Electronic structure and thermoelectric properties of narrow band gap chalcogenides

In recent years there have been a revival of interest in discovering and understanding the physical properties of novel thermoelectric (TE) systems with high figure of merit. These systems are primarily narrow band gap semiconductors. In this thesis, electronic structure calculations were carried out for several narrow band gap chalcogenide TE materials in order to understand their electronic and transport properties governing their TE characteristics. These calculations were performed within ab initio density functional theory (DFT) using full potential linearized augmented plane wave (FLAPW) method. Transport calculations were carried out using Boltzmann transport equations.; For the binary chalcogenides Bi2Se3 and Bi 2Te3 I have studied the effect of quantum confinement (QC) created by the surfaces on their bulk electronic structure. In the presence of such confinement, surface states appear, which are a consequence of the strong influence of the interlayer bonding on the bulk electronic structure of these compounds. I find that in contrast to standard belief, there is an important covalent contribution to the interlayer bonding besides the Van der Waals contribution.; (Bi2Te3)m(Sb2 Te3)n superlattices (SL) show very good TE properties at room temperature. To see how the electronic structure of Bi2Se3 and Bi2Te3 are affected by the formation of SL, I have investigated the electronic properties of (Bi 2Te3)m(Sb2Te 3)n SL as compared to those of Bi2 Se3 and Bi2Te3 bulk systems. We find that the formation of SL does not deteriorate the electronic transport properties along the cross plane direction.; Complex ternary K2Bi8Se13 system shows great potential for superior TE performance. This compound forms in two distinct phases, alpha and beta. The beta-phase, which has two sites with K/Bi disorder, is a better TE. The calculations show that alpha-phase is an indirect band gap semiconductor. For the beta-phase, we find that the atoms at the "mixed sites" are very important in determining the electronic properties. The incorporation of the K/Bi mixed occupancy at the disordered sites is crucial for the semiconductor behavior. We also find a strong anisotropy in the hole and electron effective mass.; Complex quaternary AgPbmSbTe2+ m (LAST-m) systems are excellent high temperature TE. These systems form in the rocksalt structure similar to PbTe where Ag and Sb occupy Pb sites. There are clear experimental evidences that LAST-m systems exhibit microscopic inhomogeneities rich in Ag-Sb embedded into a PbTe matrix. Our calculations show that resonant states appear near the PbTe band gap. The common feature of all Ag-Sb arrangements is that they have a more enhanced density of states (DOS) near the gap as compared to PbTe. To see how these features in the DOS affect the transport properties I have carried out transport calculations in PbTe and LAST-m systems. The results for PbTe show that the temperature dependence of the effective mass md is very important in order to have good agreement with experiment. The LAST-m systems show an enhancement of the power factor (S 2sigma) relative to PbTe. But this enhancement is not large enough to explain the experimentally observed ZT. This suggests that the reduction in the thermal conductivity caused by Ag-Sb nanostructures in PbTe matrix may be significant.