Study On Phonon Thermal Transport Properties Of Typical Two-dimensional Materials

With the rapid development of the material preparation technology,the high integration and scale miniaturization of devices have become a common trend.With the reduction of dimensions,low-dimensional materials exhibit many novel physical properties and have a wide range of applications in micro-/nano-scale devices.In the work of electronic devices,a form of energy consumption is converted into heat,and the performance of heat transport is related to the stability of electronic devices.In thermoelectric devices,the thermal transport properties of thermoelectric materials are important factors that determine their conversion efficiency.Therefore,the heat transport in low dimensional structures and materials is a very practical research topic.The study of the heat transport properties of low-dimensional materials can help discover and construct devices with certain heat transport requirements and achieve thermal management control of the devices.In order to explore the thermal conductivity of the low-dimensional materials,this thesis uses first-principles calculations based on the density functional theory to systematically study lattice thermal conductivity,phonon dispersion relations,phonon group velocity,phonon relaxation time,Gruneisen parameters,scattering phase space and other heat transport parameters of two-dimensional monolayer materials,including ZnO,1T-TaSe2,1T-NbSe2,2H-TaSe2,2H-NbSe2,graphene,C3N,penta-graphene and penta-CN2.The main results are as follows:(1)Thermal conductivity of monolayer ZnO was studied and subsequent its relation with temperature.By studying the phonon dispersion relation of monolayer ZnO,there is a wide phonon band gap in the phonon spectrum.The thermal conductivity of monolayer ZnO is 4.5 W/mK,which is lower than many other monolayer materials due to its broken phonon-phonon scattering selection rules and strongly anharmonicity.The large group velocity of LO phonon branch due to LO-TO splitting resulting from strong polar covalent bond of Zn-O and the relative increasing of specific heat capacity of the high frequency optical mode with tempareture increasing both are positively contribute to thermal conductivity,which counteracts the negative contribution to thermal conductivity from reduction of relaxation time with tempareture increasing.Finally,competion from positive and negative affects the increasion contribution from LO phonon branch to the total lattice thermal conductivity with temperature increasing.When the temperature is higher than 200 K,LO exceeds FA and plays a major role in thermal conductivity,which makes the ??T curve deviate from the ??1/T relationship curve.(2)Effects of average atomic mass and phase structure on the lattice thermal conductivity of the specific monolayer materials.The thermal conductivities of 1T-NbSe2,2H-NbSe2,1T-TaSe2,and 2H-TaSe2 materials were calculated.The softening phonon frequency and broken phonon-phonon scattering selection rules due to the low symmetry of structures result in low lattice thermal conductivities of these four monolayer materials(0.13,0.423,0.396 and 1.545 W/mK,respectively).For the MSe2(M=Ta,Nb)with both 1T and 2H structures,the relationship between the thermal conductivity and the average atomic mass was studied.Both structures show that the thermal conductivity of TaSe2(having a relatively large average atomic mass)is higher than the thermal conductivity of NbSe2(having a relatively small average atomic mass),which is different from that thermal conductivity is generally in the inversely proportional relasionship to average atomic mass.This is because the weak scattering between acoustic and optical phonons due to the large phonon band gap of TaSe2 and the weak scattering between acoustic-acoustic phonons due to the bunched acoustic phonon branches.The weak scattering intensity and small scattering phase space make TaSe2 have a long relaxation time.The thermal conductivity of 1T structure of MSe2(M=Ta,Nb)is lower than that of 2H structure.The symmetry of 1T structure is lower compared to 2H leading to its seriously borken phonon-phonon scattering selection rules and stronge anharmocity,results to the low thermal conductivity.(3)Effects of lone-pair electrons on the lattice thermal conductivity of specific monlayer materials were systematically studied.The thermal conductivity of planar hexagonal monolayer C3N(103.02 W/mK)with some C atoms replaced by N atoms in graphene is one order of magnitude lower than that of graphene(3094 W/mK).The lone-pair electrons are introduced by replaced N atoms.The overlapping wave functions of s2 lone-pair electrons from N atoms and valence electrons from adjacent C atoms induce nonlinear electrostatic forces,which lead to local distortions in lattice,resulting in strong phonon anharmonicity and the low thermal conductivity,which is consistent with previous reports.However,the thermal conductivity of penta-CN2(660.71 W/mK),which is structured from partial C atoms replaced by N atom in penta-graphene and also has lone-pair electrons,is surprisingly higher than the thermal conductivity of penta-graphene(252.95 W/mK).The lone-pair electrons have positive and negative effects on thermal transport.On one hand,if the lone-pair electrons are far retracted from the N nucleus towards the missing link of the tetrahedron,strong lattice anharmonicity will be induced,leading to negative contribution to thermal conductivity.On the other hand,lone-pair electrons distribute asymmetrically,and its electron cloud extends more widely than the bonding electrons in space.So,lone-pair electrons have strong repulsion to the bonding electrons,resulting in the reduction of the bond angle between the bonding pair of electrons,and further resulting in homogenization of bond length in penta-CN2,then the enchance of thermal conductivity.The competion of these two affects decides the higher thermal conductivity of penta-CN2 compared to penta-graphene.