Thermal Transport Properties Of Two-Dimensional Transition Metal Dichalcogenides

With the major breakthroughs made in graphene in applied research and industrial development,new two-dimensional materials with graphene-like structure have also received widespread attention.Searching for new two-dimensional materials,exploring their physical and chemical properties,regulating material properties,and promoting the development of new two-dimensional materials,have become the main work in the field of low-dimensional materials in recent years.Similar to graphene,the new two-dimensional materials also exhibit unique physical properties that differ from bulk materials.Among many new two-dimensional materials,transition metal dichalcogenides?2D-TMDs?have become hot materials in the field of two-dimensional semiconductor research due to their moderate energy band gaps and rich physical properties.To date,studies have shown that 2D-TMDs have potential to make breakthroughs in optoelectronic devices,energy storage and thermoelectric devices.Heat transport properties,as the basic physical properties of materials,are also key properties that materials consider in device applications.However,due to the limitation of experimental conditions,there are relatively few systematic studies on the heat transport properties of 2D-TMDs,especially the research on the regulation mechanism of 2D-TMDs heat transfer properties.To satisfy the application requirements of different devices for materials,exploring the phonon scattering law in the process of heat transport,and studying the effective regulation mechanism of 2D-TMDs heat transfer performance are of great significance to promote the application of materials in electronic devices.In this thesis,we combine the Boltzmann transport equation and molecular dynamics methods to systematically study the thermal transport properties of the two-dimensional transition metal dichalcogenides MX₂?M = Mo;X = S,Se?.We focus on the effects of double-layer heterojunctions and defects on the thermal transport properties of materials.The research findings provide a reference for exploring the application and development of 2D-TMDs.The main research contents and results of this thesis are summarized as follows:?1?The main carriers of thermal transport in two-dimensional transition metal sulfides are phonons.Therefore,the study of the thermal transport properties of 2D-TMDs comes down to the phonon transport problem.Here,we systematically investigate the thermal transport properties of MoS₂/MoSe₂ bilayer heterostructure?MoS₂/MoSe₂-BH?by combining first-principles calculations and Boltzmann transport theory?BTE?.Results show that the thermal conductivity of MoS₂/MoSe₂-BH at room temperature is 25.39 W/?m·K?,which is in between those of monolayer MoSe₂ and MoS₂.According to our calculated orbital-resolved phonon dispersion curves,Grüneisen parameters,phonon group velocity and relaxation time,we find that the acoustic and low-frequency optical branches below 172.65 cm^-1 have strong coupling,the phonon transport is influenced by the weak van der Waals interlayer interaction,and then change the lattice thermal conductivity of the material.?2?Using the non-equilibrium molecular dynamics method,we study the lattice thermal conductivity of monolayer MoSe₂ and its response to simulated size and defects.With the increase of sample length,the thermal conductivity of monolayer MoSe₂ nanoribbons exhibits an enhancement whereas it is insensitive to the width.At room temperature,the thermal conductivities of monolayer MoSe₂ along armchair and zigzag directions are 17.758 and 18.932 W/?m·K?,respectively,which are consistent with previous results.The impact of defects on thermal conductivity has also been studied by varying the concentration of the vacancy from 0.1% to 0.5%.The results show that an increase of the defect concentration will greatly suppress the thermal conductivity.For example,the 0.5% defect concentration with a Mo vacancy can result in a thermal conductivity reduction of ?43%.Our research also shows that the regulation of heat transfer performance can be achieved by introducing few defects.?3?Combining first-principles and molecular dynamics methods,we systematically study the electronic structure and in-plane thermal conductivity of the graphene/MoSe₂ heterojunction?G-MoSe₂?.We find that the G-MoSe₂ heterojunction has a graphene-like electronic band structure.A direct energy band gap of 2 me V is formed at a high symmetry point K.The band gap value is much smaller than the k BT value at room temperature,and it can even vanish at room temperature.Thus,the G-MoSe₂ heterojunction has graphene-like electronic properties.The molecular dynamics calculations show that the thermal conductivity of the G-MoSe₂ heterojunction at room temperature is 272.5 W/?m·K?.By adjusting the intensity factor of the interface interaction of the heterojunction,it is found that the thermal conductivity of the G-MoSe₂ heterojunction decreases with the intensity of interface interaction increasing.