Exploring strain fields, electron-phonon coupling and many-body effects in graphene via Raman spectroscopy

RWTH Publications (RWTH Aachen)

Abstract

Raman spectroscopy has become one of the most important characterization techniques for the rapidly developing research field of graphene and related 2D materials, as it allows to probe vibrational, mechanical and electronic properties rapidly, simultaneously and non-invasively. This thesis aims to further the knowledge of Raman spectroscopy in graphene, especially in regards to the effect of electron-phonon coupling, strain fields and many-body effects. We begin by thoroughly investigating the charge carrier density-dependent Raman response of graphene encapsulated in hexagonal boron nitride, which so far has been restricted by photo doping effects. By employing a metallic local gate, we are able to observe the non-adiabatic electron-phonon interaction and the anomalous phonon softening of the G-mode phonon. Enabled by the absence of detrimental strain variations, we are able to study the pristine line shape of the 2D-peak and its evolution with charge carrier density. As a result, we are able to assign the individual spectral contributions of the 2D-peak to their physical scattering processes. In the following set of experiments, we utilize the resonant coupling of the G-mode optical phonons to Landau level excitations. By analyzing these magneto-phonon resonances (MPRs) in suspended graphene we show that the electronic energy spectrum in the quantum Hall regime deviates by up to 30% from the single-particle expectation due to electron-electron interactions. This is in contrast to the diverging renormalization of the electron dispersion found at low magnetic fields, which we link to the spatial localization of the electron wave functions by the magnetic field. MPRs could further be an useful tool to study strain-gradient induced pseudo-magnetic fields. In a proof-of-principle experiment we show that the application of high amounts of strain does not limit the electronic quality of graphene and thus the observation of MPRs. Furthermore, the combination of uniaxial strain and doping-induced circular phonon dichroism allows us to change the polarization of phonons purely by electrostatic fields. To gain a more precise control over the applied strain fields we develop a novel, silicon-based MEMS platform for in situ and purely mechanically tuning of strain and strain gradients in graphene. Via scanning Raman spectroscopy, we show that our actuators reliably and reproducibly induce strain up to the breaking strength of graphene. We find a surprisingly low rupturing strength of graphene of less than 0.3% strain, which we attribute to defects located at the edges of the graphene flakes. By engineering the clamping geometry, we are able to achieve significantly higher strain values and are simultaneously able to create large strain gradients, which correspond to pseudo-magnetic fields of up to 120 mT.

Authors 1

1. Jens Sonntag corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen