Research On First Principles Of Thermoelectric Properties Of Two-dimensional HfSe₂

As a new type of material that directly converts electrical energy and thermal energy,thermoelectric materials are expected to solve two major problems:energy crisis and environmental pollution.Therefore,how to improve the conversion efficiency of thermoelectric materials has become the research focus of researchers.In this paper,the thermoelectric properties of low dimensional HfSe₂ have been studied in depth using a first principles calculation method,with particular attention paid to the electronic structure and transport characteristics of monolayer and multi-layer HfSe₂ and layered heterojunction HfS₂/HfSe₂.The changes in thermal transport properties and related physical parameters have been studied,which has certain guiding significance for relevant experimental research.The specific content of this paper is divided into the following three parts:（1）The effects of biaxial strain on the phonon,electronic,and thermoelectric properties of monomolecular layer 1T-HfSe₂ were systematically studied using a first principles calculation method combined with semi classical Boltzmann transport theory.The monolayer molecular layer HfSe₂ is a good n-type thermoelectric material,and tensile strain can cause its valence band to converge and the Seebeck coefficient to increase,leading to the transition of HfSe₂ from an n-type thermoelectric material to a p-type thermoelectric material.At the same time,due to tensile strain increasing the bond distance and weakening the bond force,phonon scattering increases,phonon group velocity decreases,and ultimately the thermal conductivity of the monolayer molecular layer HfSe₂ lattice₇7))From 5.54 Wm^-1κ^-1 at300K down to 1.16 Wm^-1κ^-1.Due to the joint improvement of electrical and thermal transport,ZT_max has significantly increased,with p-type increasing from 0.16 to 2.41 and n-type increasing from 0.53 to 1.91.Therefore,tensile strain effectively improves the electrical transport properties of the monolayer HfSe₂,reduces its thermal conductivity,and improves its thermoelectric properties.（2）From a theoretical perspective,first-principles calculations and Boltzmann transport theory are used to accurately predict the thermoelectric properties of the multi-layer intrinsic（2L,3L,and 4L）T-HfSe₂.We achieved a very high power factor of 1.2 at room temperature×10¹³cm^-3,which is due to the energy band degeneracy caused by the increase in the number of layers,improving the Seebeck coefficient and power factor.At the same time,it is found that the lattice thermal conductivity of the multi-layer intrinsic T-HfSe₂ is lower than its monolayer layer structure due to the van der Waals force.Therefore,at 300 K,T-HfSe₂ can reach a maximum p-type ZT value of～0.53,which is close to the ZT value of～0.6 of the latest Ge Te based TE material.In addition,we obtained about 1.3 n-type ZT,which is very high,close to Pb doped Sn Se～1.2.The results show that layered HfSe₂ is a very excellent thermoelectric material at room temperature.（3）The thermoelectric properties of double layered heterostructures HfS₂/HfSe₂ were studied using first principles calculations combined with Boltzmann transport theory.Due to the fact that the energy separation of the bilayer structure at CBM is smaller than that at VBM,layered HfS₂/HfSe₂ is more inclined to be n-type than HfSe₂.Therefore,we have achieved better electron transport performance in n-type doping,i.e.,the Seebeck coefficient is316μV/K,power factor up to 3.3×10¹⁹（Ωms）^-1。In the investigation of thermal transport performance,the bilayer heterojunction HfS₂/HfSe₂ with interlayer weak coupling cooperation exhibits lower thermal conductivity than the monolayer HfSe₂.Finally,at 300K,the maximum ZT value for p-type is 0.33,and the maximum ZT value for n-type is 0.98.This indicates that the bilayer heterostructure HfS₂/HfSe₂ is a very excellent n-type intrinsic thermoelectric material.However,our work has only investigated whether other heterostructures in transition metal chalcogenides,HfS₂/HfSe₂,have more excellent thermoelectric properties,which is worth further exploration.