Impact of the substrate on charge carrier mobility and interaction effects in graphene

Graphene combines exceptionally high charge carrier mobility with a strong sensitivity of its electronic properties to the environment, a combination that makes it an interesting material for a wide range of sensing and high-frequency electronics. The full potential of graphene unfolds in van der Waals heterostructures, where extrinsic disorder can be reduced and many-body interaction effects can be tuned. This thesis addresses two aspects of graphene-based van der Waals heterostructures: (i) the impact of the substrate and interface quality on the structural and electronic properties of graphene, and (ii) the role of the dielectric environment in modifying interaction effects. We begin by assessing the suitability of boron nitride (BN) produced by different growth methods as a substrate for graphene. Using cathodoluminescence, Raman spectroscopy and charge transport measurements, we analyze the structural properties of BN and the electronic properties of graphene on these BN substrates. We confirm the superior crystal quality of hexagonal BN (hBN) grown at atmospheric pressure and high temperature. At the same time, we identify nanocrystalline BN grown via physical vapor deposition as a promising scalable approach for achieving high mobility in graphene. We establish the graphene Raman 2D line width as a key parameter for benchmarking the suitability of BN as a substrate for graphene. In the second half of this thesis, we shift our focus to the role of dielectric screening in tuning interaction effects. We first study electronically-decoupled 30°-twisted bilayer graphene with interlayer spacings in the sub-nm range via magneto-Raman spectroscopy to extract the strength of electron-electron interactions and show the sensitivity of graphene to small changes in the dielectric environment. We then perform a systematic investigation of the graphene Raman 2D peak position, as a sensitive and accessible probe for characterizing electron-electron and electron-phonon interactions. For this, we use a novel theoretical framework, which we confirm by magneto-Raman and excitation-energy dependent Raman measurements. We further expand the picture of interaction effects by analyzing how the doping dependence of the Raman spectrum is modified by dielectric screening, leading us to revisit the widely used method for strain and doping and the importance of adding dielectric screening for future device characterization. The high quality of our samples allows us to further study the 2D peak shape as function of screening due to the dielectric environment and due to charge carriers. This improves the understanding of details of the Raman 2D peak process. Finally we present charge transport measurements on graphene capped with a thick top layer of WSe₂, which reveal extraordinary high room temperature mobility exceeding state-of-the-art hBN-encapsulated graphene devices. This is linked to a weaker temperature dependence of the resistivity and points towards a modification of the electron-phonon interaction in graphene.