Thermal Conductivity Modulation Of Molybdenum Disulfide Heterostructure Based On Phonon Wave-particle Duality

Recently,two-dimension transition metal dichalcogenides(TMDs)with special physical properties have become one of the most promising semiconductor candidates after graphene due to its intrinsic band gap and high Seebeck coefficient.In particular,the thermal properties of semiconductor materials are critical to the performance of electronic devices.However,the inherent lattice thermal conductivity of TMDs limits the development of its thermoelectric properties and thermal design.In this thesis,thus,based on phonon wave-particle duality,we investigated the phonon transport properties of molybdenum disulfide(MoS2)with random lattice defects,periodic arrayed nanodots and isotope nanodots embedded heterostructures,respectively.The main research contents of this paper are as follows:(1)Based on the particle property of phonons,we investigated thermal conductivity of single-layer MoS2 sheet with various random atomic-scale lattice defects using non-equilibrium molecular dynamics simulations.Simulations results indicate the thermal conductivity can be suppressed by a small amount of lattice defects.In the comparison with sulfur atom defects,molybdenum atom defects can effectively suppress the thermal conductivity of monolayer MoS2.For example,0.51%Mo atom defects can reduce thermal conductivity by 75%.Based on the lattice dynamics,it is revealed that the reduction of thermal conductivity results from the decreased phonon group velocity induced by defects,and most of phonons localized around lattice defects.Furthermore,the contributions from various frequency range of phonons to the thermal conductivity of pristine and defective MoS2 were quantified.Especially,it is found that the coupling strength of between low-frequency and high-frequency phonons gradually increases due to the introduction of atomic-scale defects.Moreover,the temperature dependence property of thermal conductivity of defective MoS2 was discussed.This part of lattice defect study is beneficial for thermal design of micro/nano electric devices and optimizing the thermoelectric properties of MoS2 based materials.(2)Manipulating coherent phonons in metamaterials such as superlattices and phononic crystals has become one of the important means of the thermal design of the microelectronic device.In this part,we investigate the controllability and visualization of the coherent phonon transport in a monolayer MoS2/MoSe2 periodic arrayed heterostructure with minimum lattice mismatching using non-equilibrium molecular dynamics simulation based on the wave duality of phonon in detail.It is found that the coherent phonon transport can be destroyed and rebuilt through adjusting the density of MoSe2 nanodot arrays.Based on lattice dynamic simulation,the phonon localization induced by the destruction of correlation is visualized directly by the spatial energy distribution of the MoS2/MoSe2.It found that most of the phonon energy were blocked in MoSe2 region.Furthermore,the phonon mode weight factor and the eigen vector diagrams provide a distinct visualization of the localized phonon modes.Besides,the correlation of phonon can be rebuilt by reducing the period length,which is verified by the enhanced group velocities extracted from phonon dispersion curves.Interestingly,the crossover from incoherent to coherent phonon transport is directly observed by the spatial energy distributions.In addition,the spectral phonon transmission coefficients of various period length also confirm the phenomenon of low frequency phonons being suppressed and then restored.Finally,the size and temperature dependence of thermal conductivity of periodic MoS2/MoSe2 arrayed heterostructure are also discussed in detail.This study of the phonon coherence and its visualizing manipulation on thermal conductivity will be beneficial to fine heat control and management in the real applications.(3)Isotopic engineering is a vital strategy to control the thermal conductivity of materials.Based on the wave-particle dualism of phonons,we investigated the thermal conductivity of the monolayer MoS2/MoSw2 and MoS2/MoSem2 periodic isotope nanodots superlattices systematically.The non-equilibrium molecular dynamics simulations results indicate that the thermal conductivity will decrease significantly,regardless of increasing or decreasing the mass of S(Se)atoms in MoSm2(MoSem2).Based on lattice dynamics,phonon participation ratios and phonon group velocities are used to reveal the phonon-isotope and phonon-boundary scattering mechanism.The localization caused by isotope-embedded can be observed directly by spatial energy distribution.According to the phonon dispersion relations,it is found that the combination of phonon localization caused by phonon-isotope scattering,phonon-boundary scattering and anti-crossing suppress the phonon transport when the MoSm2 embedded,which soften the low frequency phonon branches.Moreover,the phonon localization can be enhanced with the increasing of S isotope mass,various phonon localization mode can be observed by the eigen vector diagram.However,in addition to the hybridization impact of both phonon-isotope and phonon-boundary when the MoSem2 embedded,the resonance phenomenon is also found at low-frequency.It found that the direction of vibration component of MoSeOm2 region in opposite with MoSem2 region,and the intensity of resonance will be enhanced with the increasing of the isotopic mass.The analysis reveals that the main cause of resonance is due to the difference in lattice constants of between MoS2 and MoSe2,and isotope mass can further tune the intensity of resonance.This work has guiding significance for the regulation of thermoelectric material efficiency based on isotope engineering and the thermal design of electronic devices.