Beyond Perturbation: Role of Vacancy-Induced Localized Phonon States in Thermal Transport of Monolayer MoS₂

Sulfur vacancies in monolayer MoS₂ can provide unexpected opportunities for tailoring the properties and device applications via defect engineering. However, determining the effect of vacancies in thermal transport remains a big challenge. Using a first-principles supercell approach, we reveal the dominant role of defect-induced quasi-localized phonon states in reducing thermal conductivity of MoS₂. These states are related to flattened dispersions in phonon spectrum, which comes from perturbations in atomic mass and interatomic bonding. Although the scattering strength of each modes remains similar, the phonon group velocities are much lower near the quasi-localized modes, while the Umklapp scattering are significantly enhanced. Thus, the thermal conductivity of defective MoS₂ is severely reduced. Our results contribute to fundamental understanding of the effect of vacancies on thermal transport, and can be used to assess the defect concentrations in semiconductors quantitatively.