Reliability of Raman measurements of thermal conductivity of single-layer graphene due to selective electron-phonon coupling: A first-principles study

Abstract

Raman spectroscopy has been widely used to measure thermal conductivity (κ) of 2D materials such as graphene. This method is based on a well-accepted assumption that different phonon polarizations are in near thermal equilibrium. However, in this work we show that in laser irradiated single layer graphene, different phonon polarizations are in strong non-equilibrium, using predictive simulations based on first principles density functional perturbation theory (DFPT) and a multi-temperature model. We first calculate the electron cooling rate due to phonon scattering as a function of the electron and phonon temperatures, and the results clearly illustrate that optical phonons dominate the hot electron relaxation process. We then use these results in conjunction with the phonon scattering rates computed using perturbation theory to develop a multi-temperature model, and resolve the spatial temperature distributions of the energy carriers in graphene under steady state laser irradiation. Our results show that electrons, optical phonons, and acoustic phonons are in strong non-equilibrium, with the ZA phonons showing the largest non-equilibrium to other phonon modes, mainly due to their weak coupling to other carriers in suspended graphene. Since ZA phonons are the main heat carriers in graphene, we estimate that neglecting this non-equilibrium leads to underestimation of thermal conductivity in experiments at room temperature by a factor of 1.35 to 2.6, depending on experimental conditions and assumptions used. Underestimation is also expected in Raman measurements of other 2D materials when the opticalacoustic phonon coupling is weak.

Revisiting phonon-phonon scattering in single-layer graphene

“…While most PBTE studies used IFCs from first-principles as inputs and tried to match the measured experimental data, we employ the empirical potential to generate the interatomic force constants. This is because the thermal conductivity measurements on two-dimensional materials are still quite challenging and there is still no consensus on the accuracy of the existing measurement methods [42]. Instead, using the empirical potential, we could compare the results from PBTE studies with the thermal conductivity predictions made by MD simulations directly, and identify the influence of four-phonon scatterings and temperature dependent IFCs on phonon transport in graphene.…”

Phonon branch-resolved electron-phonon coupling and the multitemperature model

“…Electron-phonon (e-p) coupling is an important physical phenomenon that affects Joule heating, laser-matter interaction, and hot electron relaxation [1][2][3][4][5][6][7][8]. A thorough understanding of e-p coupling is crucial for researching material properties as well as advancing the electronic device technology.…”

Colloquium: Phononic thermal properties of two-dimensional materials

“…Any wrong calculation of these two factors could result in a significant systematic error. The first one has been known for many years, however, the second one has only been investigated very recently by Vallabhaneni et al (Vallabhaneni, et al, 2016). In their detailed computational study, Vallabhaneni et al pointed out that the ZA and ZO phonons that contribute the most to the thermal conductivity of single layer graphene are indeed out of thermal equilibrium.…”