Effect And Mechanism Of Vacancy On Thermal Conductivity Of Single-layered MoS₂ And MoSe₂

Single-layer MoS₂ and MoSe₂ have moderate direct bandgap,which makes them have broad prospects in electronic and optoelectronic devices.However,these devices are geometrically ultra-thin,that is,a small amount of Joule heat will lead to a substantial increase in system temperature,and thermal management becomes a key issue.The inherent structural defects（such as vacancies）in two-dimensional materials will have an important impact on the thermal transport properties of materials.Therefore,it is necessary to deeply study the influence of vacancy defects concentration on the thermal transport properties of single-layer MoS₂and MoSe₂and the underlying physical mechanism.In this paper,firstly,the effects of Mosingle vacancy and S double vacancy（Mosingle vacancy and Se double vacancy）on the thermal conductivity of single-layer MoS₂（MoSe₂）are calculated and compared by non-equilibrium molecular dynamics,and the size effect and temperature effect of thermal conductivity of defective single-layer MoS₂and MoSe₂are analyzed.The results show that the introduction of two kinds of vacancy defects will significantly reduce the thermal conductivity of single-layer MoS₂and MoSe₂,and the inhibition effect of Movacancy on thermal conductivity is stronger at the same defect concentration.The size effect results show that the thermal conductivity increases with the increase of length,and the dependence of thermal conductivity on size will be weakened when defects are added.The results of temperature effect show that the introduction of defects will weaken the dependence of thermal conductivity on temperature.Then,the physical mechanism of vacancy defect inhibiting thermal conductivity is elaborated in detail by using lattice dynamics,quantum perturbation and bond relaxation theory.The lattice dynamics results show that both vacancy defects can significantly inhibit the phonon vibration mode participation rate of single-layer MoS₂and MoSe₂.At the same defect concentration,the inhibition effect of Mosingle vacancy on the phonon vibration mode participation rate is stronger,and it is more obvious in the high frequency part.The results of spatial distribution of local modes show that the localized vibration modes are mainly distributed on the low coordination atoms around the vacancy,and the localization degree of the low coordination atoms around the Mosingle vacancy is greater than that around the S（Se）double vacancy.Combining quantum perturbation and bond relaxation theory,we propose a coordination number dependent scattering mechanism of defective atoms,that is,the scattering mechanism of phonons by low coordination atoms around vacancy defects,and it is found that phonon scattering caused by low coordination atoms around Mosingle vacancy is the main physical mechanism that affects the thermal conductivity of vacancy defect monolayer MoS₂and MoSe₂.Finally,the calculation of phonon energy density spectrum is used to verify our proposed physical mechanism.The results of phonon dispersion relationship show that the phonon dispersion curves of defective monolayer MoS₂and MoSe₂are obviously broadened,and the single vacancy of Mois especially obvious in the high frequency part.The broadening of phonon dispersion curve will shorten the relaxation time of phonon,which will lead to the decrease of thermal conductivity.The phonon energy density spectrum at the high symmetry point shows that the broadening of phonon mode increases with the increase of defect concentration,and the broadening caused by Mosingle vacancy is more obvious.In addition,Lorentz function fitting at the characteristic peak shows that the phonon relaxation time of Mosingle vacancy is less than that of S（Se）double vacancy,which verifies the correctness of our proposed physical mechanism.This paper is helpful to deepen people’s understanding of vacancy defect inhibiting thermal conductivity.