Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene.

Momchil T Mihnev,Marco Polini,Charles J Divin,John R Tolsma,Theodore B Norris,Reza Asgari,Dong Sun,Claire Berger,Walt A De Heer,Allan H Macdonald

doi:10.1038/ncomms9105

Abstract

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.

Highlights

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers
We show that Coulomb scattering between electrons in lightly doped (LD) and highly doped (HD) layers of multilayer epitaxial graphene (MEG) can provide an efficient means for interlayer thermal coupling, and provide an alternate mechanism for cooling of hot electrons in the LD layers that acts in parallel with acoustic-phonon-mediated intralayer cooling
The ratio of cooling powers Qel=Qsc is plotted in Fig. 1d as a function of electron temperature for EF,HD 1⁄4 300 meV, EF,LD 1⁄4 10 meV and various values of the LD-layer disorder mean free path

Summary

Introduction

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. That Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We show that Coulomb scattering between electrons in LD and HD layers of MEG can provide an efficient means for interlayer thermal coupling, and provide an alternate mechanism for cooling of hot electrons in the LD layers that acts in parallel with acoustic-phonon-mediated intralayer cooling We show that the calculated dynamics and trends with lattice temperature and number of epitaxial graphene layers are fully consistent with the experimental results, without the need for any fitting parameters

Objectives

Results

Conclusion