An empirical potential study of first-order Raman scattering in defective monolayer MoS2

Semiconducting monolayer molybdenum disulfide (1H phase monolayer MoS2), a famous member of two-dimensional inorganic materials, is well known by its direct bandgap. With the bandgap, it makes unique opportunities possible for (opto-)electronic applications and heat transport applications. Defects, the inevitable members of solid materials, can play an important role in tailoring various properties. Raman spectroscopy is known as an essential and versatile tool for searching crystallinity, quality of materials under defects, and strain condition. We studied Raman spectra for various types of defective monolayer MoS2 containing vacancies and antisites. We monitored the effect of each type of defect on prominent modes as a function of the defect density. We showed that the in-plane Raman mode is quite sensitive to defects. Also, for some cases, the out-of-plane mode can be used as a defect indicator. Phonons at the edge of the Brillouin zone can make low-frequency Raman active modes. Moreover, we found that for some vacancies, there are characteristic peaks, which arise mainly because of the influence of vacancies on localized strain and symmetry breaking. We found that the localized strain effect is one of the most critical effects causing the primary Raman peak shifts of the defective MoS2. To explain the Raman peak shifts, we built two simple models, and through calculations, we showed that our models work well in predicting the Raman peak shift tendencies. Furthermore, through our models, we quickly got the conclusion that the mass effect and the disappearance of bonds caused by defects are the main reasons for the system's Raman peak shifts. In this thesis, the primary methods used in the simulation are Phonopy calculations combined with empirical potentials (EP), and making projections between each observed mode and the two Raman active modes of primitive monolayer MoS2. We showed that our method is economic and accurate enough when compared with first principle