Research On Thermal Transport And Thermoelectric Properties Optimization Of Novel Two-dimensional Transition Metal Dichalcogenide

In recent years,as the investigation of the excellent properties of graphene increases,a number of novel two-dimensional(2D)materials have emerged,such as hexagonal boron nitride,black phosphorus,silicene,and transition metal dichalcogenide(TMDC).2D TMDC monolayers have become a research focus owing to rich elemental abundances,structural diversity,semiconductor properties,and adjustable electronic band structure,which can be widely used in electron devices,optoelectronic devices,and other fields.As is well-known,thermal conductivity,an inherent property of materials,can describe the heat transfer capability of materials and is an important research direction in the field of nano/microscale heat transfer.Materials with high thermal conductivity can effectively transfer heat and will be used for high-efficiency heat dissipation in electronic devices,while for materials with low thermal conductivity,it will facilitate the development of thermoelectric materials and can reduce carbon emission.Therefore,it is necessary to explore thermal transport properties and thermoelectric properties of novel 2D materials,which will contribute to tackle critical technologies and achieve the carbon peak and neutrality goals.In this dissertation,molecular dynamics and first-principles calculations are applied to study thermal transport properties and thermoelectric properties of 2D TMDC monolayers.The influential factors of the thermal conductivity and the figure of dimensionless thermoelectric figure-of-merit(ZT)have been explored.This investigation has explained the underlying physical mechanism of phonon and electron transport within the TMDC monolayers and proposed some methods for optimizing thermal transport properties and thermoelectric properties of 2D materials.The contents and results of this investigation can be summarized as follows:(1)To construct the 2D fractal structure,we couple isotopically doped atoms with fractal structures for the first time.By using the molecular dynamics simulations,the effect of fractal number on the in-plane thermal conductivity of WS2 material has been evaluated in comparison with graphene(GRA).The introduction of isotopic doping atoms at the fractal pattern,i.e.,the inhomogeneous mass distribution in the system,will change the phonon vibrational frequency and localize some phonon modes,which can enhance phonon-isotope scattering.Thus,thermal conductivities of WS2-SC,WS2-RC,and GRA-SC structures first decrease and then slightly increase with the increasing fractal number.Moreover,phonon behaviors have been investigated by using lattice dynamics simulations.It can be found that the phonon group velocities and phonon participation ratios of the WS2-SC fractal structure are smaller than those of the WS2-RC fractal structure and the GRA-SC fractal structure,which leads to a lower thermal conductivity of the WS2-SC fractal structure.Additionally,the phonon wave packet trajectories at different interfaces indicate that there are more continuous reflected waves in the WS2-SC0+WS2-SC2 system,which reveals the pronounced phonon-isotope scattering in the WS2-SC structure in comparison with the WS2-RC structure.This chapter can analyze influencing factors of thermal conductivities of 2D fractal structures,and reveal the phonon scattering mechanism of 2D fractal structures,which provides a new perspective for regulating thermal transport properties of 2D materials.(2)The effect of biaxial strain on the thermal transport properties of the WS2 monolayer has been investigated systematically by conducting the first-principles calculations and solving the Boltzmann transport equation.It is demonstrated that the lattice thermal conductivity of WS2 monolayer exhibits isotropic properties and both biaxial tensile and compressive strains can reduce the lattice thermal conductivity of WS2 monolayer.The corresponding maximum reduction ratios of lattice thermal conductivity are 27.4%and 17.3%.respectively.Furthermore,by analyzing the phonon harmonic and anharmonic characteristics of WS2 monolayer under different biaxial strains,two phenomena can be observed as follows:(1)biaxial tensile strain can decrease the phonon heat capacity,the acoustic phonon group velocity and the phonon lifetime,which results in the lower lattice thermal conductivity.(2)although biaxial tensile strain increases the phonon heat capacity,and the acoustic phonon group velocity,there has a substantial decrease in the phonon lifetime.Hence,the comprehensive competition among these factors can decline the lattice thermal conductivity of WS2 under biaxial compressive strain.This chapter can prove the effect of biaxial strain on the lattice thermal conductivity of the WS2 monolayer,and reveal underlying reasons for impacting thermal transport in 2D materials,which further lays advantageous foundations for broadening the 2D flexible electronic devices field.(3)Based on the TMDC materials,the 2D GRA-WS2 heterobilayer has been constructed.In contrast to GRA-C3N heterobilayer,the in-plane thermal conductivity and out-of-plane interfacial thermal resistance of GRA-WS2 heterobilayer are calculated by using the molecular dynamics method,and the phonon vibration properties of the corresponding materials are analyzed by employing the lattice dynamics simulations.The magnitudes of both phonon group velocity and phonon participation ratio are in the order of GRA>C3N>WS2,revealing that C3N and WS2 have more localized phonon modes and stronger phonon scattering,which can suppress phonon thermal transport.Thus,the order of thermal conductivity is GRA>C3N>WS2.Moreover,at 300 K,the thermal conductivities of both GRA-WS2 and GRA-C3N heterostructures monotonically increase with the increasing system size,and the infinite length thermal conductivities of the corresponding heterobilayers are 675.18 W m-1 K-1 and 1112.59 W m-1 K-1,respectively.Furthermore,both high temperature and large coupling strength can reduce the thermal conductivities of two heterobilayers.Besides,the thermal rectification phenomenon can not be observed in these heterobilayers by tuning system area,temperature,or coupling strength.The larger phonon mismatch between WS2 and GRA can results in a higher interfacial thermal resistance of GRA-WS2 heterostructure in comparison with GRAC3N heterostructure.In addition,high temperature and larger coupling strength will enhance the Umklapp phonon scattering and the energy transmission probability induced by phonon coupling,which generates lower thermal resistances of two heterostructures.This chapter investigates the factors influencing the in-plane and out-of-plane thermal transport properties of 2D heterobilayers,which illustrates the phonon vibration properties of 2D heterobilayers and will play an important role in guiding the thermal management of 2D electronic devices.(4)We focus on thermoelectric properties of TMDC materials and establish WS2-WSe2 superlattices with different periodic lengths.The thermoelectric properties of WS2,WSe2,SL1,and SL2 monolayers are evaluated for the first time by using the first-principles calculations.The lattice thermal conductivities of WS2 and WSe2 are isotropic,while the values of SL1 and SL2 are anisotropic.The inhomogeneous mass distribution in superlattices causes lower phonon group velocities and smaller phonon lifetimes,drastically decreasing the lattice thermal conductivity.In contrast to WS2 and WSe2 monolayers,the reduction ratio of lattice thermal conductivities of SL1 and SL2 monolayers are approximately 75.8%/40.0%and 83.4%/58.8%,respectively.Moreover,all monolayers have direct electronic band gaps,proving that they can be used as thermoelectric materials.The ZTmax of p-type doped WS2,WSe2,SL1,and SL2 monolayers at 1000 K can reach 0.43,0.37,0.95,and 0.66,respectively.The ZT of the SL1 monolayer is 2.18 times and 2.52 times larger than those of WS2 and WSe2 monolayers.This chapter explores phonon and electron transport properties of different superlattices,and analyses the underlying reasons for affecting thermoelectric properties of TMDC materials,which also expounds the critical position of superlattice to improve thermoelectric properties of 2D materials.(5)Based on the SL1 monolayer with higher thermoelectric properties,five WS2-WSe2 phononic crystals(PCH1,PCH2,PCH3,PCH4,and PCH5)are constructed.The thermoelectric properties of these structures are first investigated by combing the firstprinciples calculations with Boltzmann transport theory.It can be found that the first four structures are stable,and the PCH5 structure may have a phase transition.Moreover,only PCH1 and PCH2 structures have semiconducting properties and are suitable for thermoelectric materials.By introducing extra phonon-defect scattering,PCH1 and PCH2 structures have lower phonon group velocities and phonon lifetimes,which can further reduce the lattice thermal conductivity compared to the SL1 monolayer.At 1000 K,the p-type doped PCH1 and PCH2 structures have ZTmax of 1.17(0.96)and 2.53(4.54)along the x(y)direction.The values of the PCH2 structure are 2.66 and 6.75 times higher than those of the SL1 monolayer.This chapter can reveal the special phonon scattering mechanism in phonon crystals and propose a new strategy to optimize thermoelectric properties of 2D materials.(6)The superlattice structure,Janus structure,and nanoribbon structure are first coupled with each other to model different WS2-WSe2 nanoribbons.By employing the first-principles calculations,the thermoelectric properties of WS2-WSe2 nanoribbons(WS2,WSe2,SL,and JA nanoribbons)with ribbon widths of 5-7 are studied for the first time.All nanoribbons are structurally stable and exhibit semiconducting properties,which are lower than the values of corresponding monolayers owing to the disordered edge effect.Besides,the ribbon width can be used to adjust electronic band gaps of all nanoribbons.WS2-A5,SL-A5,and JA-A5 have high carrier mobility and large electron relaxation times.Furthermore,compared to the other nanoribbons,SL-A5 and JA-A5 have larger electrical conductivities and power factors,which are beneficial to increase the thermoelectric properties of nanoribbons.In addition,these nanoribbons have extremely low lattice thermal conductivities on account of the extra phononboundary scattering.Finally,the ZTmax of WS2,WSe2,SL,and JA nanoribbons are 1.26,1.97,5.47,and 4.13,respectively,which outclass the thermoelectric properties of the corresponding monolayers.Especially,the ZTmax of SL nanoribbon are higher than the results of the aforementioned chapters(superlattices and phononic crystals).This chapter elucidates phonon and electron transport properties of low-dimensional nanoribbon structures and points out the effect of ribbon width on thermoelectric properties of nanoribbons.Our work can improve the guideline for optimizing the thermoelectric properties of 2D materials,which is crucial to develop a new generation of high-performance thermoelectric materials.In summary,this dissertation focuses on thermal transport properties and thermoelectric properties of novel 2D transition metal dichalcogenide materials by using molecular dynamics simulations,lattice dynamics simulations,and first-principles calculations,which will promote the process of basic research on energy transport in two-dimensional materials.Our research results can regulate and optimize thermal transport properties and thermoelectric properties of TMDC.For the first time,we calculate in-plane thermal conductivities and outof-plane interfacial thermal resistances of TMDC with different structures and strains.Moreover,the phonon behaviors such as phonon heat capacity,phonon group velocity,and phonon lifetime are analyzed.Furthermore,we also reveal multiple phonon scattering mechanisms,for example,phonon-phonon scattering,phonon-isotope scattering,phonondefect scattering,phonon-electron scattering,and phonon-boundary scattering.Additionally,variation tendencies of thermoelectric properties of TMDC with different structures(i.e.,superlattice,phononic crystal,and nanoribbon)have been investigated,which can propose new strategies to efficiently improve thermoelectric properties of 2D materials and deepen our understanding for energy transport mechanisms of 2D materials.This dissertation will provide a crucial basis and support for the design and development of 2D electronic devices and thermoelectric materials.