First-principles Study Of The Structural, Electronic And Thermoelectric Properties Of InSe Nanotubes And In₄Se_3-x Single Crystal

Energy is the basis for development of modern industrial society, energy supply relates to the people’sdaily life, the national economic development and the national security. As matters stand, fossil fuels suchas petroleum, natural gas and coal are still our major source of enegy. Due to the fossil fuels’unregeneration, uneven distribution and the very strong demand for it, energy is the main cause ofinternational tensions. The extensive use of fossil fuels is leading to greenhouse effect and environmentalcrisis. Therefore, exploration for new energy and research of new energy materials are a focus for concern.Thermoelectric material is a new kind of energy material, it can convert energy between thermal energy andelectrical energy. Home heating, automotive exhaust, and industrial processes all generate an enormousamount of unused waste heat that could be converted to electricity by using thermoelectrics. Asthermoelectric generators are solid-state devices with no moving parts, they are silent, reliable and scalable,making them ideal for small, distributed power generation. Thermoelectrics have long been too inefficientto be cost-effective in most applications.The efficiency of thermoelectric materials is given by the dimensionless figure of merit ZT=S₂σT/（k_e+k_l）, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature,and keand klare the electric and lattice thermal conductivity, respectively. Since the three physicalparameters S, σ, and K are coupled with each other, it remains a major difficulty to further enhance theZT value in bulk thermoelectric materials. High ZT has been achieved in many thermoelectricnanocomposites. For example, peak ZT is increased from1.0in bulk to1.4in p-type Bi_xSb_2-xTe₃nanocomposites, from0.65in bulk to0.95in p-type Si₈₀Ge₂₀nanocomposites, and from0.9in bulk to1.3 in n-type Si₈₀Ge₂₀nanocomposites. Bi-based low-demensional materials such as Bi_1-xSb_xthin film,nanowires, and nanotubes have been successfully produced, and enhanced thermoelectric performanceshave been found in them. InSe nanotubes extracted from corresponding bulk In₄Se₃may also have highthermoelectric performance. In this work, we aim to achieve a better thermoelectric performance forInSeNTs by contacting the low-demensional system of In₄Se₃. We investigate the structural, electric, andthermoelectric properties of InSeNTs. We have employed first-principles calculations and Boltzmanntransport theory to predict InSeNT structures and expore their thermoelectric properties. After fullrelaxation,（2,2） InSeNT is the most stable one among the studied InSeNTs.（3,3）,（4,4）,（6,6）, and （8,0）InSeNTs are semiconducting. Other studied InSeNTs are metallic. The filling degree of the Se4p orbitalplays a key role in determining whether these studied InSeNTs are semiconducting or metallic. The Seatom in the semiconducting InSeNTs is coordinated by two In atoms and that in the metallic InSeNTs is bythree In atoms. The S₂σ/τ of （2,2） InSeNT is much larger than that of other InSeNTs and is nearly10times larger than that of BiSb nanotubes around the Fermi level. It is found that an appearance of light andheavily bands around the Fermi level results in its high Seebeck coefficient and reasonable electricalconductivity. The current research proposes new types of nanotubes to design high-performancethermoelectric materials. Moreover, it is possible to further improve the thermoelectrial performance ofInSeNTs by optimization of tube sizes, doping, or tuning the surface structure in future work.Recently, most of the research efforts about thermoelectric material In₄Se₃crystal focus onSe-deficient polycrystalline compounds of In₄Se_3-x. First, interface efforts of polycrystal can effectivelyreduce thermal conductivity. Second, the fabrication of polycrystalline In₄Se_3-xcompounds is easier thangrowth of Se-deficient In₄Se_3-xsingle crystal. It is difficult to control the position of Se-deficient byexperiment, only to research how does the Se defects concentration affect the thermoelectric properties of polycrystalline compounds of In₄Se_3-x. The main purpose of this paper is to study the influence of positionof Se defects and defects concentration on the anisotropy and the trend of changes of thermoelectriccoefficients. We studied the difference of change regularity of positive and negative Seebeck coefficients.We also calculated the density of states, electronic band structure of In₄Se_3-xsingle crystal with same defectconcentration and different Se-deficient position, and analyse effects of Se-deficient positions.