First-principles Study On The Thermoelectric Properties Of Several Bulk And Low-dimensional Materials

Energy crisis and environmental pollution are serious challenges for human being.To solve the problem,a study on thermoelectric materials that can convert heat into electricity and vice versa has attracted great attention.People usually use the dimensionless figure-of-merit to characterize the thermoelectric conversion efficiency,which is defined as ZT= S2?T/(?l+?e),where S,?,T,?l and ?e are the Seebeck coefficient,electrical conductivity,absolute temperature,lattice and electronic thermal conductivity,respectively.The larger ZT indicates a better thermoelectric performance.However,S,? and ?e are generally coupled with each other,which makes it hard to increase ZT significantly.Theoretical and experimental studies indicate that the thermoelectric performance of low-dimensional material is superior to that of the bulk one.On one hand,the quantum confinement effect of low-dimensional system could improve the density of states near the Fermi level,leading to a larger Seebeck coefficient.On the other hand,the enhanced phonon boundary scattering could reduce the lattice thermal conductivity.Based on these reasons,we study the thermoelectric performance of several low-dimensional materials,including MoS2 nanoribbons,PbTe layer and Bi2Te3/PbTe heterostructure.We also analyze the electrical and heat transport properties of the bulk thermoelectric material BiCuSeO,which has attracted many attentions in the past few years.Besides,using SiGe compound as a prototype,we investigate the correction of electron-phonon coupling on lattice thermal conductivity as well as the thermoelectric performance.The main contents are as follows:We study the electrical and thermoelectric properties of several armchair MoS2 nanoribbons with different width.Due to the dangling bonds,there are strong distortions on the edge structure,where the narrower nanoribbon corresponds to stronger change.Since the states near the Fermi level mainly come from edge atoms,the distortion significantly affects the band gap and electrical transport properties.Finally,we find that the narrower MoS2 nanoribbon(N=4)shows the best thermoelectric performance,where the optimized room temperature ZT for n-type and p-type systems are 2.5 and 3.4,respectively.Bulk PbTe is a well-known high performance thermoelectric material.We study the electronic and phonon transport properties of a two-dimensional PbTe with the space group P-3m1.Due to the three-fold rotational symmetry,the band structure of PbTe layer has six valleys on the valence band,which indicates that the p-type system could have a better thermoelectric performance.Since the component atoms are heavy,PbTe layer has a low cutoff phonon frequency and goup velocity,leading to a low lattice thermal conductivity.Our calculation predicts that the ZT value of p-type PbTe could reach 2.0 at 700 K,which demonstrates its potential application in thermoelectrics.Van der Waals heterostructure has attracted great concerns due to the extraordinary electronic and photonic properties.To explore its possibility as thermoelectric materials,we first construct the heterostructure Bi2Te3/PbTe by choosing component materials with intrinsic low lattice thermal conductivity,appropriate band gap and good thermoelectric properties themselves.However,the calculated band structure shows a very small band gap(0.013 eV),leading to a much lower Seebeck coefficient.By applying isoelectronic substitution and 5%compressive strain,we find the band gap of Bi2Se3/PbSe could reach 0.43 eV and leads to a larger power factor.Compared with original Bi2Te3/PbTe heterostructure,band engineering promotes its thermoelectric performance as much as two times.Owing to the good thermoelectric properties,oxide material BiCuSeO has attracted much attention recently.However,their unique electronic and phonon properties are still under debate and a complete understanding is quite necessary.By a detailed analysis of the three-dimensional energy dispersion relationship in the whole Brillouin Zone,we find there are eight valleys with almost the same energy on the valence band,which could promote the carrier concentration without decreasing the Seebeck coefficient.Besides,their large density of states effective mass could well explain the measured low mobility and relatively larger Seebeck coefficient.Combining the phonon spectrum analysis and first-principles molecule dynamic simulations,we believe that the strong anharmonicity mainly originates from Cu atom,rather than prevailingly believed Bi atom.Interestingly,we find that the charge transport of BiCuSeO is mainly governed by the(Cu2Se2)2-layer while the(Bi2O2)2+ layer plays a major role in the heat transport.Such unique characteristic could be used to independently manipulate the electronic and phonon transport properties so that the thermoelectric performance of BiCuSeO could be further enhanced.Using SiGe compound as a model system,we study the effects of electron-phonon coupling on lattice thermal conductivity and the figure-of-merit.It is noted that the electron relaxation time has a weak dependence on the carrier concentration while the phonon relaxation time quickly decreases as the carrier concentration increases.This fact indicates that the electron-phonon coupling could have an important effect on lattice thermal conducvitity when the carrier concentration is relatively high.Our computational results demonstrate that the ZT of SiGe compound obtains an increase of 23%when considering the correction of electron-phonon coupling on lattice thermal conductivity at 1200 K.Our study not only provides a new insight in understanding the lattice thermal conductivity in heavily doped thermoelectric materials,but also emphasizes the importance and necessity of considering electron-phonon coupling in accurately predicting their figure-of-merit.