## 1 Introduction  
Two-dimensional (2D) molybdenum disulfide (MoS2), one of the semiconductor members of the 2D layered transition-metal dichalcogenide (TMDC) group, has been paid attention for fundamentally interesting phenomena such as indirect-to-direct crossover of the band structure in the monolayer (ML) limit, deep excitonic energy levels, strongly coupled valley, spin degree of freedoms, etc. [1; 2; 3]. Besides the basic scientific interests, as a promising alternative to conventional semiconductors in contemporary electronics for overcoming the fundamental limit of down-scaling, diverse electronic device applications have been developed actively from elementary device elements to complex digital circuits [4; 5]. Moreover, the direct bandgap of ML-MoS2 enables miniaturizing photonic and optoelectronic integrated circuits [6]. In addition, 2D MoS2 structures are expected to be suitable for sensor applications since they show, as surface states themselves, extremely high surface area-to-volume ratio, outstanding sensitivity to the environment, and excellent chemical and mechanical stability [7; 8; 9; 10]. Moreover, integrating the interactive 2D MoS2 layer into hetero-structures composed of other 2D layers or functional materials of other dimensions realizes novel synergetic applications such as flexible electronics and sensors,memory devices, and energy conversion systems [11; 12; 13; 14; 15]. However, it is still challenging to replace conventional materials whose synthesis, doping, and device fabrication processes are well-established in the modern industry. Therefore, for feasible applications, it would be required to enhance the functional capability of 2D MoS2 by tuning its properties controllably. For these purposes, various approaches have been developed: substitutional doping, nanostructure decoration, mechanical strain application, chemical surface functionalization, organic material coating, plasma and laser thinning, etc. [10; 16; 17; 18; 19; 20; 21; 22]. One of the traits that make several TMDC MLs, including MoS2, WS2, MoSe2, and WSe2, distinguishable from conventional semiconductor materials such as Si is that excitonic transitions are active even at room temperature (RT) in ambient conditions due to extraordinarily huge binding energy and long lifetime of excitons [2; 7; 23]. In the case of ML-MoS2, its optical and optoelectronic properties can be governed by the transition between neutral excitation (A) and negatively charged exciton (A\({}^{-}\)), also referred to as [2; 7; 23]. The effects of the A-A\({}^{-}\) transition are controllable by charging/discharging ML-MoS2 via various methods such as electrostatic gating, interplay with phase change materials, gas molecule physisorption, metal nanoparticle coating, etc. [2; 7; 20; 24]. Further, the charge transfer between ML-MoS2 and physisorbed gas molecules could occur more actively with defects [7; 25; 26]. The sulfur (S) vacancies are known to exist in ML-MoS2 as one of the primary native point defects based on the direct observation of transmission electron microscopy (TEM) as well as theoretical calculations [27; 28; 29]. These point defects tend to transfer electrons to physisorbed reactive molecules such as O2 and H2O and specific organic molecules, including single-layer titanyl phthalocyanine, leading to p-type doping as well as the transition from A\({}^{-}\) to A in ML-MoS2[7; 25; 26; 29; 30; 31]. Therefore, defect engineering, specifically controlling the amount of S vacancies, could be an efficient method to alter the optical, optoelectronic, and electrical properties of ML-MoS2 and also sensitivity to environmental conditions [25; 27; 28; 32; 33; 34; 35; 36; 37; 38; 39]. The defects, including S vacancies, have been shown to be created in diverse ways: thermal annealing, plasma treatment, irradiation with electrons, ions, and laser, etc. [25; 32; 33; 35; 36; 37; 38]. Above all, laser heating has been broadly employed for materials processing including deposition, selective melting, microstructural modification, fusion, etc.