) layers because of weaker van der Waals forces between (0001) layers. The 3R phase contains three layers per unit cell, stacking in rhombohedral symmetry with trigonal prismatic coordination, \(a=3.16\) A and \(c=18.33\) A. A third metastable phase, 1T-MoS\({}_{2}\) with a tetragonal symmetry, was also artificially produced by intercalating 2H-MoS\({}_{2}\) with alkali metals [27; 28; 29]. The artificial 1T phase and natural 3R phase are metastable at room temperature and transform to the 2H phase on heating [27] or IR irradiation [30] or microwave irradiation [31]. This work focuses on the 2H MoS\({}_{2}\) phase. 1T and 3R polytypes will not be discussed. Molybdenum disulfide (2H-MoS\({}_{2}\)) is a transition-metal dichalcogenide with a melting point of 2375 \({}^{\circ}\)C. The bulk material is chemically stable in dilute acids and oxygen and insoluble in water. Figure 1a shows the crystallographic structure of 2H-MoS\({}_{2}\) bulks. There are two S-Mo-S layers per unit cell stacked in hexagonal symmetry. Each S-Mo-S layer consists of two hexagonal planes of sulfur atoms and an intermediate hexagonal plane of molybdenum atoms. The sandwiched molybdenum atom plane is coordinated through ionic-covalent interactions with the sulfur atoms, forming a stable MoS\({}_{2}\) mono-layer as shown in Figure 1b. The MoS\({}_{2}\) mono-layers are held together by weak van der Waals interactions. The forces between the mono-layers are not very strong [32], and the interatomic forces within a mono-layer are sufficient for thermodynamical stability [33]. Therefore, 2H-MoS\({}_{2}\) has been widely used as a dry lubricant under high temperature due to the microscaled chemical properties. Figure 2 shows the calculated band gap of MoS\({}_{2}\) bulks and that of MoS\({}_{2}\) few-layers. The bulk 2H-MoS\({}_{2}\) crystal is an indirect band-gap semiconductor with a band gap of 1.23-1.29 eV [32; 34; 35; 36]. MoS\({}_{2}\) bulks can be employed as cathode materials in batteries [29]. MoS\({}_{2}\) bulks show very poor catalytic activities for hydrogen evolution reactions. The band gap of MoS\({}_{2}\) is a function of layer thickness as shown in Figure 2b-d, and unperturbed by substrate interactions. The mono-layer MoS\({}_{2}\) has a direct band gap of 1.8 eV [37]. The band structure changes from the indirect-gap in bulks to the direct-gap in MoS\({}_{2}\) mono-layers, being confirmed from experiments [38; 39]. The thickness-dependent electronic structure change is closely associated with many corresponding changes in chemical and physical properties. For example, the transition for indirect-to-direct band gap is correlated with changes in mechanical strength, spin density, bond energy, electrical conductivity, and the properties of transistor and sensor devices. MoS\({}_{2}\) nanoparticles can adsorb various compounds, including tetrahydrothiophene, thiophene, benzothiophene, dibenzothiophene, and their derivatives [40]. The chemical [29; 41] and physical properties [42; 43; 44] of MoS\({}_{2}\) few-layers were recently reviewed. These novel properties of MoS\({}_{2}\) 2D materials make them suitable 2D candidates in environmental applications [45; 46], catalysts (such as electrocatalysts [47; 48; 49; 50; 51] and photocatalysts [48; 49; 51; 52; 53]), energy storage (such as lithium ion batteries [48; 49; 54], supercapacitors [49; 54], elastic energy [12], and solar cells [49]), as well as sensing [55].