Raman Spectroscopy on Graphene Encapsulated in Hexagonal Boron Nitride

In this thesis, magneto-Raman spectroscopy and laser-induced doping experiments on graphene samples are shown. The graphene crystals are encapsulated via dry, purely mechanical transfer processes in van der Waals heterostructures, where hexagonal boron nitride and tungsten diselendie are used as substrate materials. The intrinsically low charge carrier doping and high mobilities that result from the fabrication process manifest themselves in the observability of a coupling of optical phonons to Landau level excitations at low magnetic fields. From these magneto-phonon resonances, we analyze the significance of many-body interaction effects for the magneto-optical spectrum of graphene. In fact, the measurements show that the excitation energies of Landau level transitions in our structures deviate from the values expected from a single particle picture by up to 15%. Furthermore, we identify characteristic life times of the Landau level excitations which depend on the magnetic field and/or the Landau level indices of the involved states. By utilizing the magnetic field to suppress the coupling of the Raman G peak of graphene to the electronic system, we identify strain variations within the laser spot to be a major broadening mechanism of the relevant Raman peaks. As a consequence, Raman spectroscopy is a highly suitable tool for sample characterization in graphene research, since such short-ranged strain variations have been shown to be a limiting factor for the charge carrier mobilities in high-quality graphene samples. In the second part of the thesis, we focus on the implementation of functional graphene devices in van der Waals heterostructures exploiting a recently reported strong photo-induced doping effect in heterostructures of graphene and hexagonal boron nitride. By employing a focused, green laser we are able to obtain any desired doping profile in our samples with this technique. The doping profiles can be fully erased and rewritten. The technique is noninvasive for the electronic quality of the sample and the spatial resolution is solely limited by the dimensions of the laser spot. The laser induced doping technique can be highly relevant for the implementation of graphene based electron-optics devices and can be likely extended to other van der Waals heterostructures, where hexagonal boron nitride is used as a substrate material