Analysis and computational design of thermoelectric materials using low-dimensional structural properties

Abstract

We have studied the electronic structure of thermoelectric materials using the first-principles electronic structure calculation based on density functional theory (DFT). In the transport calculation, Boltzmann transport equation (BTE) are used. This thesis consists of two parts: the first part discussed the potential of the A2Sb4Se8 (A = K, Rb, Cs) to be good thermoelectric (TE) materials, and second part suggested that a strategy to improve TE properties of MoS2 monolayer through the formation of MoS2-graphene heterostructure.

Part Ⅰ. We investigated the thermoelectric properties of the layered ternary selenides A2Sb4Se8 (A = K, Rb, Cs) and the lattice thermal conductivity of K2Sb4Se8 on the basis of DFT calculations, to find that these selenides are a high-performance n-type thermoelectric material. The Seebeck coefficients and power factors calculated for the electron carriers of A2Sb4Se8 (A = K, Rb, Cs) are greater than those of the well-known thermoelectric material Bi2Te3 and PbTe. The lattice thermal conductivity klatt of K2Sb4Se8 is comparable to that of PbTe, well-known for its low lattice thermal conductivity. In terms of both electronic and phonon structures, the structural parts of the A2Sb4Se8 (A = K, Rb, Cs) phases crucial for the thermoelectric properties are the Se2 dimer units of their conformationally-flexible Sb-Se-Se-Sb bridges.

Part Ⅱ. We investigated the electronic and thermoelectric (TE) properties of MoS2-graphene heterostructure. The electronic structures of MoS2-graphene heterostructure are mainly contributed from graphene and MoS2 monolayer for the valence band maximum and conduction band minimum, respectively. Because of the change in the electronic structures near the Fermi level, the calculated Seebeck coefficients S and electrical conductivity sigma of MoS2-graphene are largely affected from those of graphene and MoS2 monolayer. Its power factor S2sigma is increased compared to those of graphene and MoS2 monolayer at the electron concentration of 1011~1012 cm-2, which corresponds to a three-dimensional concentration of ~ 3×1018~3×1019 cm-3. We also demonstrated that MoS2 monolayer shows the p-type TE behavior while MoS2-graphene heterostructure is given to n-type TE material. Current study provides a strategy to improve TE properties of MoS2 monolayer through the formation of MoS2-graphene heterostructure.