Theoretical Studies On The Thermoelectric Properties Of Bismuth Based Bulk And Low-Dimensional Structures

Thermoelectric materials are capable of directly converting heat into electricity as well as providing cooling via heat pumping. Thermoelectric devices are compact, scalable, reliable and silent, with no moving parts. There has been growing interest in the search for high performance thermoelectric materials, which are believed to play an important role in meeting the demand for renewable and clean energy in the future. The efficiency of a thermoelectric material is evaluated by the dimensionless figure of merit, i.e., the ZT value (=S2σT/κ). A good thermoelectric material requires a high ZT value, one therefore must try to increase the power factor (S2σ) and/or decrease the thermal conductivity. As the transport coefficients in the ZT value are coupled with each other and they all depend on the carrier concentration, it is usually very difficult to achieve a high ZT value, which limits the energy conversion efficiency of thermoelectric materials and thus hinders their wide-spread applications. In1993, Hicks et al. first predicted that low-dimentional structures could have significantly enhanced thermoelectric performance compared with their bulk counterparts. In recent years, improved fabrication technologies have been developed which make it possible to synthesize well-controlled nanostructures. Using low-dimensional structures has been considered a promising way to obtain high performance thermoelectric materials.Bismuth (Bi) based compounds are found to be promising thermoelectric materials which have a ZT value of about1at room temperature. It is expected that significantly enchanced thermoelectric performance can be achieved by nanostructuring the Bi-based compounds. In this dissertation, we used a combination of density functional theory (DFT), nonequilibrium Green’s function (NEGF) method, semi-classical Boltzmann theory, and molecular dynamics (MD) simulations to investigate the structural, electronic, phonon, and transport properties of Bi-based bulk and low-dimensional structures.For the bulk system, we calculated the electronic properties of (Sb0.7sBi0.25)2Te3compound, from which the transport coefficients were evaluated as a function of chemical potential. To optimize the power factor, the possible doping atoms and the corresponding optimal doping concentrations are predicted. By inserting an averaged thermal conductivity obtained from the experiments, we predicted that the room temperature ZT value of (Sb0.75Bio.25)2Te3compound can be increased to1.8by appropriate p-type doping.As for the low-dimensional structures, we first focused on the BiSb nanoribbons (BSNRs) with different widths and edge configurations. It was found that the pristine BSNRs with armchair edges (ABSNRs) are semiconducting and the band gaps exhibit a width dependent odd-even oscillation. In contrast, the pristine BSNRs with zigzag edges (ZBSNRs) are found to be metallic. When all the edge atoms are passivated by hydrogen, both the ABSNRs and ZBSNRs become semiconducting and the corresponding band gaps decrease monotonically with the increasing width. If, however, the edge atoms are partially passivated, the ABSNRs can be either semiconducting or metallic. Moreover, local magnetism appears when all the edge Sb atoms are passivated and there are one or more unsaturated Bi atoms. We found that all the investigated odd-numbered ABSNRs have almost the same peak value of the power factor around the Fermi level. This is not the case for the even-numbered ABSNRs, where the peaks are twice that of when they are n-type doped. Our calculations indicated that by appropriate doping, the room temperature ZT value of BSNRs can be enhanced by about7times compared with the corresponding bulk value, which makes them very promising candidates for thermoelectric applications.We also considered the nanotube structures of BiSb (including one kind of hexagonal and two gear-like structures), which are similar to those of carbon nanotubes. Our calculations indicated that the gear-like nanotubes (g-NTs) are energetically more favorable than their hexagonal counterparts (h-NTs). All the g-NTs are found to be semiconducting. At room temperature, the Seebeck coefficients exhibit large peaks near Fermi level and their absolute values increase monotonically with increasing band gaps of the nanotubes. The MD simulations show that the investigated BiSb nanotubes have very low lattice thermal conductivity. By appropriate n-type doping, the ZT value at room temperature is about5times larger than that of the bulk BiSb, suggesting that the BiSb nanotubes are also excellent candidates for the thermoelectric applications.Finally, we examined the thermoelectric properties of a series of [110] and [210] Bi2Te3nanowires with different widths. Our calculated results indicated that the nanowires with [110] orientation are energetically more favorable than those with [210] orientation. All the investigated Bi2Te3nanowires are found to be semiconducting and the band gaps of [110] nanowires monotonically increase with the decreasing cross-sectional width. For the [210] nanowires, however, the band gaps exhibit an interesting width-dependent even-odd oscillation. Due to relatively larger Seebeck coefficients and smaller lattice thermal conductivities, the [210] nanowires show a better thermoelectric performance than the [110] nanowires. By appropriate n-type doping, the ZT value can reach as high as2.5at350K for the [210] nanowire with the width N=5.