# Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers
## 1 Introduction  
With the discoveries of zero-dimensional buckyballs in the 1980s [1] and one-dimensional carbon nanotubes in the 1990s [2; 3], various nanomaterials have been synthesized and characterized. Their physical and chemical properties are unique compared with counterpart bulks (graphite, diamond, and amorphous carbon) because of size-induced quantum effects, enabling their great potential in various fields. Two-dimensional (2D) graphene was reported in the 2000s [4] and excellent electrical behaviors were claimed. Since then investigations of 2D layers were initialized and rapidly developed in recent decades. At present, the 2D layer family spans conductors (such as graphene), insulators (such as hexagonal boron nitride), and semiconductors. Transition-metal dichalcogenide (TMDC) materials, including MoS2, WS2, MoSe2, and WSe2, are layered semiconductors and have been fabricated into 2D semiconducting layers. MoS2 2D layers have novel potential applications in novel nano-optoelectronics, optical sensors, catalysts, energy storages, and environments. Additionally, MoS2 2D layers show thickness-dependent semiconducting behaviors, being the most interesting and important among TMDC 2D layers. Bulk MoS2 is a semiconductor with an indirect energy band gap of 1.2 eV while mono-layer MoS2 is a direct band-gap semiconductor with an energy band gap of 1.9 eVdue to 2D quantum confinement. The MoS\({}_{2}\) layers have also important applications in valleytronics--a new type of electronics where information is encoded by wave quantum number of electrons.  
MoS\({}_{2}\) low-dimensional materials have been investigated over the past 50 years. MoS\({}_{2}\) few-layers (4-5 layers) were first prepared by a peeling process from bulk crystals in the 1960s [5]. Tribological properties of MoS\({}_{2}\) films have been studied later in a vacuum or dry air as solid lubricants since the 1990s [6; 7]. Compared with bulks, the friction coefficient of MoS\({}_{2}\) films was reduced, and the wear life of the films was enhanced. Catalytic properties of MoS\({}_{2}\) nanoparticles [8] and films [9; 10] have been investigated from the 1980s. It is believed that edge sites of MoS\({}_{2}\) nanomaterials play important roles in electrochemical and catalytic behaviors. MoS\({}_{2}\) atomically thin materials, especially mono-layer MoS\({}_{2}\), have been widely investigated in recent years because of their sizable direct band gap, high charge-carrier mobility, and excellent mechanical flexibility [11; 12]. The materials have potential applications in the next generation flexible electronics [11; 13; 14], elastic energy storage[12], field-effect transistors [15; 16], electronic switches [13], electronic devices such as chemical biosensors [16; 17], and optoelectronic devices [18] such as photodetectors and solar cells [19]. It was demonstrated that chemically active defects, such as sulfur vacancies, significantly control or tune their catalytic activities [20], electronic transports [21; 22; 23], and optical properties [24].  
Various forms of irradiation and particle fluxes modulate the density of sulfur vacancies of chemical-vapor deposition (CVD) grown and mechanically exfoliated MoS\({}_{2}\) sheets, affecting their chemically active defects. Therefore, irradiation effects have been carried out in recent years. Additionally, these MoS\({}_{2}\) devices and sensors have been used in harsh environments, and it is necessary to investigate their tolerance under irradiation. Radiation effects on two-dimensional graphene were recently reviewed [25; 26]. Defect engineering and defect-induced properties of other atom-thick mono-layers were summarized as well [26]. The impacts of irradiation on 2H MoS\({}_{2}\) two-dimensional materials are reviewed here.