First Principles Study On Thermoelectric Transport Properties Of Novel Low Dimensional Pentagonal Structural Materials

In recent years,the increasing energy consumption and the increasingly serious environmental problems have caused wide public concern.In order to cope with the coming energy crisis,it is of great significance to improve the efficiency of energy utilization and develop sustainable and eco-friendly energy resources.Thermoelectric materials can realize the environmentally friendly conversion between heat and electricity,and the thermoelectric devices made from them do not need any transmission parts and work without noise and waste.They are the best candidate materials to solve the energy crisis and help to achieve the goal of carbon neutrality.Although the thermoelectric properties of traditional inorganic and organic materials have been improved significantly in the past few decades,the problem of low conversion efficiency still limit the large-scale application of thermoelectric materials.Therefore,if the conversion efficiency of thermoelectric materials can be improved,the thermoelectric materials will have a broader application prospect.High performance thermoelectric materials need to have excellent electrical transport properties and low thermal conductivity.In low-dimensional materials,the quantum confinement effect can increase the density of states near the Fermi surface,which may increase the power factor of the material.At the same time,the quantum size effect can also enhance the scattering of phonons,thus limiting the heat transport of materials.It is shown that most of the layered 2D materials possess special band structures and suitable band gaps,which make them promising for thermoelectric materials applications.Since the pentagonal graphene was predicted by theoretical calculations,this new structure of layered materials has received a lot of attention.At present,PdSe₂ of the pentagonal structure can be synthesized by experiments,which further confirms the stability of the pentagonal structure.Therefore,this paper focuses on exploring new pentagonal layered thermoelectric materials.In this paper,the structure of penta-Sb₂X（X=Si,Ge,Sn）and penta-Bi₂X（X=Si,Ge,Sn）monolellers with pentagonal structure is optimized by first principles calculation based on density functional theory.Combined with the electron and phonon Boltzmann transport equation,the electric transport properties,thermal transport properties and thermoelectric properties are calculated theoretically,which provides theoretical guidance for the exploration of high performance laminated thermoelectric materials.The main research contents include:1.We propose a new penta-Sb₂X（X=Si,Ge,Sn）monolayer material with penta-graphene’s pentagonal structure and excellent thermoelectric transport properties.We find that energy band degeneracy leads to high Seebeck coefficient.The results show that the N-doped penta-Sb₂Si has excellent electrical conductivity and high electron mobility of 4010 cm²V^-1s^-1.Penta-Sb₂Ge and penta-Sb₂Sn have very low lattice thermal conductivity,2.34 and 0.94 Wm^-1K^-1 at 300K,respectively.The results show that monolayer penta-Sb₂X has excellent thermoelectric properties and is expected to be used in the field of thermoelectric.2.We extended the pentagonal structure to bismuth-based compounds and found that penta-Bi₂X（X=Si,Ge,Sn）monolayers have ultra-low lattice thermal conductivity.Calculations show that the lattice thermal conductivities of Bi₂Ge and Bi₂Sn at 300 K are 0.3 and 0.03 Wm^-1K^-1,respectively,giving them very high ZT values for both p-and n-type doping.For the n-type doped Bi₂Si,both the ultra-high electron mobility and the large Seebeck coefficient indicate that it has very good electrical transport properties.The results of this calculation indicate that penta-Bi₂X offers new possibilities for bismuth base-like thermoelectric materials.3.Due to the characteristics of high mobility and suitable band gap,penta-Bi₂Si is expected to be applied to high-performance electronic devices,so we further explore penta-Bi₂Si.We find,using first principles and ab initio molecular dynamics calculations,that the layered tetragonal Bi₂Si is a stable semiconductor with a fairly large indirect band gap of 1.30 eV and electron mobility in excess of 17,000 cm² V^-1s^-1 at 300 K.Interestingly,the electron mobility of the bilayer Bi₂Si is 10 times higher than that of the monolayer due to the layer coupling.The mobility of Bi₂Si significantly exceeds that of Si,indicating the possibility of achieving excellent performance in field-effect devices based on electron transport in Bi₂Si,while the layered structure may facilitate deposition based synthesis and device fabrication.