First-principles Study Of New Low-dimensional Janus Structure Flexible Thermoelectric Materials

The development of the Internet of Things requires devices to be light,wireless,and wearable,but the lack of cable power and battery space and time constraints make it a challenge to supply these devices conveniently and continuously.Wearable thermoelectric devices are expected to address this challenge by utilizing surrounding heat to harvest electrical energy for devices via the so-called thermoelectric effect.The thermoelectric technology,as a kind of energy conversion technology that can directly convert heat into electrical energy,has the advantages of green and pollution-free,safe and reliable,can reuse waste heat and has no limitation of environment.However,most of the traditional thermoelectric materials have either poor mechanical stability or low room-temperature conversion efficiency,obstructing their wide applications in wearable electronics.In order to solve above problems,in this thesis,based on first principles calculations in the framework of density functional theory(DFT)combined with Boltzmann transport equation,we propose a new strategy to achieve high room-temperature thermoelectric conversion efficiency by utilizing Janus structured transition metal chalcogenides(TMDs),expound the synergistic mechanism of the electric dipole and Rashba effects in the system,screen and receive high room temperature thermoelectric conversion efficiency,high flexibility and mechanical stability of the material.In this thesis,12 most promising H-phase monolayer Janus structure transition metal chalcogenides MXY(M=Zr,Mo,Hf,W;X?S,Se,Te)were selected as candidates for wearable thermoelectric materials.By the systematic study of the crystal structure and electronic structure characteristics of these 12 systems,we selected monolayer WSTe as the most promising candidate among these systems,and the Rashba band splitting characteristics and its influence on electron energy density and electron spin orientation in WSTe were found.We further performed comprehensive DFT and Boltzmann transport calculations to study the thermoelectric parameters,i.e.,electrical conductivities,thermal conductivities and Seebeck coefficients,under different temperatures and doping concentrations.We found that the synergistic effects between Rashba type spin-orbit coupling and external electric dipole play a critical role on the thermoelectric performance,in which the Power factor of monolayer WSTe system was effectively increased.In particular,the room-temperature a power factor 7.94m W/m K² with a doping concentration of 2×10²⁰ cm^-3.Based on density functional perturbation theory,we found the out-of-plane acoustic mode(“breathing”mode)exhibits obvious anharmonicity,indicating strong couplings with other phonon modes.Within three-phonon scattering approximation,we found that the anharmonicity is originated from the out-of-plane perturbated electric dipole.The coupling between“breathing”mode and long-range Coulomb interaction largely suppresses the lattice thermal conductivity,which was 2.74 Wm^-1K^-1 at room temperature,one order of magnitude smaller than that of monolayer Mo S₂.Finally,the maximum value of ZT of WSTe system reached 0.41 at room-temperature with the doping concentration of 1.5×10²⁰ cm^-3,which was one of the best room-temperature thermoelectric materials in two-dimensional TMDs,and the dimensionless figure of merit ZT value is commonly used to estimate the energy conversion efficiency of thermoelectric material.In addition,we also demonstrated the excellent flexibility and mechanical properties of monolayer WSTe,which was comparable to that of monolayer Mo S₂.Also,under external strain(-2%?4%),the power factor of monolayer WSTe material can be stabilized around(8 m W/m K²)its potential application prospect in wearable thermoelectric materials.