First Principles Study On Thermoelectric Properties Of Layered Materials

Along with the development of human society,huge demanding of energy is in following.Thus,exploring techniques and materials in relation with producing and converting energy is important.Thermoelectric?TE?materials that could directly and reversibly convert the waste heat and electrical power is one of the most promising approaches to solving current energy crises and environmental pollution.However,the TE materials are currently confronted with two key bottlenecks:low power converting efficiency and high production cost.Thus,searching the TE materials of high figure of merit?ZT?have become an important goal in materials science.Recently,layered materials have attracted much attention due to their wonderful optical,thermal and electronic properties.In addition,the TE performance can be further enhanced by band structure engineering and dimensionality reduction.In this paper,based on density functional theory?DFT?together with semi-classical Boltzmann transport equation,we systematically study the electronic structure,lattice thermal conductivity and transport properties of monolayer Sn P₃,XSe?X=Ge,Sn,and Pb?and CaP₃,which offer theoretical reference for exploring TE materials of high performance.1. Based on first-principles and Boltzmann transport equation,we have calculated the TE properties of monolayer Sn P₃.Results show that it has a low intrinsic lattice thermal conductivity4.97 W m^-1K^-1,mainly originating from its small average acoustic group velocity?1.18 km/s,large Grüneisen parameters-7.09,strong dipole-dipole interaction and strong phonon-phonon scattering.The exhibiting mode mixture between in-plane and out-plane vibrations enhances the complexity of phonon phase space,which greatly suppresses lattice thermal conductivity.A highly twofold degeneracy appearing at K point gives a highSeebeck coefficient.Our calculated ZT for optimal p-type doping at 500 K can approach3.46 along armchair direction,which is better than the theoretical value of 1.94 reported in the well-known TE material SnSe.2. We also explore the electronic and phonon transport properties of group-IV selenides XSe.Calculated results show that monolayer PbSe exhibits a highSeebeck coefficient of?1150?V/K and an ultra-low thermal conductivity of?0.50 W m^-1 K^-1at room temperature.Additionally,the lowest optical branch is softening and overlapping with the acoustic branches for the three sheets,leading to strong acoustic-optical interactions and highly nonlinear dispersion curves.Meanwhile,we also find that the lattice thermal conductivities decrease with the decreasing frequency of lowest optical branch decreasing.Using our calculated transport parameters,large values of ZT of 1.76,2.32,and 4.71 can be obtained at effective temperature range under p-type doping for GeSe,SnSe,and PbSe,respectively.This work may offer promising candidates for applications of TE and motivate further experimental exploration.3. We systematically study TE properties of layered material CaP₃ from the three-dimensional?3D?bulk form to two-dimensional?2D?monolayer.It is found that the lattice thermal conductivity of monolayer CaP₃(?0.65 W m^-1 K^-1)is much lower than that of its bulk form(?2.14 W m^-1 K^-1)at room temperature.More remarkably,theSeebeck coefficients of monolayer exhibit a dramatic enhancement compared to its bulk structure.The maximum ZT for p-type doping at 700 K can reach 6.37,which is enhanced nearly 10times compared to its bulk form?0.63?.Collectively,this work sheds light that the low-dimensional nanostructure technology can effectively improve the thermal-electricity conversion of this class of materials.