The in-plane thermal and electrical conductivities of several suspended polycrystalline gold nanofilms with thickness of 20.0–54.0 nm have been measured simultaneously at 100–310 K. Both the thermal and electrical conductivities drop greatly compared to the corresponding bulk value, and the electrical conductivity reduction is larger. Fits to the temperature-dependent electrical conductivity confirm that the scattering of electrons by softened phonons is significant and cannot be reconciled with the classical size-effect model considering only surface and grain boundary. Taking into account the enhanced electron-phonon scattering, the electrical conductivity is well predicted over the whole temperature range and the obtained Debye temperature agrees well with the calculated value from the elastic continuum model. Furthermore, a new model on the thermal transport of metallic nanofilm is proposed based on the Energy Conservation Law, in which the electron-phonon scattering induced electron energy decrease is supposed to be counteracted by the phonon energy increase. The present model greatly improves the prediction of thermal conductivity in thin films compared to the corresponding result directly from electrical thermal analogy applied to bulk metals.

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