Transport Properties of Nanoscale Systems with Nanogap for Silicon Clathrate on Density Functional Theory and Non-Equilibrium Green’s Function Method

Recently, nanostructured materials or nanocomposites, rather than thin films or superlattices, are of increasing interest in creating a new material with high thermoelectric figure of merit. The approach of nanoscale control of materials by introducing nanostructures, such as nanoinclusions, nanointerfaces, etc., may have a significant influence on the transport properties due to the energy filtering effect of the potential barrier at interfaces, or strong scattering effect of phonons and/or carriers at interfaces, whose density increases with decreasing size of structure. On the other hand, it is also of importance to elucidate the mechanism of enhancement in the Seebeck coefficient for nanostructured material systems. The density functional theory (DFT) using non-equilibrium Green’s function (NEGF) method may be a powerful tool to calculate the transport properties of nanoscale systems. There are, however, few studies of nanoscale system for thermoelectric clathrates by DFT using NEGF method. Thus, we adopt the DFT using NEGF method to calculate the transport properties of nanoscale system of Ba8Au6Si40/nanogap/Ba8Au6Si40, where nanogap acts as a potential barrier, as a model of nanoscale clathrate Ba8Au6Si40 system, to investigate the effect of nanointerface on the transport properties of thermoelectric clathrates. For Ba8Au6Si40/nanogap/Ba8Au6Si40 system, the Seebeck coefficient value at EF is greatly enhanced. The electrostatic potential difference was found to be large at the nanogap for Ba8Au6Si40/nanogap/Ba8Au6Si40 system. The calculation suggests that the potential barriers at nanointerface, such as grain boundary, have a significant influence on the Seebeck coefficient. We also discuss the effect of nanointerface on the electron and thermal conductance.