Thermoelectric Transport Properties Of Several Bismuth-based Bulk And Low-dimensional Materials

Thermoelectric materials have attracted a lot of attention from the science community due to their interesting transport properties and potential applications in cooling and power generation. The efficiency of a thermoelectric material is quantified by the dimensionless figure of merit:ZT=S2σ/(κe+κl), In most thermoelectric materials, the transport coefficients S, σ, and κe are interrelated in a way which makes it very challenging to achieve a high ZT value. Recently, several promising strategies have been proposed to improve the thermoelectric performance which includes maximizing the power factor (S2σ) through electronic doping and band engineering, as well as minimizing the lattice thermal conductivity through phonon scattering. Low-dimensional or nano-structured systems combine both advantages and thus are believed to exhibit significantly larger ZT values than their bulk counterparts. Recently, it is found that some of the good thermoelectric materials also belong to the newly discovered state of material, the topological insulator. As a heavy element with strong spin-orbit coupling strength, bismuth is a common constitute of traditional thermoelectric materials and many well-known topological insulators. It is thus interesting to study the thermoelectric properties of bismuth based topological insulators and to elucidate whether the topologically protected surface/edge states in topological insulators could also be utilized to enhance the thermoelectric performance. In this dissertation, we combined first-principles calculations with electrons and phonons Boltzmann equations and equlibrillum molecular dynamics to investigate the thermoelectric performance of several bismuth based materials, including bulk Bi2Te3, two-dimensional Bi (111) and Bi (110) layers and the one-dimensional Bi nanoribbons.We first studied the structural, electronic, and transport properties of bulk Bi2Te3 within density functional theory taking into account the van der Waals interactions (vdW) and the quasiparticle self-energy corrections. It is found that the optB86b-vdW functional can well reproduce the experimental lattice constants and interlayer distances for Bi2Te3. Based on the fully optimized structure, the band structure of Bi2Te3 is obtained from first-principles calculations with the GW approximation and the Wannier function interpolation method. The global band extrema are found to be off the high-symmetry lines, and the real energy band calculated is in good agreement with that measured experimentally. In combination with the Boltzmann theory, the GW calculations also give accurate prediction of the transport properties, and the calculated thermoelectric coefficients of Bi2Te3 almost coincide with the experimental data.We then studied the structural and electronic properties of bismuth (111) monolayer. It is found that the monolayer forms a stable low-buckled hexagonal structure, which is reminiscent of silicene. The electronic transport properties of the monolayer bismuth are then evaluated by using electrons Boltzmann equation with the relaxation time approximation. By fitting first-principles total energy surface, a modified Morse potential is constructed, which is used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. The room temperature ZT value of the bismuth (111) layer is estimated to be 2.1 and 2.4 for the n-and p-type doping, respectively.We have also investigated the thermoelectric properties of the distorted bismuth (110) layer using first-principles calculations combined with the Boltzmann transport equation for both electrons and phonons. To accurately predict the electronic and transport properties, the quasiparticle corrections with the GW approximation of many-body effects have been explicitly included. It is found that high thermoelectric performance could be achieved in the distorted bismuth (110) layer, which is essentially stemmed from the weak scattering of electrons. Moreover, we demonstrate that the distorted Bi layer remains high ZT values(ZT>2.0) at relatively broad regions of both temperature and carrier concentration. This study confirms that the deformation potential constant charactering the electron-phonon scattering strength is an important paradigm for searching high thermoelectric performance materials.We finnally investigate the thermoelectric properties of the Bi nanoribbon with and without non-trivial edge states based on first-principles calculations combined with the Boltzmann transport theory. We found that there is a competition relation between the edge and bulk transports in the nanoribbon with non-trivial edge states and a high ZT value exceeding 3.0 could be achieved for both p-and n-type carriers at 300 K in this nanoribbon when the relaxation time ratio between the edge and the bulk states is 1000. Our study indicates that the utilization of topological non-trivial edge states might be a promising approach to cross the threshold of the industrial application of thermoelectrics.