Unraveling the properties of defective WS₂ monolayers in a controlled gas atmosphere: a theoretical study

Two-dimensional materials are emerging as the ideal candidates for innovative electronic and optoelectronic devices, since they intrinsically push the scaling process down to the atomic level, overcoming common technologic limits. Among the plethora of two-dimensional materials, Transition Metal Dichalcogenide (TMD) monolayers, such as the direct-bandgap semiconducting WS₂, have been intensively studied since they exhibit a unique combination of atomic-scale thickness, optoelectronic and mechanical properties, resulting suitable for a wide range of applications. Moreover, Chemical Vapour Deposition (CVD) techniques allow for a fairly easy synthesis of large flakes. However, WS₂ properties are strongly influenced by the adsorption of chemical species in the surrounding environment due to the large surface-volume ratio in the monolayers. This process is enhanced in CVD samples due to the high defect density. Identifying the impact of adsorbed molecules on the material properties is fundamental for the design of effective technological applications since environmental conditions, such as humidity or the presence of oxidizing agents, can alter the performances in an unpredictable way. For that reason, this thesis focuses on ab initio Density Functional Theory (DFT) simulations of molecule adsorption on WS₂ monolayers, analysing the influence of defects in this process. Initially, the electronic properties of common accidental defects, such as Cr as W substituent, O as S substituent and S vacancies, are analysed in order to identify their main geometrical and electronic features. Then, the adsorption of common gas molecules on the defect sites is studied to quantify the interaction strength and understand how the adsorbed molecules alter the characteristics of the system. According to simulations, molecular oxygen results to have the major influence on the material properties among the different analysed gases. Physisorption is the most common process also in presence of defects, but O₂ chemisorption can also occur on the S vacancy site. The main effects of the latter process are the decrease of the band gap value and the passivation of the in-gap defect states that mediate non-radiative recombination mechanisms. Hence, the O₂ interaction has also beneficial effects in view of the photon emission enhancement. For that reason, the accidental gas exposure and a partial material oxidation can become an intentional strategy to passivate the reactive sites of the S vacancies and ‘repair’ the flakes.