Defects in bulk and monolayer 2D transition-metal dichalcogenides

Point defects in transition-metal dichalcogenides (TMDs) significantly influence their electronic and optical properties. Despite extensive research over the past decade, identifying these defects in layered two-dimensional materials remains challenging and often controversial. Through first-principles calculations, we re-examine the roles of chalcogen vacancies and hydrogen impurities in bulk TMDs, providing insights into formation energies and thermodynamic and optical transition levels. Our findings reveal that sulfur vacancies can account for the recently observed cathodoluminescence spectra in MoS2 flakes and predict similar optical levels in other bulk TMDs. We find hydrogen impurities more stable at interstitial sites within the Mo plane, acting as shallow donors and potentially explaining the frequently observed n-type conductivity in some bulk TMDs. We also predict the local vibration mode frequencies for hydrogen impurities, facilitating their identification through Raman or infrared spectroscopy.

Additionally, our results indicate that chalcogen vacancies are deep acceptors and do not contribute to n-type or p-type conductivity in monolayer TMDs. The (0/-1) and (-1/-2) transition levels occur within the band gap, resulting in paramagnetic charge states S=1/2 and S=1, respectively, in a collinear-spin representation. We discuss trends based on band alignments among TMDs, which can guide future experimental studies on vacancy behavior.