First-principles Study Of Thermoelectric Properties Of Single Layer （Ti,Zr,Hf）X₂（X=S,Se）

With the increasingly serious environmental pollution and energy shortages,thermoelectric conversion materials that can make the best of heat energy are receiving more and more attention.Exploring new high-efficiency thermoelectric materials and optimizing their performances by means of dimension,stress,doping,nano-composites and so on,is the main issue of current research of thermoelectricity.In this work,the density functional theory calculations combined with the semi-classical Boltzmann transport theory are used to investigate the effects of crystal structure,chemical composition and stress on the thermoelectric performances of single-layer transition Metal compounds（Ti,Zr,Hf）X₂（X=S and Se）.The main results are as follows:1,1T-ZrX₂（X=S and Se）single layer have low lattice thermal conductivity(κ_ph)and ultra high power factor（PF）,and the optimal ZT value of P-type is about 0.8.Theκ_ph of 1T-ZrS₂ and 1T-ZrSe₂ at room temperature（300K）are 1.43Wm^-1K^-1 and 0.97 Wm^-1K^-1,respectively.And the power factor（PF）of 1T-ZrS₂and 1T-ZrSe₂ at room temperature（300K）are 11.95 mW·K^-2m^-1 and 13.58 mW·K^-2m^-1,respectively.Due to their ultra high conductivity,the thermal conductivity of lattice is much smaller than the electronic thermal contribution in the optimum carrier concentration region(¹0²¹cm^-3).So the electron only thermoelectric figure-of-merit(ZT_el)can be considered as the performance index.2,Based on the atomic orbital characteristics and their interaction,we analyzed the variations of band gaps,electron and hole mobilities,and thermoelectric properties of this series with different crystal structure and stress.The Sulfur（Selenium）atoms have two kinds of orbitals with the in-plane（p_x,p_y）and the p_z pointing to the out-layer.These two orbitals have different bonding characteristics（s,p）on a two-dimensional plane,and different hybridization with the d-orbitals of the transition Metal atoms in the 1T and2H structures,so the energy level order and the band valley distribution of 1T and 2H structures are different response to the stress.The tensile strain can realize the convergence of the light and heavy hole bands of the1T structure,and the compressive strain can realize the band convergence of 2H structure,so that the thermoelectric figure of merit is multiplied.3,The electronic structure and the thermoelectric properties of 1T-MS₂（M=Ti,Zr,Hf）alloys are investigated by varying the chemical composition.The calculation results show that the different（ZrHf）S₂ compositions has only small effect on the valance band structure,but the band structure of Ti-based alloys changes greatly.,As for the P-type thermoelectric materials,（Zr,Hf）S₂ have the highest Power Factor,and the mass fluctuations can scatter phonons stronly,reducing the lattice thermal conductivity,and optimizing the thermoelectric performances.