Mechanisms of nonequilibrium electron-phonon coupling and thermal conductance at interfaces

Abstract

We study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an $3 nm Ti adhesion layer between the Au film and the nonmetal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate with temperature, which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electroninterface scattering is shown to have negligible influence on the Kapitza conductances between the Au/Ti and the substrates at longer time scales when the electrons and phonons in the metal have equilibrated. These results suggest that only during highly nonequilibrium conditions between the electrons and phonons (T e ) T p ) does electron-phonon scattering at an interface contribute to thermal boundary conductance. V C 2015 AIP Publishing LLC. [http://dx.

Thermal interface conductance across metal alloy–dielectric interfaces

“…To determine the size of the error bars, the FDTR-measured phase-lag vs. heating frequency data was fit to the heat diffusion equation to extract the deviations in G when At a metal-dielectric interface, electrons that carry heat within the metal transfer it to phonons that transmit the energy across the interface in a process known as electron-phonon coupling [41]. Electron-phonon coupling has been modeled as a thermal resistance process that is in series with the phonon energy transmission across the interface [42][43][44]. The values of G reported in Figure 2 represent the composite G due to the electron-phonon coupling conductance (G e-p ) and the phonon transmission conductance (G p ).…”

A Review of Experimental and Computational Advances in Thermal Boundary Conductance and Nanoscale Thermal Transport across Solid Interfaces

“…These pathways are i) metal electron–metal phonon coupling at the metal/nonmetal interface ( h e−mp ), ii) phonon–phonon coupling across the interface ( h p−p ), and iii) metal electron–nonmetal phonon coupling directly across the interface ( h e−nmp ). At a metal/nonmetal interface, electronic contributions to interfacial conductance have been mostly suggested to be nonexistent under equilibrium conditions . The interest in this topic was triggered by the seminal work from Stoner and Maris where they reported measurements of h K between a series of metals and nonmetals to which they compared with various phonon–phonon interface models and found discrepancies between theory and experiments .…”

“…However, recent theoretical works based on the two‐temperature model have suggested that strong electron–phonon coupling in the metal can lead to an increase in the phonon–phonon thermal conductance, while for weak electron–phonon coupling in the metal, this resistive pathway could become significant . There have also been several experimental works that have argued that under time scales where there is strong nonequilibrium between electrons and phonons in the metal, h e−nmp could potentially increase the rate of energy exchange at the interface . For example, Giri et al studied electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump–probe technique.…”

Thermal Conductivity Reduction in a Nanophononic Metamaterial versus a Nanophononic Crystal: A Review and Comparative Analysis

“…The notion of a surface oxidized membrane acting as an NPM—with the disordered oxygen atoms covering the membrane surface acting as the nanoresonators—was proposed by Xiong et al NPMs in the form of branched nanoribbon materials composed of molybdenum disulfide (MoS 2 ) were studied by Liu et al Giri and Hopkins and Yang and co‐workers extended the NPM concept to carbon nanotubes and graphene sheets, respectively. Another intriguing NPM architecture is based on a graphene sheet with branching fullerene nanoresonators . Other than thermal conductivity reduction, the impact of nanopillars on the heat capacity was investigated by Iskander et al using both theory and experiments .…”