Research On The Thermal Regulation And Thermoelectric Performance Mechanism Of Several Typical Two-dimensional Materials

As electronic devices become more and more miniaturized and integrated,the amount of heat generated per unit area has also increased,and it is extremely important to achieve efficient and reasonable control of heat.In addition,facing the upcoming energy crisis and environmental problems,the development of clean,pollution-free new energy sources is urgent.Thermoelectric materials have received widespread attention because they can convert thermal energy into electronic energy.This dissertation has carried out a series of researches on the thermal energy control,thermoelectric performance conversion and regulation of several typical two-dimensional materials.Through reasonable structural design,we can achieve efficient control of thermal energy,which provides a way of thinking for the design of new thermal functional materials.And by applying strain,surface functionalization,and forming heterojunctions,we have significantly improved the thermoelectric properties of thermoelectric materials.The specific research is as follows:Firstly,we used molecular dynamics methods to study the thermal conductivity of twelve types of nanoporous graphene,and found that the thermal conductivity of nanoporous graphene is significantly lower than that of graphene,which is one to two orders of magnitude lower in value.So far,the structure of metamaterials is mainly based on the macroscopic composite structure of metal/polymer.In this work,we extend the concept of thermal metamaterials to two-dimensional graphene-based systems,mainly because graphene has a fast response speed,which is in contrast to traditional three-dimensional metamaterials.Through the artificial composite material composed of graphene and porous graphene,we designed three metamaterials with special physical significance,and simulated the heat conduction process of these structures by finite element method.The results show that some strange thermal phenomena such as effective shielding,aggregation and rotation of heat flow can be observed in the original/nanoporous graphene composites.The main reason is that the thermal conductivity of such two-dimensional systems has significant anisotropy.Moreover,by introducing external point heat sources,we can find that these thermal phenomena are not very sensitive to external interference.This research provides new guidance for the design of thermal metamaterials and can be used to search for other two-dimensional thermal functional materials.Secondly,based on first-principles calculations,we used the Boltzmann's transport equation to study the thermoelectric properties of the new two-dimensional ((PbX)₂(X=S,Se and Te).The interaction of various transport parameters restricts the improvement of thermoelectric performance to a large extent,and is the bottleneck that is eager to be solved at present.Here we apply biaxial tensile strain to ((PbX)₂(X=S,Se and Te)and find that the thermoelectric properties of these materials are significantly improved.The results showed that the room temperature ZT values of (PbS)₂,(PbSe)₂ and (PbTe)₂ increased to about 2.5(1.7),2.84(1.65)and 3.06(2.99)along the zigzag(armchair)direction,respectively.Theoretical analysis shows that this is mainly caused by the slight increase in Seebeck coefficient and the significant decrease in phonon thermal conductivity.Moreover,the sharp drop in phonon thermal conductivity is mainly due to the increase in phonon scattering rate caused by strong anharmonicity.The excellent ZT value of ((PbX)₂ single layer shows its potential application in the thermoelectric field and the application of strain has a good prospect in improving thermoelectric performance.Then,using first-principles calculations combined with Boltzmann's transport equation,we studied the thermoelectric properties of the dumbbell-shaped silicon structure.The results show that the structure has isotropic thermoelectric properties,and the ZT value in both directions at room temperature is about 0.7.Subsequently,we used Cl,Br,and I to functionalize the surface and continue to study the thermoelectric properties of these structures,and found that surface functionalization can effectively improve the thermoelectric properties of the structure.The thermoelectric properties of Cl-Si,Br-Si,and I-Si along the armchair direction and zigzag direction can be increased to about 1.16(0.92),1.4(1.12),and 0.92(0.89),respectively.This is mainly caused by the slightly reduced power factor and significantly reduced thermal conductivity.Further analysis we find that the decrease in thermal conductivity can be attributed to the enhancement of phonon scattering.In addition,we have also found that the thermoelectric performance is significantly improved with the increase of temperature,which has potential application value for high temperature thermoelectric.This research provides an effective strategy to improve the thermoelectric properties of two-dimensional materials.Finally,we use density functional theory combined with non-equilibrium Green's function to study the thermoelectric properties of MoS₂/MoSe₂ lateral heterojunction and van der Waals heterojunction.Compared with pure MoS₂,the thermoelectric performance of the MoS₂/MoSe₂ lateral heterojunction is significantly increased due to the sharply decreased thermal conductivity and slightly decreased power factor.In addition,the thermoelectric performance of the MoS₂/MoSe₂ van der Waals heterojunction can be further improved.The ZT value at room temperature can reach3.5,which is 3 times and 6 times of MoS₂/MoSe₂ lateral heterojunction and pure MoS₂,respectively.This is because the strongly localized electronic state and phonon state lead to the ultra-low thermal conductivity of the MoS₂/MoSe₂ van der Waals heterojunction.In addition,we also found that the thermoelectric performance of the MoS₂/MoSe₂ van der Waals heterojunction is not very sensitive to the contact area due to the competitive relationship between the power factor and the total thermal conductivity.The current research provides an effective strategy to improve the thermoelectric performance of two-dimensional heterojunctions,which can also be extended to other materials for different applications.