Different effects of grain boundary scattering on chargeand heat transport in polycrystalline platinum and goldnanofilms

The in-plane electrical and thermal conductivities of severalpolycrystalline platinum and gold nanofilms with differentthicknesses are measured in a temperature range between the boilingpoint of liquid nitrogen (77K) and room temperature by using thedirect current heating method. The result shows that both theelectrical and thermal conductivities of the nanofilms reducegreatly compared with their corresponding bulk values. However, theelectrical conductivity drop is considerably greater than thethermal conductivity drop, which indicates that the influence of theinternal grain boundary on heat transport is different from that ofcharge transport, hence leading to the violation of theWiedemann--Franz law. We build an electron relaxation model based onMatthiessen's rule to analyse the thermal conductivity andemploy the Mayadas & Shatzkes theory to analyse the electricalconductivity. Moreover, a modified Wiedemann--Franz law is providedin this paper, the obtained results from which are in good agreementwith the experimental data.