Structural Design And Thermoelectric Properties Of New Black Phosphorus-based Materials

Along with the development of social civilization,human society has an increasing demand for energy.At present,the energy consumed in industrial production and daily life is still mainly from fossil fuels such as coal and oil.Due to the limitation of the conversion efficiency of the heat engine,a large amount of energy is directly discharged into the environment with the form of waste heat,resulting in energy waste and serious environment pollution.Therefore,to find a new pollution-free energy conversion method is of great significance to solve the energy crisis and environmental problems.Thermoelectric materials,as a kind of functional materials which can realize the direct conversion to heat energy and electric energy,showing great potential in the field of industrial waste heated recovery.The conversion efficiency of thermoelectric materials measured by material-dependent figure of merit ZT=S²σT/κ,where S,σandκare the Seebeck coefficient,the electrical conductivity and the thermal conductivity,respectively.These three thermoelectric parameters are interrelated,so it is difficult to realize independent control and optimization,which is also an important bottleneck to limit the efficiency of thermal power conversion in traditional thermoelectric material system.Currently,commercial thermoelectric materials,such as Pb Te,Bi₂Te₃ system,mostly contain toxic heavy metals or expensive rare elements（Pb,Ag,Te,Bi,etc.）,and the experimental synthesis process is strict and the manufacturing cost is high,which has been greatly limited in practical application.Therefore,it is very important to find economic,environment-friendly and high-performance thermoelectric materials.In recent years,as a new type of semiconductor material with comparable performance to graphene,layered black phosphorus has become a research hotspot in the field of functional materials.The unique folded honeycomb structure makes the electrical and thermal properties of black phosphorus show a high degree of anisotropy and high carrier mobility within the layer.In addition,it is safe and non-toxic,and the synthetic elements are low,so it is expected to become a high-performance thermoelectric material.However,due to the high thermal conductivity of black phosphorus and its easy oxidation and degradation in the air,the improvement of the thermoelectric merit of black phosphorus and its practical application in the field of thermoelectric materials are severely restricted.To solve the above problems,the electronic structure and phonon structure of multi-dimensional black phosphorus system are studied in detail by first principles calculation.And the thermoelectric transport properties are studied by combining with Boltzmann transport theory.In order to deepen the understanding of the relationship between the microstructure and the macroscopic physical properties of the system,we explored the synergistic effects of various control methods（including the size effect of nano dimension,superlattice structure,surface stress,carrier transport orientation,doping,etc.）on the electronic structure and thermoelectric properties of materials.Innovative achievements have been made in the following aspects.1.The substitution doping of the fifth main group elements can effectively improve the thermoelectric properties of black phosphorus bulk.By systematically studying the thermoelectric properties of black phosphorus and VA-doped black phosphorus（N,As,Sb and Bi）,we found that the VA doping improves the energy band anisotropy of the black phosphorus.At the same time,the coupling between the p states of the intrinsic elements and impurity elements near the Fermi surface is enhanced,which effectively adjust band structure and improves its anisotropy of black phosphorus.So that the electric transport performance of black phosphorus along the zigzag direction is significantly improved.At 300 K,the optimal ZT values of p-type BiP₇ and n-type NP₃are as high as 1.21 and 0.87,which are 17.7 and 9.8 times higher than those of black phosphorus.This work provides a pathway to improve the thermoelectric conversion efficiency of black phosphorus.2.Structure design and thermoelectric transport properties of black phosphorus nanotubes.The electrical and thermal properties of black phosphorus are intralayer anisotropic.To make full use of this unique advantage and explore the different orientation of carrier conduction to obtain the best thermal transport properties,we constructed black phosphorus nanotubes along different crystal axes（1,0）,（0,1）,（1,1）,（1,2）and（2,1）orientations.It is found that there is a strong dependence between the electronic structure and the orientation of the crystal axis.The results show that the surface electronic states are distributed along the（1,1）crystal axis.The electronic structure has the band characteristic of multi-energy valley,which is beneficial to the improvement of electrical transport performance.Further calculation show that the carrier mobility of nanotube structure can reach 2430 cm²V^-1s^-1 at room temperature,which is about 2.5 times that of black phosphorene.The thermoelectric merit at room temperature is about one order of magnitude higher than that of black phosphorene.This work shows that the directional control of carrier transport orientation can significantly improve the thermoelectric properties of anisotropic materials,which opens up a new research direction for finding high-performance flexible thermoelectric materials.3.Design of high-performance thermoelectric materials for black phosphorus-like GeP₃ system.GeP₃ system with black phosphorus like layered structure has high carrier mobility,but its narrow band gap leads to low Seebeck coefficient,which is not conducive to the regulation and improvement of thermoelectric transport performance.To solve this problem,we intercalated hexagonal boron nitride（h-BN）in layered GeP₃to construct superlattice heterostructures to broaden the band gap,regulate the band structure,and improve the thermoelectric potential.The calculations show that GeP₃/h-BN heterostructures have unique multi-valley band structure,and exhibit excellent thermoelectric properties.The ZT value of p-type materials can reach 5.13 at room temperature.In addition,by applying 5%tensile strain,the thermoelectric merit value of n-type doping can be improved to 1.15 at room temperature.This work expands the research space of high-performance black phosphorus based thermoelectric materials,and provides the necessary theoretical basis and guarantee for the experimental preparation and exploration of new environment-friendly thermoelectric materials.