Study of Punctual Defects in Monolayer WS2: Evidence of Correlations Between Raman and Photoluminescence Spectroscopy

The investigation of defects in transition-metal dichalcogenides (TMDs) are of capital interest for future applications because they strongly change the optical, electronic, or vibrational properties of such materials. In this sense, spectroscopic techniques, such as Raman and photoluminescence, are powerful tools to investigate the optoelectronic properties of crystal defects in two-dimensional TMDs. In this work, we observed that defect-activated Raman modes and bound exciton emissions can be strongly correlated. Specifically, we investigated the impact that sulfur vacancies, produced by focused helium ion beam, have on the electronic and phononic properties of WS2 grown by chemical vapor deposition. The photoluminescence spectra show two new emission peaks related to defects in the crystal structure. The defective nature of these bands were corroborated by density functional theory calculations, which showed new electronic states in the band gap associated with sulfur vacancies. Furthermore, by monitoring the evolution of the Raman spectra as a function of the defect concentration, we observed two new defect activated modes. These bands are explained by a second-order double resonant Raman process, similar to the D band of graphene. Finally, we found out that the defect-related Raman peaks become fully resonant for high defect concentrations. It turns out that degenerate electronic states split in two separated levels for high defect concentrations that are involved in the resonance of the Raman processes.