Sulfur Adatom and Vacancy Accelerate Charge Recombination in MoS₂, but by Different Mechanisms: Time-Domain Ab Initio Analysis

Two-dimensional transition metal dichalcogenide performance depends strongly on defect morphology. Calculations predict that sulfur adatom and vacancy are among the most energetically favorable defects in MoS₂. By performing ab initio quantum dynamics calculations we demonstrate that both defects accelerate nonradiative recombination, but this happens through different mechanisms. Suprisingly, holes never significantly populate the shallow trap state created by the sulfur adatom, since the trap is stronly localized and decoupled from free charges. Charge recombination bypasses the hole trap. Instead, it occurs directly between free electron and hole. The recombination is faster than in pristine MoS₂, because the adatom strongly perturbs the MoS₂ layer, breaks its symmetry, and allows more phonon modes to couple to the electronic subsystem. In contrast, the sulfur vacancy accelerates recombination by the traditional mechanism involving charge trapping, followed by recombination. This is because the hole and electron traps created by the vacancy are much less localized than the hole trap created by the adatom. Because the sulfur adatom accelerates charge recombination by a factor of 7.9, compared to 1.7 due to vacancy, sulfur adatoms should be strongly avoided.