Rearch On Thermoelectric Transport Properties Of Two-dimensional Layered Materials

With the rapid development of the electronics industry,traditional metallic materials such as copper and aluminum can no longer meet the current heat dissipation requirements of highly integrated electronic devices;additionally,the world’s energy consumption is becoming increasingly serious,and improving energy efficiency has become a top priority.The thermal transport properties of materials influence whether electronic devices can perform reliably at high temperatures for an extended period of time,and the thermoelectric properties of materials determine their energy conversion efficiency.Since two-dimensional layered materials have good thermal transport properties and thermoelectric properties,it is the key to solve the above problems to carry out deeply research on the thermoelectric transport properties of two-dimensional layered materials.By First-principles,Density Functional Theory,Boltzmann transport equation,and Deformation Potential theory,the thermal transport and thermoelectric properties of five representative two-dimensional layered materials（including typical monolayer WS₂,monolayer ZrSe₂,monolayer HfSe₂,and novel monolayer Janus materials ZrTeSe and Hf TeSe）are investigated in depth in this dissertation.With an emphasis on both harmonic and anharmonic effects,the effects of phonons on thermal transport properties and the regulatory mechanism are thoroughly discussed,and the thermoelectric merits of the materials are calculated.The research findings can provide significant reference and theoretical guidance for the thermal and thermoelectric design of microelectronic devices.The main researches of this thesis are as follows:（1）The thermoelectric transport properties of typical monolayer WS₂were investigated.The dispersion curves,relaxation times,and phonon mean free path of monolayer WS₂ phonons were calculated,and the lattice thermal conductivity of monolayer WS₂ is 149.12 W/（m·K）at 300 K and decreases with increasing temperature.It is found that the effect of the acoustic phonon branches on the thermal conductivity is more significant than that of the optical phonon branches（LA phonon branch has the maximum impact）.Based on the three-phonon scattering as well as the phonon mean free path,a method to adjust its lattice thermal conductivity by controlling the frequency and changing the structure size is proposed.In addition,it is found that monolayer WS₂ is direct bandgap semiconductor,whose bandgap is calculated to be 2.30 e V.And the ZT value of monolayer WS₂ at p-type carriers is larger and increases with temperature,indicating its better performance of thermoelectric effect at high temperature.（2）The thermoelectric transport properties of monolayer ZrSe₂ and monolayer HfSe₂ were investigated.The lattice thermal conductivities of monolayer ZrSe₂ and HfSe₂ were 3.23 W/（m·K）and 4.50 W/（m·K）at 300K,respectively.The phonon spectra indicate that there is a strong coupling between the acoustic and optical branches of monolayer ZrSe₂,so the lattice thermal conductivity of monolayer ZrSe₂ is lower.Meanwhile,the contribution ratios of each phonon branch to the total thermal conductivity of monolayer ZrSe₂ and HfSe₂ at different temperatures were calculated,and both were found that the acoustic phonon branch plays a major role in the whole thermal transport process（TA phonon branch has the biggest impact）.Finally,the thermoelectric properties of monolayer ZrSe₂ and HfSe₂ were studyed,and it was shown that both are indirect bandgap semiconductors with bandgaps of 1.55 e V and 1.57 e V,respectively.Monolayer ZrSe₂ and HfSe₂ exhibit better thermoelectric properties at n-type compared to p-type.At 600 K,the optimal ZT values for n-type of both are 3.27 and 4.45,respectively.Under the same conditions（temperature and carrier concentration）,monolayer HfSe₂ has a higher ZT value than ZrSe₂,whice demonstrate that monolayer HfSe₂ is a excellent n-type thermoelectric material.（3）The thermoelectric transport properties of the novel monolayer double sided materials ZrTeSe and Hf TeSe were predicted.It is calculated that both monolayer ZrTeSe and monolayer Hf TeSe have extremely low lattice thermal conductivity（lower than most two-dimensional layered materials）,with values of 0.004 W/（m·K）and 1.71 W/（m·K）,respectively.The main reason for the low thermal conductivity of monolayer ZrTeSe is that its acoustic phonon scattering rate is much higher than that of monolayer Hf TeSe.At the same time,it is found that both LA acoustic phonon branches play a major role in their entire thermal transport processes（the reason is that the LA phonon branches of both have larger phonon group velocities and phonon lifetimes）.Furthermore,the optimal ZT values of monolayer ZrTeSe and Hf TeSe at 300 K are 0.82 and 0.90 for n-type and 1.82 and 1.22 for p-type,respectively,and their ZT values decrease continuously with the increase of temperature.In summary,monolayer HfSe₂ and monolayer ZrSe₂ are more suitable as thermoelectric materials.Monolayer HfSe₂ has a higher ZT value,proving that it is a excellent n-type thermoelectric material.For the novel monolayer ZrTeSe and Hf TeSe,both are more suitable as thermal insulation materials,where monolayer ZrTeSe has a very low lattice thermal conductivity,indicating its better performance in thermal insulation properties.In this dissertation,the thermoelectric transport properties of five two-dimensional materials are investigated,which can provide theoretical reference and reference for the application,design and performance prediction of two-dimensional layered materials in nanoelectronics devices.