Influence of grain boundary scattering on the electrical and thermal conductivities of polycrystalline gold nanofilms

The electrical and thermal conductivities of polycrystalline gold nanofilms have been measured simultaneously by a direct current heating method, and the measured results are compared with the Mayadas and Shatzkes theory. It is found that the reduced electrical and thermal conductivities of gold nanofilms are strongly dominated by grain boundary scattering. The reflection coefficient of electrons striking the grain boundaries for charge transport is 0.7, which agrees well with a previous scanning tunneling potentiometry study. The reflection coefficient for thermal transport, however, is only 0.25. The Lorenz numbers for the polycrystalline gold nanofilms, which are calculated from the measured electrical and thermal conductivities, are much greater than the value predicted by the Wiedemann-Franz law for the bulk material. The results indicate that the electron scatterings on the grain boundaries impose different influences on the charge and heat transport in the polycrystalline gold nanofilms. A model of effective density of conduction electrons has been utilized to interpret the violation of the Wiedemann-Franz law in polycrystalline gold nanofilms.