First-Principles Study Of Interlayer Coupling,Point Defect And Thermal Transport In Layered Thermoelectric Materials

Since the successful mechanical exfoliation of graphene by Novoselov and Geim in2004,more and more attention has been paid to layered materials,including boron nitride,transition metal dichalcogenides,black phosphorus,etc.The interlayer binding of some layered materials is mainly of weak van der Waals type,while for some materials,other type of binding,i.e.Coulomb interaction,is also important.These different types of interlayer interactions are important in material fabrication and physical property characterization.Based on first-principles calculations,we have studied the interlayer coupling,defects,and thermal transport properties of several layered materials.The results are summarized as follows:Firstly,we have studied the phonon thermal transport properties of Bi₂O₂X,BiCuOX and Bi₂OX₂?X=S,Se,Te?.We find that they have low lattice thermal conductivity,below 2W/mK at room temperature.Through detailed analysis of their phonon dispersion relations,Grüneisen parameter,relaxation time,electron localization function,we can ultimately correlate their low thermal conductivity to the strong anharmonicity of interlayer chemical bonding.Normally,from S to Te,we expect the lattice thermal conductivity to decrease with increasing atomic mass for materials with similar lattice structures.However,for Bi₂O₂X?X=S,Se,Te?,we notice that Bi₂O₂S shows lower lattice thermal conductivity than Bi₂O₂Se.This is due to the lattice distortion of Bi₂O₂S.Secondly,it has been predicated that mono-and few-layer SnSe show promising thermoelectric performance.In order to fabricate mono-or few-layer SnSe,it is important to study the interlayer coupling of SnSe.By comparing with graphite,MoS₂ and black phosphorus,we analyze the interlayer coupling from different points of view,including binding energy,phonon dispersion relation,layer-breathing phonon mode and interlayer charge transfer by using first-principles calculations.The results indicate that SnSe has the strongest interlayer coupling,and black phosphorus is weaker but still much stronger than graphite and MoS₂.Moreover,the charge transfer renders the interlayer coupling of SnSe not of pure van der Waals type.There is also Coulombic interacion induced by interlayer charge transfer.Due to the much stronger interlayer interaction,we anticipate that it is rather difficult to mechanically exfoliate mono-or few-layer SnSe.Finally,1T'phase of transition metal dichalcogenides WTe₂ has attracted much attention recently.We have studied the single atom points defects in 1T'-WTe₂.Various defects including vacancy,anti-site,and interstitial defects are considered.All the considered defects are non-magnetic.The formation energy calculations show that,the interstitial defects have the lowest formation energy,while the anti-site defects have the largest.Therefore,the interstitial defect is easier to form.We also simulate the scanning tunneling microscopy?STM?images of pristine and defected samples based on the electronic structure calculation,since STM is a powerful tool to investigate these point defects.These simulated STM images may be useful in future experimental investigation of1T'-WTe₂.We also study the defect-induced localized states,some of which lie near the Fermi level.These states may influence the electronic transport property of the defected structures.